Toward Circular and Sustainable Urban Wastewater Treatment: Integrating Adsorption and Advanced Oxidation Processes

Abstract

Wastewater treatment is fundamental to sustainable urban development, and recent European legislation now requires quaternary treatment of final effluent at wastewater treatment plants. Among the technologies evaluated for this purpose, adsorption and Advanced Oxidation Processes (AOPs) have demonstrated the highest removal efficiencies, and the ongoing shift toward more circular and sustainable urban wastewater management, positioning them as two of the most strategically significant technologies in the field. Quaternary treatments with ozonation and activated carbon adsorption (PAC/GAC) achieve median micropollutant removal above 80%. GAC is generally associated with the highest costs, followed by PAC and ozonation, typically in the range of approximately 0.035 to 0.3 € per cubic meter in European settings. This review presents a systematic comparison of adsorption and AOPs for the removal of urban wastewater pollutants, with emphasis on removal efficiency, energy requirements, carbon footprint, and operational limitations. It delineates the conditions under which each technology demonstrates superior performance and highlights its complementary strengths across different contaminant groups and treatment objectives. Beyond conventional performance indicators, the review frames these technologies as key enablers of circular wastewater treatment through material regeneration, resource recovery, and process integration.

1. Introduction

Urban areas are now widely recognized as major contributors to global environmental pressures, making the sustainability of urban systems a central element of the international sustainability agenda [1]. Although the field has expanded substantially, conceptual debates remain active in the academic literature, and cities continue to face numerous complex challenges in practice [2]. Key pressures on urban regions worldwide intensify demands on water resources [3]. Water forms a critical nexus at the center of sustainable development [4] because it supports socio-economic progress and influences the achievement of all Sustainable Development Goals (SDGs) [5].

With accelerating urbanization, the problem of urban domestic wastewater has become increasingly severe [6]. Additionally, the combined effects of rapid urban growth, insufficient solid waste management, and industrial discharges have intensified water pollution, threatening aquatic environments and reducing access to safe water for human consumption [7]. A significant portion of global water sources is now heavily polluted, largely due to anthropogenic activities originating from both point and diffuse pollution sources, such as industrial outflows and sewage contamination [8]. Urban wastewater treatment plants are key hotspots for the release of contaminants of emerging concern, and they play a major role in the dissemination of antibiotic resistance [9]. Consequently, urban wastewater frequently contains emerging pollutants, including pharmaceuticals, pesticides, heavy metals, pathogens, organic compounds, PCBs, PFASs, and related substances that are not effectively removed by conventional treatment technologies [10]. Their occurrence within wastewater treatment plants serves as a clear indicator of the increasing urbanization of the water cycle [11]. Given their ecotoxic characteristics, many urban effluents exhibit toxic effects on aquatic organisms [12]. Scientific findings have increased stakeholder and policy maker awareness of the risks linked to releasing contaminants of emerging concern into the environment [13].

To address these challenges, cities are increasingly considering treated wastewater reuse as a way to ease pressure on freshwater resources and strengthen the resilience and sustainability of urban water systems [3]. Water reuse is gaining increasing prominence within circular economy frameworks [10]. Although circular economy principles originated in industrial ecology and production systems, they are now receiving considerable attention in urban contexts [14], and the recovery and reuse of treated wastewater have been identified as essential priorities for developing sustainable and resilient water systems [15].

Although treatment methods vary by region [16], conventional urban wastewater treatment plants still release bioavailable contaminants, creating an ongoing challenge [17]. The ongoing discharge of nutrients, especially nitrogen and phosphorus, contributes to eutrophication in receiving water bodies [18], particularly in low-order streams that experience intermittent flow conditions [19]. The revised Urban Wastewater Treatment Directive (UWWTD), which entered into force in November 2024 [20], introduces mandatory additional treatment, referred to as quaternary treatment, alongside enhanced nutrient removal requirements for nitrogen, potassium, and phosphorus, which are essential for crop productivity and soil health [21], as well as more stringent limits on micropollutants [22]. These limits, which are primarily implemented at wastewater treatment plants, target the removal of 80% of concentrations of contaminants of emerging concern and are verified through monitoring a selected set of representative micropollutants [23]. Considerable research efforts have focused on synergistic combinations of treatment technologies, including hybrid advanced oxidation processes (AOPs) [24,25] and hybrid AOPs integrated with biological treatment systems [26]. However, among the technologies assessed for meeting UWWTD removal targets, adsorption and ozonation have demonstrated the highest removal efficiencies [23]. The cost of advanced wastewater treatment varies substantially depending on the initial configuration, scale, and operating conditions of the treatment plant. In European settings, estimates of capital and operational expenditures indicate that granular activated carbon typically represents the most expensive option, followed by powdered activated carbon and ozonation, with reported treatment costs in the range of approximately 0.035–0.05 €·m⁻³ of treated wastewater [27]. In other regional or operational contexts, however, significantly higher costs have been reported. For example, a techno-economic assessment conducted in France estimated ozonation costs of 0.1–0.2 €·m⁻³ and activated carbon costs of 0.2–0.3 €·m⁻³, depending on plant size, initial design, operational conditions, and reagent supply chains [28]. More recent evidence from full-scale applications indicates that the costs of ozonation for contaminant removal have declined, with current estimates approaching 0.25 €·m⁻³ at an applied ozone dose of approximately 9.5 g O₃·m⁻³ [27]. Astrid Fischer emphasized that the evaluation of removal efficiency studies would benefit from a structured framework for assessing scientific and technical evidence, including clearly defined criteria for relevance and reliability across technologies [29]. Despite encouraging performance reported in the literature [6,30], integrated frameworks for circular wastewater treatment remain limited, constraining the systematic investigation of synergistic treatment strategies. Moreover, techno-economic assessments continue to be complex and resource-intensive, as they depend on multiple interrelated parameters. Within this context, the absence of unified evaluation criteria and the divergence in technical and economic characteristics among treatment technologies represent an opportunity to identify synergistic potential, reveal complementary functionalities, and guide the transformation of urban wastewater treatment toward improved sustainability.

This review offers new insights into these two processes by analyzing the impact of various operational conditions. Building on this foundation, this work aims to evaluate the synergies between adsorption and advanced oxidation in quaternary urban wastewater treatment, while identifying their technical and economic limitations under the new urban wastewater treatment directive (UWWTD) requirements.

The review is organized into several thematic sections beyond the introductory context. Section 2 provides an overview of the major pollutant classes in urban wastewater and the key parameters used for their characterization. Building on this, Section 3 evaluates conventional treatment technologies, highlighting their operational limitations and exploring emerging opportunities for wastewater reuse in urban systems. Section 4 and Section 5 then focus on adsorption and AOPs, detailing their underlying mechanisms, methodological approaches, performance constraints, and recent technological advancements. Section 6 expands the discussion to the roles of adsorption and advanced oxidation processes (AOPs) in promoting circular urban wastewater management within urban wastewater management. Section 7 discusses existing knowledge gaps and provides recommendations for future research, while the final section presents the overall conclusions of the study.

Methodology of the Literature Review

An automated search strategy was applied due to the broad research scope and large volume of relevant studies [31]. Scopus was selected as the primary database because of its wide disciplinary coverage and its support for systematic and reproducible searches [32,33]. A predefined search string targeting adsorption and advanced oxidation processes in quaternary treatment of urban wastewater was applied to titles, abstracts, and keywords, with searches completed in November 2025. Retrieved studies were subsequently screened using predefined inclusion criteria based on Scopus metadata, retaining only English language research articles published within the defined study period, while excluding reviews and other publication types.

2. Urban Wastewater Pollutant Types

Pharmaceuticals, personal care products, PFAS, microplastics, engineered nanomaterials, and pathogens have become prominent contaminant groups because of their persistence, their capacity for bioaccumulation, and their detrimental impacts on ecological systems and human health [34]. Figure 1 presents the major pollutant categories typically found in urban wastewater. The figure lists nutrients, pathogens, heavy metals, organic pollutants, and suspended solids, with labels indicating associated environmental and public health risks.

Table 1. Concentration ranges in urban effluents.

Detected Compounds	Concentration Range
TSS [36].	190–537 mg/L
Nitrogen [37].	4 and 74 mg/L
Phosphates [37].	4–14 mg/L
Organophosphates, fluorescent whitening agents, phthalates, phenolic substances [35].	~10–1000 ng/L
(PAHs) and UV filters [35].	>10 ng/L
Per- and polyfluoroalkyl substances (PFAS) [35].	0.1 to 5 ng/L
Metals [38].	Copper 19.8 to 541 µg/L,
	Nickel 2.4 to 17 µg/L,
	0.41 to 4.60 µg/L Cobalt
	38 to 670 µg/L Zinc
	18 to 24 µg/L Cadmium
	1.07 to 10 µg/L Lead
	0.05 to 6.5 µg/L Silver
	0.20 to 0.86 µg/L Gold
	0.08 to 11 µg/L Mercury
Antibiotics [38].	1.66–1.38 × 104 µg/L Beta-lactam antibiotics
	6.4 × 104 µg/L cephalosporins,
	6 × 103–7 × 103 µg/L streptomycin,
	0.46–5.6 × 103 µg/L sulfonamides,
	0.74–7.9 × 103 µg/L quinolones,
	3.16 × 10−3–2.2 × 103 µg/L tetracyclines,
	0.13–1.0 × 104 µg/L macrolides,
	1.4 × 103–2.9 × 103 µg/L ciprofloxacin,
	6.5 × 102–7.3 × 102 µg/L norfloxacin,
	4.2 × 102–6.5 × 102 µg/L ofloxacin and
	0.06–7.9 × 103 µg/L metronidazole
Non-antibiotic pharmaceuticals [38].	0.1–3.0 µg/L ketoprofen,
	0.7–344 µg/L acetaminophen
	60–389 µg/L caffeine,
	0.04–0.96 µg/L metoprolol,
	0.04–1.4 µg/L N,
	N-diethylaniline,
	0.06–2.5 µg/L carbamazepine,
	0.8–0.7 µg/L domperidone,
	0.03–2.8 µg/L benzotriazole
	0.01–0.2 µg/L gatifloxacin,
	0.8–1.3 µg/L irbesartan,
	16–19 µg/L valsartan,
	3.6–9.2 µg/L metformin
	0.2–4.1 µg/L fexofenadine
MP [38].	0.1 to 0.5 mm

presents concentration ranges in urban effluents. The variability in effluent data arises from multiple factors, including differences in sampling strategies, analytical methodologies, and wastewater treatment plant (WWTP) characteristics at both facility and national levels. Plant-specific parameters that govern influent composition and removal efficiency include the number of connected inhabitants and population equivalents relative to the treated wastewater volume, the type of WWTP, such as combined or separate sewer systems, the treatment configuration and level, as well as operational conditions including temperature, seasonality, and diurnal fluctuations. Depending on their predominant sources within society, the concentrations of certain compounds in effluents scale with the number of connected inhabitants, whereas others are more strongly influenced by the proportion of industrial wastewater and the types of industries discharging into the system. In addition, socioeconomic factors, including economic prosperity, purchasing power, and associated consumption patterns, can substantially affect the emission profiles of chemical contaminants [35].

2.1. Organic Compounds

Xenobiotic organic compounds, including pharmaceuticals, personal care products, pesticides, and flame retardants, constitute contaminants of emerging concern (CECs) that are continuously released into the environment [39], often at ng/L to μg/L levels, yet capable of causing measurable toxic effects [40,41]. Pesticides represent one of the most significant contaminant groups affecting aquatic ecosystems globally [42]. They are extensively applied in agriculture, industry, and various sectors to manage pest populations and unwanted vegetation. Entry pathways into aquatic environments include agricultural runoff, soil leaching, and atmospheric deposition. Per- and poly-fluoroalkyl substances (PFAS) are highly persistent anthropogenic compounds widely used in industrial processes, firefighting foams, and consumer products, leading to their ubiquitous presence in wastewater and the environment, along with potential health risks [34]. Treated effluents from wastewater treatment plants frequently contain PFAS due to the limited removal capability of conventional treatment processes, making these facilities important pathways for PFAS emissions into aquatic environments. Documented sources of PFAS entering urban wastewater systems include consumer product disposal, stormwater runoff, the use of aqueous film-forming foams, and atmospheric deposition [43].

Although PFAS research in urban wastewater is extensive, existing studies mainly focus on the occurrence and fate of PFAS during wastewater treatment processes [44,45]. Pharmaceutical compounds and their transformation products constitute another major group of emerging organic contaminants because of their extensive global use and the potential risks they pose to human and ecosystem health. Pharmaceuticals enter aquatic environments via treated and untreated wastewater from households, industrial facilities, livestock, aquaculture, and hospitals [46]. Antibiotics are of particular concern due to their contribution to antibiotic resistance [47]. Although organic micropollutants are typically present at low concentrations (nmol/L to μmol/L), they pose significant risks to ecosystems and human health, with studies showing that antidepressants can alter fish behavior [48], and non-steroidal anti-inflammatory drugs like diclofenac can impair kidney function and disrupt ovulation in fish [46].

2.2. Heavy Metals

Human activities, including industrial operations, urban development, and infrastructure expansion, are major sources of heavy metals such as mercury, lead, cadmium, and arsenic in aquatic environments, often contributing one to three orders of magnitude more than natural sources [49].

2.3. Microplastics & Nanoparticles

Microplastics (typically 1–5 mm) and nanoplastics (usually <1 µm) pose significant environmental risks, with urban wastewater treatment plants acting as major release points due to a lack of regulatory requirements [9,50]. Synthetic microfibers, primarily polyester (PET) from textiles, domestic laundry, and commercial washing, are the most commonly detected microplastics in wastewater effluents [51]. Their environmental release is particularly concerning because microplastics and microfibers can be unintentionally ingested and cause adverse effects in a wide range of organisms, including through trophic transfer within the food chain [52]. Although wastewater treatment plants receive these particles through sewage and are not specifically engineered to remove them, they still achieve partial removal across multiple treatment stages. Concentrations of microplastics (MPs) and nanoplastics (NPs) in treated effluents remain elevated, ranging from 24 to 209% of influent levels, partly due to MP fragmentation during treatment processes. Both MPs and NPs are susceptible to fragmentation because weakened particle surfaces may fail under mechanical or chemical stress. Their formation and fragmentation pathways, physicochemical properties, and occurrence in aquatic environments are closely linked to interactions between nano and microplastics and water and wastewater treatment plant processes. The presence of NPs and MPs in water, therefore, raises concerns regarding the role of wastewater treatment processes in promoting MP fragmentation. In addition, most studies investigating NP and MP pollution in water focus exclusively on particles larger than 10 μm, without distinguishing agglomerates from individual particles or addressing how agglomeration and dispersion influence particle size distributions and concentration measurements [53]. MPs can adsorb heavy metals, persistent organic pollutants, and pharmaceuticals, posing risks to aquatic organisms by affecting feeding, nutrient uptake, and ecosystem stability, while also facilitating contaminant transfer across trophic levels. [34]. Nanoparticles also represent an emerging class of pollutants of concern in aquatic environments. These materials can bioaccumulate through multiple exposure pathways and pose significant risks to both human health and aquatic organisms [54]. Endocrine disrupting compounds (EDCs) likewise present serious ecological hazards because they interfere with hormonal regulation, leading to reproductive impairments, endocrine dysfunction, and developmental abnormalities in aquatic species [55].

2.4. Pathogens

Antibiotic-resistant bacteria (ARB) pose significant risks to human and animal health, with six pathogens, Escherichia coli, Staphylococcus aureus, Klebsiella pneumoniae, Streptococcus pneumoniae, Acinetobacter baumannii, and Pseudomonas aeruginosa, responsible for over 71% of AMR-related deaths [56]. Urban wastewater treatment plants are recognized as major sources of antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs) released into the environment [57]. However, studies addressing the simultaneous removal of organic matter and pathogens from real secondary treated wastewater remain limited. As wastewater treatment strategies increasingly align with circular economy principles, the ability to remove multiple classes of contaminants within a single treatment train is essential to reduce energy consumption, capital investment, and operational costs [58]. The secondary treatment stage in urban wastewater treatment plants is primarily designed for organic matter removal, during which ARG reduction occurs mainly through adsorption and biodegradation mechanisms. Adsorption is often facilitated by extracellular polymeric substances, whose cross-linked structure can selectively retain extracellular ARGs, while biodegradation involves enzymatic activity, such as nucleases produced during microbial metabolism, that can degrade genetic material [57]. Tertiary and advanced treatment processes are generally more effective in the removal or inactivation of ARBs and ARGs, including oxidation and disinfection methods such as ozonation and ultraviolet irradiation, as well as membrane filtration, which can degrade, inactivate, or physically retain resistant bacteria and their genetic material, thereby reducing their release in treated effluents [57]. Since primary and secondary treatment processes are not designed for pathogen inactivation, their effectiveness in removing antibiotic resistance is limited compared with advanced treatment technologies. The overall performance of advanced treatment systems depends on the specific types of ARBs and ARGs present, as well as operational parameters such as reaction time and applied dosage. Consequently, the integration of multiple treatment processes is necessary to achieve optimal removal, as combined systems can compensate for individual limitations and enhance overall ARB and ARG abatement [57].

Removal efficiencies achieved by adsorptive and oxidative treatment methods can differ substantially depending on the specific physicochemical properties of the target compounds [59]. For example, the presence of dissolved organic matter (DOM) in wastewater effluents can hinder the removal of organic micropollutants (µPs) by competing for adsorption sites and causing pore blockage in adsorbents [59]. The wide variability of micropollutants (µPs) and stricter discharge standards make activated carbon adsorption or ozonation alone insufficient for effective µP removal [60], so these processes are often used complementarily [59]. In combined schemes, activated carbon is typically applied post-ozonation, as ozonation reduces effluent organic matter’s hydrophobicity, molecular weight, and aromaticity, enhancing adsorption [59]. However, challenges remain: most studies use synthetic wastewater and non-representative conditions, limiting large-scale applicability; comprehensive evaluations of process sequencing or simultaneous application are lacking; integrating ozonation and adsorption into existing wastewater treatment infrastructure is difficult; assessments often ignore additional benefits such as by-product reduction and nutrient removal; and real-time monitoring strategies using surrogates for process optimization are underdeveloped, with research mainly focused on standalone systems [61].

3. Conventional Methods of Treating Urban Wastewater and Challenges

Water scarcity and pollution have emerged as critical global challenges, underscoring the urgent need for innovative and sustainable approaches to wastewater management. Although conventional wastewater treatment processes effectively remove many traditional contaminants, they face ongoing limitations that include high energy demands, substantial sludge generation, and reduced efficiency when addressing newly identified pollutants. These constraints have encouraged the adoption of advanced treatment technologies that can overcome such shortcomings and support long-term environmental sustainability. Technological advancements are crucial in wastewater treatment, offering efficient chemical, physical, and biological methods to remove contaminants from water sources [62].

Plants typically apply primary, secondary, and in some cases tertiary processes, which include nutrient removal, sand filtration, and disinfection. However, these treatments are generally ineffective for eliminating contaminants of emerging concern (CECs) [27]. To reduce the discharge of these substances into the environment, and to limit their accumulation in soils and crops when treated effluent is reused for irrigation, advanced treatment steps must be integrated into existing systems [63]. The European Commission has introduced the concept of quaternary treatment, referring to supplementary processing of urban wastewater aimed at removing the broadest spectrum of micropollutants [64]. A conventional wastewater treatment plant consists of primary mechanical treatment, secondary biological treatment, and tertiary advanced treatment, with the specific combination chosen based on contaminant type, pollutant concentration, and the intended use of the treated water [65].

• Primary treatment involves screening to remove large materials such as wood, cloth, and paper, thereby preparing the water for subsequent biological and advanced processes while preventing operational failures.
• Secondary treatment relies on microbial activity to reduce biological oxygen demand.
• Tertiary treatment provides further purification and is critical for determining the final water quality.

Nonetheless, secondary and tertiary stages in urban wastewater treatment plants are insufficient for removing many contaminants [63].

Conventional urban treatment plants remain major point sources for the release of emerging contaminants because they were not originally designed to target their removal. A significant concern relates to the combined effects of multiple pollutants in aquatic systems. In contrast to traditional contaminants with clearly defined toxicity thresholds, emerging contaminants interact with other chemicals, dissolved organic matter, and microbial communities, which can produce complex and unpredictable ecological impacts. Prolonged exposure to low levels of pharmaceuticals and pesticides can cause sublethal effects in aquatic organisms, and the incomplete removal of emerging contaminants in wastewater makes treated effluents a major source of pollution, as confirmed by national monitoring studies detecting antibiotics, synthetic hormones, and microplastics in sediments [34]. Although conventional treatment technologies are effective for fundamental purification, they exhibit notable limitations in efficiency, operational costs, and removal of emerging contaminants, which ultimately constrain opportunities for safe and sustainable water reuse.

Conventional wastewater treatment effectively removes solids, organic matter, and nutrients, often achieving 80–90% removal for organics and 60–90% for nutrients. However, it is often insufficient for emerging contaminants, whereas tertiary or advanced treatments can boost micropollutant removal to over 85%, sometimes reaching or exceeding 90% [66]. Integrated flow constructed wetlands combine multiple constructed wetland configurations and have demonstrated improved removal efficiencies compared with single wetland units, particularly for nutrients and other pollutants [67]. Choi [68] investigated the performance of Phragmites australis and Triarrhena sacchariflora, reporting sulfadiazine removal efficiencies of up to 81.86 percent, sulfamethoxazole removal of up to 85 percent, sulfamethazine removal of up to 49.43 percent, and trimethoprim removal of up to 2.32 percent at a hydraulic retention time of 48 h. In addition, Chrysopogon zizanioides has been applied for the removal of antibiotics, especially tetracycline and ciprofloxacin, from secondary wastewater effluents under high nutrient conditions, specifically nitrogen and phosphorus. Reported removal efficiencies ranged from 89 to 100 percent for tetracycline and from 60 to 94 percent for ciprofloxacin, with an observed inverse relationship between nutrient concentration and antibiotic removal efficiency [69]. Despite these improvements, conventional urban wastewater treatment plants remain largely ineffective in removing most contaminants of emerging concern, including antibiotics, antibiotic-resistant bacteria, and antibiotic resistance genes [70]. Moreover, conventional treatment approaches typically require substantial land area and are associated with considerable capital and operational costs. They also show limited effectiveness in separating emerging contaminants, persistent organic pollutants, and other recalcitrant substances. Additional concerns relate to the environmental footprint of conventional technologies, particularly the high energy demand associated with aeration in activated sludge systems [62].

Assessing Treatment Challenges and Reuse Opportunities Across Urban Wastewater Streams

Building on the limitations of conventional treatment methods, it is evident that different wastewater streams, such as gray water, black water, stormwater, and seawater [71] present distinct challenges that directly influence their potential for reuse.

Regarding rainwater or stormwater, this category represents both a supplementary water resource and a mechanism for flood mitigation in urban environments [72]. Household-scale treatment approaches, including boiling, filtration, and solar disinfection, have been shown to improve water quality and substantially reduce microbial loads, making them suitable options for small-scale applications [71].

Table 2. Overview of Major Wastewater Streams, Their Contaminants, Treatment Challenges, and Reuse Potential.

Wastewater Stream	Main Contaminants	Key Treatment Challenges	Reported Removal Efficiencies	Reuse Suitability	Ref
Greywater	Biodegradable organics, nutrients (N, P), pharmaceuticals, health & beauty products, aerosols, pigments, heavy metals (Pb, Ni, Cd, Cu, Hg, Cr), faecal coliforms, Salmonella	Highly variable composition; emerging contaminants; pathogens; reuse-specific quality standards differ regionally	COD removal: 73.7–82.9% (bio-photocatalyst)	Non-potable applications (irrigation, toilet flushing) after moderate treatment	[73,74]
Blackwater	High COD (~50% of domestic wastewater), N (~91%), P (~78%), suspended solids, pathogens (E. coli, Salmonella, Shigella, enterovirus, hepatitis A)	Extremely high organic/nutrient load; pathogenic microorganisms; requires multi-barrier treatment (anaerobic digestion + aerobic treatment + membranes + electrochemical/adsorption)	Phosphate: reduced from 0.27 mg/L to <0.05 mg/L; ammonia & total N reduced >50% (oxygen-loaded adsorbents)	Restricted non-potable reuse; resource recovery (biogas, fertilizers) after intensive treatment	[75,76]
Marine/Industrial effluents	Nutrients, heavy metals, plastics/microplastics, chemicals, oil, pathogens	Complex pollutant mix; large spatial scale; integrated treatment required; diverse pollutants require physical, chemical, and biological treatment	A combined Fenton and hydrodynamic cavitation treatment of dairy effluent achieved 89% COD degradation in 60 min, increasing to 93% after 2 h.	Limited direct reuse; mainly treated for safe discharge or industrial applications	[77,78]

summarizes the main types of wastewater, their typical contaminants, key treatment challenges, reported removal efficiencies, and potential reuse applications.

Blackwater primarily consists of a mixture of feces, urine, and flushing water, and is characterized by high concentrations of organic matter, accounting for approximately half of the chemical oxygen demand (COD) load of domestic wastewater, as well as elevated levels of suspended solids. Approximately 51% of the COD, 91% of nitrogen, 78% of phosphorus, and the majority of pathogenic microorganisms present in domestic wastewater originate from blackwater. In water-saving sanitation systems, such as vacuum toilets, pollutant concentrations can exceed those of conventional flushing toilets by more than an order of magnitude. Fecal matter contains a wide range of pathogenic microorganisms, including Escherichia coli, Salmonella, Shigella, enteroviruses, and hepatitis A virus, which are responsible for the transmission of numerous waterborne diseases. Inadequate treatment of blackwater, particularly in areas with insufficient sanitation infrastructure, can therefore pose significant risks to environmental and public health. Conventional blackwater treatment relies primarily on anaerobic digestion technologies, including septic tanks and high-rate anaerobic reactors, which offer advantages such as low operational costs and high organic matter removal efficiency. Enhanced nitrogen and phosphorus removal can be achieved by integrating anaerobic processes with aerobic treatment systems. The incorporation of ecological treatment approaches, such as constructed wetlands and vertical greening systems, can further improve treatment performance while providing additional landscape and aesthetic benefits. To improve the retention and removal of organic matter and micropollutants, membrane separation technologies are frequently coupled with biological treatment processes. Given the high load of pathogenic microorganisms in blackwater, electrochemical treatment technologies are being investigated as rapid disinfection options. Following advanced treatment, blackwater can be further processed through chemical precipitation and microalgae cultivation, enabling resource recovery and supporting closed-loop nutrient recycling [75]. The principal challenges in blackwater treatment include extremely high organic and nutrient loads, the predominance of pathogenic microorganisms, and the need for robust, multi-barrier treatment systems to ensure safe discharge or reuse. Urban black-odorous water, caused by hypoxia and nutrient overload, was treated using oxygen-loaded adsorbents (activated carbon, Attapulgite, Phoslock, Muscovite). Oxygen-loaded coal-based activated carbon (OCC) and Muscovite (OM) raised dissolved oxygen above 6 mg/L on day one, with OCC maintaining high ORP (+327 mV) for 15 days. Phosphate was reduced from 0.27 mg/L to <0.05 mg/L, and ammonia and total nitrogen decreased by over 50%, primarily due to microbial activity dominated by Dechloromonas. These oxygen-loaded adsorbents show promise for rapid urban water remediation [76].

Marine pollution represents a complex and multifaceted mixture of contaminants, including solid waste, nutrients, chemical substances, heavy metals, sewage, oil residues, noise, and thermal pollution. Among these, solid waste, particularly plastic debris, constitutes the dominant fraction of marine litter, while nutrient enrichment, largely originating from agricultural runoff, contributes to eutrophication and associated ecological impacts. In parallel, industrial discharges introduce substantial quantities of heavy metals and hazardous chemicals into marine ecosystems. It is estimated that more than 8.3 million tons of solid waste are discharged into the oceans annually, with plastics accounting for the majority of this input. Intensive agricultural practices, rapid industrial development, and high population densities in urban areas are the primary sources of nutrient loading, chemical contamination, sewage inputs, and heavy metal pollution in the marine environment. The increasing complexity and volume of wastewater discharges necessitate the development and implementation of robust, integrated treatment systems capable of addressing diverse pollution sources and variable contaminant loads. Effective pollution mitigation strategies require a comprehensive understanding of wastewater composition, including nutrients, heavy metals, microplastics, pharmaceuticals, organic contaminants, and pathogenic microorganisms. Each pollutant class demands tailored treatment approaches, such as physical separation, chemical oxidation, or biological degradation. Consequently, achieving optimal pollutant removal efficiency relies on the adoption of comprehensive, multi-stage treatment schemes that integrate multiple complementary technologies [77]. Key challenges in marine pollution management arise from the heterogeneity and diffuse nature of contaminant sources, the large spatial scales involved, and the need for integrated treatment strategies capable of simultaneously addressing multiple classes of pollutants.

In the case of gray water, this refers to wastewater produced from kitchen sinks, dishwashers, washbasins, washing machines, bathtubs, and showers [79]. Gray water represents a major fraction of urban wastewater and contains various emerging contaminants that can negatively impact both ecosystem integrity and public health. A detailed understanding of gray water characteristics and pollutant concentrations is essential for developing an effective and safe recycling system [80]. The term gray water is also associated with the visible change in water color, which often turns grey during or shortly after generation, such as during laundry processes. In practice, the term is used for domestic wastewater that excludes toilet effluent [81]. This exclusion is due to the fact that black water from toilets contains elevated levels of organic matter, nutrients, and pathogenic microorganisms [82]. Greywater characteristics and generation volumes exhibit substantial variability, largely influenced by water use practices, lifestyle patterns, and settlement type. Greywater is typically rich in readily biodegradable organic matter and basic constituents originating primarily from domestic activities. These include nutrients such as nitrates and their derivatives, phosphorus compounds, as well as xenobiotic organic compounds (XOCs) and microbial contaminants, including faecal coliforms, Salmonella spp., and other hydrochemical constituents. In addition, several studies have reported the presence of pharmaceuticals, personal care and cosmetic products, aerosols, pigments, and toxic heavy metals, including Pb, Ni, Cd, Cu, Hg, and Cr, at appreciable concentrations in greywater. The occurrence of these substances reflects the increasing complexity of greywater composition. Most existing treatment technologies have been designed to target specific pollutants rather than to provide comprehensive greywater treatment. Furthermore, water quality requirements vary depending on the intended reuse application, while greywater composition and production rates differ significantly across locations. Consequently, treatment systems should be tailored to specific reuse objectives, accounting for regional variability and compositional complexity, to ensure that treated effluents comply with applicable quality guidelines [83]. The primary challenges in greywater treatment therefore stem from its highly heterogeneous composition, the presence of emerging contaminants and pathogens, and the necessity to meet region-specific, reuse-oriented water quality standards. Advanced Oxidation Processes (AOPs) are effective for greywater treatment, and a gCuFe₂O₄@Chitosan bio-photocatalyst was developed for this purpose. Characterization confirmed its uniform quasi-spherical nanoparticles, chemical stability, magnetic retrievability (Ms = 17.13 emu/g), and thermal stability up to 600 °C. Under UV-activated persulfate, optimal conditions (pH 3, 2 mM persulfate, 0.8 g/L photocatalyst) achieved COD removal of 82.9% for synthetic and 73.7% for natural greywater, following pseudo-first-order kinetics with sulfate and hydroxyl radicals driving degradation. The photocatalyst demonstrated high reusability, supporting sustainable greywater treatment applications [73]. Rising water demand and droughts have intensified water scarcity, highlighting the need for greywater reuse. This study evaluated a pretreatment system combining ozone and solar irradiation for greywater from university dining hall dishwashing. Key results included 91.8% overall colour removal via sedimentation and filtration, surfactant removal of 50.8% using dried compost (better than fly ash or zeolite), and 84.1% oil and grease removal using a solar photoreactor with vertical ozonation. The processes are effective for pollutant removal, though further optimization is needed to improve efficiency [74].

4. Adsorption Process

To overcome the persistent pollutants and treatment challenges associated with various wastewater streams, advanced techniques such as adsorption provide effective and targeted solutions. Adsorption removes contaminants through various mechanisms, including electrostatic attraction, van der Waals forces, π–π interactions, hydrogen bonding, hydrophobic interactions, and acid–base reactions [84].

4.1. Mechanism

Adsorption is a key process in environmental and industrial applications, driven by multiple mechanisms. Physisorption involves weak, reversible interactions like van der Waals forces, while chemisorption forms stronger, often irreversible chemical bonds. Additional mechanisms, such as ion exchange and surface complexation, influence adsorption capacity and selectivity. The mechanisms underlying adsorption are diverse and complex [85]. Effective adsorption processes often show a decline over time due to pore-filling on the adsorbent surface [86]. Additionally, adsorption efficiency can be adversely affected by the presence of competing ions in solution. Hybrid treatment approaches have demonstrated higher removal efficiencies [87]. Functional groups of the adsorbent material play a vital role in determining the adsorption mechanism. The greater the number of functional groups in the adsorbent material, the greater the possibility of fluoride ions binding in different ways and, therefore, the greater the number of interactions [88].

Figure 2 illustrates the removal of toxic metals via adsorption onto a high-surface-area polymeric adsorbent, highlighting both monolayer and multilayer adsorption mechanisms driven by physical and chemical interactions [89].

Physisorption is a reversible adsorption process driven by weak, non-specific van der Waals forces, including London dispersion, dipole–dipole interactions, and hydrogen bonding, and is favored at lower temperatures and higher pressures. The nature of the target pollutants strongly influences the adsorption mechanism. Electrostatic interactions, for example, are ineffective for neutral species, as the ionic charge of the solute must be opposite to that of the adsorbent surface. The ionization of specific functional groups, which is often characterized by the pKa, determines the surface charge, making this mechanism highly pH-sensitive [91]. Van der Waals forces can also contribute to the attraction of ions to the adsorbent surface. Materials such as activated carbon, clay, and zeolites have been observed to remove fluoride ions predominantly through physisorption [88].

Chemisorption involves the formation of strong, specific chemical bonds, often with electron transfer, and is generally irreversible. In chemisorption processes, ions are replaced by other exchangeable ions, typically hydroxide or chloride, on the adsorbent surface. This results in ion binding facilitated by electrostatic interactions [88]. Adsorption can sometimes display both physisorption and chemisorption characteristics, with the dominant mechanism influenced by environmental conditions, affecting capacity and selectivity [89]. Ion exchange adsorption involves replacing ions on the adsorbent surface with ions from the solution, driven by electrostatic interactions and ion affinity for charged sites [89]. Surface complexation involves chemisorption, in which specific ligands interact with surface functional groups to form multi-atom complexes [92,93].

Precipitation adsorption involves the formation of solid precipitates on the adsorbent surface, especially for inorganic contaminants [92]. Biosorption utilizes biological materials, including microorganisms, algae, or plant matter, to remove pollutants or ions from aqueous solutions [94]. Hydrophobic adsorption involves the preferential association of non-polar molecules with hydrophobic adsorbent surfaces [95]. Chiral adsorption allows for the selective binding of enantiomers to chiral adsorbent surfaces [96]. Electrostatic adsorption is driven by the attraction between charged functional groups on the adsorbent surface and oppositely charged molecules in the solution [97].

Evidence from studies employing real urban wastewater indicates that nonspecific interactions, particularly electrostatic attraction and surface complexation, dominate adsorption mechanisms. For instance, the study by Andreia F. Santos demonstrated that phosphorus removal from urban wastewater is primarily governed by electrostatic attraction, representing an important advance by elucidating both the performance of calcined eggshells in real wastewater matrices and the mechanisms through which phosphorus binds to the adsorbent. Identifying these mechanisms is essential for the rational design of full-scale wastewater treatment systems and for assessing the potential reuse of the spent adsorbent as a fertilizer [62]. Similarly, Maria Cristina Collivignarelli et al. reported that metal adsorption onto biochar derived from agricultural waste was controlled mainly by surface complexation and electrostatic interactions. These results further highlight the critical role of charge-driven adsorption processes in real wastewater treatment applications [98].

4.2. Batch vs. Column Experiments

Batch adsorption is an effective method for removing pollutants from wastewater, especially at small volumes and low contaminant levels, with process parameters like stirring, temperature, contact time, pH, and adsorbent dosage carefully controlled. After equilibrium is reached, the adsorbent is separated from the treated water by removing the solution. Batch adsorption is appreciated for its simplicity, ease of operation, and cost-effectiveness, and it is commonly employed by researchers to evaluate the performance of various adsorbent–adsorbate systems. The adsorption parameters obtained from these experiments provide important insights into the efficiency of the system. However, these findings may not directly apply to continuous column systems, where shorter contact times often prevent the attainment of equilibrium [99]. A range of adsorption configurations can be employed to investigate mass transfer between an adsorbate and an adsorbent, including batch systems, continuous moving bed reactors, fixed bed columns, fluidized bed reactors, and pulsed bed systems. Batch adsorption is widely used in laboratory-scale studies but has limited applicability in large-scale wastewater treatment. In contrast, fixed bed column systems are considered more practical and scalable, as they allow continuous operation and improved pollutant removal efficiency [100]. Continuous adsorption processes are primarily applied at large scale and in industrial settings, although they require more complex equipment. These systems offer more stable adsorbate concentrations, longer residence times, and enhanced mass and heat transfer [101]. The performance of fixed bed columns is influenced by multiple parameters, including adsorption equilibrium characteristics such as isotherms and capacity, kinetic factors such as diffusion and convection coefficients, hydraulic hold up, column geometry, and flow distribution within the bed [100]. Higher influent concentrations increase the concentration gradient between the adsorbent surface and the bulk solution, thereby enhancing the driving force for mass transfer. As the mass transfer driving force increases, the adsorbent becomes saturated more rapidly, which can result in higher apparent adsorption capacity. Variations in flow rate can also influence adsorption performance, as higher flow rates may enhance capacity by reducing external mass transfer resistance or reduce capacity by shortening residence time [102]. Adsorbent particle size is frequently reduced to enhance adsorption capacity in aqueous systems. However, very small particle sizes can lead to poor bed packing and excessive pressure drop in fixed bed columns. Therefore, an effective adsorbent must exhibit sufficient mechanical stability and structural rigidity to withstand operational stresses. An optimal balance between particle size and mechanical strength is required to minimize bed voidage while preventing particle abrasion and crushing caused by fluid flow and column loading. Abrasion studies on granular activated carbon used in aqueous treatment have shown that surface roughness at the microscale decreases as abrasion progresses and that particles become increasingly rounded. Mini column adsorption experiments further indicated that, for the attrition levels examined, abrasion did not have a measurable effect on the mass transfer coefficient [103].

4.3. Adsorption Isotherms

Adsorption isotherms describe the relationship between the adsorbent and the equilibrium concentration of solutes in solution [104]. Adsorption isotherms predict the capacity of an adsorbent and show how solutes, such as antibiotics, distribute on its surface at different equilibrium concentrations. Common models include Langmuir [105], Freundlich [106], Temkin [107], Tóth [108], Fritz [109] and Dubinin–Radushkevich (D–R) isotherms [110]. Jordana Georgin et al. stressed that isothermal models provide insights into adsorption under constant temperature conditions, allowing the exploration of mono- or multilayer adsorption mechanisms and operational optimization [111]. Such models are particularly valuable in industrial applications, enabling accurate predictions of maximum adsorption capacities (q_max) and appropriate dosing requirements. The Freundlich model often applies to composite adsorbents exhibiting heterogeneous surfaces, whereas carbon-based adsorbents generally conform to the Langmuir model due to their more homogeneous surfaces. The Redlich-Peterson model, a three-parameter equation, addresses the limitations of conventional two-parameter isotherms. Among less commonly applied models, the Dubinin-Radushkevich (D-R) isotherm has been shown, in some studies such as by Antonelli et al. [112], to better fit multilayer adsorption on heterogeneous surfaces, emphasizing micropore volume filling rather than layer-by-layer adsorption. Non-linearized isotherms are generally preferred to minimize errors and improve the reliability of thermodynamic analyses [111,113].

4.4. Adsorption Kinetics

Adsorption kinetic models describe the rate and time-dependence of adsorption, aiding system design and adsorbent selection. The development of adsorbents with high affinity for pollutants is a key research focus. Adsorption kinetics describe the rate and mechanisms of pollutant removal, typically modelled using established kinetic equations [111]. Understanding adsorption kinetics is essential for optimizing adsorbent performance, including synthesis, surface modification, and operational parameters. Factors such as the nature of the adsorbent, the adsorbate, and the mixing conditions influence the adsorption rate [114]. Recent studies have favoured plant-based adsorbents due to their rapid synthesis kinetics [91]. While kinetic modelling provides valuable insights, it should be complemented by multiple models to achieve a comprehensive understanding of the adsorption process [111]. The finite number of available adsorption sites on the adsorbent surface leads to saturation, and assuming linear adsorption under such conditions may overestimate capacity [115]. Common models include pseudo-first-order (PFO), based on adsorption–desorption equilibrium, and pseudo-second-order (PSO), which accounts for chemisorption and adsorbent–adsorbate interactions [116].

4.5. Adsorbents

Adsorption technologies use materials like activated carbons, biochar, graphene oxide, and metal–organic frameworks, leveraging their high surface area and tunable chemical properties to efficiently capture and remove pollutants [117]. Figure 3 presents bimetallic metal–organic frameworks (BMOFs) for the remediation of wastewater contaminated with toxic metals [118].

4.6. Factors Controlling Adsorption Efficiency

Adsorption efficiency is influenced by solution pH, adsorbent dosage, initial pollutant concentration, and contact time, which together affect surface charge, available active sites, mass transfer, and equilibrium attainment. Ionic strength and temperature further impact adsorption by modifying electrostatic interactions and affecting molecular mobility, which in turn influence adsorption capacity and affinity. Therefore, a comprehensive understanding of these parameters is essential for accurately evaluating and interpreting the performance of an adsorbent [119].

Functional Groups: They can be engineered on adsorbent materials to target specific contaminants, enhancing both selectivity and adsorption capacity. However, these modifications can require complex chemical treatments and may affect the material’s stability and reusability [89].

Morphology: the morphology and surface functionalization of nanomaterials, as well as temperature, have been shown to significantly impact adsorption kinetics, isotherms, adsorption rates, and underlying mechanisms [120].

pH: Optimizing solution pH can improve adsorption efficiency and enable pH-responsive behavior, but precise control is needed, limiting its use in environments with variable pH [89]. Solution pH critically affects adsorption by influencing the adsorbent’s surface charge and the ionization state of pharmaceuticals. Key parameters include the point of zero charge (pH_pzc), which determines whether the surface is positively or negatively charged, the pharmaceutical’s pK_a, and the zeta potential (ζ), which reflects surface charge and stability. The isoelectric point (IEP), where ζ equals zero, is especially important for charged adsorbents like clays [119].

Temperature: significantly affects pollutant adsorption by influencing reaction kinetics, adsorbent expansion, ion mobility, and solid–liquid interface properties. Higher temperatures can enhance adsorption capacity by increasing molecular motion and diffusion. Thermodynamic parameters (ΔG⁰, ΔH⁰, ΔS⁰) help interpret the process in terms of spontaneity, energy changes, and entropy. Negative ΔG⁰ denotes spontaneity, positive ΔH⁰ suggests endothermic adsorption, with ΔH⁰ values distinguishing physisorption (<50 kJ/mol) from chemisorption (>50 kJ/mol), while positive ΔS⁰ reflects increased disorder during ion exchange [121].

Contact time: longer contact time generally enhances adsorption and allows the system to reach equilibrium, although it may reduce throughput and increase operational costs [89]. Contact time also affects both the economic efficiency of the process and the adsorption kinetics, making it a key factor in governing adsorption performance [122]. Shorter contact times allow more batches to be treated, improving efficiency and reducing costs. Adsorption capacity increases rapidly initially due to abundant vacant sites, then slows as sites fill, eventually reaching equilibrium [123]. The efficiency of adsorption processes is governed by contact time, specifically hydraulic contact time for powdered activated carbon (PAC) and empty bed contact time (EBCT) for granular activated carbon (GAC). Wenling Shi et al. reported that a minimum EBCT of 10 min is critical to mitigate risks associated with incomplete removal of contaminants. When GAC doses exceed 100 mg/L, pesticide and transformation product (TP) removal rates surpass 90% [62]. Although higher ozone doses can enhance the removal of parent pesticides, they may increase TP-related risks unless combined with adequate contact time or subsequent GAC adsorption. Ellen Edefell et al. observed EBCT values in GAC filters ranging from 16 to 21 min, with an average of 19 min [124]. In another full-scale study, parallel rapid gravity biofilters were optimized for the removal of dissolved organic matter (DOM) from urban and agriculturally impacted river water using a commercial, non-adsorptive expanded-clay medium. The experiment manipulated EBCTs across four biofilters, maintaining contact times at 15, 30, 50, and 80 min by adjusting hydraulic loading. Results demonstrated that increasing EBCT improved the removal of organic matter fractions, including dissolved organic carbon (DOC) and fluorescent components such as microbial humic-like (F290/420) and protein-like (F280/340) substances [125]. For PAC applications, relatively short hydraulic contact times of 18–30 min [126] up to 0.7–3 h [127] are generally sufficient. However, PAC can remain in the reactor longer when recirculated to the contact tank, resulting in extended residence times ranging from a minimum of 12 h to several days [27].

Competing ions: they can enhance selective adsorption and mimic real wastewater conditions but may reduce overall adsorption capacity, requiring careful design in mixed-contaminant systems [89]. In real wastewater, pollutants rarely occur in isolation and often coexist with other contaminants. Competitive adsorption typically arises among species with similar surface charge characteristics [123]. Interfering ions can lower adsorption efficiency by competing for sites and altering adsorbent functional groups, with high-charge-density anions typically having a greater impact than low-charge-density cations [99]. In practical wastewater systems, adsorption efficiency is strongly affected by both the structural stability of the adsorbent and the presence of coexisting inorganic species. While certain metal–organic frameworks (MOFs) demonstrate good hydrothermal stability, many are prone to degradation under strongly acidic or alkaline conditions, which can result in framework collapse and the release of toxic metal nodes (e.g., Cd, Cr, Co, Ag) or hazardous organic linkers [128]. Moreover, natural contaminated waters contain abundant geochemical ions, including anions such as chloride (Cl⁻), nitrate (NO₃⁻), arsenite (AsO₂⁻), and sulfate (SO₄²⁻), as well as cations including potassium (K⁺), sodium (Na⁺), calcium (Ca²⁺), and magnesium (Mg²⁺), which may enhance or inhibit the adsorption of target heavy metals [129]. For instance, Yang et al. [130] demonstrated that coexisting ions (Cl⁻, SO₄²⁻, and PO₄³⁻) competed with Cr(VI) for active sites on MnUiO-66, whereas Li et al. [131] observed that a silver-triazole MOF-1-NO3 preferentially adsorbed Cr₂O₇²⁻ over SO₄²⁻, NO₃⁻, ClO₄⁻, and Cl⁻. Consequently, the presence of competing ions and natural organic matter can reduce adsorption efficiency and shorten the material’s operational lifespan. Addressing these limitations requires the design of robust frameworks with strong metal–ligand bonds and the development of hybrid composites to enhance structural integrity [128].

Adsorbent Dosage: Adsorbent dosage is a key factor in the adsorption process, as it defines the adsorbent-to-contaminant ratio and directly impacts the adsorption capacity at a given initial concentration. According to Kroeker’s rule, increasing adsorbent dosage enhances overall pollutant removal by increasing available surface area, while the specific adsorbed amount per unit mass decreases due to incomplete site utilization at higher dosages [132].

Initial concentration: higher initial contaminant concentrations can enhance the driving force for adsorption; however, at very high concentrations, the adsorption sites may become saturated, leading to a decline in removal efficiency [89]. As the initial concentration rises, the overall sorption capacity increases. At low contaminant concentrations, the abundance of available adsorption sites results in higher removal efficiency. Conversely, at elevated concentrations, the rapid saturation of these sites reduces the percentage of contaminants removed, even though the total amount of adsorbed substance continues to grow [133].

Miscellaneous: Aging can markedly change the shape, morphology, size, and physicochemical properties of adsorbents, thereby impacting their adsorption performance [134]. For instance, aged microplastics often display adsorption rates that differ from those of pristine microplastics [135]. Particle size also affects removal efficiency [136]. Regeneration has increasingly been reconceptualized as a resource-recycling strategy aimed at enhancing both economic and environmental sustainability through repeated adsorption–desorption cycles [137]. Reusability is a key consideration for practical applications, as it depends on the adsorbent’s desorption and regeneration capabilities [138].

4.7. The Factor of Cost in the Adsorption Equation

Although cost remains a decisive criterion in evaluating adsorption technologies, a notable gap persists in the availability of reliable data describing the full economic footprint of individual adsorbents. Most studies focus on synthesis and preparation costs, as these are critical factors in assessing the potential for adsorbent application, while variability in precursor costs makes it difficult to generalize cost evaluations [139,140]. As a result, cost considerations continue to impede the large-scale deployment of high-performance adsorbents, while the economic implications of regeneration processes remain insufficiently studied and poorly documented in the current literature.

Table 3. Reported costs of selected adsorbents.

Adsorbent	Adsorbate	Cost ($/Kg)	Cost Analysis	Ref
Eggshell incorporated agro-waste adsorbent pellets (C72 composite system)	Orthophosphate	3.15	The cost analysis accounts for the direct cost of the raw materials, but it does not account for other unit operations in the process (e.g., labour, energy inputs, drying, etc.).	[141]
TiO2/ZnO nanocomposite-modified biochar	Acetaminophen	170	The cost was estimated using the material and operational costs	[142]
Lupine seed (Lu-SP) biomass	Methylene Blue	0.92	Material costs, energy costs, overhead costs	[143]
Pumpkin seed shells (PSSP) biomass	Methylene Blue	0.87	Material costs, energy costs, overhead costs	[143]
Date seed biochar (DSBC)	Pesticides	1.16	Accounting for all major inputs and operational expenses associated with the production process.	[144]
Oxidized hydrochar	Tetracycline	4.71	The production cost of adsorbent (collection of samples, size reduction, and preparation of adsorbent, carbonization, activation, and reusability)	[145]
Hydrochar-derived activated carbon	Tetracycline	3.47	The production cost of adsorbent (collection of samples, size reduction, and preparation of adsorbent, carbonization, activation, and reusability)	[145]
Melia azedarach-derived phosphoric acid-treated AC (MA-AC400)	Reactive Orange 16	8.36	Cost analysis based on activities	[146]

presents reported cost estimates for selected adsorbents used for the removal of different contaminants. The table lists the adsorbent type, target adsorbate, cost per kilogram, and the cost components considered in each study. Most existing cost analyses of adsorbents are focused primarily on activated carbon, reflecting its established role in treatment processes while highlighting the necessity for more cost-effective alternatives. Many biomass-derived adsorbents can be produced at costs below $5 kg⁻¹; however, certain nanomaterial-based systems illustrate the potential for substantially higher expenses. For example, TiO₂/ZnO nanocomposite-modified biochar is estimated at approximately $170 kg⁻¹, whereas an eggshell–agro-waste composite is around $3.15 kg⁻¹ (excluding several operational steps), demonstrating how increased material complexity and omission of unit operations can significantly raise the overall economic burden.

4.8. Adsorptive Removal of Pollutants from Urban Wastewater

Recent data underscore the versatility of adsorption processes across different contaminants, demonstrating their adaptability in complex urban wastewater matrices. Table 4 provides a summary of recent studies highlighting the effectiveness of adsorption-based technologies for the removal of various pollutant classes from urban wastewater. For example, the study by Santos et al. investigated adsorption as a phosphorus (P) removal strategy. Biogenic calcium carbonate from eggshell waste was used as an adsorbent in batch experiments treating real wastewater with 40 mg P-PO₄³⁻/L at pH 7.33 and 100 rpm stirring, with the material characterized by SEM-EDS, XRD, and FTIR-ATR before and after adsorption.

In the study by Santos et al., biogenic calcium carbonate derived from industrial eggshell waste exhibited a promising maximum adsorption capacity of approximately 4.57 mg P g⁻¹ for phosphorus under real urban wastewater conditions (original pH ~7.33). However, batch experiments revealed that high concentrations of sulfate ions (≈2689 mg L⁻¹) reduced the adsorbent’s selectivity for phosphorus, highlighting the impact of competitive anions on performance in real wastewater matrices. From a techno-economic perspective, although eggshell waste is a low-cost raw material and the calcination conditions (700 °C, 60 min) were chosen to balance phosphorus removal and energy consumption, the energy requirements for calcination and the moderate adsorption capacity observed in real wastewater indicate that further optimization and detailed cost analysis are required prior to full-scale implementation. Moreover, the presence of sulfate and other competing anions, such as chloride, in complex wastewater streams may necessitate pretreatment adjustments or the development of modified adsorbents to ensure consistent performance under variable field conditions [62].

Table 4. Application of adsorption processes for pollutant removal in urban wastewater.

Adsorbent	Adsorbate	pH	Operating Conditions	Equilibrium Time	Performance (Removal Efficiency%, Adsorption Capacity mg/g)	Ref
Biogenic calcium carbonate from industrial eggshell	Phosphorus	7.33	5, 10, and 20 g/L at 25 °C	120 min	4.57 mg/g	[62]
Zeolite (0.5–1.0 mm) filled columns	Ammonium	7.6	flow rate (2.4 L h−1)	30 min	9.23 mg/g	[147]
Rice husk biochar RHBHCl	Fe, Mn, and Se	7.6	0.25 g biochar and 50 mL wastewater under agitation at 200 rpm	540 min	0.071 mg/g Fe 0.032 mg/g Mn 0.011 mg/g Se	[98]
Rice husk biochar RHBNaOH	Fe, Mn, and Se	7.6	0.25 g biochar and 50 mL wastewater under agitation at 200 rpm	540 min	0.198 mg/g Fe 0.077 mg/g Mn 0.022 mg/g Se	[98]
Red oak activated carbon	Methylene Blue	10	dosage = 0.25 g/50 mL, initial dye concentration = 10.0 mg L−1, temperature = 45 °C, mixing rate = 175 rpm	120 min	97.18%	[148]

Table 4. Application of adsorption processes for pollutant removal in urban wastewater.

Adsorbent	Adsorbate	pH	Operating Conditions	Equilibrium Time	Performance (Removal Efficiency%, Adsorption Capacity mg/g)	Ref
Biogenic calcium carbonate from industrial eggshell	Phosphorus	7.33	5, 10, and 20 g/L at 25 °C	120 min	4.57 mg/g	[62]
Zeolite (0.5–1.0 mm) filled columns	Ammonium	7.6	flow rate (2.4 L h−1)	30 min	9.23 mg/g	[147]
Rice husk biochar RHBHCl	Fe, Mn, and Se	7.6	0.25 g biochar and 50 mL wastewater under agitation at 200 rpm	540 min	0.071 mg/g Fe 0.032 mg/g Mn 0.011 mg/g Se	[98]
Rice husk biochar RHBNaOH	Fe, Mn, and Se	7.6	0.25 g biochar and 50 mL wastewater under agitation at 200 rpm	540 min	0.198 mg/g Fe 0.077 mg/g Mn 0.022 mg/g Se	[98]
Red oak activated carbon	Methylene Blue	10	dosage = 0.25 g/50 mL, initial dye concentration = 10.0 mg L−1, temperature = 45 °C, mixing rate = 175 rpm	120 min	97.18%	[148]

A recent study evaluated zeolite-filled columns for ammonium (NH₄⁺) recovery from treated wastewater, testing two particle size ranges (0.5–1.0 mm and 2.0–5.0 mm) under three flow rates (1.2, 1.6, and 2.4 L/h) to determine optimal conditions for NH₄⁺ removal. Following adsorption, a desorption experiment evaluated the zeolite’s regeneration capability. Results indicated that the highest flow rate enhanced the adsorption capacity of both zeolite sizes by approximately 29% compared to the lowest flow rate. Furthermore, the smaller 0.5–1.0 mm zeolite adsorbed roughly 60 mg more NH₄⁺ than the 2.0–5.0 mm particles, highlighting the impact of particle size on adsorption efficiency. Zeolite demonstrated rapid desorption, releasing 44–78% of adsorbed NH₄⁺ within 30 min, with the lowest flow rate achieving up to 123–148% higher recovery. This indicates zeolite’s effectiveness for NH₄⁺ recovery from treated wastewater, supporting nutrient recycling and circular economy strategies [147].

Collivignarelli et al. studied three rice husk biochar variants, unmodified (RHB), HCl-modified (RHBHCl), and NaOH-modified (RHBNaOH), for manganese, iron, and selenium removal from urban wastewater. RHBNaOH showed the highest efficiencies (Mn 76%, Se 66%, Fe 66%), while RHBHCl and RHB were less effective. Adsorption followed the Langmuir model, with kinetics fitting both pseudo-first- and pseudo-second-order models (R² > 0.9), demonstrating biochar’s potential as an effective metal adsorbent [98].

4.9. Limitations

The adsorption performance of a wide range of established adsorbent materials, including activated carbons, zeolites, and biochar-based adsorbents, has been extensively investigated for the removal of diverse pollutants. Each class of adsorbents exhibits distinct strengths and limitations with respect to removal efficiency, versatility, stability, selectivity, and cost, leading to inherent tradeoffs among these performance criteria. Although many of the adsorbents discussed, particularly carbon-based materials and zeolites, have demonstrated promising pollutant removal capabilities, several technological barriers continue to limit their large-scale application in real wastewater treatment systems. As discussed in Section 4.5 and Section 4.8, carbon-based materials typically provide the highest surface areas and adsorption capacities for a broad spectrum of pollutants. Consistently, Section 4.8 identifies carbon materials as exhibiting superior adsorption performance. However, their practical application is often constrained by challenging regeneration processes, which frequently necessitate adsorbent replacement. Disposal via thermal treatment requires additional energy input and may generate secondary environmental impacts. Furthermore, the production of high-quality and high-purity carbon adsorbents is energy-intensive and costly [119]. Consequently, achieving an optimal balance between environmental benefits and production costs remains a major challenge. Across the literature, many “novel” or “smart” adsorbent materials are often described as low cost, sustainable, or green without the support of comprehensive life cycle or economic assessments. This is reflected in the limited number of studies that report detailed cost evaluations, with rigorous economic analyses largely restricted to activated carbon-based materials and a small subset of biomass-derived sorbents. Even within these studies, cost assessments frequently vary in scope, with some considering only raw material costs while neglecting critical operational factors such as energy consumption, labor requirements, and processing steps, thereby limiting the comparability and reliability of economic claims. Although the development of sustainable and low-cost synthesis routes is essential to overcome these economic challenges, no single adsorbent type can be considered universally superior for the removal of all pollutants. Adsorption processes generally require substantial quantities of adsorbent, which must be either regenerated or disposed of after use [62]. Moreover, laboratory-scale studies often fail to accurately reflect pilot-scale or full-scale performance, due to the highly complex and variable nature of real wastewater matrices. The difficulty in realistically simulating wastewater composition under laboratory conditions hinders accurate assessment of adsorbent performance under practical operating conditions [62]. A further limitation is the progressive decline in adsorption capacity over successive regeneration cycles. The extent of performance loss is strongly dependent on the regeneration method applied, underscoring the need to optimize regeneration strategies that balance desorption efficiency, structural integrity, and environmental safety. Beyond regeneration efficiency, the broader environmental and life cycle impacts associated with adsorbent recycling and disposal must also be critically evaluated [62].

5. Advanced Oxidation Process (AOPs)

While adsorption efficiently removes many contaminants, Advanced Oxidation Processes (AOPs) complement adsorption by degrading persistent organic pollutants that cannot be captured through adsorption alone. AOPs have become recognized as highly effective treatment methods, supported by a growing body of research [149]. Since their development in the 1990s [150], AOPs have relied on the non-selective oxidation potential of hydroxyl radicals (•OH), which react rapidly with organic pollutants (reaction rates generally between 10⁸ and 10¹¹ M⁻¹ s⁻¹). In addition to •OH (hydroxyl radicals), other reactive oxygen species (ROS), such as SO₄•⁻ (sulfate Radical), O₂•⁻ (superoxide radical), ¹O₂ (singlet oxygen), and HO₂• (hydroperoxyl radical), can also degrade contaminants [151]; however, hydroxyl radicals remain the primary species responsible for pollutant removal. Organic compounds typically react with •OH via hydrogen abstraction or addition, generating carbon-centered radicals [150]. AOPs generally occur in two stages: first, radical generation, and second, the reaction of these radicals with micropollutants in the wastewater. In situ radical production drives these reactions [152]. The generated •OH radicals degrade organic molecules and pathogenic substances primarily through hydrogen abstraction mechanisms [153]. The contaminant degradation occurs specifically during the homogeneous photolysis of H₂O₂ into •OH radicals. For olefins and aromatic compounds, an alternative degradation pathway involves the direct addition of the radical to the molecular structure [153].

5.1. Categorization of AOPs

For practical purposes, Advanced Oxidation Processes (AOPs) can be grouped into four main categories [103]. Chemical AOPs employ a chemical reagent together with a catalyst. Photochemical AOPs rely on solar or ultraviolet (UV) energy to generate reactive species. Electrochemical AOPs employ an electrical current to produce radicals, while sonochemical AOPs utilize ultrasound in the initial phase to induce radical formation [152].

5.1.1. Chemical Processes

The chemical approach generates reactive oxygen species (ROS) using reagents like hydrogen peroxide and ozone, primarily via the Fenton and Peroxone processes. Specific temperature, pressure, and pH conditions are required, limiting its large-scale application for pollutant degradation [154].

• Fenton

Fenton technology represents one of the most effective AOPs for environmental contaminant degradation. The system requires the presence of hydrogen peroxide (H₂O₂) and ferrous ions (Fe²⁺) and is widely used due to its high efficiency in generating •OH radicals, cost-effectiveness, and operational simplicity. Fenton reactions demand an optimal concentration of Fe²⁺ and an acidic solution (approximately pH 3) for effective radical formation. The Fenton process operates optimally at an acidic pH of approximately 3, which is significantly lower than that of typical wastewater. Consequently, pH adjustment prior to and following treatment is required, resulting in increased chemical consumption and operational costs. In addition, elevated concentrations of organic contaminants can promote rapid hydrogen peroxide decomposition, thereby further increasing reagent demand. To address these limitations, research efforts have focused on the development of catalysts that remain effective under near-neutral pH conditions, as well as on the use of chelating agents, such as EDTA, citrate, or oxalate, to sustain process efficiency while minimizing the need for extensive acidification [155]. Following the degradation of organic pollutants, the solution pH must be adjusted to neutral, which leads to the precipitation of iron as hydroxides, resulting in substantial iron sludge and non-recyclable iron ions [156].

The main advantages of this method include high efficiency, low operational costs, effective degradation of hazardous organic pollutants [25], operation under ambient conditions (room temperature and atmospheric pressure) and the use of readily available, easily stored, and easily handled chemicals. Additionally, the uniform reaction structure minimizes mass transfer limitations, simplifying reactor design. However, two main drawbacks exist: self-decomposition of H₂O₂ and oxidant loss due to the radical scavenging effect of H₂O₂ [157]. Other limitation is the requirement for acidic conditions, as the Fenton reaction is optimal at a pH around 3. In many cases, the pH of the wastewater needs to be adjusted before treatment. Subsequently, to precipitate excess iron and manage the resulting solid sludge, the pH must be increased after the reaction. Hydrogen peroxide concentration plays a critical role in this process. Higher H₂O₂ concentrations generally enhance the removal of organic compounds, but excessive amounts may induce toxicity in the aqueous system. To maintain proper control, a continuous H₂O₂ dosage during the oxidative treatment is recommended. Additionally, increasing the temperature can accelerate the reaction rate. For low concentrations of organic pollutants, a slight temperature rise is sufficient, as the reaction between H₂O₂ and the catalyst is exothermic [155].

• Ozone-Based (O₃) processes

Ozone acts as a powerful oxidizing agent, participating in various chemical reactions with both organic and inorganic compounds. The ozonation process can be classified as an advanced oxidation process (AOP) under specific pH conditions, as hydroxyl radicals are generated during ozone decomposition. In ozonation, two pathways are defined by pH: under acidic conditions (pH < 4), the direct pathway involves reactions between molecular ozone and dissolved compounds, whereas under alkaline conditions (pH > 10), the indirect pathway occurs via hydroxyl radicals generated from ozone decomposition reacting with the compounds. As ozone generates hydroxyl radicals in water, the process is considered an advanced oxidation process [158]. Ozone, a strong oxidant (redox potential 2.07 V), degrades organic pollutants via direct electrophilic attack or indirectly through hydroxyl radicals. Its reactivity varies by pollutant and involves four main pathways: redox reactions, cycloaddition forming ozonides, electrophilic attack on nucleophilic sites, and nucleophilic reactions with carbonyl or C–N bonds [159].

Ozonation in wastewater treatment provides multiple advantages, such as reducing sludge production and eliminating persistent organic contaminants. Ozone promotes sludge solubilization, lowering overall biomass production. The efficiency of ozone mass transfer depends on hydrodynamic and physicochemical factors. However, ozone is inherently unstable, rapidly decomposing into molecular oxygen, which limits its utilization. Alone, ozone may not fully oxidize some refractory organic compounds due to its relatively low reaction rate. To enhance treatment efficiency, ozone is often combined with hydrogen peroxide (H₂O₂), UV light, catalysts, photocatalysts, or ultrasound to boost hydroxyl radical generation [159]. Although effective, ozonation has significant limitations, including high energy demands and the limited stability of ozone, which contribute to increased operational costs. Additionally, in waters containing bromide, ozonation can lead to bromate formation, generating potentially carcinogenic bromated organic compounds. This risk is particularly significant in seawater desalination and drinking-water treatment, and to a lesser extent in wastewater effluent polishing [160].

A more accurate assessment of advanced treatment efficacy on effluent toxicity can be achieved through the use of bioassays, which provide a more comprehensive characterization than monitoring a limited set of indicator compounds. Ecotoxicological studies show that bromate formation during ozonation is minimal at specific ozone doses up to 0.4 g O₃/g DOC, increasing nearly linearly at higher doses, but remaining below 3% for typical wastewater doses of 0.4–0.6 g O₃/g DOC. In municipal wastewater with bromide ≤100 μg/L, effluent bromate stays below the WHO guideline of 10 μg/L; higher bromide levels require careful evaluation, and ozonation may be unsuitable [29]. Schindler Wildhaber et al. [161] recommend assessing post-ozonation water quality using five complementary bioassays, the Ames test, Yeast Estrogen Screen (YES), algae assay with SPE samples, Ceriodaphnia dubia reproduction assay, and fish embryo toxicity test with Danio rerio, to capture highly polar compounds. Temporary toxicity increases from labile byproducts can be mitigated by subsequent biologically active treatments such as sand or biologically activated carbon filters [27]. Plant-based bioassays are also valuable for monitoring wastewater effluent quality, particularly when effluent is reused for agricultural irrigation. These assays provide multiple evaluation endpoints, including root growth, shoot growth, and germination rate, and are cost-efficient, though they typically require 4–6 days to yield results [162]. Life cycle assessment studies indicate that greenhouse gas emissions associated with ozonation, mainly from energy and reagent consumption, typically range between 0.15 and 0.3 kg CO₂ per cubic meter of treated wastewater [163,164].

5.1.2. Photochemical Processes

Photochemical processes are highly efficient methods, environmentally friendly, simple, and cost-effective for pollutant removal. In these processes, UV radiation is combined with strong oxidants, generating hydroxyl radicals (•OH) that accelerate pollutant degradation [165].

• Photocatalysis

Photocatalysis uses light-activated redox reactions of photocatalysts and is valued for being eco-friendly, efficient, simple, low-cost, and producing non-toxic by-products [25]. Photocatalysis occurs when light energy excites electrons in a photocatalyst, forming electron–hole pairs that generate reactive species like hydroxyl radicals to oxidize organic pollutants into harmless products. Titanium dioxide (TiO₂) is widely used due to its high surface area and efficient charge separation, though its UV-only absorption limits natural-light efficiency, prompting modifications for visible-light responsiveness [166].

• UV/H₂O₂ (Ultraviolet/Hydrogen peroxide processes)

The UV/H₂O₂ system generates hydroxyl radicals (•OH) by introducing hydrogen peroxide under UV light irradiation. Its main advantage is that UV simultaneously disinfects by inactivating microorganisms and promotes peroxide photolysis, generating highly reactive radicals. This photochemical advanced oxidation process is well established for water and wastewater treatment [167]. UV can also degrade pollutants through photochemically assisted oxidant generation and catalytic processes. However, microorganisms can sometimes repair partially damaged DNA, leading to potential reactivation depending on UV dose, disinfectant stability, contact time, pH, temperature, and microbial type and load. Moreover, water quality factors, including turbidity, hardness, suspended solids, iron, manganese, and humic substances, can impede UV light penetration. Studies show that although UV treatment alters microbial community composition, total bacterial counts can return to untreated levels within days, emphasizing the need for careful adjustment of operational parameters to ensure effective treatment [63].

• Photo-assisted Fenton-like processes

Photo-assisted Fenton-like processes combine heterogeneous Fenton catalysis with UV or visible light to enhance hydroxyl and sulfate radical production. Light helps regenerate Fe²⁺ from Fe³⁺, overcoming slow Fe²⁺ regeneration and narrow pH limitations of conventional Fenton systems, enabling effective operation near neutral pH. Recent studies demonstrate that visible-light-assisted Fenton-like systems can effectively degrade persistent pharmaceuticals, such as venlafaxine (VEN), via synergistic photocatalytic and redox mechanisms. Overall, combining photochemical and Fenton-like pathways provides higher oxidation potential, lower chemical consumption, and improved catalyst reusability, making this approach promising for the sustainable removal of antidepressants and other micropollutants from wastewater [93].

5.1.3. Electrochemical Processes

• Electrochemical Oxidation

Electrochemical oxidation is an effective advanced oxidation method for removing pharmaceutical contaminants from wastewater, using electrical currents to generate reactive species that degrade pollutants via direct anodic oxidation, indirect mediated oxidation, and cathodic reduction. It offers high efficiency, operational control, scalability, and reduced chemical use, though high energy costs and electrode limitations are challenges. Combining with catalysts or using three-dimensional reactors can enhance hydroxyl radical production and overall pollutant removal [168].

Electrical advanced oxidation processes (AOPs) are often viewed as environmentally advantageous due to their efficient conversion of electrical energy into hydroxyl radicals (•OH) and secondary oxidants like H₂O₂ and O₃, while operating without added chemicals. Nonetheless, additional energy is required to transport contaminants to the reactive electrode surface, and chloride oxidation can produce active chlorine species, which may subsequently form chlorate and perchlorate [169]. From an environmental perspective, it is particularly important that AOPs minimize chemical and energy consumption, promote synergistic effects, avoid the formation of toxic byproducts such as halogenated or nitro-compounds, and achieve maximal contaminant degradation efficiencies [170].

• Anodic oxidation

Anodic oxidation generates hydroxyl radicals (•OH) on the anode surface via water oxidation, operating without the need for added chemical reagents. Pollutants adsorb onto the anode surface and are subsequently oxidized. During wastewater treatment, organics undergo distinct behaviors on the anode, which can be categorized as follows:

-

Certain anodes, such as platinum or graphite, perform soft oxidation, leading to polymer and refractory by-products. These anodes have a low oxygen evolution potential (OEP).

-

For anodes with high OEP, hydroxyl radicals are weakly adsorbed on the surface, enabling them to react more freely with pollutants, promoting degradation and complete mineralization.

The efficiency of anodic oxidation depends on factors such as pollutant and electrolyte concentration, applied current intensity, pH and temperature of the solution, electrode material properties, and the rate of mass transfer between the wastewater and the anode [154].

5.1.4. Sonochemical Processes

• Sonochemical oxidation

Sonochemical oxidation uses cavitation to degrade pollutants. When sound waves create microscopic bubbles in a liquid, their collapse generates high temperatures and pressures that break chemical bonds in pollutants. This process operates at ultrasonic frequencies (15 kHz to 1 MHz), with higher power intensities speeding up degradation. There is also a synergistic effect when combined with photocatalytic oxidation, as ultrasonic cavitation produces free radicals and enhances photocatalysts’ ability to degrade pollutants [25].

• Sonocatalysis

Sonolysis is a safe, clean, and versatile method that uses ultrasound waves to generate radicals in aqueous solutions through acoustic cavitation. This involves the formation and collapse of microbubbles, but sonolysis alone produces too few hydroxyl radicals for efficient degradation. To improve the process, it is often combined with catalysts like UV light or TiO₂, creating sonocatalysis or sonophotocatalysis, depending on the catalyst used [152].

5.1.5. Other Processes

• Sulfate radical (SR)-based advanced oxidation processes

Sulfate radical advanced oxidation processes (SR-AOPs) use sulfate radicals, either alone or with hydroxyl radicals, to degrade organic pollutants. These radicals are generated by activating oxidants like peroxymonosulfate (PMS) or peroxydisulfate (PDS), similar to the process used in hydroxyl radical-driven AOPs [171]. Without external energy input, PMS and PDS exhibit minimal reactivity. However, the reduced structural symmetry of PMS allows it to be activated more readily than PDS under equivalent catalytic environments [172]. Common approaches for activation include the use of metallic or non-metallic catalysts, together with thermal, UV–vis, microwave, ultrasonic, electrochemical, alkaline, and photocatalytic (e⁻/h⁺) processes [173].

• Catalytic wet air oxidation process

Wet air oxidation (WAO) employs hydroxyl radicals generated at high temperature and pressure to partially oxidize pollutants into less toxic and more degradable small molecules. However, WAO alone rarely achieves complete mineralization; oxygen-containing organics often resist full conversion to CO₂ and H₂O, and nitrogen-containing compounds may not fully convert to nitrogen gas. To address these limitations, catalysts such as Mn²⁺, Co²⁺, or Pt are introduced to enable deeper oxidation. Compared to WAO, catalytic WAO (CWAO) can achieve more complete oxidation at lower temperatures and pressures [25].

• Supercritical water oxidation

SCWO is a versatile technique for treating a wide range of industrial wastewaters. Unlike landfill or other disposal methods, SCWO is a true destruction process for organic matter. Oxidation-based destruction methods include biological treatment, incineration, AOPs, WAO, and SCWO. The choice of treatment depends on the organic content of the wastewater: for concentrations up to 1%, biological or AOP treatments are suitable, while highly concentrated wastewaters (1–20% organic content) are better treated by SCWO rather than incineration due to lower toxic gas emissions and cost considerations [174].

Two key factors are essential for the industrial adoption of a treatment technology: technical feasibility and implementation cost. Numerous studies have demonstrated that advanced oxidation processes (AOPs) are technically feasible for wastewater treatment. However, comprehensive assessments of their financial viability at an industrial scale remain scarce [175]. The costs associated with pH adjustment, particularly the addition of bases in highly buffered waters, limit the practical application of ozonation at elevated pH to a restricted range of source and wastewater types, while also increasing the risk of bromate formation [169]. Furthermore, ozone generation is energy-intensive, making it comparatively expensive. Photocatalytic ozonation systems incur higher costs than simple photocatalysis and may face commercial limitations. Accurate energy accounting, where the energy consumed is assigned to the number of degraded contaminants, is critical for a precise techno-economic evaluation of these oxidation systems. For example, the removal of dichloroacetic acid using catalytic ozonation and photocatalytic oxidation has been reported to be approximately 2 and 15 times more costly, respectively, than using photocatalytic ozonation alone. Some AOPs are comparatively more cost-effective. Fenton’s process benefits from inexpensive reagents such as Fe²⁺ and H₂O₂. TiO₂ photocatalysts offer stability and reusability, enhancing cost efficiency. Ultrasound is relatively low-cost compared with other physical AOPs, whereas homogeneous Fenton requires significant chemical input, with treatment costs ranging from 0.2 to 17.7 € m⁻³ depending on conditions [176]. Electrochemical oxidation faces economic constraints primarily from electrode production, maintenance, and power supply costs. While Fenton processes rely on low-cost iron catalysts, photo-Fenton and electro-Fenton treatments are more energy intensive, making energy a major factor in their overall cost. Chlorine-based methods offer lower energy demand due to rapid reaction kinetics but rely on complex chemical precursors, whereas UV-based processes are highly energy demanding. Ozone production itself requires approximately 0.3 kWh per m³ of treated wastewater, comprising roughly 0.05 kWh for ozone generation from oxygen and 0.25 kWh for oxygen preparation [27]. Bertanza et al. [177] reported that a tertiary ozonation stage using 8 g m⁻³ ozone consumed 0.05–0.08 kWh m⁻³ (excluding energy for liquid oxygen production). Electro-Fenton processes are characterized by substantial energy consumption and require operation at low pH, necessitating additional chemicals for acidification and neutralization. In comparison, solar photo-Fenton and solar photoelectro-Fenton exhibit roughly one order of magnitude lower environmental impact. For example, degradation of -methylphenylglycine under solar photo-Fenton conditions resulted in 4–6.8 kg CO₂-eq m⁻³, compared with 28–60 kg CO₂-eq m⁻³ for solar photoelectro-Fenton. Electrochemical oxidation is considered among the most environmentally friendly OH-based AOPs, with a reported carbon footprint approximately 30 times lower than UV/TiO₂ photocatalytic oxidation of olive mill wastewater under similar conditions [170].

In specific applications, concurrent or sequential dosing can be advantageous, such as to mitigate competition with ozone-reactive constituents or to maximize direct ozone effects (e.g., disinfection) before rapid ozone decomposition in the AOP. For heterogeneous catalytic ozonation, reactor design is critical. Experiments must consider ozone interactions with the catalyst, ozone decay into •OH in the absence of a catalyst, and potential sorption of probe compounds. Control experiments under identical conditions, including non-catalytic references, are necessary. Probe compounds with varying affinities may be used to assess adsorption and catalytic effects. Stability of catalytic activity must also be evaluated, as •OH can be generated through finite reactions, for instance from ozone interaction with activated carbon [169]. When employing radiometry or opaque chemical actinometers, it is necessary to consider the reactor’s geometric complexity and hydraulic flow patterns. In multi-pass systems, factors such as the choice of sampling points, recirculation velocity, feed tank volume, and mixing play a crucial role in obtaining meaningful results, especially during pilot-scale studies. Reactor design and operational conditions, including hydraulic behavior and energy efficiency (for example, the calorimetric efficiency of transducers in sonochemical processes), are key for accurate performance assessment [169].

5.2. Pollutants Removal from Urban Wastewater Through AOPS

Advanced oxidation processes have emerged as powerful treatment technologies capable of degrading a wide spectrum of persistent pollutants in urban wastewater through highly reactive oxidative pathways. Table 5 presents recent advancements in the application of advanced oxidation processes (AOPs) for the removal of priority pollutants from urban wastewater. Corpa et al. assessed ozonation, photocatalysis, and electrooxidation for treating urban WWTP effluent spiked with 17 cytostatic drugs (25 μg/L), a concentration chosen to allow direct mass-spectrometric detection and kinetic evaluation. Given the toxicity and persistence of cytostatics and their poor removal by conventional WWTPs, the study highlights the need for additional treatment processes. Ozonation (200 mg O₃/L) removed over 90% of ten compounds but achieved less than 60% removal for seven others. TiO₂ photocatalysis (500 mg/L) degraded four compounds by more than 90%, while the remaining thirteen showed removal below 75%. Electrooxidation was the most effective, eliminating 14 of the 17 compounds, with an average removal of 75% and over 94% elimination for eight compounds. The study highlighted that:

• No single technique achieves high removal for all contaminants,
• Each compound was removed by at least one treatment,
• Most laboratory studies fail to predict kinetic behavior in complex wastewater.

Table 5. Application of AOPs for removing pollutants from urban wastewater.

AOPs	Pollutants	Catalyst/Electrode	Findings	Ref
Ozonation/Photocatalysis with TiO2/Electrooxidation	17 cytostatic compounds	TiO2 AEROXIDE-powder/Boron-doped diamond (BDD/Si NeoCoat) electrode	Ozonation removed over 90% of 10 compounds but less than 60% of 7, photocatalysis achieved over 90% removal for 4 compounds but under 75% for the rest, and electrooxidation effectively removed 14 of 17 compounds with an average removal of 75% and 94% for 8 compounds.	[178]
Electrochemical oxidation—Sodium persulfate process	Ampicillin	Boron-doped diamond anode	Combining EO/SLR/SPS enhanced AMP degradation in wastewater was limited and toxicity was not fully eliminated. Anodic oxidation generated high hydroxyl radical concentrations, promoting pharmaceutical mineralization in the effluent.	[179]
Anodic Oxidation	30 pharmaceuticals	BDD (anode)	The combined EO/SLR/SPS process accelerated AMP removal in wastewater, while anodic oxidation promoted mineralization, though complete detoxification was not achieved.	[180]
Electrochemical oxidation	17β-estradiol, 17α-ethinylestradiol, sulfamethoxazole, bisphenol A, oxybenzone, diclofenac, triclosan, caffeine, carbamazepine	MMO/Ti or Pt/Ti (anodes), MMO/Ti (cathode)	Using MMO electrodes improved CEC removal by 20–50%, achieving over 90% removal of CECs and E. coli within 2 h. Current intensity had little impact, and EO proved eco-friendly with low treatment costs (~1.1 €/m3).	[181]

Table 5. Application of AOPs for removing pollutants from urban wastewater.

AOPs	Pollutants	Catalyst/Electrode	Findings	Ref
Ozonation/Photocatalysis with TiO2/Electrooxidation	17 cytostatic compounds	TiO2 AEROXIDE-powder/Boron-doped diamond (BDD/Si NeoCoat) electrode	Ozonation removed over 90% of 10 compounds but less than 60% of 7, photocatalysis achieved over 90% removal for 4 compounds but under 75% for the rest, and electrooxidation effectively removed 14 of 17 compounds with an average removal of 75% and 94% for 8 compounds.	[178]
Electrochemical oxidation—Sodium persulfate process	Ampicillin	Boron-doped diamond anode	Combining EO/SLR/SPS enhanced AMP degradation in wastewater was limited and toxicity was not fully eliminated. Anodic oxidation generated high hydroxyl radical concentrations, promoting pharmaceutical mineralization in the effluent.	[179]
Anodic Oxidation	30 pharmaceuticals	BDD (anode)	The combined EO/SLR/SPS process accelerated AMP removal in wastewater, while anodic oxidation promoted mineralization, though complete detoxification was not achieved.	[180]
Electrochemical oxidation	17β-estradiol, 17α-ethinylestradiol, sulfamethoxazole, bisphenol A, oxybenzone, diclofenac, triclosan, caffeine, carbamazepine	MMO/Ti or Pt/Ti (anodes), MMO/Ti (cathode)	Using MMO electrodes improved CEC removal by 20–50%, achieving over 90% removal of CECs and E. coli within 2 h. Current intensity had little impact, and EO proved eco-friendly with low treatment costs (~1.1 €/m3).	[181]

Combining processes such as electrooxidation and ozonation is recommended to boost WWTP removal efficiency and satisfy tighter regulatory standards [178].

Frontistis and coworkers examined ampicillin degradation via electrochemical oxidation on a BDD anode with sodium persulfate. AMP removal followed pseudo-first-order kinetics, with degradation rates increasing at higher SPS levels, higher current densities, and lower ampicillin (AMP) concentrations. While anions in bottled water had no measurable effect, 10 mg/L humic acid reduced the degradation rate by 40%. In secondary effluent, process efficiency increased nearly 3.5-fold due to the electrogeneration of active chlorine species, enhancing indirect oxidation. The electrochemical oxidation/sodium persulfate (EO/SPS) process outperformed SPS activation by simulated solar irradiation/sodium persulfate (SLR/SPS), and coupling the two processes electrochemical oxidation/simulated solar irradiation/sodium persulfate process (EO/SLR/SPS) showed cumulative AMP degradation. However, rapid AMP removal was accompanied by limited mineralization and incomplete toxicity elimination when tested in wastewater [179].

Calzadilla et al. evaluated anodic oxidation of 30 pharmaceuticals on a boron-doped diamond (BDD) anode across sulfate, chloride, and mixed electrolytes at natural pH and current densities of 6–40 mA/cm², achieving >85% mineralization in all cases. Depending on the medium, 25 transformation products and various inorganic ions were formed, with chloride enhancing absorbance through intermediate generation. In secondary effluent spiked with the pharmaceuticals, AO achieved ~90% mineralization after 300 min at 6 mA/cm², with an energy cost of 18.95 kWh/m³ (~2.90 USD/m³) [180].

Realistic evaluation of wastewater treatment must account for mass-transfer limitations and matrix effects, as chemical behavior is strongly influenced by water matrix, pH, and dissolved organic carbon, with some transformation products potentially more toxic than parent compounds. Electrooxidation efficiency depends on cell design, mass transport, matrix composition, electrode material, and applied current, with boron-doped diamond (BDD) anodes performing best. Matrix constituents like chloride, carbonate, and phosphate can inhibit ozonation, while sulfate can enhance removal. Dissolved ozone remained below 0.1 mg/L due to rapid reactions, and high ozone doses showed significant matrix quenching. Combining treatments, such as ozonation and electrooxidation, improves removal of diverse compounds. Large-scale treatment cannot be reliably predicted from ultrapure water studies, as ozone doses need to be 10–20 times higher and kinetics are 2.5–18 times slower, whereas photocatalysis and electrooxidation show minimal differences in reaction rates between real and ultrapure water [178]. Wastewater treatment combining H₂O₂ and UV irradiation enables the degradation of organic pollutants that are poorly reactive toward ozone and hydroxyl radicals but exhibit high photoactivity. Given the low solubility of ozone in water and the limited effectiveness of conventional ozonation, which can generate toxic by-products, catalytic ozonation has emerged as an alternative to improve radical generation and mineralization efficiency. The presence of certain metal ions has been shown to enhance ozone decomposition and radical formation in both homogeneous and heterogeneous catalytic systems. Electrochemical treatments have also demonstrated effectiveness in degrading recalcitrant organic pollutants [170]. Several studies indicate that oxidation efficiency is optimized under alkaline conditions, where indirect ozonation predominates, producing higher concentrations of hydroxyl radicals via the self-decomposition of ozone in the presence of OH⁻ ions. Consequently, ozonation is particularly suitable for wastewater streams with alkaline characteristics. Fenton-like reactions can also achieve effective degradation under alkaline conditions. Additionally, sulfate radicals retain reactivity over a wide pH range, including acidic (pH 2) and alkaline (pH 8) environments [170]. The risk of generating toxic bromate (BrO₃⁻) is mitigated in the presence of natural organic matter (NOM) at typical concentrations. Therefore, this critical challenge in ozonation can be addressed through the use of UV and sulfate radical–based advanced oxidation processes [170].

5.3. Limitations

Ozonation is limited by the formation of potentially toxic oxidation byproducts, including bromate in bromide-containing waters, and by its inability to achieve complete mineralization when used alone. Full mineralization would require impractically high ozone doses, making standalone ozonation economically and operationally unfeasible for most applications [182]. Beyond treatment performance, ozone presents significant operational and safety challenges. As a highly reactive and toxic gas, ozone poses health risks even at low exposure levels, particularly through inhalation. Its strong oxidative properties also lead to corrosion of many conventional construction materials, necessitating the use of ozone-resistant materials such as stainless steel, polytetrafluoroethylene, or glass-lined reactors. Consequently, industrial implementation requires stringent safety measures, including continuous ozone monitoring, effective ventilation, and ozone destruction systems. These requirements substantially increase system complexity and operational costs, thereby limiting the widespread adoption of ozonation in industrial settings [182].

6. Catalyzing Circular Urban Wastewater Management: The Rising Role of Adsorption and AOPs

6.1. Comparative Role Between Adsorption and AOPs in the Regulatory Context

The selection of appropriate water and wastewater treatment processes depends on multiple factors, including the local regulatory framework, treatment costs, crop type in reuse applications, and social acceptance [183]. Compliance with established water quality standards is essential to address public health concerns and to build and maintain societal trust [184]. Consequently, the objective of the relevant Directive extends beyond improving effluent quality to also minimizing the environmental footprint of treatment processes, notably through the introduction of energy neutrality targets for wastewater treatment plants. This policy direction aims to support the restoration of water quality. In parallel, the European Water Resilience Strategy reinforces these goals by framing water resilience as both an environmental priority and an economic opportunity for European industry. Within this broader context, micropollutants represent one of the most significant challenges in water pollution control, as they are often inadequately removed by conventional biological treatment processes in wastewater treatment plants [185].

Adsorption is widely regarded as a simple and highly effective method for the rapid removal of contaminants from aqueous systems [186]. Among available adsorbents, activated carbon has been widely recognized as one of the most effective materials for wastewater purification, owing to its high porosity, large specific surface area, non-toxic nature, and broad availability.

Activated carbon is typically applied either as powdered activated carbon, introduced as a slurry into contact reactors, or as granular activated carbon, operated in fixed or packed bed filtration systems [23]. Stand-alone powdered activated carbon processes have demonstrated performance comparable to, or in some cases superior to, other established treatment technologies, particularly in terms of cost efficiency and carbon emissions. Moreover, powdered activated carbon allows faster and more controllable micropollutant removal and requires a smaller physical footprint compared to granular activated carbon systems [187]. Granular activated carbon, characterized by its high specific surface area, enables the adsorption and physical removal of a wide range of micropollutants from treated effluents [187]. Granular activated carbon filtration is a mature and robust technology, widely regarded for its operational simplicity, and has consistently achieved micropollutant removal efficiencies exceeding 80 percent in both laboratory-scale and pilot-scale studies [188]. Nevertheless, its performance may vary depending on influent water characteristics and the physicochemical properties of the target micropollutants [185]. Compared to powdered activated carbon, granular activated carbon offers the advantage of reactivation and reuse, which can result in a lower overall carbon dioxide footprint. However, regeneration is associated with high energy demands for desorption of accumulated organic compounds. Regeneration can be achieved through thermal treatment or by passing concentrated chemical solutions through the packed bed, both of which generate exhausted regeneration streams, either gaseous or liquid, that are classified as hazardous waste and require costly and complex management. As a result, the sustainability of granular activated carbon filters is strongly influenced by the frequency of adsorbent replacement or regeneration. In contrast, powdered activated carbon cannot be regenerated and must be separated from the treated water prior to disposal. Its service life can be extended through recirculation into aerobic activated sludge systems, which enhances the removal of contaminants of emerging concern but also increases sludge production. In some countries, such as Switzerland, recycled powdered activated carbon is ultimately removed and incinerated together with excess sludge [23]. At full scale, the combined use of ozone and activated carbon within a single reactor remains uncommon. Powdered activated carbon is typically applied as a single-use dose, generally in the range of 10 to 20 mg/L [27]. However, significantly higher dosages, ranging from 100 to 500 mg/L or more, are often required to effectively initiate or enhance treatment performance [189]. A fundamental limitation of adsorption processes is that pollutants are transferred from the aqueous phase to the solid phase without degradation, which may lead to secondary pollution risks [190]. Additionally, adsorption is inherently a relatively slow process, and the need for frequent regeneration or replacement of spent adsorbents is considered a major drawback [191].

Advanced oxidation processes are based on the generation of highly reactive radical species capable of oxidizing organic contaminants [191]; Photocatalytic advanced oxidation processes can overcome several limitations associated with alternative technologies. For instance, ozonation exhibits selective reactivity toward carbon–carbon multiple bonds, while membrane processes generate concentrated waste streams with elevated pollutant loads. Photocatalytic approaches generally enhance overall treatment efficiency compared to non-photolytic advanced oxidation processes, offering greater robustness and adaptability for full-scale implementation. Among these processes, the photo-Fenton reaction has been identified as a particularly promising option due to its high hydroxyl radical production, resulting in rapid micropollutant degradation kinetics. Although traditionally operated under acidic conditions, several studies have demonstrated effective photo-Fenton treatment at near-neutral pH through the use of chelating agents, achieving removal efficiencies exceeding 80 percent. This feature makes homogeneous photo-Fenton processes especially attractive for wastewater treatment plant applications, as pH adjustment is not required. In contrast, UV hydrogen peroxide systems generate lower concentrations of hydroxyl radicals and therefore exhibit slower reaction kinetics, but they remain appealing due to the absence of iron and pH constraints. While UV hydrogen peroxide systems show limited efficiency in complex matrices such as industrial wastewater, they can achieve satisfactory removal in less complex municipal wastewater matrices [185]. Despite their advantages, advanced oxidation processes have been reported to promote the formation of toxic transformation products through side reactions. For example, the degradation of organic contaminants in the presence of nitrogen can lead to the formation of nitrogen-containing byproducts [151].

Table 6. Comparison of key operational and environmental parameters for ozonation and activated carbon treatment.

Criterion	Ozonation (O3)	Activated Carbon (GAC)
Recovered resources	Nutrient recovery [192].	Nutrient recovery [193].
Energy demand	~0.3 kWh/m3 total (0.05 kWh/m3 for ozone generation + ~0.25 kWh/m3 for oxygen preparation) [27]; alternatively 0.05–0.08 kWh/m3 excluding liquid oxygen production [177].	0.040 kWhel m−3 [194]
GHG emissions	0.2 and 0.3 kg CO2-eq/m3 [195].	0.15–0.3 kg CO2e/m3 treated wastewater [196]
Treatment cost	0.1–0.2 €/m3 [28].	0.2–0.3 €/m3 [28]
Contact time/HRT	10–14 min recommended at low ozone dosage [163].	EBCT 20–30 min for effective MP removal [62]. 5–20 min for effective pesticide removal [62].
Operational constraints	Reactor configuration and applied ozone dosage play a critical role in determining treatment performance [163]	Requires low DOC and suspended solids (<20 mg/L) to avoid clogging and excessive backwashing [197]
TRL		Operates between Technology readiness level (TRL) 1–5 [198].

compares ozonation and GAC treatment in terms of energy demand, GHG emissions, costs, hydraulic retention times, operational constraints, and technology readiness, highlighting the trade-offs and practical considerations for micropollutant and CEC removal. Ozonation generally exhibits faster treatment kinetics than activated carbon, requiring shorter contact times of approximately 10 to 14 min, compared with empty bed contact times of 20 to 30 min for granular activated carbon. However, ozonation is more energy intensive, with energy requirements ranging from 0.05 to 0.30 kWh m⁻³, depending on oxygen production conditions. In contrast, activated carbon processes typically consume less energy, although their performance is strongly influenced by influent dissolved organic carbon and suspended solids. With respect to operating costs, activated carbon systems are generally more expensive, with costs of approximately 0.2 to 0.3 € m⁻³, compared with 0.1 to 0.2 € m⁻³ for ozonation. Life cycle assessment studies further reveal notable differences in greenhouse gas emissions, estimated at 0.15 to 0.30 kg CO₂e m⁻³ of treated wastewater for activated carbon and approximately 0.2 to 0.3 kg CO₂e m⁻³ for ozonation. These findings highlight that trade-offs between energy consumption, operating cost, and treatment efficiency must be carefully evaluated when selecting the most appropriate advanced treatment technology.

Considering the wide diversity of micropollutants in terms of concentration, charge, molecular size, and hydrophobicity, as well as the likelihood of increasingly stringent discharge standards, reliance on activated carbon adsorption or ozonation as stand-alone processes is insufficient to fully address micropollutant removal challenges [61]. Limitations associated with individual treatment technologies can be mitigated through process integration. Accordingly, hybrid treatment approaches have been extensively investigated to enhance biodegradability or to ensure compliance with discharge regulations. Even when a single process achieves adequate removal, upgrading an existing treatment plant may not be economically feasible, thereby necessitating the implementation of hybrid or sequential advanced treatment processes to ensure acceptable effluent quality while maintaining cost effectiveness [199]. Hybrid processes refer to treatment technologies whose performance is enhanced by combining multiple treatment mechanisms within a single unit, whereas integrated processes involve the sequential application of two or more advanced treatment techniques [199].

In this context, coupling advanced oxidation processes with adsorption has been shown to reduce the toxicity of treated effluents [200]. Adsorption generally exhibits superior performance in micropollutant removal, whereas advanced oxidation processes are more effective in reducing total organic carbon and chemical oxygen demand. This difference in performance is largely attributed to competition from background dissolved organic matter in wastewater and to low ratios of biodegradable organic carbon relative to total organic carbon. Furthermore, larger activated carbon particles may promote pore blockage, leading to reduced organic removal efficiency [199].

As a result, combined ozone and activated carbon systems have attracted increasing attention, offering advantages such as controlled ozone and powdered activated carbon dosages to limit bromate formation, reduce residual sludge production, and extend granular activated carbon operational lifetimes. Stand-alone ozonation is known to produce bromate, a compound associated with potential carcinogenic risks. The complementary mechanisms of oxidation and adsorption enhance overall micropollutant removal efficiency when these processes are combined [61].

Table 7. Selected micropollutants and CECs, treatment technologies, and reported removal efficiencies.

Micropollutant/CEC	Technology	Dose	Removal Efficiency	Ref
μP in secondary effluent	O3 + PAC	O3 0.54 mg/mg DOC + PAC polishing	80%	[61]
Selected CECs	O3	0.3–1.5 g O3/g DOC	>80%	[201]
Pharmaceuticals and transformation products	O3 + GAC	0.28 g O3/mg DOC + GAC	For pharmaceuticals (>99%) and for oxidation transformation products (>60%).	[202]

links selected micropollutants and contaminant-of-emerging-concern (CEC) groups with advanced treatment technologies commonly considered to meet regulatory objectives for wastewater effluent quality. The table summarizes reported operational conditions, including oxidant dose, alongside typical removal efficiencies achieved using ozonation-based processes and adsorption polishing (PAC/GAC). By presenting representative performance ranges from the literature, the table illustrates how different technology combinations are applied to achieve pollutant removal under realistic treatment conditions.

Nevertheless, a comprehensive evaluation of different operational scenarios, including treatment sequence options such as adsorption followed by ozonation, ozonation followed by adsorption, or simultaneous ozone and activated carbon dosing, remains limited [61]. Available evidence suggests that incorporating an adsorption stage downstream of advanced oxidation processes can improve the removal of toxicity, trace organic contaminants, and disinfection byproducts, although reported outcomes remain partially contradictory and warrant further investigation [199].

Figure 4 illustrates two common advanced treatment configurations for micropollutant removal, showing the flow of influent through ozone reactors, adsorption units, and subsequent effluent or sludge handling processes.

The hybrid system combining O₃ with granular activated carbon (GAC) offers several advantages. Post-ozonation combined with GAC effectively removes byproducts and odor-causing residues, enhances the removal of recalcitrant and emerging contaminants, enables the potential reuse of existing infrastructure such as retrofitted sand filters, and benefits from ozone-assisted regeneration that preserves the pore structure and improves micropollutant removal.

Bio granular activated carbon combined with ozonation has been applied to enhance the effectiveness of subsequent ozone treatment by removing a fraction of effluent organic matter and other ozone scavengers, thereby improving overall oxidation efficiency [203]. The ozone and granular activated carbon process further benefits from the potential reuse of existing infrastructure, such as retrofitted sand filters. Life cycle assessment studies indicate that, for waters containing moderate pesticide concentrations below 5 μg L⁻¹, this combined approach can reduce total treatment costs by approximately 20 to 30% [204]. A novel treatment strategy coupling granular activated carbon adsorption with ozone-based regeneration has been developed to enable long term water decontamination. Granular activated carbon demonstrated excellent adsorption performance for atrazine removal, achieving efficiencies of up to 99.9%, while ozone regeneration ensured the reusability of the adsorbent. Different regeneration pathways were evaluated to support sustained adsorption efficiency, with ozone micro nano bubbles showing superior regeneration performance compared with conventional ozone application, while simultaneously inhibiting bromate formation. Atrazine adsorption was governed by pore filling, hydrogen bonding, and π–π electron donor-acceptor interactions. Surface phenolic hydroxyl groups, carboxyl groups, and pyridinic nitrogen sites facilitated ozone activation, promoting reactive oxygen species generation and effective regeneration of the granular activated carbon surface. Long-term pilot-scale operation confirmed the robustness of the combined adsorption and regeneration process, demonstrating enhanced removal of organic micropollutants, UV254 absorbance, and permanganate index. In addition, intermittent ozone micro-nano bubble regeneration enabled effective decontamination within the pore structure while preserving its integrity. For practical applications, this approach was estimated to reduce water production energy costs by approximately 0.63 kWh m⁻³ [205]. A study evaluated the combined use of ozonation and granular activated carbon (GAC) filtration to mitigate chemical and biological risks from pesticides and their transformation products. Bench-scale tests showed ozonation removed 70–90% of benzimidazole and 20–70% of triazine pesticides within 10–15 min, but some byproducts posed new toxicological risks. Subsequent GAC filtration (>100 mg L⁻¹) removed over 90% of both parent compounds and byproducts, substantially reducing toxicity. Full-scale validation confirmed the approach’s effectiveness, though GAC capacity declined after 7–9 years, suggesting replacement every two years is needed for optimal performance [62]. Laboratory-scale experiments were carried out to evaluate the effectiveness of ozonation alone and in combination with granular activated carbon adsorption for treating agricultural leachate from leek fields in Tehran, Iran. Oxidation of the granular activated carbon surface with ozone prior to treatment enhanced the removal of recalcitrant contaminants. Based on ozone concentration and granular activated carbon dosage, the removal of total dissolved solids, chemical oxygen demand, and biochemical oxygen demand was evaluated for both ozonation alone and integrated ozone granular activated carbon adsorption. The combined process achieved removal efficiencies of 46% for total dissolved solids, 44.81% for chemical oxygen demand, and 44.13% for biochemical oxygen demand. Among the investigated treatment strategies, integrated ozone granular activated carbon adsorption using ozone modified granular activated carbon demonstrated superior performance, with removal efficiencies of 55.2% for chemical oxygen demand, 54.4% for total dissolved solids, and 56.5% for biochemical oxygen demand, compared with ozonation alone, which achieved removals of 25.1%, 25.7%, and 43.6%, respectively [206].

Although combining ozonation with powdered activated carbon offers potential benefits, several challenges persist. GAC regeneration is energy-intensive, produces hazardous waste, and experiences declining capacity over time, necessitating replacement and increasing operational costs. Similarly, the use of powdered activated carbon (PAC) in a bioreactor with post-ozonation or in an O₃ + PAC configuration presents significant benefits. Recirculating PAC into the biological treatment stage prolongs its effective lifetime and enhances the removal of contaminants of emerging concern, with pilot-scale studies such as PACAS plus ozonation and ozone plus PAC confirming the feasibility of this approach. Nonetheless, many studies rely on synthetic wastewater and apply dosages that do not reflect real conditions; the optimal sequencing of PAC and ozone treatment has not been established, full-scale evaluations are limited, and integration with existing treatment infrastructure remains challenging.

Notably, many studies use synthetic wastewater rather than real secondary effluent, limiting their relevance to full-scale applications. In addition, ozone and powdered activated carbon dosages, as well as micropollutant concentrations, are frequently not representative of practical operating conditions, and reactor configuration and operational aspects are often overlooked. Second, a systematic comparison of different operational scenarios, including the sequence of application such as adsorption prior to ozonation, ozonation prior to adsorption, or simultaneous dosing of ozone and activated carbon, is currently lacking. At present, combined ozonation and powdered activated carbon technologies under pilot scale evaluation include PACAS plus ozone in the Netherlands and ozone plus powdered activated carbon in Sweden. However, the optimal strategy for integrating both processes following secondary treatment has yet to be fully established. More importantly, the effective integration of the complementary mechanisms of ozonation and powdered activated carbon, namely oxidation and adsorption, particularly within existing municipal wastewater treatment plant infrastructure, remains a significant challenge for targeted upgrades and retrofitting. Furthermore, evaluations should extend beyond micropollutant removal efficiency to include additional polishing effects of secondary effluent, such as the mitigation of byproduct formation and improvements in nutrient removal. Finally, integrated monitoring and control strategies based on surrogate parameters, particularly those employing spectral techniques such as UV-visible absorbance and excitation emission matrix fluorescence spectroscopy, remain underdeveloped for combined ozone and powdered activated carbon systems. Most existing studies focus on standalone ozonation or powdered activated carbon processes, highlighting the need for real-time monitoring approaches that enable optimization and control of ozone and powdered activated carbon dosing in integrated systems [61].

6.2. Redefining Urban Wastewater Treatment Toward a Circular and Sustainable Future

The circular economy (CE) framework can steer innovations in water technologies and regulations that promote water reuse and recycling. As a critical resource for food production, water is central to CE discussions, with nearly all sectors, including agriculture and aquaculture, depending on it. Embedding energy generation and resource recovery, such as nutrient and water recycling, into urban wastewater treatment can substantially enhance circular urban wastewater management. As a result, future WWTPs are anticipated to evolve into environmentally sustainable centers for water reuse and recovery [207]. The shift toward circular and sustainable urban wastewater management has positioned adsorption and advanced oxidation processes (AOPs) as two of the most strategically important technologies. While both target the removal of recalcitrant contaminants, they differ in mechanisms, operational costs, and resource recovery potential.

Table 8. Contrasting roles of adsorption and AOPs in Circular Urban Water Management.

Parameters/Technology	Mechanism	Efficiency	Cost	In Keeping with the Tenets of the Circular Economy	Sustainability	Key Milestone: The Evolving Research Direction
Adsorption	Simple design [208]. Regenerable adsorbent [209].	Reported removal efficiencies reaching up to 99.9% [210].	Low-cost approach [137]	Dynamic operation enabling recycling and reuse of spent adsorbents [211].	Regeneration ensures economic and environmental sustainability [139].	Regenerated adsorbents as a resource reuse strategy [139].
AOPs	AOPs exhibit high oxidation efficacy and do not produce any secondary pollutants [212].	Effective and emerging approaches for pollutant degradation and mineralization [168].	Many advanced oxidation processes (AOPs) are complex and costly, with photocatalysis being relatively cost-effective, Fenton-like systems moderately economical, and electrochemical oxidation and ozonation considered high-cost [168].	AOPS supports circular-economy principles via catalyst recycling, the use of waste-derived materials, and resource recovery [213].	Heterogeneous Fenton technologies offer high catalyst stability and reusability [214]. Sustainable catalysts are characterized by renewability, low energy demand, recyclability, non-toxicity, strong activity and selectivity, water compatibility, and cost-effectiveness [168].	AOPs enable the adoption of a waste-to-resource approach using low-cost and sustainable catalysts [168].

provides a comparative summary, highlighting their strengths, limitations, and evolving roles in next-generation urban water treatment systems.

Moreover, adsorption and advanced oxidation processes (AOPs) have undergone a transformative evolution, emerging as essential drivers of circular and sustainable wastewater treatment. Their combined ability to remove persistent pollutants, regenerate materials, and recover valuable resources transforms wastewater management from a linear disposal model to a regenerative, resource-efficient approach. By producing ultra-clean effluents suitable for reuse while minimizing chemical use and secondary waste, these technologies advance low-carbon, future-ready urban water systems. AOPs contribute to circular economy objectives through catalyst recycling, incorporation of waste-derived materials, and integration with resource recovery strategies. Approaches such as Fe²⁺ regeneration in Fenton systems and environmentally friendly catalyst production reduce environmental impacts and improve cost efficiency [213]. Meanwhile, adsorption promotes the reuse of regenerated adsorbents, serving as a key strategy to enhance both economic and environmental sustainability [139].

6.3. Adsorption–AOP Hybrids

Existing knowledge gaps and fragmented research hinder the identification of a single optimal treatment capable of achieving over 80% removal across the diverse range of emerging contaminants. These contaminants are present at low concentrations, exhibit diverse chemical structures, and are persistent, making effective removal challenging. Implementing quaternary treatments in wastewater treatment plants (WWTPs) would considerably raise operational costs [178]. Evaluation of treatment technologies centers on cost-effectiveness, energy efficiency, carbon emission reduction, and broad-spectrum micropollutant removal, criteria that indicate technological maturity, environmental sustainability, and commercialization potential.

Ozonation and homogeneous photo-driven processes, including UV/H₂O₂ and solar photo-Fenton, have been effectively explored as quaternary treatment options for pathogen inactivation and emerging contaminant removal [9]. In particular, ozonation and UV/H₂O₂ are established full-scale technologies for urban wastewater treatment [62]. However, alongside primary reactions with target pollutants, side reactions involving primary and secondary reactive oxygen species (ROS) can generate a variety of by-products, some of which may be more toxic than the original contaminants. Many studies on AOP-based wastewater treatment do not fully address this risk. When intermediate products undergo cross-reactions or dimerization, complete pollutant removal becomes unfeasible unless synergistic combinations with other techniques are applied [151]. Advanced oxidation processes (AOPs) can promote the formation of toxic by-products via side reactions; for example, degradation of organic pollutants in AOP systems in the presence of nitrogen can generate nitrogen-containing derivatives [151]. Ozonation, in contrast, oxidizes organic compounds, particularly aromatics, through both direct ozone reactions and indirect radical-mediated pathways. However, ozonation faces limitations, including low solubility and stability of ozone, the need for high dosages, and the potential formation of undesired by-products such as bromate, other brominated disinfection byproducts (DBPs), and various aldehydes or organic acids. These constraints can complicate the oxidation process and may lead to the accumulation of smaller degradation products, as reflected in UV absorbance variations [215].

Adsorption technologies may play a crucial role in meeting European Union urban wastewater directives. They also enable resource recovery, such as nitrogen and phosphorus, through adsorption–desorption–based separation and concentration. As regulations tighten and priorities shift from pollutant removal to resource recovery, the demand for advanced adsorbents is expected to grow [216]. Various strategies have been investigated to regenerate adsorbent materials for repeated use, with in situ regeneration in fixed-bed columns representing a key advancement for scale-up applications. In this context, few studies have explored the heterogeneous Fenton process as a continuous-mode regeneration approach using carbon-based adsorbents [217]. For instance, in the study by Julia Nieto-Sandoval et al., granular activated carbon (GAC) functionalized with magnetite nanoparticles (Fe₃O₄/GAC) enabled in-situ regeneration of saturated adsorbent through H₂O₂ addition via heterogeneous Fenton oxidation. The system was evaluated in a fixed-bed column under continuous operation, targeting the pharmaceutical diclofenac (DCF). The adsorbent was synthesized by incorporating 5 wt.% iron into commercial GAC via incipient wetness impregnation, followed by calcination and reduction. Immobilization of the magnetite nanoparticles did not significantly affect the specific surface area (~1000 m² g⁻¹) or the primary properties of the GAC, and the adsorption capacity remained essentially unchanged (~400 mg g⁻¹). Notably, the addition of H₂O₂ restored the adsorbent’s capacity at 25 °C using 3–6 g L⁻¹ H₂O₂ over 20 h. In situ regeneration was successfully demonstrated over three consecutive adsorption–regeneration cycles for treating 100 mg L⁻¹ DCF, with consistent breakthrough curves. Iron leaching during operation was minimal, remaining below 2 wt.% of the solid throughout regeneration. As a proof of concept, the system was tested for a representative DCF concentration (500 µg L⁻¹), achieving complete removal over 10 days and effective regeneration after saturation within just 3 h [218]. Stand-alone PAC adsorption offers performance comparable or slightly superior to established treatments, with advantages in cost and carbon footprint. It provides faster, more controllable micropollutant removal than GAC and avoids the by-product formation typical of ozonation [60].

To achieve multi-functional water treatment, research has focused on integrating multiple treatment technologies to create closed-loop systems for recovering acids, water, and salts. Successful integration depends on technological compatibility and wastewater composition. Complementary processes can act synergistically when combined, whereas incompatible technologies may interfere with one another if not properly aligned [219].

Figure 5 illustrates the key process-specific factors influencing micropollutant removal. Biological treatment performance is primarily determined by compound biodegradability (Figure 5a), while ozonation efficiency is governed by ozone reaction rate constants and applied ozone dose (Figure 5b). Activated carbon adsorption achieves high removal for most compounds, with performance largely influenced by pKa rather than hydrophobicity (Figure 5c), which explains the persistence of Candesartan and Irbesartan. Biological degradability strongly correlates with median removal in biological treatment (Figure 5a), where Irbesartan (IRB) and Benzotriazole (BTR) are most effectively degraded and Candesartan (CND) and Carbamazepine (CBZ) remain the most persistent. During ozonation, median removal aligns with ozone reaction rate constants (kO₃) (Figure 5b). Diclofenac (DCF) and CBZ show high removal, whereas IRB, 4,5-MBT, and BTR exhibit lower removal. Ozone dose also displays a strong positive correlation with treatment efficiency. In activated carbon adsorption, most micropollutants, except CND and IRB, reach median removals above 80%, with only minor dependence on log K_ow. Instead, pKa plays a dominant role, as CND and IRB, which have the lowest pKa values despite high K_ow, show the poorest adsorption performance [220].

The combined application of adsorption and Fenton processes is highly relevant for urban wastewater treatment. Due to the specific characteristics of urban sewage, selecting appropriate materials is critical for practical implementation. Activated carbon, biochar, and minerals or clays are most commonly used because of their effectiveness and safety [221]. For example, impregnating granular activated carbon (GAC) with iron ions has proven efficient for degrading antibiotics in urban wastewater [221], while biochar is also employed for the extraction of organic contaminants [222]. Zhang et al. [61] reported that coupling ozonation with activated carbon adsorption is essential to limit the formation of transformation by-products. Assessments show that ozonation- and AC-based treatments, including combined configurations, effectively meet performance requirements and are now widely implemented across Europe [61]. Common setups include O₃ + sand filtration, O₃ + GAC, in-tank dosing of powdered activated carbon within a conventional activated sludge process (PACAS), and PAC dosing after biological treatment with or without sedimentation followed by filtration [187]. Among these, ozonation, and especially PAC-based systems, are the most dominant, serving over 90% of the wastewater-treated population [61]. Table 9 presents recent studies highlighting the synergistic integration of adsorption and advanced oxidation processes (AOPs) for pollutant removal from wastewater.

This study assessed several laboratory-scale treatment processes for four common CECs—CBZ, SMX, BTR, and MBTR—under varying operational conditions. A comparison of ozonation and adsorption based on reported data highlights distinct removal behaviors for the compounds studied. Ozonation achieved rapid elimination of CBZ and SMX, reaching approximately 90% removal within 5 min and complete degradation after 15 min. MBTR required a longer contact time (30 min) for full removal, while BTR showed limited degradation, with only 50% reduction after 60 min. Conversely, adsorption experiments revealed a different affinity sequence (BTR > MBTR >> CBZ > SMX), with adsorption capacities reflecting this trend in both batch and continuous-flow systems. Freundlich isotherm modelling indicated favourable adsorption, with KF values of 74.13 and 97.09 for BTR and MBTR, respectively, compared to lower values for CBZ (18.90) and SMX (18.65). Overall, while ozonation provided faster removal of CBZ and SMX, adsorption displayed greater affinity for BTR and MBTR, emphasizing the compound-specific performance differences between the two treatment processes [23].

Photocatalytic materials integrating agricultural waste–derived biochar with TiO₂ nanoparticles were developed for combined adsorption–photocatalytic reduction in Cr(VI). TiO₂ supported on straw biochar (TBC-3, 0.025 g BC) achieved 99.9% Cr(VI) removal under sunlight within 25 min—2.9–3.5 times higher than TiO₂ or biochar alone—and maintained ~93% efficiency after four cycles, demonstrating strong stability. The enhanced performance arises from the synergistic interaction between biochar adsorption and TiO₂ photocatalysis, and a corresponding Cr(VI) removal mechanism was proposed [223].

A literature assessment of 12 key micropollutants showed that conventional biological treatment is largely inadequate, with median removal generally below 50% except for benzotriazole and irbesartan [220].

A 3D carbon aerogel containing Fe-doped carbonitrides (Fe-NC/CAG) was evaluated for combined adsorption and in situ AOP regeneration. Characterization (SEM, BET, XRD) confirmed a highly porous structure (518.7 m²/g) with Fe⁰ and Fe₃O₄ phases, and adsorption followed pseudo-second-order kinetics. After pollutant adsorption, immersion in PMS enabled in situ degradation and material regeneration. Under optimal conditions, the system achieved over 90% removal of antibiotics, phenolics, and dyes, maintaining stable performance across six cycles. Table 10 summarizes the advantages and disadvantages of Fe-NC/CAG [224].

Table 9. Pollutants removal from wastewater through synergistic integration of AOPS and adsorption.

Adsorption/AOPs Synergistic Treatment	Pollutants	Performance	Operational Conditions	Scalability	Ref
Adsorption-ozonation	Carbamazepine (CBZ, antiepileptic)	Ozonation Ozonation rapidly removed CBZ and SMX, achieving 90% elimination within 5 min and complete removal by 15 min. In contrast, MBTR and BTR required 30 min and 60 min, respectively, to reach 50% reduction. To achieve an 80% removal target and mitigate ozonation by-products, a subsequent adsorption step is recommended following ozonation.	PAC C0 = 505 ng/L Matrix = tertiary effluent   GAC PICACTIF TE (PICA) C0 = 1 µg/L Matrix = WW T = 23 °C; pH = 7.5–7.9 DOC: 3.0–5.4 mg/L   GAC Calgon Filtrasorb 400 (F400) C0 = 1 µg/L Matrix = WW T = 23 °C; pH = 7.5–7.9 DOC: 3.0–5.4 mg/L   GAC C0 = 1.00 mg/L Matrix = deionized water	The results, obtained at laboratory scale, need adaptation for full-scale applications, considering wastewater characteristics and treatment setup. Potential risks, especially toxic by-product formation during ozonation, require further evaluation, as only one test addressed this in the study.	[23]
Adsorption-ozonation	Sulfamethoxazole (SMX, antibiotic)	Ozonation Ozonation rapidly removed CBZ and SMX, with 90% reduction achieved within 5 min and complete removal by 15 min. In contrast, MBTR and BTR reached only 50% reduction after 30 and 60 min, respectively. To meet the 80% removal target and manage ozonation by-products, it is recommended to follow ozonation with an adsorption step.	Core shell AC C0 = 5–10–30–50–100 mg/L Matrix = water T = 25 °C; pH = 5.6   PAC C0 = 5–10–30–50–100 mg/L Matrix = water T = 25 °C pH = 5.6   PAC C0 = 269 ng/L Matrix = tertiary effluent   alfalfa-derived biochar C0 = 10–100 mg/L Matrix = deionized water pH = 5   Activated biochar C0 = 5–50 mg/L Matrix = distilled water T = 30 °C – T = 50 °C pH = 5.4		[23]
Adsorption-ozonation	1H-Benzotriazole (BTR) and 5-Methyl-1H-Benzotriazole (MBTR)	Ozonation Ozonation rapidly removed CBZ and SMX, achieving 90% elimination within 5 min and complete removal by 15 min. In contrast, MBTR and BTR required 30 and 60 min, respectively, to reach a 50% reduction. To attain the 80% removal target and mitigate ozonation by-products, a subsequent adsorption step is recommended.	PAC C0 = 100 μg/L Matrix = deionized water   PAC C0 = 5.28–6.7–9.8–7.5 μg/L Matrix = WW   GAC C0 = 1.00 mg/L Matrix = deionized water		[23]
Adsorption-ozonation	12 MPs	Quaternary treatment combining ozonation and activated carbon adsorption significantly enhances micropollutant removal in WWTPs, achieving median efficiencies exceeding 80% for all target compounds, with candesartan remaining the most persistent.	-		[220]
Adsorption-photocatalysis	Cr(VI)	Cr(VI) removal reached 99.9% under sunlight, while TBC-3 retained around 93% efficiency after four cycles, indicating excellent stability and reusability.	Under sunlight irradiation for 25 min	-	[223]
Adsorption-in situ AOPs	antibiotics, phenolics, and dyes	Under optimal conditions, the integrated Fe-NC/CAG process achieves over 90% removal of antibiotics, phenolics, and dyes, while maintaining stable performance over six consecutive cycles.	T: 25 °C, SMX: 100 mg/L, adsorbent: 2.5 g/L.	-	[224]

Table 9. Pollutants removal from wastewater through synergistic integration of AOPS and adsorption.

Adsorption/AOPs Synergistic Treatment	Pollutants	Performance	Operational Conditions	Scalability	Ref
Adsorption-ozonation	Carbamazepine (CBZ, antiepileptic)	Ozonation Ozonation rapidly removed CBZ and SMX, achieving 90% elimination within 5 min and complete removal by 15 min. In contrast, MBTR and BTR required 30 min and 60 min, respectively, to reach 50% reduction. To achieve an 80% removal target and mitigate ozonation by-products, a subsequent adsorption step is recommended following ozonation.	PAC C0 = 505 ng/L Matrix = tertiary effluent   GAC PICACTIF TE (PICA) C0 = 1 µg/L Matrix = WW T = 23 °C; pH = 7.5–7.9 DOC: 3.0–5.4 mg/L   GAC Calgon Filtrasorb 400 (F400) C0 = 1 µg/L Matrix = WW T = 23 °C; pH = 7.5–7.9 DOC: 3.0–5.4 mg/L   GAC C0 = 1.00 mg/L Matrix = deionized water	The results, obtained at laboratory scale, need adaptation for full-scale applications, considering wastewater characteristics and treatment setup. Potential risks, especially toxic by-product formation during ozonation, require further evaluation, as only one test addressed this in the study.	[23]
Adsorption-ozonation	Sulfamethoxazole (SMX, antibiotic)	Ozonation Ozonation rapidly removed CBZ and SMX, with 90% reduction achieved within 5 min and complete removal by 15 min. In contrast, MBTR and BTR reached only 50% reduction after 30 and 60 min, respectively. To meet the 80% removal target and manage ozonation by-products, it is recommended to follow ozonation with an adsorption step.	Core shell AC C0 = 5–10–30–50–100 mg/L Matrix = water T = 25 °C; pH = 5.6   PAC C0 = 5–10–30–50–100 mg/L Matrix = water T = 25 °C pH = 5.6   PAC C0 = 269 ng/L Matrix = tertiary effluent   alfalfa-derived biochar C0 = 10–100 mg/L Matrix = deionized water pH = 5   Activated biochar C0 = 5–50 mg/L Matrix = distilled water T = 30 °C – T = 50 °C pH = 5.4		[23]
Adsorption-ozonation	1H-Benzotriazole (BTR) and 5-Methyl-1H-Benzotriazole (MBTR)	Ozonation Ozonation rapidly removed CBZ and SMX, achieving 90% elimination within 5 min and complete removal by 15 min. In contrast, MBTR and BTR required 30 and 60 min, respectively, to reach a 50% reduction. To attain the 80% removal target and mitigate ozonation by-products, a subsequent adsorption step is recommended.	PAC C0 = 100 μg/L Matrix = deionized water   PAC C0 = 5.28–6.7–9.8–7.5 μg/L Matrix = WW   GAC C0 = 1.00 mg/L Matrix = deionized water		[23]
Adsorption-ozonation	12 MPs	Quaternary treatment combining ozonation and activated carbon adsorption significantly enhances micropollutant removal in WWTPs, achieving median efficiencies exceeding 80% for all target compounds, with candesartan remaining the most persistent.	-		[220]
Adsorption-photocatalysis	Cr(VI)	Cr(VI) removal reached 99.9% under sunlight, while TBC-3 retained around 93% efficiency after four cycles, indicating excellent stability and reusability.	Under sunlight irradiation for 25 min	-	[223]
Adsorption-in situ AOPs	antibiotics, phenolics, and dyes	Under optimal conditions, the integrated Fe-NC/CAG process achieves over 90% removal of antibiotics, phenolics, and dyes, while maintaining stable performance over six consecutive cycles.	T: 25 °C, SMX: 100 mg/L, adsorbent: 2.5 g/L.	-	[224]

Table 10. Summary of the advantages and disadvantages of Fe-NC/CAG.

Aspect	Reported Advantages	Reported Limitations
Removal efficiency	Over 90% removal of antibiotics, phenolics, and dyes; stable performance across six cycles	-
Adsorption capacity	Maximum adsorption capacities: 137.7 mg g−1 (SMX), 103.3 mg g−1 (BPA), 129.2 mg g−1 (AR1)	Improvement of adsorption capacity remains a challenge
pH applicability	Effective over a wide pH range (1–12)	-
Effect of inorganic ions	Effective in the presence of various inorganic ions	-
Material characterization	SEM, BET (518.7 m2/g), XRD confirmed highly porous structure with Fe0 and Fe3O4 phases	-
Regeneration capability	Adsorbed pollutants degraded in situ by immersion in PMS, enabling catalyst regeneration	Improvement of catalytic oxidation ability remains a challenge
Reactive species generation	•OH, SO4•−, O2•−, and 1O2 identified in the Fe-NC/CAG/PMS system	-
Real wastewater treatment	COD removal efficiency up to 92.7% in actual sewage after four cumulative adsorption cycles	-
Integration advantage	Combines adsorption and in situ AOPs, maintaining stable performance even after six cycles	-

Table 10. Summary of the advantages and disadvantages of Fe-NC/CAG.

Aspect	Reported Advantages	Reported Limitations
Removal efficiency	Over 90% removal of antibiotics, phenolics, and dyes; stable performance across six cycles	-
Adsorption capacity	Maximum adsorption capacities: 137.7 mg g−1 (SMX), 103.3 mg g−1 (BPA), 129.2 mg g−1 (AR1)	Improvement of adsorption capacity remains a challenge
pH applicability	Effective over a wide pH range (1–12)	-
Effect of inorganic ions	Effective in the presence of various inorganic ions	-
Material characterization	SEM, BET (518.7 m2/g), XRD confirmed highly porous structure with Fe0 and Fe3O4 phases	-
Regeneration capability	Adsorbed pollutants degraded in situ by immersion in PMS, enabling catalyst regeneration	Improvement of catalytic oxidation ability remains a challenge
Reactive species generation	•OH, SO4•−, O2•−, and 1O2 identified in the Fe-NC/CAG/PMS system	-
Real wastewater treatment	COD removal efficiency up to 92.7% in actual sewage after four cumulative adsorption cycles	-
Integration advantage	Combines adsorption and in situ AOPs, maintaining stable performance even after six cycles	-

6.4. Limitations

Despite the potential synergistic benefits of combining ozonation with powdered activated carbon, several challenges remain. First, many published studies are not representative of full-scale applications, as they frequently employ synthetic wastewater rather than real secondary effluent, apply ozone and powdered activated carbon dosages that do not reflect practical conditions, and overlook critical aspects of reactor configuration and operation. Second, comprehensive comparisons of different operational strategies, including the sequencing of adsorption and ozonation or their simultaneous application, are largely absent. Currently, combined ozonation and powdered activated carbon technologies under pilot scale evaluation include PACAS combined with ozonation in the Netherlands and ozone plus powdered activated carbon systems in Sweden. However, the optimal strategy for integrating both processes after secondary treatment remains unresolved. More importantly, the effective integration of the complementary mechanisms of oxidation and adsorption within existing municipal wastewater treatment plant infrastructure, particularly in the context of targeted upgrades or retrofitting, continues to present significant technical challenges. In addition, evaluations should consider not only micropollutant removal but also byproduct control and nutrient removal, while highlighting that surrogate-based spectral monitoring approaches remain insufficiently developed for combined ozonation and powdered activated carbon systems. Most existing studies focus on standalone ozonation or adsorption processes, highlighting the need for real-time monitoring tools that support process control, optimization, and reliable performance assessment in integrated treatment configurations [61].

6.5. Adsorption and Advanced Oxidation Processes for Nutrient and Energy Recovery

Several studies have investigated the use of adsorption and advanced oxidation processes (AOPs) for nutrient and energy recovery.

Ji Wu et al. demonstrated a synergistic treatment combining short-term ozonation with ion exchange to degrade algal organic matter and recover nutrients. Brief ozonation rapidly transformed organic fluorophores, while quaternary ammonium and La(OH)₃-loaded resins selectively removed nitrate and phosphate with high efficiency, showed strong reusability, and enabled effective adsorbent regeneration, highlighting a promising integrated strategy for treating algae-rich waters with simultaneous pollutant removal and resource recovery [192]. It has also attracted attention for nutrient recovery applications, as it can gradually release phosphorus into soil, making it a more effective soil amendment than raw biomass [225]. However, not all phosphorus retained in hydrochar is readily bioavailable to plants. In some cases, hydrothermal carbonization of specific feedstocks, such as digestate, may even reduce plant-available phosphorus, which is already limited in the untreated solid digestate. Consequently, phosphorus-laden hydrochar may function as an inefficient phosphorus fertilizer under certain conditions. An alternative and more effective strategy involves applying ash derived from hydrochar combustion to soil, which can enhance edaphic phosphorus availability. This improvement is attributed to the tendency of phosphorus to bind with basic oxides present in the ash, forming more crystalline and soluble hydroxyl apatite phases that are more readily absorbed by plants [193]. Surface redox functional groups on hydrochar play a key role in enhancing methane production during anaerobic digestion. Sludge-derived hydrochar, rich in oxygen-containing groups, particularly C–O functionalities, showed the greatest promotion of methane generation, while specific C(OH)=O groups in kitchen waste hydrochar enhanced proton transfer and interspecies electron transfer. Additionally, microbial immobilization on hydrochar surfaces improved microorganism–functional group interactions, further increasing biogas yields [193].

Nutrient recovery represents a transformative shift in wastewater treatment, moving beyond traditional removal to generate valuable resources for other urban sectors, such as local food production [226]. Similarly, energy recovery contributes to increased energy self-sufficiency within wastewater treatment plants (WWTPs). A key question remains regarding the optimal recovery strategy, incremental, radical, or a combination, that maximizes local resource availability while minimizing environmental impacts. Addressing this question can guide the redesign of WWTPs and support the prioritization of circular strategies from an environmental perspective. While life cycle assessments integrating both energy and nutrient recovery are increasingly recommended, many pilot-scale recovery solutions still lack a comprehensive evaluation of their impacts and benefits [227]. Despite growing interest in “nexus thinking,” this concept largely remains theoretical and has yet to translate into practical strategies. Advancing to “nexus doing” is essential for implementing radical actions and transforming them into concrete projects. Moreover, in a circular water economy, nexus doing must align with environmental and economic objectives while ensuring that the redistribution of recovered resources does not generate environmental injustices affecting certain urban sectors or social groups [62].

6.6. Economic Considerations and Barriers in Urban Wastewater Treatment and Reuse

From an economic perspective, conventional single technologies, such as the Fenton oxidation process, benefit from the use of basic equipment, moderate operating conditions, and relatively safe Fenton reagents. However, the main drawback of homogeneous Fenton is the generation of significant amounts of undesirable iron (III) sludge, which incurs additional treatment and disposal costs, estimated at 10–50% of total operational expenses [228]. Mousset et al. [229] valuated the cost of different AOPs for wastewater treatment using accumulated oxygen equivalent criteria (AOCD), comparing their cost efficiency for phenol removal under optimal operating conditions. The cost assessment—which included sludge management, chemical consumption, and electricity—revealed several trends. UV-based processes showed poor performance (17% phenol mineralization in 160 min) and were excluded from cost comparisons. For 50% mineralization, TOC removal followed the order Fenton > Electro-Fenton > Photo-Fenton > Photo-electro-Fenton > Ozonation, making Fenton the most cost-competitive for pre-treatment. At 75% mineralization, the order shifted to Electro-Fenton > Photo-Fenton > Photo-electro-Fenton > Fenton > Ozonation, and at 99% mineralization to Electro-Fenton > Photo-electro-Fenton > Photo-Fenton > Fenton > Ozonation. Overall, Electro-Fenton emerged as the most versatile option across mineralization targets, while Fenton remained cost-efficient due to low iron-catalyst expenses. In contrast, low-quantum-yield UV-based AOPs became increasingly expensive at higher mineralization levels [229].

A comprehensive techno-economic evaluation of quaternary treatments must integrate sustainability considerations [230]. Ozonation is energy-intensive and can generate harmful by-products. PAC use requires continuous carbon dosing and creates additional sludge that often must be incinerated. GAC offers the advantage of regeneration, reducing carbon demand and waste, but regeneration itself is energy-intensive and can produce residual streams requiring incineration [230]. Economic assessments of Capital Expenditures (CAPEX) and Operating Expenses (OPEX) for three quaternary treatments highlight key cost drivers: ozone dosage, GAC regeneration frequency, and PAC dosing/disposal. Sensitivity analyses reveal that operational parameters significantly influence costs, sometimes affecting the ranking of treatment technologies. Per capita treatment costs change little with WWTP size, except between 10,000 and 100,000 PE. PAC adsorption is typically the most expensive option because continuous dosing and lack of regenerability increase sludge management costs, whereas GAC regeneration provides longer-term economic advantages. Ozonation efficiently degrades many micropollutants and improves biodegradability but may produce bromate and other by-products, often requiring additional polishing steps such as sand filtration or BAC, which raise overall costs. Activated carbon adsorption avoids by-product formation but concentrates pollutants on the carbon, necessitating regeneration or disposal. PAC provides comparable removal performance but must be continuously applied, cannot be regenerated, and generates more sludge, leading to higher operational expenses [220].

Ozonation has gained popularity due to reduced production costs over the past decade and environmental advantages over chlorine. It is effective for treating wastewater with organic-complexed metals [159]. For photocatalytic oxidation and catalytic wet air oxidation (CWAO), cost-effective and efficient catalysts are critical for saving expenses and ensuring treatment performance. However, high-temperature and high-pressure conditions in CWAO increase costs and demand robust reaction vessels. Combining different AOPs can enhance organic degradation while reducing operational costs [25].

Reusing secondary treated wastewater for agricultural, industrial, municipal, and recreational applications mitigates water scarcity [231]. Quaternary-treated wastewater can promote a circular economy by recovering nutrients (K, N, P) for fertigation, reducing the need for synthetic fertilizers and associated environmental impacts [232].

Urban wastewater reuse has led to up to 54% reductions in freshwater demand, facilitated by advanced technologies such as membrane bioreactors, reverse osmosis, and AOPs, ensuring safe water for diverse applications. Decentralized systems offer localized treatment, reducing transport losses and adapting to community needs, improving efficiency, and lowering costs [233]. Effective wastewater management also supports urban agriculture and energy sectors through waste-to-biomethane conversion, aligning with SDG 6 principles and promoting a regenerative circular economy [234].

Barriers to widespread adoption of wastewater reuse are categorized as economic, technical, and social. Decentralized reuse systems are increasingly viable in areas where centralized infrastructure is impractical, offering reduced water losses and community empowerment [235]. Integrating modular equipment and innovative technologies such as UV disinfection enhances operational efficiency, economic feasibility, and urban water resilience, particularly under increasing environmental pressures [236,237].

7. Knowledge Gaps and Future Recommendations

Advanced urban wastewater treatment technologies, including activated carbon adsorption using both powdered and granular forms, as well as ozonation, have demonstrated high effectiveness in the removal of contaminants of emerging concern from municipal effluents. The selection of an appropriate quaternary treatment stage should not be based solely on pollutant removal efficiency, but should also rely on a comprehensive sustainability assessment that considers adaptability to varying influent conditions, economic feasibility, environmental impacts, and social factors. In this regard, advanced oxidation processes often emerge as more promising options than granular activated carbon adsorption, owing to their capacity for real-time adjustment to water quality fluctuations, lower material consumption, and reduced secondary waste generation.

The limited number of systematic comparative studies evaluating activated carbon adsorption, ozonation, and emerging treatment technologies constrains the robust identification of the most effective and cost-efficient solutions for advanced urban wastewater treatment. Moreover, site-specific constraints, such as land availability, access to solar energy, and electricity costs, may lead to different optimal technology choices across treatment plants. Importantly, comparative assessments should be designed to address multiple relevant endpoints for safe effluent discharge or reuse, including abatement of contaminants of emerging concern, effluent toxicity reduction, microbial and bacterial inactivation, minimization or elimination of transformation byproducts, control of antibiotic resistance, and overall treatment costs.

Looking ahead, the development of responsive or “smart” composite materials that react to factors such as pH, temperature, or light represents a promising research frontier. These materials enable controlled adsorption and potential self-regeneration. Furthermore, integrating adsorption with catalytic degradation processes, such as advanced oxidation technologies, offers the potential for complete mineralization of persistent organic pollutants, thereby providing more comprehensive remediation strategies.

Finally, sustainable regeneration approaches and rigorous life-cycle assessment are essential for future implementation. Regeneration methods must be environmentally benign, economically viable, and non-toxic. In parallel, comprehensive cradle-to-grave life-cycle assessments are urgently required to substantiate the claimed environmental benefits of biopolymer-based adsorbents relative to established materials such as activated carbon.

8. Conclusions

Water scarcity has become a major global challenge, intensified by population growth and climate change. Ensuring effective wastewater treatment is therefore vital for safeguarding public health and the environment. However, efforts to improve water quality often struggle to keep pace with the rapid expansion of urban communities. Adsorption can substantially enhance the performance of physicochemical treatment systems for urban wastewater, particularly when combined with advanced oxidation processes (AOPs). When applied sequentially or in tandem, these technologies work synergistically, enabling the removal of recalcitrant pollutants, material regeneration, and resource recovery, thereby shifting wastewater management from a linear disposal model to a regenerative approach. AOPs support circular economy principles through catalyst recycling, while adsorption contributes by redefining regenerated adsorbents as a resource reuse strategy. By producing ultra-clean effluents suitable for reuse and minimizing chemical use and secondary waste, the integration of these technologies accelerates the transition toward low-carbon, resource-efficient, and resilient urban water systems. The integration of AOPs with adsorption significantly amplifies the removal of persistent and emerging contaminants while driving the evolution of more sustainable, high-efficiency water treatment systems. These synergistic approaches are set to become pivotal in the next generation of wastewater treatment, meeting both environmental sustainability imperatives and the growing demands of modern society.