A Comprehensive Review on Various Phases of Wastewater Technologies: Trends and Future Perspectives

Abstract

Wastewater Treatment Plants (WWTPs) encompass a range of processes from preliminary to advanced stages. Conventional treatments are increasingly inadequate for handling emergent pollutants, particularly organic compounds with carcinogenic potential that pose risks to aquifers. Recent advancements prioritize integrating Advanced Oxidation Processes (AOPs) and adsorbents with conventional methods to effectively retain organic pollutants and enhance mineralization. There is a growing preference for non-chemical or minimally chemical approaches. Innovations such as combining ozone and other biological processes with photo-sono-assisted methods, alongside integrating AOPs with adsorbents, are promising. These approaches leverage catalyst-assisted reactions to optimize oxidation efficiency. This review aims to provide a holistic perspective on WWTP processes, spanning wastewater intake to the production of potable water, highlighting key technologies, operational challenges, and future trends. The focus is on advancing sustainable practices and enhancing treatment efficacy to safeguard water quality and address evolving environmental concerns effectively.

1. Introduction

Urbanization and global population growth have drastically increased the demand for water resources, placing unprecedented pressure on natural water sources. Only 0.5% of the water of Earth exists in an accessible liquid form in rivers, lakes, ponds, and groundwater, intensifying concerns about water scarcity [1,2,3]. This is exacerbated by the fact that 40% of the world population resides in severely water-stressed regions, experiencing a lack water for at least one month each year [4,5]. Over the past four decades, global water usage has been mounting at a rate of 1% annually, with emerging economies expected to sustain this trend until 2050 [6]. As a consequence, water bodies such as rivers and lakes are being increasingly contaminated by industrial, domestic, and agricultural effluents—with organic and inorganic pollutants—resulting in environmental and water related issues [7].

In response to these challenges, wastewater treatment plants (WWTPs) play a pivotal role in safeguarding water quality and mitigating environmental impacts on wastewater. On the United Nations Sustainable Development Goals, WWTPs are recognized as a critical infrastructure to remove contaminants from effluents in order to ensure public health, ecological integrity, and increase water reuse [8]. However, current conventional WWTPs primarily focus on removing suspended particles and nutrients, often falling short in effectively eliminating emerging contaminants such as pharmaceuticals and personal care products, termed as organic micro-pollutants (OMPs) [9]. The high toxicity and carcinogenicity of these pollutants pose a significant challenge, necessitating the breakdown of refractory molecules into smaller, more amenable forms for further oxidation through biological methods [10]. The inadequate removal of OMPs poses significant environmental and human risks, as these pollutants persist in water bodies and can disrupt aquatic ecosystems. Moreover, seasonal variations in WWTP performance, influenced by factors like flow rates and treatment residence times, further impact the efficiency of pollutant removal [11]. As a result, emerging contaminants continue to spread through the water cycle, contaminating natural water sources and requesting more robust treatment approaches.

The introductions of new advanced oxidation processes (AOPs), regarding WWTP conventional processes, have emerged as promising technologies capable of degrading and mineralizing a wide range of organic pollutants. AOPs harness highly reactive hydroxyl radicals to oxidize and break down complex organic molecules into simpler, less harmful by-products [12]. These processes represent a critical advancement in wastewater treatment, offering improved efficiency in removing emerging contaminants and enhancing overall water quality, with studies reporting a Chemical Oxygen Demand (COD) degradation efficiency above 90% in the treatment of several organic compounds [13]. In addition to focusing solely on AOPs, it is crucial to recognize that integrating various AOPs can significantly enhance treatment efficiency [14]. Emerging approaches are also being developed to remove contaminants of emerging concern effectively. These include combining new biological treatments with biofilms, bioreactors, and membrane technologies. Furthermore, the use of organic and natural adsorbents in bioreactors, along with nanofiltration and reverse osmosis membranes, is showing promise [15,16,17].

Furthermore, this introduction of new processes, and consequently new technologies, meets the ongoing efforts in Europe and globally with respect to enhancing capabilities of WWTPs to meet stringent water quality standards and achieve carbon neutrality goals [18]. These underscore the growing importance of integrating sustainable practices into wastewater management, including resource recovery and energy efficiency improvements [19].

This review aims to comprehensively explore the technologies and processes currently employed in wastewater treatment plants. Adopting a holistic perspective, the manuscript evaluates treatment efficiency, identifies key processes, and examines the associated technologies, providing both an in-depth explanation and a critical analysis of their advantages and limitations. Particular attention is given to emergent contaminants, which are typically addressed in tertiary and quaternary treatment stages. However, the review also considers innovative technologies emerging across all treatment phases, which have the potential to improve energy efficiency and overall treatment performance. This analysis offers a thorough assessment of the current state of wastewater treatment while also projecting future trends aimed at enhancing both energy efficiency and pollutant removal, emphasizing the challenge posed by emerging contaminants.

2. Conventional Wastewater Treatment Plant

WWTPs are crucial facilities designed to treat wastewater by removing contaminants, making it safe for disposal or reuse. These facilities primarily target pathogens, nutrients, and pollutants to safeguard water resources and protect aquatic ecosystems. However, they face challenges such as maintenance, monitoring, emerging contaminants, efficiency issues, and managing sludge disposal, all of which increase costs significantly [20,21,22]. Wastewater originates from diverse sources categorized into rainwater run-off from impermeable surfaces, domestic and industrial wastewater, and agricultural run-off. Each source introduces varied pollutants—including hazardous/non-hazardous, as well as organic/inorganic, requiring tailored treatment approaches [22,23,24,25].

Treatment typically involves five stages: preliminary, primary, secondary, tertiary and advanced treatment, and sludge management, as can be seen in Figure 1. These stages employ physical, chemical, and biological methods to remove solids, organic matter, nutrients, and contaminants from effluents [23,26,27]. The selection of techniques depends on effluent characteristics, costs, and additional impurities [5]. In the preliminary treatment stage, physical processes such as fine and coarse screens and grit chambers are employed. While these methods result in minimal reductions in the COD, Biochemical Oxygen Demand (BOD), Total Nitrogen (TN), and Total Phosphorus (TP) levels, they effectively remove around 25% of the organic load and nearly all non-organic solids, preventing large debris from damaging equipment downstream [22]. Primary treatment integrates physical sedimentation with initial chemical coagulation to remove suspended solids, achieving a typical reduction of one-third in the BOD and two-thirds in Total Suspended Solids (TSS) [2,4,27]. Secondary treatment employs biological reactors to further degrade organic matter, followed by sedimentation to separate biological flocs from the treated wastewater [28]. Tertiary treatment then targets the remaining nutrients and pollutants, refining the water for reuse, with advanced processes focusing on the elimination of toxic compounds to meet stringent water quality standards [5,27,28].

Regarding the removal of Emerging Contaminants, conventional WWTPs typically achieve a 20–50% reduction during primary treatment, 30–70% during secondary treatment, and around or more than 90% reduction following tertiary treatment [29].

3. Preliminary Treatment

Preliminary wastewater treatment focuses on physically treating effluent to remove larger suspended solids and floating materials such as rags, paper, strings, and fibers. This step aims to refine the effluent to minimize the risk of equipment malfunction due to solid presence. Key components include screens, grinders, and grit chambers [27].

The treatment sequence typically begins with a trash rack to capture large debris and garbage, followed by a bar and fine screen, grinder, and grit chamber [27]. The initial steps involve circular or rectangular openings, where retention is achieved manually or mechanically through bar racks and screens. Coarse screens use metal bars set at an angle of approximately 60 degrees to the flow direction [30]. Mechanical screens vary in design, including chain-driven, reciprocating rake, catenary, and continuous belt for coarse screening, as well as static wedge-wire, stair, and drum screens for fine screening [27].

The process is followed by grit chambers, which are used in order to remove mineral particles with diameters of 0.15–0.20 mm. Chambers are designed to settle particles with a specific gravity, which is calculated as the ratio between the particle and water densities—similar to sand (2.65) and with a settling velocity of 1.3 cm/s. Types include horizontal flow, aerated, and vortex grit chambers. Horizontal flow chambers are classic solutions, while aerated chambers minimize head loss but consume more power, require intensive labor, and may produce odors. Vortex chambers have a compact design with minimal head loss but can face challenges with grit compaction and proprietary designs [27]. The removal efficiency of this treatment is notably low for COD, Suspended Solids (SSs), Inorganic Suspended Solids (ISSs), TN, and TP, as it primarily focuses on removing large debris before preliminary sedimentation. A comprehensive study led by He et al. [31] reported removal efficiencies of just 2.36%, 6.68%, 4.91%, 5.14%, and 6.45%, respectively, for these parameters after using grit chambers. In general, with respect to preliminary treatment, it is expected to remove almost all non-organic solids and more than 25% of the organic load [22].

4. Primary Treatment

The primary treatment phase is essential for removing large suspended solids, floating materials, and organic matter following preliminary treatment. In this stage, the primary treatment processes combine physical methods, such as sedimentation or gravity settling, with the addition of coagulants or flocculants to enhance the removal of hydrophobic compounds in order to remove 60% of grease and oil, 50% of BOD over 5 days, and 70% of SSs [9,22]. Floating substances like oil, grease, and rags are skimmed off as scum from the water surface. This stage precedes secondary/biological treatment and focuses on separating solids from the liquid [4,26].

Table 1. Advantages and disadvantages of main primary treatment in a Wastewater Treatment Plant.

Process	Advantages	Disadvantages
Coagulation/Flocculation [22,23,32,33,34]	Process simplicity	Requires non-reusable materials
	Integrated physicochemical process	Physicochemical monitoring required
	Wide range of chemicals available	Increased sludge volume generation
	Inexpensive and easily accessible materials	Low removal of arsenic, dissolved impurities, ions, and pathogens
	Good sludge settling	Further processing is required
Flotation [23,35]	Integrated physicochemical process	High initial capital costs
	Non-ionic or ionic collectors	High energy, maintenance, and operation costs
	Efficient removal of small and low-density particles	pH dependent
	Low retention time	Cannot remove colloidal or dissolved solids and nutrients

provides a comparison of the advantages and disadvantages of the most-used processes on primary treatment.

4.1. Coagulation and Flocculation

The coagulation and flocculation of dissolved solids are integral to enhancing the sedimentation process in wastewater treatment [27], as seen in Figure 2. Coagulation involves destabilizing particles through charge neutralization using positively charged ions, while flocculation promotes the collision and aggregation of these particles into larger flocs through agitation [5,36]. These flocs subsequently solidify and settle to the tank bottom, aiding sedimentation [5,33,36,37]. Additional processes like filtration clarify the treated wastewater [37,38].

Commonly used coagulants in wastewater treatment include cationic inorganic metal salts [33] like aluminium sulfate [39], ferric chloride [40], and ferrous sulfate [41], as well as long-chain non-ionic or anionic polymers that include flocculants like Polyacrylamides [42] and Non-Ionic Polyacrylamides [43]. Coagulation can remove up to 40% of organic materials and nitrogen, primarily sediment-suspended pollutants through particle agglomeration [34,44]. Key parameters affecting coagulation efficiency include coagulant dose, settling time, and pH [37]. The process follows four main steps: neutralization, sweep coagulation, bridging, and patch flocculation [37].

However, small flocs formed during coagulation are unstable and can break apart rapidly. To enhance the process and ensure the sedimentation of formed flocs, flocculant agents are added. These agents agglomerate slow-settling micro-flocs, facilitating their aggregation and settling [33,37].

Commonly, coagulants and flocculants are classified into chemical and natural types, with inorganic salts and synthetic polymers being predominant in conventional applications, while natural sources and plant-based materials are gaining popularity due to their environmental friendliness and specific functional groups aiding in coagulation [34,38,45]. However, natural coagulants face challenges such as higher costs associated with industrial-scale production [34].

Hybrid coagulants are emerging as promising alternatives, offering superior efficiency and lower costs compared to conventional options. These materials combine the advantages of both inorganic salts and organic polymers, providing enhanced flocculation and minimizing environmental impacts such as sludge formation [38]. Nonetheless, further research is needed to fully understand their operational parameters and environmental implications [38].

In terms of flocculants, polymeric types like polyacrylamide and polyamine are preferred for their high efficiency in facilitating separation processes. Synthetic polymeric flocculants, while effective, pose environmental concerns due to their non-biodegradability and reliance on non-renewable resources [33,37]. Bioflocculants, derived from natural sources like chitosan and tannins, show promise due to their biodegradability and potential for cost-effectiveness, although their current scale-up remains limited [33,37].

The primary challenges are due to complex extraction processes, limited skilled professionals, and the fluctuating availability of raw materials. While abundant, natural resources must be carefully managed to avoid competition with food, hindering mass production. Moreover, the results of treating low-turbidity water have been insufficient, and the use of micro-organism-based flocculants may increase the bacterial load in the effluent, further complicating the treatment process. Most experiments remain at the laboratory scale, and the economic feasibility of scaling up industrial applications is yet to be fully explored. To address these issues, research should simplify extraction methods, utilize waste as raw material, and optimize bioflocculants for real-world applications. Efforts should also explore ways to eliminate bacteria from effluent and investigate key factors like molecular weight, dosage, and pH to enhance pollutant removal. Solutions such as developing new micro-organisms, improving flocculation mechanisms, and applying grafted bioflocculants on a larger scale will be essential for overcoming existing limitations [37,46,47,48,49].

Both organic coagulants, such as PolyAMINEs, PolyDADMACs, melamine formaldehydes, tannins, and various natural-based products, as well as inorganic coagulants, including aluminum sulfate, aluminum chloride, polyaluminum chloride, aluminum chlorohydrate, ferric sulfate, ferrous sulfate, and ferric chloride, have been extensively utilized in diverse treatment processes and sludge management. The selection of appropriate coagulants is critical, as highlighted in numerous studies [50,51,52,53], which have compared their effectiveness and associated side effects. The choice of coagulant has significant implications for optimizing energy recovery and reducing the energy demands of sludge management processes. For instance, certain coagulants have been found to inhibit methane production, potentially undermining the decarbonization efforts of WWTPs [50].

4.2. Flotation

Flotation is employed to remove low-density materials, particularly in industries like petroleum (oil and grease) and domestic water treatment (fats, oils, and grease) [54]. In Figure 2, the processes of coagulation, flocculation, and flotation of the primary method are evident, followed by a subsequent sedimentation stage at the end.

Gas flotation technologies include dissolved gas flotation (DGF), induced gas flotation (IGF), and electrolytic flotation (EF). DGF is widely used in oily wastewater treatment, where gas is injected through needle valves to create fine bubbles by reducing pressure. It typically operates in full-flow, split-flow, or recycled-flow configurations, with the recycled-flow setup being the most common. In this setup, up to 30% of the effluent is recycled back to the flotation tank after being pumped out, promoting efficient pollutant adhesion and aggregation with micro-bubbles. The tank is typically divided into two compartments by a baffle; the first compartment facilitates bubble–pollutant interaction, forming a foam layer that is skimmed off in the second compartment [54,55]. However, gas flotation systems have drawbacks such as long retention times, high footprint and operational costs, temperature sensitivity, and potential foam generation [54,55].

IGF operates by mechanically inducing gas bubbles through high-speed impellers or diffusers, primarily in industrial wastewater treatment. This method offers lower retention times compared to DGF, leading to a reduced carbon footprint. However, IGF systems often incur higher maintenance costs due to mechanical wear and tear during operation [55]. Despite the significant potential of these processes, there is a notable lack of economic feasibility assessments, as operational costs and maintenance expenses must be carefully weighed against the benefits of improved effluent quality. While DGF systems are effective, they can be cost-prohibitive due to their energy demands. In contrast, IGF systems may present a more economically viable option when integrated with other treatment processes, such as biological treatment or filtration [23,35].

Electrolytic flotation utilizes the electrolytic decomposition of aqueous solutions to generate hydrogen and oxygen bubbles. This method offers advantages such as scalability, no need for additional chemicals, and flexible operational control over conditions. Challenges include high electricity costs, limited pH control capability, and potential anode degradation due to oxygen oxidation near the anode, increasing operational expenses [55].

Recent trends in flotation technology focus on utilizing micro-bubbles and nano-bubbles for efficient oil removal from wastewater, mainly from emulsions. Various methods for generating micro- and nano-bubbles include pressurized dissolution, Venturi-based devices, swirl flow, porous membranes, ultrasonic irradiation, and electrolysis [55].

Flotation processes offer advantages such as the selective removal of small particles, metal ions, and turbidity through physicochemical interactions with non-ionic and ionic collectors. However, they involve high implementation, energy, and maintenance costs, and they rely on chemical inputs. When combined with flocculation (floc-flotation), flotation efficiency improves, enhancing turbidity removal and overall recovery rates [23,54].

5. Secondary Treatment

Secondary treatment in WWTPs are primarily centered on biological processes aimed at degrading organic matter and reducing solids, which are often followed by additional processes like nutrient removal or chemical treatment as necessary [27].

Two main types of secondary treatment processes exist: suspended or attached growth. Suspended growth processes involve micro-organisms suspended in the bioreactor, where they degrade organic matter through natural purification processes facilitated by mixing techniques such as activated sludge, sequencing batch reactors, and lagoons [27,56]. The growth of microorganisms in suspended growth processes typically progresses through phases, including lag, exponential growth, and stationary and death phases.

In contrast, attached growth processes involve micro-organisms that adhere to inert surfaces (like rock, gravel, or slag) within the reactor. As wastewater flows through these reactors, it interacts with these surfaces, facilitating the conversion and removal of organic substances. Examples of attached growth processes include trickling filters, biotowers, and rotating biological contactors (RBCs) [57]. Attached growth processes generally offer higher removal efficiencies, better reaction kinetics, operational flexibility, increased microbial density, and reduced sludge generation and operational costs compared to suspended growth processes [27,56].

Table 2. Advantages and disadvantages of main secondary processes in a Wastewater Treatment Plant.

Process	Advantages	Disadvantages
Activated Sludge [22,58]	Lower installation cost	Long hydraulic retention time
	High removal of organics and pathogens	Higher operation cost
	Low area needed	Large amount of sludge
	Applicable to large- and small-scale WWTPs	Shock loads impact stability
Sequencing Batch Reactor [35,59,60,61]	Can be fully automated	Sludge bulking issues
	Short aeration time	Time required for sludge settling
	Low area requirements and manpower	High CAPEX and OPEX
	Low energy consumption
Membrane Bioreactor [22,35,58,62,63,64]	Lower footprint	High operational cost
	Effective for pathogen, solids, and biological waste	Membrane fouling
	Higher efficiency than AS	Short operational life of the membrane
	No chemical usage	Requires skilled manpower
	Direct recycling of effluent	High energy consumption
Biofilm-Based [1,65,66,67,68]	High treatment efficiency	Low adaptability under varying conditions
	No sludge recirculation	Low efficiency when clogged or fouling occurs
	Low energy consumption	Regular maintenance required
	Cost-effective	Limited full-scale implementation
	Effective for high-strength wastewater under extreme conditions

outlines the advantages and disadvantages of the secondary treatment processes commonly used in wastewater treatment, focusing on those widely or increasingly used.

5.1. Activated Sludge

Activated sludge is a predominant process in wastewater treatment plants, typically configured as a continuous-flow stirred tank reactor (CSTR). The feasibility of activated sludge technology in municipal settings is significantly influenced by its operational efficiency and relatively low maintenance costs, making it economically viable over the long term despite a substantial initial capital investment [22,58]. This process involves three key components: a biological reactor, a sedimentation tank or clarifier, and a recycle system, as can be seen in Figure 3. Micro-organisms in suspension within the bioreactor are aerated to maintain aerobic conditions for biological treatment. During aeration, atmospheric air or oxygen is introduced to stimulate microbial activity, facilitating the breakdown of biodegradable components in the influent. Factors such as BOD, temperature, COD, and oxygen availability significantly influence the efficiency of organic matter degradation in this phase [27,58,69].

The treated wastewater then proceeds to the secondary clarifier for effluent clarification and the thickening of settled solids [27,58,69]. Mixed liquor from the primary treatment phase is transferred to the secondary phase, with the treated wastewater discharged into natural water bodies or subjected to further treatment. The thickened solids, known as underflow, are separated, with a portion disposed of as waste activated sludge. The remaining portion (20% to 50%) is recirculated to the aeration tank as return activated sludge, maintaining a high concentration of active biomass necessary for effective treatment [27,58,69].

In plug flow reactors, an alternative configuration for activated sludge treatment, wastewater flows through narrow tanks, allowing all particles to spend an equal amount of time in the reactor during each cycle. While this approach can be more efficient than CSTRs under optimal conditions, achieving these conditions consistently is challenging due to longitudinal dispersion caused by aeration and mixing, which disrupts true plug flow. CSTRs are more resilient to shock loads and offer better management of variable influent conditions compared to staged reactors in series [58].

In aerobic processes, oxygenation is crucial to ensure that sufficient oxygen is available for the degradation of organic matter. The oxygen supply should match the rate of oxygen utilization and maintain a slight excess in the tank to support aerobic metabolism continuously. Oxygen is typically introduced through air diffusers or mechanical aerators. Air diffusers, positioned along the sides or bottom of the tank, inject air in the form of bubbles ranging from coarse (up to 25 mm) to fine (as small as 2.5 mm) in diameter. Fine bubble diffusers offer higher oxygen transfer efficiency due to their larger surface area per volume. Still, they require more maintenance and energy compared to coarse bubble diffusers, which also have lower head loss [27].

Mechanical aerators, such as impellers and brush-type aerators, are also utilized to induce turbulence and promote air–water interaction. High-speed impellers introduce air into small volumes of water, creating mixing through velocity gradients, whereas brush-type aerators in oxidation ditches enhance air entrainment and provide momentum to wastewater [27].

In the pursuit of addressing emerging challenges like the removal of pharmaceuticals and personal care products, novel variations of activated sludge processes are being proposed and developed. Among these advancements are sequencing batch reactors (SBRs) and membrane bioreactors (MBRs) [58].

5.2. Sequencing Batch Reactor

SBRs, a modified form of the Activated Sludge Process (ASP), integrate aeration and sedimentation within a single tank, eliminating the need for separate units and sludge return. Unlike conventional systems, the SBR operates through distinct phases—filling, reaction, settling, drawing, and idling—within the same tank [60]. This design is particularly advantageous for small-scale installations or sites with limited space, as it consolidates the entire treatment process of the ASP into one tank. The flexibility offered by its four primary operation modes allows for the easy adjustment of process conditions, thereby enhancing its capability to meet stringent effluent quality standards [58,59].

In Figure 4, it is possible to see the sequence of functioning of the ASP. During the fill phase, raw wastewater is introduced into the tank to contact the active biomass from the previous cycle. The tank configuration can vary between static, mixed, or aerated, with high concentrations of micro-organisms reducing the treatment duration in this phase. In the react phase, biological reactions for organic degradation and nutrient removal are completed under aerated or mixed conditions, with no inflow or outflow. The settle phase mimics a batch clarifier, with no influent or effluent flow, allowing biomass settlement. During the draw phase, treated supernatant is decanted using a fixed or floating decanter after biomass settlement. The idle phase serves as a buffer between the draw and fill phases, facilitating biomass conditioning and sludge wasting as per the operating strategy [59,60,61]. Key operational factors include sludge retention time, organic loading rate, aeration rate, temperature, and mixed liquor suspended solids concentration [60].

Beyond its traditional applications in domestic and industrial wastewater treatment, SBR technology has been increasingly adapted for effluents containing high concentrations of recalcitrant pollutants in recent years [60]. To optimize resources and enhance the circular economy of WWTPs, recent studies have highlighted advances in the production of biodegradable plastics [70]. For instance, Castagnoli et al. [71] investigated the production of polyhydroxyalkanoates (PHAs), a class of biodegradable plastics, using sludge from a lab-scale SBRs. Other researchers [72,73,74] are also exploring the potential of utilizing activated sludge for producing these bioplastics, thereby optimizing resource recovery within WWTPs. This approach aims to address the challenge of recycling or reusing sludge, reducing the environmental footprint of bioplastics and diminishing the reliance on food crops, sugar cane, and vegetable oils. These traditional feedstocks are associated with high carbon emissions and significant impacts on the global food supply.

5.3. Membrane Bioreactor

The MBR combines activated sludge and membrane technologies, enhancing municipal and industrial wastewater treatment by independently adjusting the solids retention time (SRT) and hydraulic retention time (HRT) while maintaining high biomass concentrations. It integrates activated sludge biodegradation with solid–liquid separation via membrane filtration, replacing conventional secondary settling tanks and sand filtration systems with ultrafiltration (UF), micro-filtration (MF) membranes, or Reverses Osmosis (RO) [75]. The mechanisms for pollutant removal in MBRs include volatilization, size exclusion, electrostatic repulsion, or adsorption [75]. Studies report more than 95% of efficacy regarding the removal of Pharmaceutical and Personal Care Products (PPCPs) using the combination of MBR-RO/NF [76] and around 90% on COD and BOD removal [77,78].

MBRs are classified into two types based on the membrane position: side stream and immersed (or submerged) configurations [79]. Side stream setups employ tubular membranes externally, whereas immersed systems incorporate flat-sheet or hollow-fiber membranes internally. This distinction extends to operational mechanics, with immersed MBRs operating under vacuum-induced flow dynamics, keeping the mixed liquor exterior to the membrane, whereas submerged membranes within the aeration tank improve blending, maintain material suspension, and reduce biofouling [58,62,63,80].

Membranes themselves can be categorized into flat-sheet and cylindrical shapes. Flat-sheet membranes are simpler and typically used in submerged MBRs, but they have a low packing density and a large footprint. In contrast, cylindrical membranes offer a high packing density and space economy, and they are available in both hollow-fiber (HF) and multi-tube configurations [58]. MBRs can operate in aerobic, anoxic, or anaerobic modes depending on oxygen availability and wastewater characteristics. Aerobic conditions facilitate organic carbon and ammonia conversion, whereas anoxic conditions support nitrate reduction to nitrogen gas. In anaerobic states, biochemical processes assist in phosphorus removal due to the absence of dissolved oxygen and other oxidants [62,81].

The application of membranes in MBRs extends to various types tailored for specific purposes: Biomass Separation Membrane Bioreactors (BSMBRs) focus on biomass separation, Membrane Aeration Bioreactors (MABRs) use pressurized air or oxygen to enhance biofilm formation for efficient oxygen diffusion, Extractive Membrane Bioreactors (EMBRs) selectively transport specific recalcitrant organic compounds, and Ion Exchange Membrane Bioreactors (IEMBRs) utilize concentration-gradient-driven processes for targeted pollutant removal under anoxic conditions [62]. Critical membrane requirements include high resistance to acidic and basic conditions, chemical and mechanical durability over at least five years, and the ability to withstand a wide pH range (1 to 12). Polymers such as Polysulfones (PSFs), Polyvinylidene Difluoride (PVDF), Polytetrafluoroethylene (PTFE), and Cellulose Acetate (CA) are commonly used due to their suitability for these conditions and operational longevity [82,83].

However, membrane fouling remains a significant challenge, which is characterized by the accumulation of undesired materials on or within membrane pores. It progresses through stages of initial short-term transmembrane pressure (TMP) rise during conditioning, which are followed by a longer-term linear or exponential TMP increase, as well as potentially sudden, rapid TMP spikes. This leads to reduced permeate flux, increased cleaning/backwash frequency, and shortened membrane lifespan. Control strategies involve in situ (physical) and ex situ (chemical) cleaning methods, with full-scale MBR plants commonly employing coarse bubble scouring and cross-flow velocity adjustments to mitigate fouling. Despite advancements, fouling management remains a significant challenge in MBR operations, resulting in elevated maintenance costs and making it difficult for this process to be as suitable for WWTP applications as activated sludge systems [62,79,80,81,84,85].

5.4. Biofilm-Based

Biofilm-based technologies focus on micro-organism populations or units attached to a surface. The process can be divided into three stages. The first is transient attachment, where micro-organisms initially make contact and attach to a surface. Next, during the adaptation stage, micro-organisms adjust and settle, forming a stable biofilm. However, some biofilm can sporadically detach from the surface, which is known as irregular detachment. This technology addresses the limitations of conventional anaerobic–aerobic wastewater treatment systems and encompasses processes such as Rotating Biological Contactors (RBCs) and Moving Bed Biofilm Reactors (MBBRs) [1].

RBCs are fixed-film biological wastewater treatment systems consisting of large flat or corrugated discs or drums filled with lightweight packed supports mounted on a horizontal shaft. These discs or drums are submerged in wastewater and are typically made of corrosion-resistant materials that support microbial growth, forming biofilms. The shaft of the RBC rotates at a constant speed, 1–2 revolutions per minute, facilitating oxygen transfer during aerobic processes and ensuring that the biomass remains in the optimal aerobic conditions essential for effective wastewater treatment. As wastewater flows over the surface of the rotating discs or drums, micro-organisms metabolize the organic materials present in the wastewater. The rotation also induces turbulence, aiding in removing of excess solids from the media, thereby enhancing overall system efficiency. Clarification processes are subsequently employed to remove these solids from the system, ensuring continued functionality and effectiveness in wastewater treatment [1,65,66,86,87].

The conventional RBC concept can be enhanced by combining it with membrane filtration, resulting in the Membrane Rotational Biological Contactor (MBRC). Rotating disks generate shear rates near the membrane surface, aiding fouling control by removing foulants, increasing membrane permeability, and reducing energy consumption. Disk filtration and rotary filter systems have successfully minimized cake layer formation by generating high shear forces on the membrane surface through disk rotation. This scouring action effectively eliminates the need for additional fouling control techniques [67,86].

In a similar approach, the MBBR combines the advantages of both suspended and attached growth processes, with micro-organisms growing on plastic carriers (polyethylene or polypropylene) as biofilm. The support material for the biofilm in reactor can be moved by aeration in aerobic processes or mechanical stirrers in anoxic/anaerobic processes, as shown in Figure 5. They include small carrier modules that move within the reactor along with the water. The system uses an aeration mechanism to keep carriers afloat, ensuring optimal interaction between the substrate, biomass, and carrier. The biomass growth occupies the entire tank volume, providing a large protected surface area for biofilm attachment and development, and the biofilm on carriers has anaerobic inner layers and aerobic outer layers, enabling nutrient removal in a single reactor and reducing land area requirements [1,87,88,89]. Increasing the amount of floating material or using media with a large specific biofilm contact area improves the tolerance of MBBR to toxicity and overall effectiveness [88].

However, both technologies present several challenges for effective wastewater treatment applications. Apart from the evident issues related to fouling and biofilm management, RBCs face limitations in scalability due to space constraints and the complexity of operating larger systems, which require larger shafts and contactors. This complexity can hinder their application in full-scale WWTPs and results in high capital costs, ongoing maintenance expenses, and significant energy consumption. Similarly, MBBRs encounter challenges in scale-up projects, as they also require complex designs and are sensitive to specific contaminants. This sensitivity can lead to difficulties in effectively removing certain pollutants, making MBBRs less viable options in terms of capital costs compared to more established technologies. Ultimately, these factors raise concerns about the practicality and economic feasibility of RBCs and MBBRs in municipal wastewater treatment applications [1,65,66,67].

Current techniques in the secondary or biological stage present various issues. Despite its widespread use in wastewater treatment plants, the activated sludge process has drawbacks such as large sludge production, high energy consumption, and problems with secondary clarifiers like foaming and coloring. The microbial species involved are highly dependent on the physicochemical parameters of the effluent [58]. Additionally, improving the removal of emerging pollutants requires high SRT or sequential anoxic-aerobic phases [58]. SBRs still present drawbacks similar to activated sludge processes, such as sludge bulking and high suspended solids content. Some configurations, like special bioreactors that favor the growth of floc-forming heterotrophic bacteria over filamentous ones, can mitigate these issues [59]. However, these solutions require complex control systems, sophisticated maintenance, flow equalization tanks for high flow rates, and they pose a risk of solids escaping during effluent withdrawal [61].

On the side of technologies with more capability to remove emergent pollutants, MBRs face significant challenges, including high costs due to constant efforts required to extend membrane life and reduce depreciation. These systems need to enhance specific flux and minimize energy consumption, but they are not optimized for low-strength wastewaters. However, they are crucial for maximizing energy production and for the removal of nitrogen and phosphorus [82]. Additionally, these processes suffer from high operational and capital costs, membrane fouling, and high energy demands. While nano-filtration can address issues with low molecular weight pollutant removal, it increases costs and alters effluent flux [62,79,81,90]. Biofilms face challenges due to their sensitivity and adhesion issues, which are not always optimal under non-ideal conditions. High capital costs for reactor setup limit their application in large-scale wastewater treatment. Moreover, there is insufficient research on how chemicals affect biofilm microbial communities and diversity, which further complicates their adoption in industrial wastewater treatment [68].

These drawbacks and limitations in biological treatment justify implementing tertiary treatment and developing new processes and technologies to enhance overall efficiency. This includes addressing emerging pollutants, as highlighted in the future trends section.

6. Tertiary Treatment and Advanced Oxidation Processes

The effluent from secondary treatment requires further disinfection for several uses. This stage aims to reduce organics, nutrients, turbidity, nitrogen, phosphorus, heavy metals, bacteria, and viruses. Initially, treated water can be used for irrigation and non-potable uses. After complete disinfection, it becomes suitable for consumption [22]. Various processes are employed in this phase, including nutrient removal, disinfection, ion exchange, membrane processes, filtration, and more [1]. It is important to note that numerous other processes can be employed to maximize pollutant removal. Current disinfection methods include ultraviolet (UV) disinfection, ozone disinfection, and chlorine disinfection. The effectiveness of virus and pathogen deactivation varies among these methods and holds promise for removing antibiotic-resistant bacteria (ARB) and antibiotic-resistance genes (ARGs) [34,91].

Biological and physicochemical treatment methods alone are inadequate for completely eliminating contaminants of emerging concern (CECs) and pathogens from water. AOPs generate highly reactive radicals (${\mathrm{HO}}^{·}$, ${\mathrm{H}}_2$${\mathrm{O}}_2$, ${\mathrm{O}}_2$, and ${\mathrm{O}}_3$) that are extremely effective in degrading a wide range of CECs and inactivating pathogens [92]. These processes are characterized by rapid oxidation reactions, high reaction rates, and short treatment times, making them highly efficient for wastewater treatment. Hydroxyl radicals can abstract hydrogen atoms from organic contaminants, transfer electrons to initiate oxidation reactions, or add to double bonds or aromatic rings, initiating degradation processes [93]. AOPs can be categorized into light-based processes that combine UV light with other agents and dark oxidative processes that do not involve light sources. They can also be classified by the type of oxidants used, including chemical oxidants, Fenton and Photo-Fenton processes, photocatalytic processes, and ultrasound treatment [92,94].

Table 3. Advantages and disadvantages of main advanced oxidation processes (AOPs) for tertiary and post-tertiary wastewater treatment.

Process	Advantages	Disadvantages
Ozonation [58,94,95,96,97]	High efficiency for a variety of pollutants	Low effectiveness for heavy metals
	No sludge production	Highly toxic gas
	Possible to combine with various catalysts	Generation of toxic by-products
	Strong oxidation ability	High capital and operating costs
	Contribution of oxygen to water after disinfection	Short half-life
UV [22,58,94,95,96,98,99,100]	No formation of disinfection by-products	Efficiency dependent on suspended particles
	Short retention time	Non-effective for antibiotic-resistant bacteria
	Effective on a wide range of resilient viruses	Cannot remove soluble impurities
	Economical	Photoreactivation post-UV exposure
	Produces hydroxyl radicals
	No chemical usage and compact
Ultrasound [34,101,102,103]	Compact	Requires high energy
	Environmentally friendly	Need for supplemental oxidants
	Produces hydroxyl radicals	Not commercially applicable yet
	No chemical usage
Fenton [94,102,104,105,106,107]	Not expensive	pH dependent
	Efficient for organic pollutants	High amount of Fenton agents required
	Total mineralization	Difficulties in transporting chemicals like ${\mathrm{H}}_2$${\mathrm{O}}_2$
	Simple implementation	High amounts of iron sludge
	Environmentally friendly
	Shortest reaction time among AOPs
Electro-Chemical [108,109,110,111,112,113]	Degrades a wide range of contaminants	Initial cost of electrode materials
	Simple setup and operation procedures	Formation of secondary pollutants
	Easily combined with other AOPs	Expensive and inefficient electrodes
		Production of toxic intermediate products
Adsorbents [108,109,110,111,112,113,114]	Cost-effectiveness	Adsorption capacity tends to decrease
	Versatility	Non-specific binding
	Environmental friendliness	Difficult to recover and reuse some adsorbents
	High efficiency in removing heavy metals	Use, disposal, and management of adsorbents
	Sustainability and long-term cost-effectiveness

presents the advantages and disadvantages of each disinfection process treatment.

6.1. Chloronation and Ozonation

Chlorine has been widely utilized for microbial inactivation in wastewater disinfection prior to discharge. It has been the most commonly used chemical disinfectant in water and wastewater treatment globally, working by oxidizing and destroying the cellular material of target micro-organisms [115,116]. However, the production of mutagenic and carcinogenic disinfection by-products (DBPs) due to reactions with organic compounds in wastewater, along with its ineffectiveness in controlling ARB and ARGs, has led to the adoption of alternative chemical methods like ozonation [96,97,98,115].

Ozonation, is highly effective in the disinfection of organic and inorganic compounds, with the advantage of decomposing into harmless oxygen, thereby eliminating the formation of unwanted by-products [97]. It is considered an AOP due to its generation of hydroxyl radicals that deactivate pathogens in drinking water and to remove color, taste, and odor, as well as to reduce the presence of trace organic chemicals [4,117,118,119,120].

Ozone decomposition results in the creation of hydroxyl radicals through two main mechanisms: direct pathway reaction (pH < 4), involving dissolved compounds reacting with molecular ozone, and indirect pathway reaction (pH > 10), involving hydroxyl radicals generated from ozone breakdown reacting with dissolved compounds [4,120].

Ozone formation through generators requires the dissociation of oxygen into atoms. Veremeychik et al. [121] discuss various ozone production technologies, highlighting photochemical, barrier discharge, corona discharge, and electrolytic methods as the most common approaches. The photochemical method, which occurs naturally, produces most ozone using mercury and excimer xenon lamps [122]. Barrier discharge, occurring between dielectrics or a dielectric and metal using alternating current (AC), carries heat away with the oxygen and ozone flow, eliminating the need for cooling at low capacities [123]. Electrolytic synthesis uses aqueous perchloric or sulfuric acid solutions with platinum electrodes, relying on water decomposition. Ionizing radiation forms ozone through processes involving the excitation of oxygen molecules by light or electric fields [121]. Corona discharge, in Figure 6, the most reliable and efficient method, widely used industrially, involves a high-voltage and grounded electrode separated by a gap and dielectric, with ozone formed from oxygen dissociation by electron energy [124,125]. This method involves passing a dry oxygen-bearing gas through a corona discharge. Common feed gases include oxygen, air, and recycle streams with oxygen, nitrogen, argon, carbon dioxide, and other diluents [125,126,127].

Economically, ozone treatment poses challenges due to its high energy demand, particularly when ozone is generated from air or oxygen, resulting in significant operational costs and limiting its widespread application in large-scale WWTPs. Currently, its use is more feasible in industrial or smaller treatment facilities. However, given the high potential of ozone technology, ongoing advancements in ozone generation methods may improve its future economic viability for larger-scale applications [58,94,96,97]. Current research focuses on combining various discharge methods, connecting surface and barrier discharges in series, enhancing the source of the gas, employing pulsed corona discharges, and increasing the energy efficiency of the systems [121]. Hafeez et al. [127] suggested integrating dielectric barrier discharge (DBD) with corona discharge, as DBD prevents arcing, allowing higher input power and more uniform discharge compared to corona discharge.

6.2. UV Disinfection

UV disinfection is recognized as an effective alternative for inactivating many waterborne pathogens without producing harmful DBPs or chemical residuals [91,97]. UV radiation deactivates microorganisms through several mechanisms. It primarily forms pyrimidine dimers in DNA and RNA, inhibiting the ability of viruses to replicate. Additionally, UV can react with ${\mathrm{H}}_2$${\mathrm{O}}_2$ to produce hydroxyl radicals, which efficiently disinfect by transforming organic molecules. UV light is directly absorbed by nucleic acids, particularly DNA, disrupting transcription or replication processes and leading to microbial death or inactivation [98,128]. UV waves are classified into Ultraviolet A (315–400 nm), Ultraviolet B (280–315 nm), Ultraviolet C (200–280 nm), and vacuum Ultraviolet (100–200 nm). The optimal wavelength for UV disinfection is 253.7 nm [95].

UV light can be emitted from mercury lamps, including low-pressure (LP) lamps that emit nearly monochromatic light at 254 nm and medium-pressure (MP) lamps that emit polychromatic spectra. However, these lamps are fragile, contain toxic mercury, require large energy inputs, and have a relatively short lifespan. This has led to the development of alternatives such as UV light-emitting diodes (UV LEDs). UV LEDs offer advantages such as mercury-free operation, small size, and fast startup time, and they can emit radiation from 210 nm to visible light. However, their limitations include low radiation flux and energy efficiency [94,128].

The incorporation of UV technology holds significant potential, particularly with the advancement of energy-efficient LED lamps. These lamps offer reduced energy consumption, making UV systems more cost-effective and sustainable. Additionally, the scalability of UV technology enhances its suitability for both small and large WWTPs, offering flexibility in various operational contexts. This adaptability, combined with its environmental benefits and effectiveness in pathogen removal, positions UV as a promising solution for widespread application in modern WWTPs [22,58,95,96].

6.3. Fenton

The Fenton-type AOP can be categorized into two main branches: homogeneous Fenton and heterogeneous Fenton. Homogeneous Fenton involves catalysis throughout the liquid phase, while heterogeneous Fenton occurs on the surface of solid catalysts [12].

The homogeneous Fenton reaction requires the use of a light source, iron, and hydrogen peroxide. It involves the reaction of hydrogen peroxide (${\mathrm{H}}_2$${\mathrm{O}}_2$) with ferrous iron (${\mathrm{Fe}}^{2+}$) to form hydroxyl radicals (${\mathrm{HO}}^{·}$), which oxidize organic and inorganic compounds. During this process, ${\mathrm{Fe}}^{2+}$ is oxidized to ferric iron (${\mathrm{Fe}}^{3+}$), decomposing ${\mathrm{H}}_2$${\mathrm{O}}_2$ into hydroxyl radicals. ${\mathrm{Fe}}^{3+}$ can be reduced back to ${\mathrm{Fe}}^{2+}$ by reacting with excess ${\mathrm{H}}_2$${\mathrm{O}}_2$, forming additional radicals and hydroperoxyl radicals (^·${\mathrm{O}}_2$H), albeit at a slower rate than the core Fenton reaction [106,107].

Heterogeneous Fenton methods have emerged to address the limitations of homogeneous Fenton methods, utilizing solid catalysts to enhance efficiency [12,104]. Sustainable metal-based heterogeneous catalysts improve the process by adsorbing ${\mathrm{H}}_2$${\mathrm{O}}_2$ on surface sites, decomposing ${\mathrm{H}}_2$${\mathrm{O}}_2$ and generating hydroxyl radicals to destroy pollutants. However, current Fenton processes face challenges, including the need for high oxidant dosages and significant energy input, which reduce their cost-effectiveness and limit their application in WWTPs, making more suitable industrial wastewater streams. Advanced techniques for the selective removal of target compounds are necessary [129]. Natural iron-containing materials, such as goethite, hematite, and magnetite, have proven effective as heterogeneous Fenton catalysts. Additionally, carbon materials like graphene can enhance the oxidation potential of Fe(III) in Fenton reactions. Despite their effectiveness, the synthesis methods for iron minerals on supporting materials are often expensive and time-consuming [130].

6.4. Ultrasound

Sonolysis encompasses an advanced treatment process involving ultrasonic cavitation induced by ultrasound, and it is considered one of the greener methods, as it does not use or produce any chemicals [104]. Although not yet widely applied in large-scale projects, most laboratory tests show promising results, typically using the horn configuration for the acoustic radiator. The primary drawback of ultrasound technology is its high energy consumption, which increases the overall energy footprint of WWTPs. However, its application can significantly enhance process efficiency, contributing to improved pollutant removal and overall treatment performance. Acoustic cavitation occurs when high-frequency sound waves emitted by ultrasound devices create alternating compressive and tensile phases in a liquid medium, reaching a critical threshold where the distance between adjacent molecules becomes sufficient to induce cavitation [131,132,133,134]. The cavitation generates ${\mathrm{HO}}^{-}$ and ${\mathrm{H}}^{+}$ radicals through pyrolysis, with dimerization of ${\mathrm{HO}}^{·}$ forming ${\mathrm{H}}_2$${\mathrm{O}}_2$, which then attacks and oxidizes the pollutants [135,136,137]. Typically, ultrasonic frequencies vary between 20 and 40 kHz, with some studies referencing higher frequency levels in the ${10}^4$ range [132,138,139].

The ultrasound treatment equipment mainly consists of an ultrasound generator device and a chamber as shown in Figure 7. Ultrasound waves are introduced into the liquid medium either via direct contact with the ultrasonic source (direct sonication) or by immersing a vessel containing the solution to be treated (indirect sonication). Indirect sonication consists of a setup with a transducer and a vibrating plate for the bath method [140]. The horn system comprises an ultrasonic electrical generator, an electromechanical/piezoelectric transducer, a waveguide, an ultrasonic horn or acoustic radiator, a mounting flange, a reactor chamber, and working liquid inlets and outlets, as shown in Figure 7. In some cases, additional components such as a cooling system with respective pipes are included [141].

The ultrasound system includes a piezoelectric transducer with electrically active piezoelectric elements that transform electrical energy into mechanical energy [142,143,144]. This system comprises a converter, a booster, a waveguide, and finally, the horn or acoustic radiator. Depending on the desired resonance frequency and transducer frequency, the converter and booster can have different configurations to achieve the desired frequency output on the acoustic radiator. The acoustic radiator or horn is responsible for propagating the ultrasound in wastewater. Different configurations are possible, with propagation occurring either over the entire cylinder or only at the bottom part. For optimal propagation of the ultrasound wave, the horn device should ideally include a synergy between a perforated geometry on the top and a solid shape on the bottom to enhance cavitation [145].

6.5. Electrochemical

Electrochemical Advanced Oxidation Processes (EAOPs) are recognized for their high efficiency and straightforward operation, and they can oxidize contaminants in two primary ways: direct and indirect electrochemical oxidation. Direct oxidation involves the direct process on the surface of the anodic electrode in the electrochemical cell, where contaminants are oxidized upon direct contact with the electrode surface. On the other hand, indirect electrochemical oxidation involves the generation of Reactive Oxygen Species (ROS) from water oxidation on the anode surface. These ROS, such as ${\mathrm{HO}}^{·}$, can diffuse into the bulk solution, thereby oxidizing contaminants indirectly. The diffusion of ROS into the bulk solution enhances the process efficiency by mitigating mass transfer limitations [110,113].

The electrochemical setup consists of an anode and a cathode. Direct oxidation occurs on the anode surface during electrolysis, while ROS generated at the anode can migrate into the bulk solution to facilitate indirect oxidation. This dual mechanism increases the overall degradation efficiency of contaminants [110,113,146,147]. Direct oxidation at the anode involves the direct reaction of pollutants at the anode surface, forming stable oxidants. This is followed by the mediated production of ${\mathrm{HO}}^{·}$ radicals, which can rapidly transform into other oxidants like ${\mathrm{H}}_2$${\mathrm{O}}_2$, enhancing the oxidation of pollutants. Additionally, hydrogen peroxide production at the cathode is achieved by the reduction of oxygen on the cathode surface, with the process being enhanced using gas diffusion cathodes that ensure contact between the cathode, water, and oxygen [111,146].

The advanced oxidation mechanism can also be related to anodic oxidation, where heterogeneous (${\mathrm{HO}}^{·}$) radicals generated at the anode surface promote both direct oxidation of pollutants at the anode and the formation of oxidants that extend the oxidation process into the bulk solution [111]. Other processes can be combined with electrochemical oxidation, including electro-Fenton, photo-electro-Fenton, and sono-electro-chemistry. Electro-Fenton (EF) involves the production of homogeneous (${\mathrm{HO}}^{·}$) radicals in the bulk solution by the reaction of ${\mathrm{H}}_2$${\mathrm{O}}_2$ with ferrous iron (${\mathrm{Fe}}^{2+}$). Photo-electro-Fenton (PEF) enhances the EF process by using UV light to photolyze ${\mathrm{H}}_2$${\mathrm{O}}_2$, increasing the production of (${\mathrm{HO}}^{·}$) radicals. Sono-electro-chemistry (SE) combines ultrasound with electrochemical processes, promoting cavitation and thereby increasing the production of (${\mathrm{HO}}^{·}$) radical [111,112,146].

6.6. Adsorption

Adsorption is widely recognized as a cost-effective and reliable method for wastewater treatment due to its efficiency in removing pollutants from aqueous solutions [4]. Adsorption involves the transfer of solutes from a liquid phase onto the surface of a solid phase through physicochemical interactions. The substance being adsorbed is known as the adsorbate, and the material onto which it is adsorbed is called the adsorbent [32], as shown in Figure 8. The process involves three main phases: migration of the adsorbate, intra-particle diffusion, and adsorption/desorption. During migration, adsorbate molecules move towards the surface of the adsorbent material. Once at the surface, the molecules diffuse into the pores of the adsorbent. Finally, adsorption occurs when the molecules adhere to the surface via chemical bonds or physical forces. Desorption is the reverse process of releasing adsorbed pollutants from the adsorbent surface back into the solution phase [119,148].

In adsorption, heavy metal ions and other pollutants from the aqueous phase are attracted and bound to the surface of a solid material. Adsorption is driven by factors such as the surface area, pore size, and surface chemistry of the adsorbent. Desorption is the reverse process where adsorbed pollutants are released from the adsorbent surface back into the solution phase [119,148].

Physical adsorption occurs due to non-specific van der Waals forces between the adsorbate and the adsorbent surface. These connections are weakly specific and reversible, involving minor thermal effects. Chemical adsorption, on the other hand, involves chemical reactions between the adsorbate and the adsorbent, leading to the formation of covalent or ionic bonds and requiring higher activation energies [32,149]. The adsorption efficiency is highly dependent on factors such as temperature, pH, stirring duration, and initial concentration of the contaminants. Higher temperatures generally increase adsorption rates, optimal pH levels vary depending on the specific adsorbent and contaminant, prolonged contact times enhance adsorption, and higher concentrations of contaminants typically result in higher adsorption rates [32,34].

Adsorbents can be divided into three main groups: low-cost sorbents, nano-adsorbents, and biosorbents. Low-cost sorbents include natural adsorbents, agricultural wastes, and industrial wastes. Nano-adsorbents encompass carbon-based nano-materials, metal oxide-based nano-materials, nano-composites, and boron nitride materials. Biosorbents are based on biological waste [4].

Activated carbon is one of the most widely used adsorbents due to its high adsorption capacity and efficiency. It has a well-developed porous structure, a large specific surface area, and a variety of surface functional groups. Activated carbon can be sourced from coal, wood, coconut shells, and agricultural waste. However, powdered activated carbon is difficult to separate from the solution and can be expensive. Commercial activated carbons are available in granular, powdered, cloth, and fibrous forms [32,149]. In addition to commonly used adsorbents like activated carbon and zeolites, polymer-based adsorbents such as grafted Nylon-6 fibers with Polydimethylaminoethylmethacrylate (PDMAEMA) have been shown to effectively remove heavy metal ions and anions from wastewater [150].

Zeolites are crystalline aluminosilicates with high ion exchange capacity, large surface area, and hydrophilicity. They can be natural or industrially produced. Clay materials, such as kaolinite, smectites, and mica, also play a significant role due to their high surface area, excellent physical and chemical properties, and good cation exchange capacity. These materials are widely used for heavy metal removal due to their availability and cost-effectiveness, though they generally have lower adsorption capacities compared to zeolites. Biomaterials, such as non-living biomass, algal biomass, and microbial biomass, are highly effective in adsorbing heavy metals, have a low cost, and are renewable [32,149,151,152]. Magnetic adsorbents are gaining attention due to their easy recovery and high adsorption potential. Lignin copper ferrite, for instance, has shown remarkable efficiency in removing crystal violet dye from wastewater under mild conditions, with adsorption driven by a physical process and supported by pseudo-first-order kinetics [153].

Agricultural wastes, including rice husks, sawdust, coconut shells, sugarcane bagasse and fruit peels, are abundant, renewable, and often discarded as waste, making them cost-effective and sustainable adsorbents [148,154]. In the realm of nanomaterials, fullerenes, carbon nano-tubes (CNTs), and graphene-based materials are among the most prominent for pollutant removal [32,124,130].

6.7. Hybrid Processes

AOPs hold significant potential for the future of WWTPs, especially in addressing the growing need to remove emerging pollutants [155]. However, their widespread application is currently hindered by high energy requirements and complex mechanisms, which pose challenges for scaling up these technologies. Numerous studies have demonstrated the benefits of combining AOPs with other treatment processes, leading to reduced energy consumption, minimized use of raw chemicals and materials, and enhanced pollutant removal efficiency. As shown in Table 4, the integration of different processes can significantly improve the removal efficiency of various pollutants.

Combining AOPs with MBR technologies, for instance, has shown great promise in mitigating membrane fouling by approximately 40% [156] while also enhancing the removal of pharmaceutical pollutants [157,158]. In processes such as ozonation, where concerns about economic viability for large-scale applications persist, integrating AOPs with technologies like ultrasound, Fenton reactions, and UV irradiation can reduce costs and simultaneously improve disinfection and degradation rates. This is achieved through the optimization of degradation efficiency, as demonstrated in recent studies [159,160,161,162].

Furthermore, the combination of AOPs with adsorbents, particularly biosorbents, offers a dual benefit: enhancing both the adsorption rate and capacity and improving the reusability of adsorbents [104,163,164,165]. This approach presents a sustainable and efficient solution for WWTPs by reducing the reliance on conventional technologies that may be inadequate for the removal of emerging pollutants. The integration of AOPs with other processes represents a viable path forward for WWTPs, as it not only reduces energy consumption but also improves water quality by targeting pollutants that conventional technologies struggle to remove. Through continued research and optimization, these hybrid approaches could pave the way for more sustainable and effective wastewater treatment solutions in the future.

Table 4. Hybrid combination between AOPs with physical and chemical wastewater treatment.

Wastewater	Process	Experimental Conditions	Removal Efficiency
Synthetic	Adsorption/Fenton [166]	5000 $\mathsf{\mu }$M ${\mathrm{K}}_2$${\mathrm{S}}_2$${\mathrm{O}}_8$; 0.05 g AG; pH 3	SMT: 78% (60 min)
	Adsorption/US [167]	0.5 g/L FeCS; 40 kHz and 300 W US (pre-treatment)	MB: 98% (10 min)
	Electro-Fenton/UV [168]	$1.42·{10}^4$ mg/L ${\mathrm{Na}}_2$${\mathrm{SO}}_4$; 15.19 mg/L ${\mathrm{FeSO}}_4·7$${\mathrm{H}}_2$O; pH 3; 0.6 L/min air flow; Felt graphite anode and cathode; 3 mA/${\mathrm{cm}}^2$ current density	MB: 99% (20 min); TC: 62% (20 min); MG and ${\mathrm{AO}}_7$ 90% (14 min)
	US/Fe(0)/S(IV) [169]	0.05 mmol/L S(IV); 0.05 g/L Fe(0); 40 kHz US	TBP: 89.6% (30 min)
	US/${\mathrm{O}}_3$/MC [161]	100 W US; pH 9; 8.6 kV Discharge Voltage; 15 mL/min flow rate; 0.8 mm microchannel width	MB: 92.7% (14 min)
Municipal	Adsorption/US [170]	1 mg Cu(BDC)@Wool; 0.5 mL/min flow rate; 7.5 mm bed height; pH 2	RIF: 98.6% (120 min)
	${\mathrm{Cl}}_2$/${\mathrm{O}}_3$/UV [171]	465 mJ/${\mathrm{cm}}^2$ UVC; 3 mg/L ${\mathrm{Cl}}_2$; 3 mg/L ${\mathrm{O}}_3$	FLU: 80%; GMF: 90%; PRM: 50%; CBZ, TMP, SMZ: >99% (continuous)
	MBR/${\mathrm{O}}_3$ [156]	5 g/${\mathrm{Nm}}^3$ ${\mathrm{O}}_3$ inlet; NF-90 polyamide membrane	CBZ and SMZ: 100% (15–20 min); TB: 100% (>30 min), APAP, TET: 100% (5 min)
	MBR/UV/PS [157]	PVDF flat UF membranes; 254 nm UV; 0.06 mmol/L PS	OMP: 100% (150 min)
	US/UV/${\mathrm{H}}_2$${\mathrm{O}}_2$ [162]	100 W and 40 kHz US; UVC; 0.5 mM ${\mathrm{O}}_3$; 7.8 mM ${\mathrm{H}}_2$${\mathrm{O}}_2$	E. aerogenes: 98.6%; E. coli: 99.1%; Other coliforms: 96.2%; Total coliforms: 98.1%; COD: 91.1% (10 min)
Industrial	(A/O)MBR/Fenton [172]	Alumina microporous membrane; 30 mmol/L ${\mathrm{H}}_2$${\mathrm{O}}_2$; 6 mmol/L ${\mathrm{FeSO}}_4$·${\mathrm{H}}_2$O	COD: 90%, AOX: 79%, ${\mathrm{NH}}_4^{+}$: 88% (continuous)
	Adsorption/Fenton [173]	Wood biochar adsorbent; 7 cm bed depth; 15 Ml/min flowrate; 10 mM ${\mathrm{Fe}}^{2+}$; 15 mM ${\mathrm{H}}_2$${\mathrm{O}}_2$; pH 3	COD: 94.5%; Sulphide: 97.4%; ${\mathrm{NH}}^{3-}$N: 96.2%, ${\mathrm{NO}}^{3-}$: 83.1%; ${\mathrm{PO}}_4^{3-}$: 79.3%; Cr(VI): 96.9% (120 min)
	Coagulation-AC/UV/Fenton [165]	UVC; 160 ppm/L alum coagulant; 1:100 AC; 1:300 ${\mathrm{FeSO}}_4$·7${\mathrm{H}}_2$O:${\mathrm{H}}_2$${\mathrm{O}}_2$	COD: 87.49%; BOD: 87.02%; TSS: 72.45%; Zn: <99%; Cu: 64%; Pb: 96%; Fe: 35% (60 min)
	Electroadsorption [174]	50 mM ${\mathrm{K}}_2$${\mathrm{SO}}_2$ electrolyte; 0.2 mM Fe (II); pH 3; coconut shell cathode; Iridium and Ruthenium coating anode	TOC: 87% (120 min)
	Electro-Fenton [175]	pH 5.95; 1.5 mL ${\mathrm{H}}_2$${\mathrm{O}}_2$; 1.8 ${\mathrm{H}}_2$${\mathrm{O}}_2$/${\mathrm{Fe}}^{2+}$; Al anode; Iron cathode; 2 A and 24 V current density	COD: 95.8% (60 min)
	EC-UV-Fenton [176]	Al anode; Stainless Steel cathode; 120 A/${\mathrm{m}}^2$ current density; pH 6.87; UVC 32 W; 0.4 ${\mathrm{Fe}}^{2+}$/HP	COD: 75.1%; Color: 93.3%; TSS: 82.0%; Aromatic compounds: 89.8% (30 min)
	US/Electro/Fenton [177]	0.2 mM ${\mathrm{FeSO}}_4$; 10 mA/${\mathrm{cm}}^2$ current density; 100 W US; 4.3 kWh/kg SEC; 0.2 mM ${\mathrm{FeSO}}_4$	COD: 91.04%; Turbidity: 84.62%; Phenols: 91.67% (56 min)
	US/UV/Fenton [160]	2 g/L FA load; pH 3; 576 kHz US	COD: 40%; Colour: 36.8%; Aromatic Compounds: 50.8% (60 min)

Ciprofloxacin; MB: Methylene Blue; TC: Tetracycline; MG: Malachite Green; ${\mathrm{AO}}_7$: Acid Orange 7; TBP: Tributyl Phosphate; RIF: Rifampicin; FLU: Fluoxetine; GMF: Gemfibrozil; PRM: Paracetamol; CBZ: Carbamazepine; TMP: Trimethoprim; SMZ: Sulfamethoxazole; AOX: Adsorbable Organic Halogens; FA: Flash Ash; SEC: Specific Energy Consumption.

Table 4. Hybrid combination between AOPs with physical and chemical wastewater treatment.

Wastewater	Process	Experimental Conditions	Removal Efficiency
Synthetic	Adsorption/Fenton [166]	5000 $\mathsf{\mu }$M ${\mathrm{K}}_2$${\mathrm{S}}_2$${\mathrm{O}}_8$; 0.05 g AG; pH 3	SMT: 78% (60 min)
	Adsorption/US [167]	0.5 g/L FeCS; 40 kHz and 300 W US (pre-treatment)	MB: 98% (10 min)
	Electro-Fenton/UV [168]	$1.42·{10}^4$ mg/L ${\mathrm{Na}}_2$${\mathrm{SO}}_4$; 15.19 mg/L ${\mathrm{FeSO}}_4·7$${\mathrm{H}}_2$O; pH 3; 0.6 L/min air flow; Felt graphite anode and cathode; 3 mA/${\mathrm{cm}}^2$ current density	MB: 99% (20 min); TC: 62% (20 min); MG and ${\mathrm{AO}}_7$ 90% (14 min)
	US/Fe(0)/S(IV) [169]	0.05 mmol/L S(IV); 0.05 g/L Fe(0); 40 kHz US	TBP: 89.6% (30 min)
	US/${\mathrm{O}}_3$/MC [161]	100 W US; pH 9; 8.6 kV Discharge Voltage; 15 mL/min flow rate; 0.8 mm microchannel width	MB: 92.7% (14 min)
Municipal	Adsorption/US [170]	1 mg Cu(BDC)@Wool; 0.5 mL/min flow rate; 7.5 mm bed height; pH 2	RIF: 98.6% (120 min)
	${\mathrm{Cl}}_2$/${\mathrm{O}}_3$/UV [171]	465 mJ/${\mathrm{cm}}^2$ UVC; 3 mg/L ${\mathrm{Cl}}_2$; 3 mg/L ${\mathrm{O}}_3$	FLU: 80%; GMF: 90%; PRM: 50%; CBZ, TMP, SMZ: >99% (continuous)
	MBR/${\mathrm{O}}_3$ [156]	5 g/${\mathrm{Nm}}^3$ ${\mathrm{O}}_3$ inlet; NF-90 polyamide membrane	CBZ and SMZ: 100% (15–20 min); TB: 100% (>30 min), APAP, TET: 100% (5 min)
	MBR/UV/PS [157]	PVDF flat UF membranes; 254 nm UV; 0.06 mmol/L PS	OMP: 100% (150 min)
	US/UV/${\mathrm{H}}_2$${\mathrm{O}}_2$ [162]	100 W and 40 kHz US; UVC; 0.5 mM ${\mathrm{O}}_3$; 7.8 mM ${\mathrm{H}}_2$${\mathrm{O}}_2$	E. aerogenes: 98.6%; E. coli: 99.1%; Other coliforms: 96.2%; Total coliforms: 98.1%; COD: 91.1% (10 min)
Industrial	(A/O)MBR/Fenton [172]	Alumina microporous membrane; 30 mmol/L ${\mathrm{H}}_2$${\mathrm{O}}_2$; 6 mmol/L ${\mathrm{FeSO}}_4$·${\mathrm{H}}_2$O	COD: 90%, AOX: 79%, ${\mathrm{NH}}_4^{+}$: 88% (continuous)
	Adsorption/Fenton [173]	Wood biochar adsorbent; 7 cm bed depth; 15 Ml/min flowrate; 10 mM ${\mathrm{Fe}}^{2+}$; 15 mM ${\mathrm{H}}_2$${\mathrm{O}}_2$; pH 3	COD: 94.5%; Sulphide: 97.4%; ${\mathrm{NH}}^{3-}$N: 96.2%, ${\mathrm{NO}}^{3-}$: 83.1%; ${\mathrm{PO}}_4^{3-}$: 79.3%; Cr(VI): 96.9% (120 min)
	Coagulation-AC/UV/Fenton [165]	UVC; 160 ppm/L alum coagulant; 1:100 AC; 1:300 ${\mathrm{FeSO}}_4$·7${\mathrm{H}}_2$O:${\mathrm{H}}_2$${\mathrm{O}}_2$	COD: 87.49%; BOD: 87.02%; TSS: 72.45%; Zn: <99%; Cu: 64%; Pb: 96%; Fe: 35% (60 min)
	Electroadsorption [174]	50 mM ${\mathrm{K}}_2$${\mathrm{SO}}_2$ electrolyte; 0.2 mM Fe (II); pH 3; coconut shell cathode; Iridium and Ruthenium coating anode	TOC: 87% (120 min)
	Electro-Fenton [175]	pH 5.95; 1.5 mL ${\mathrm{H}}_2$${\mathrm{O}}_2$; 1.8 ${\mathrm{H}}_2$${\mathrm{O}}_2$/${\mathrm{Fe}}^{2+}$; Al anode; Iron cathode; 2 A and 24 V current density	COD: 95.8% (60 min)
	EC-UV-Fenton [176]	Al anode; Stainless Steel cathode; 120 A/${\mathrm{m}}^2$ current density; pH 6.87; UVC 32 W; 0.4 ${\mathrm{Fe}}^{2+}$/HP	COD: 75.1%; Color: 93.3%; TSS: 82.0%; Aromatic compounds: 89.8% (30 min)
	US/Electro/Fenton [177]	0.2 mM ${\mathrm{FeSO}}_4$; 10 mA/${\mathrm{cm}}^2$ current density; 100 W US; 4.3 kWh/kg SEC; 0.2 mM ${\mathrm{FeSO}}_4$	COD: 91.04%; Turbidity: 84.62%; Phenols: 91.67% (56 min)
	US/UV/Fenton [160]	2 g/L FA load; pH 3; 576 kHz US	COD: 40%; Colour: 36.8%; Aromatic Compounds: 50.8% (60 min)

Ciprofloxacin; MB: Methylene Blue; TC: Tetracycline; MG: Malachite Green; ${\mathrm{AO}}_7$: Acid Orange 7; TBP: Tributyl Phosphate; RIF: Rifampicin; FLU: Fluoxetine; GMF: Gemfibrozil; PRM: Paracetamol; CBZ: Carbamazepine; TMP: Trimethoprim; SMZ: Sulfamethoxazole; AOX: Adsorbable Organic Halogens; FA: Flash Ash; SEC: Specific Energy Consumption.

7. Future Trends

The dual necessity of removing emerging pollutants while simultaneously reducing the carbon and energy footprint of WWTPs poses significant challenges for their future. These two main objectives, along with existing technological issues, highlight several key trends for the future.

• Coagulants and flocculants pose challenges due to their sourcing and disposal issues. Research should focus on developing eco-friendly biocoagulants and flocculants to address these concerns, prioritizing sustainable raw materials that avoid competition with food supply chains. Reducing extraction complexity and increasing studies on bio-based alternatives are essential. Testing these materials in real-scale projects and improving additive efficiency can help lower energy demands, contributing to more sustainable treatment processes:
• As the demand for removing emerging pollutants increases, there is a shift towards environmentally friendly physical processes that reduce reliance on chemical treatments, enhancing the effectiveness of both and striving for optimal synergy between them. Garrido-Cardenas et al. [178] observed a surge in publications on AOPs since 2015, with future trends focusing on scaling up and implementing these processes in real-world conditions. Current research primarily involves pilot-scale and batch-mode studies, but practical application necessitates a transition to continuous flow evaluations with the integration of fine-tuning operational factors and techno-economic analyses [11,12,101].
• The improvement of MBRs should prioritize enhancing membrane material stability and activity, modifying existing systems, and exploring novel sustainable materials [179,180,181,182]. Coupling MBRs with AOPs or other processes can boost removal efficiency and reduce fouling. Research must shift to real wastewater and pilot-scale testing, focusing on optimizing reactor structures, minimizing energy consumption, and reducing operational costs. Effective membrane cleaning and regeneration are essential for extending lifespan and lowering environmental impacts [102].
• Regarding US technology, sono-chemical oxidation, requires further investigation to understand its potential and integration into WWTPs. Current research focuses on conventional ultrasound devices, but future studies should explore diverse chamber designs and cavitation effects on pollutant removal. Combining ultrasound with other treatments has shown significant potential, and understanding synergies, mechanisms, and optimal configurations will be crucial for improving its application in wastewater treatment.
• Traditional Fenton limitations can be addressed by integrating other AOPs [12,183]. The shift toward heterogeneous Fenton is essential due to the impracticality of the homogeneous process. Future trends focus on using materials like zero-valent iron, iron (hydr)oxides, iron-based metallic glasses, and loaded iron-based materials to improve catalyst efficiency and reduce energy costs [183,184]. Research should optimize catalyst performance, lower operational costs, and explore synergies with other AOPs to enhance treatment efficacy [183].
• Ozone technology has advanced but still faces high energy demands and potential pollutant by-products. Scaling up requires dosage optimization, particularly when combined with Fenton, ultrasound, and UV light. Future research on ozone-based processes should prioritize economic evaluations, cost-effectiveness, reactor designs, and degradation efficiency [163]. Conducting pilot- and field-scale studies is essential to assess feasibility, while a deeper understanding of reaction kinetics and the development of robust models will enhance the optimization of ozonation-based AOPs.
• AC remains a key adsorbent in wastewater treatment, with future trends focusing on optimizing configurations [174,185]. Nanomaterials and bioadsorbents also show promise, though improvements are needed in biosorbent production and scalability for pilot applications in WWTPs. Research should prioritize combining AC with other methods to enhance adsorption capacity, reusability, and removal efficiency. Developing novel adsorbents like carbon nanotubes and metal-organic frameworks, alongside leveraging machine learning and AI for adsorption kinematics study, is crucial for advancing eco-friendly and high-capacity adsorption technologies [114].
• To enhance UV disinfection efficacy and mitigate drawbacks, combining UV with oxidants like hydrogen peroxide, ozone, persulfate, chlorine, and chlorine dioxide is promising. Future research should focus on the mechanisms behind these synergies to improve microorganism removal and reduce by-products [99]. Another key trend is using solar energy as a power source, requiring continued research on optimal conditions, pilot project implementation, and reactor development to maximize solar efficiency.
• Yuan et al. [146] and Li et al. [147] highlight trends in Electrochemical-AOPs (EAOPs) aimed at improving Reactive Oxygen Species efficiency. Key strategies include creating confined micro-environments with metal nano-particles and porous graphite for targeted antibiotic degradation, optimizing cathode properties and addressing low antibiotic concentrations. Future research focuses on developing cost-effective anode materials like carbon and graphite, enhancing toxicity assessment methods, and integrating EAOPs with UV light, ozone, membranes, and biological treatments to boost efficiency.

8. Conclusions

The recent development of WWTPs has led to an urgent need for new technologies to address emerging pollutants. Despite significant research in recent years, advancements in WWTP technologies are seldom integrated into full-scale plants. A primary challenge with current technologies is their high energy demand and the difficulty in finding sustainable alternatives to reduce the reliance on non-biodegradable chemicals. The processes used in primary and secondary treatments require considerable energy due to the need for physical and chemical methods that separate large quantities of inorganic and organic materials from water. Although continuous innovation in various processes for these treatment stages is ongoing, traditional methods remain highly effective and economically attractive. While these conventional processes are more developed due to current knowledge, it is essential to identify solutions that enhance them by reducing the need for chemicals and energy while exploring new sustainable materials and additives.

Simultaneously, the growing demand for eliminating a wider range of pollutants has led to the proliferation of various technologies, particularly for tertiary and post-tertiary treatment. However, challenges remain in transitioning from laboratory-scale studies to real-world implementation. One such example is AOPs, which are crucial for achieving sustainable and effective wastewater treatment solutions. These approaches emphasize large-scale implementation, continuous flow evaluations, and the need to address emerging contaminants like ARBs. Integrating AOPs with MBRs, ultrasound, and adsorption materials can significantly improve treatment efficiency while reducing energy consumption and minimizing harmful by-product formation. This integration enhances pollutant removal across diverse water matrices and aligns with new directives for comprehensive pollutant degradation mechanisms.

Future research should optimize operational parameters, be conducted thorough techno-economic assessments, and advance mechanistic understanding to ensure practicality and affordability. Innovations in materials science, catalysis, and process integration are crucial for overcoming current limitations and achieving sustainable water management goals. Furthermore, developing specific, cost-effective materials to remove emergent pollutants underscores the importance of integrating cutting-edge biological and material-based approaches. Utilizing solid wastes and industrial by-products promotes resource efficiency and environmental sustainability, supporting a shift towards green processes like ultrasound, photo-catalysis, and bioadsorbents.

Ultimately, wastewater treatment trends advocate for the holistic integration of advanced technologies throughout the treatment process. This approach promises enhanced efficiency, reduced costs, and improved environmental stewardship, ensuring sustainable water management practices that safeguard global water resources and public health.