Advanced Oxidation Process in the Sustainable Treatment of Refractory Wastewater: A Systematic Literature Review

Abstract

More than 4 billion people yearly suffer from global water scarcity amid climate change, rapid population growth, and growing industrial activity. Due to the high concentrations of recalcitrant organic compounds, refractory wastewater is highly resistant to conventional biological treatment and represents a critical obstacle for water reuse and sustainable water management. A systematic literature review of 35 peer-reviewed articles published from 2010 to 2025 is provided to evaluate the utilization and sustainability potential of advanced oxidation processes (AOPs) for treating recalcitrant wastewater. Using the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) framework, the review assesses numerous AOPs, such as ozonation, UV/H₂O₂, Fenton reactions, and photocatalysis, while also evaluating their performance, efficiency, and integration ability. The results show that AOPs demonstrate pollutant removal rates often greater than 96%, reduce sludge formation, and improve effluent biodegradability. They can be applied at different treatment stages, combined with any renewable energy systems, and therefore can scale and be sustained, thereby aligning with UN Sustainable Development Goal 6. AOPs provide a technically feasible and eco-friendly solution for higher quality wastewater treatment. In the face of increasing pressure on global water resources, and the urgent need for sustainable water resource management, this study offers valuable insights for policymakers and practitioners aiming to adopt resilient and circular strategies for water.

1. Introduction

Water is an essential natural resource that supports nearly every life form on the Earth’s surface. Although water covers over 70% of the Earth’s surface, only 2.5% is available for human use [1]. Further breakdown indicates that only 1% of the available freshwater is accessible and can be used to support domestic needs and commercial purposes [1]. Studies indicate that freshwater is becoming increasingly scarce, and more than 2 billion people globally have limited access to safe drinking water [1,2]. In 2022 alone, more than 4 billion people faced water shortages at least once in 30 days [1,2]. This number would increase significantly if natural freshwater sources were either depleted or become inaccessible. A sustainable solution to the water shortage problems lies in the use of advanced technologies to convert more wastewater into a valuable economic and social resource.

Advanced wastewater treatment is a multidimensional approach aimed at purifying wastewater and making it available worldwide for either domestic or commercial reuse. Advanced wastewater treatment relies on sophisticated methods and technologies to remove organic contaminants and other complex compounds that would otherwise be discharged back into circulation for reuse [2]. These advanced processes can be considered tertiary because they purify water that has undergone primary and secondary treatment processes [2,3]. However, these advanced treatment processes are not just designed for removing complex compounds of wastewater. According to Amor et al. [3], advanced wastewater treatment methods and technologies are also designed to meet stringent environmental standards to protect ecosystems from potentially harmful carbon compounds. A study by Mousset et al. [4] found that sustainability is one of the key considerations behind advanced treatment processes to enhance efficiency and reliability.

This study explores one of the most sustainable ways of treating refractory wastewater to make it more viable for reuse. Refractory wastewater contains organic compounds that are highly resistant to various biodegradation mechanisms [2,4]. This makes it difficult to remove using conventional wastewater treatment mechanisms, where bacteria are mostly used to break down organic compounds. Refractory organics mostly originate from pharmaceutical factories, paper-making factories, textiles factories, and even oil refineries [4]. Refractory organic compounds can also be dangerous when released into landfills or nearby rivers and lakes. For instance, when absorbed into lakes, refractory organics can release harmful chemicals that destroy marine vegetation and could significantly cause irreparable harm to aquatic life [2,4,5]. The removal of refractory organics calls for innovative and sustainable mechanisms to prevent their causing potential environmental damage.

Biological and physicochemical systems are often insufficient when applied to the treatment of industrial effluents with persistent pollutants, despite conventional wastewater treatment advances. Conventional processes like activated sludge and trickling filters are mainly meant for biodegradable organics and lack the ability to break down high-strength, chemically complex constituents [6]. These limitations lead to incomplete treatment, excessive sludge production, and an inability to meet stringent regulatory discharge standards. Additionally, traditional methods are not flexible enough to respond to changing contaminant loads and newly identified pollutants, limiting the effectiveness of wastewater treatment approaches and impeding progress towards water security on a global scale [7].

AOPs overcome some of these limitations effectively by generating ·OH during non-selective oxidation mechanisms to mineralize pollutants [8]. AOPs mineralize recalcitrant species to carbon dioxide and water and improve the quality and biodegradability of outcoming effluents [9]. At the same time, high removal efficiencies of pollutants and a decrease in the production of sludge have been achieved with technologies such as ozonation, UV/H₂O₂, Fenton reactions, and photocatalysis [10]. Thus, AOPs can contribute to meeting the main objectives of Sustainable Development Goal 6 (Clean Water and Sanitation), through effective strategies for reducing toxic material emissions from point sources, enhancing wastewater reuse, and removing chemical contaminants to achieve water quality targets set by international sustainability agendas [11].

Challenges such as high capital and energy costs, operational complexity, and the need for advanced monitoring hinder largescale implementation of AOPs [12]. The performance of EC is also affected by the water matrix composition and the selection of the reagent [13]. These barriers highlight the need to consider AOPs not only in terms of their technical efficiency, but also their economic and environmental sustainability, especially as countries seek to adopt long-term strategies consistent with SDG 6 [11].

The treatment of refractory wastewater can occur through disposal filters and membranes, electrochemical removal, and AOPs. Disposal filters and membranes are often used to remove micro-organic compounds that may still be present after wastewater undergoes primary and secondary treatment processes [4]. Mechanisms such as reverse osmosis can be applied to a selective barrier to allow smaller molecules to pass while retaining larger ones [4,5]. Wastewater then undergoes other examination processes to remove other potential contaminants. Depending on the types of suspected molecules in wastewater, membrane filtration can take many forms, including microfiltration, ultrafiltration, and reverse osmosis [5]. Nanofiltration often targets the tiniest molecules that cannot be removed through microfiltration or ultrafiltration [2,5]. Membrane filtration is often used in cleaning wastewater from food and beverage factories, municipal wastewater, and seawater desalination.

Electrochemical removal involves passing wastewater through electric currents to remove organic and inorganic contaminants. The treatment process often occurs through various steps, including electrocoagulation, electro-flotation, electrodialysis, and electrolytic metal separation [3,4,5]. Electrocoagulation is where precipitates of dissolved ions can be moved downstream as solids or liquids to separate them from other water particulates. Electro-flotation helps in separating microbubbles of solids from water by suspending or floating them using electric currents [5]. The materials that remain in situ can then be removed from wastewater using chlorine as a disinfectant, a process known as hypochlorite electrolysis [14]. Electrochemical removal has several advantages, including the ability to adapt quickly to wastewater fluctuations by switching to lower or upper currents depending on the water volume [6,7]. However, electrochemical treatment is a high-energy consumption process and may generate intermediate metabolites that may not be safe for consumption.

One of the most common mechanisms used in treating refractory wastewater is AOPs. The study by Spiniello et al. [14] describes advanced oxidation as a chemical process that breaks down complex contaminants using highly reactive oxygen species such as hydroxyl radicals (·OH). The core principle behind advanced oxidation processes (AOPs) is the generation of a highly reactive species (hydroxyl radicals) that has proven more effective against highly resistant organic and inorganic contaminants [2,5,14,15]. This gives AOPs greater advantages over conventional wastewater treatment processes because they are capable of removing even the most stubborn organic compounds, including pesticides and contaminants from oil refineries [15]. Moreover, AOPs consume less energy compared to the electrochemical treatment processes, making it easier to integrate into modern wastewater treatment facilities used by municipalities to clean wastewater.

Unlike other wastewater treatment methods, advanced processes are designed to target the most complex or challenging contaminants that would be impossible to eliminate using conventional methods. Examples of these complex contaminants include personal care products, pesticides, pharmaceuticals, and endocrine-disrupting chemicals [15]. These contaminants are often embedded into water particles, making it difficult to remove using traditional wastewater treatment mechanisms. AOPs are designed to eliminate these stubborn contaminants and make refractory wastewater safer for commercial consumption or reuse at home [5,15,16]. AOPs have been hailed for their efficiency, minimal chemical use, and reduced sludge production [16]. Moreover, AOPs can be integrated into existing water treatment plants as tertiary mechanisms to remove the most persistent organic contaminants [15,16]. Some of the most common AOPs that will be discussed in this study include ozonation, Fenton reactions, UV/H₂O₂ combination, photocatalysis, wet air oxidation (WAO), and ultrasonic irradiation.

The number of people reporting severe water scarcity has been growing steadily due to climate change and pollution. While water makes up more than 70 percent of the Earth’s surface, only a small portion is accessible for human consumption or industrial use [1]. Ocean water, for instance, must undergo a complex desalination process to make it available for human and commercial use. The United Nations estimates that more than 700 million people globally will be displaced from their homes due to severe water scarcity [1]. Moreover, the United Nations estimates that at least 1 in every 4 children will be living in areas affected by acute water shortages by 2040 [1]. If not addressed, water scarcity may also expose a huge portion of the human population to various diseases and premature deaths.

Demand-driven water scarcity is a common problem usually experienced in densely populated areas such as cities and major urban centers. However, population-driven water scarcity often occurs when the number of people living in a particular area exceeds the water supply, leading to per capita water scarcity [2,16]. While factors such as population and demand can be controlled, the world is facing a more complex challenge in the form of climate change [2,4,5]. According to Mirza et al. [15], climate-driven water shortage may occur due to increased cases of droughts and fluctuations in rainfall patterns even in places that were once considered cool and wet zones. South Africa, for instance, is a notable case of a water-scarce country. The country’s annual precipitation is down to 450 mm, significantly lower than the global average of 850 mm [1]. Apart from South Africa, there are more than 190 countries facing water scarcity challenges.

Pollution-driven water scarcity is an emerging challenge due to increased industrialization and chemical spillage on existing water bodies. Unlike climate- or demand-driven water shortages, pollution-driven water scarcity is not a quantity issue [15]. In this case, water is available in large quantities but is unsuitable for either human or industrial use. Numerous companies or factories have been accused of discharging their industrial effluents into nearby water bodies, including creeks and rivers [2,16]. Some factories discharge their waste into landfills, where they gradually dissolve into the surrounding soils, eventually polluting groundwater below [17]. Pollution-driven scarcity remains a significant concern due to its complex nature and the ability to destroy large volumes of water within a short duration. Even with strict environmental laws already enacted in various countries, pollution remains a pervasive issue that requires multifaceted solutions.

Wastewater treatment innovations offer a viable opportunity to save the world from growing water scarcity. However, due to massive population growth, industrialization, and stricter environmental laws, wastewater treatment has not achieved its full potential [1,17]. Some of the most innovative solutions have not been explored fully due to potential environmental concerns [6,15,17]. Massive population growth and industrialization mean there is ever-growing volumetric capacity of wastewater that should be cleaned and returned to circulation [17]. Whether there are technologies that can meet the massive demand for water remains debatable. Most municipal wastewater treatment facilities experience perennial capacity challenges and the inability to scale up quickly to meet urban needs [3,5]. Researchers are currently working on more sustainable wastewater treatment mechanisms to increase water supply with a minimal carbon footprint.

Wastewater treatment mechanisms also need to overcome complex contaminants, especially from chemical processing factories, such as pesticides, pharmaceuticals, textiles, and pulp or paper manufacturing [5]. Traditional wastewater treatment methods have proven less capable of removing stubborn contaminants, especially from industrial effluents [15]. Most of the complex contaminants contain high levels of grease, corrosive chemicals, and heavy metals. The high amount of sludge produced during wastewater treatment also remains a significant concern because it leads to pollution [5,17]. Removing these complex contaminants has proven not just difficult, but also costly for most municipal facilities. According to Moravvej et al. [17], this explains why AOPs are becoming popular, because they are more efficient, less costly, consume less energy, and create minimal environmental concerns. This study explores not just how AOPs work but also their advantages over conventional wastewater treatment methods because in the literature there are still limitations regarding how these topics are analyzed, compared to how analysis is performed in this research.

This study is limited in scope to AOPs and their potential advantages over traditional wastewater treatment, focusing on refractory wastewater. The author investigated this problem by conducting a review of current studies on the use of AOPs in treating wastewater. The contents of this study are organized into an introduction and background, AOPs, methodology, results, discussion, and conclusions. The findings of this study will assist researchers, regulators, administrators, and investors in identifying sustainable wastewater treatment mechanisms that can be applied in large-scale facilities to increase water supply and minimize scarcity. Additionally, this study’s findings will assist in filling gaps in the literature regarding the use of AOPs in sustainable wastewater treatment.

The overarching purpose of this study is to examine the application of AOPs in refractory wastewater treatment from a sustainability angle. The study focuses on the growing need to implement AOPs in wastewater management facilities. In doing so, municipalities are more likely to increase water supply, minimize energy consumption, reduce environmental pollution, and address other existing challenges affecting conventional wastewater treatment methods.

2. AOPs

This section briefly discusses various applications of AOPs in treating wastewater. The section outlines why using AOPs offers more advantages over conventional water treatment mechanisms. Apart from the applications, this section also outlines the most common AOPs used in the treatment of wastewater. The main argument is that AOPs are the most viable mechanism for degrading recalcitrant compounds that would be difficult or impossible to eliminate using traditional mechanisms.

Among the applications of AOPs in the treatment of refractory wastewater, the removal of persistent organic compounds is the biggest driving factor, the authors of [2] finding that AOPs have highly reactive oxygen species that can destroy diverse organic compounds. Traditional wastewater treatment mechanisms have a limited supply of these reactive species, making them unsuitable for refractory wastewater [3,4,5]. However, one of the challenges often reported is the control mechanisms needed to achieve maximum efficiency and effectiveness. The use of monitoring and feedback loops helps in achieving optimization, especially by monitoring the chemical oxygen demand (COD) level [15]. A significantly lower COD level shows that organic compounds have been significantly reduced or eliminated [2,17]. This allows refractory wastewater to be pushed to subsequent stages for further processing and treatment.

AOPs are prominent destroyers of a specific type of pollutant known as persistent organic pollutants (POPs), common in refractory wastewater [2]. AOPs achieve this critical milestone by breaking down complex organic pollutants into simpler and less toxic compounds [3,17]. One of the emerging challenges associated with traditional wastewater treatment methods is the large deposits of sludge, often exacerbating management and cost concerns. AOPs address this issue by reducing the volume and toxicity of the sludge, making it environmentally friendly and easier to dispose of, even for use as organic fertilizers [18]. AOPs also assist other traditional wastewater treatments by breaking down recalcitrant organics into simpler compounds that are biologically available [19]. This means that biological processes can be used to further destroy the remaining compounds and purify water for domestic and commercial applications.

Refractory wastewater is often aesthetically repulsive due to its bad color and odor. In most cases, the color and odor of refractory wastewater come from industrial biochemical contaminants, making the water unsuitable and potentially harmful [16,17,19]. AOPs address this problem by destroying or oxidizing the compounds associated with the bad odor and color [3,19]. The diverse applications of AOPs make them suitable for standalone or complementary wastewater treatment processes. For instance, most municipalities use AOPs as complementary treatment processes to remove the most persistent organic and inorganic contaminants [2,18]. Moreover, AOPs help traditional wastewater treatment facilities to meet strict environmental regulations [2]. Contemporary wastewater management facilities are designed to meet several goals, including the growing ecological challenges associated with industrial facility waste.

2.1. Hydroxyl Radicals Oxidation

The hydroxyl radical is the most powerful oxidizing agent used in wastewater treatment. Unlike other reactive species, OH· species have oxidation potential ranging from 2.8 V (pH 0) and 1.95 V (pH 14) [2]. This makes them not just the most powerful, but also the most common reactive agent used in wastewater treatment. Moreover, hydroxyl radicals are non-selective in behavior, making them applicable to a wide variety of wastewater treatment, including some of the most stubborn contaminants [3]. Hydroxyl radicals are often generated when hydrogen peroxide (H₂O₂) reacts with ultraviolet (UV) light to create a carbon compound that effectively destroys chemical bonds in the pollutants, eventually eliminating the organic compounds [3,4,15]. Due to their shorter lifespan, hydroxyl radicals can only be generated in situ through a combination of various oxidizing agents, including H₂O₂ and ozone (O₃), irradiation (using UV light), and catalysts such as ferrous ion (Fe²⁺) [2,3,16].

Hydroxyl radicals mainly destroy pollutants using either radical addition or hydrogen abstraction. Radical addition is commonly used because it relies on available radicals such as sulfates or OH· [2,3]. Once these radicals are produced, they are added to the organic pollutants in wastewater, eventually breaking them down through oxidation into smaller and less toxic compounds [3,5,17]. Radical addition can be used as a pre-treatment mechanism to break down persistent contaminants before proceeding to other treatment processes [19]. The fact that these reactive agents are generated naturally makes the process more cost-effective than other treatment processes. In most cases, radical addition also facilitates the mineralization of the pollutants into water and carbon dioxide gas [2,19]. The resulting compounds are less toxic and can be channeled toward reuse or disposed of without raising environmental concerns.

Hydroxyl radicals can also attack pollutants through electron transfer, hydrogen abstraction, or a combination of radicals. Hydrogen abstraction is where a hydrogen atom is extracted from an organic molecule, leaving behind a carbonated radical, thereby initializing the biodegradation process [2,3,14]. This method is often applicable to complex organic molecules that are difficult to reduce through other oxidation processes [4]. In some cases, hydroxyl radicals trigger other positive reactions by transferring an electron from an organic molecule, creating a positively charged radical that can react with other oxidizing agents. There are also cases where both hydrogen abstraction and electron transfer are combined to obtain optimal results, especially when dealing with more complex organic molecules [3,20]. Hydroxyl radicals have demonstrated significant flexibility and capacity to react with diverse organic molecules regardless of the contaminant’s source [2,3,20]. This demonstrates why future wastewater treatment processes will preferentially employ AOPs over other treatment mechanisms.

2.2. Ozone-Based AOPs

Even without indirect oxidation, O₃ itself remains a strong oxidizing agent with an oxidation potential of 2.07 V vs. SCE (saturated colonel electrode) [2,3,5]. Direct oxidation, however, is a selective reaction that limits the capacity of O₃ to break down a variety of organic compounds. Instead, the resulting reactive species can only target ionized or disassociated forms of organic pollutants, making the process less satisfactory [20]. In the presence of water and other favorable conditions, O₃ reacts to form OH· and four oxygen (O₂) atoms [2,3]. Being a more powerful reactive species, hydroxyl radicals can destroy more organic pollutants without any selective reaction [2,18,19]. Moreover, the process can be scaled up or down depending on the volume of wastewater, making the treatment process more efficient and controllable.

Apart from being a strong oxidant, ozone-based AOPs are also considered for their environmentally friendly by-products. The decomposition of O₃ often leads back to O₂ and other harmless chemical residues in the treated water [2]. O₂ is a harmless chemical that is not only environmentally friendly but also needed for respiration by both animals and plants [3,20]. Increasing the supply of O₂ in the atmosphere makes this process more productive in multiple dimensions. There are also higher chances of obtaining mineralized by-products, such as carbon dioxide and water, after the decomposition process [5,19]. The mineralized by-products are also environmentally viable and differ from other processes that generate potentially harmful carbon compounds, such as chlorofluorocarbons associated with the greenhouse effect [2,19].

According to Jamil [2], the main issue that remains a significant threat affecting AOPs is the high installation cost. The high installation cost remains a significant challenge to corporations and municipalities [2,16]. The cost barrier also favors the application of traditional wastewater treatment mechanisms that have proven less effective in the presence of recalcitrant organic compounds [17,21]. Moreover, the frequent adjustment of pH levels during the treatment process makes it difficult to achieve without sufficient monitoring devices. There is a need for automatic monitoring tools with real-time feedback to enable timely adjustments and optimal results [21]. Moreover, the general design of the AOP facility should ensure there is sufficient contact time to optimize the reaction and destruction of various contaminants.

2.3. UV Radiation

UV-based AOPs rely on UV radiation to initialize the photolysis of O₃ or H₂O₂, leading to the formation of highly reactive hydroxyl radicals [2,21]. This means UV light plays a pivotal role in decomposing H₂O₂ into constituent hydroxyl radicals. For instance, a H₂O₂ molecule is broken under UV radiation to create two OH· radicals [2,19]. At a different wavelength (less than 242 nm) OH· can also be generated through photolysis of water (H₂O) [3]. Both chemical formulae for these processes are illustrated as follows:

H₂O₂ + hv → 2OH·

(1)

H₂O + hv → OH⋅ + H⋅

(2)

In a different approach, UV irradiation can also generate hydroxyl radicals in the presence of a photocatalyst. The most common catalyst used in producing hydroxyl radicals is titanium dioxide (TiO₂). The UV light helps in breaking down TiO₂ molecules into positive and negative ions with an oxidative capacity [2]. These radicals eventually mineralize organic pollutants into less toxic by-products, such as carbon dioxide and water [11]. Photocatalysis is often valued because it is highly efficient in producing hydroxyl radicals, leading to the destruction of micropollutants [2,21]. In most municipal wastewater treatment facilities, photocatalysis is often applied as a pre-treatment or post-treatment mechanism to disinfect water and make it available for large-scale consumption.

UV irradiation has become increasingly popular in wastewater treatment plants because it is highly efficient in disinfecting water and leaves behind minimal or less harmful by-products [16,19]. Compared to chlorine, which has been used for centuries to disinfect water, UV-based processes offer significant advantages [2,21]. As a water disinfectant, the use of chlorine goes back to the 19th century or earlier. Its application in water treatment was first discovered when researchers found that chlorine can help in controlling a wide variety of waterborne diseases [2]. Moreover, years of research have also helped in discovering advanced chlorine-based solutions, including chlorine dioxide, chloramines, and sodium hypochlorite [3]. The main problem with chlorine-based solutions has been the formation of potentially toxic by-products [4,22]. This has prompted researchers to explore other innovative alternatives, including UV irradiation.

2.4. Fenton Reactions

Fenton-based AOPs rely on metals such as iron to activate H₂O₂ and produce hydroxyl radicals in water. Iron is the most popular metal chosen for this process because its reaction with H₂O₂ generates a highly reactive oxygen species [22]. The strong oxidative potential makes this process more suitable for destroying micropollutants in acidic wastewater from industrial effluents [3,22]. The Fenton process usually involves a reaction between H₂O₂ with ferrous ions (Fe²⁺) to create highly reactive species [2,19,21]. The highly reactive species are often described as hydroxyl radicals, although Fe²⁺ ions are also produced as by-products of the reaction. The conventional Fenton reaction often involves the following:

Fe²⁺ + H₂O₂ → Fe³⁺ + OH⋅ + H⁻

(3)

Classical Fenton reactions often occur in acidic conditions and are usually applied in the pre-treatment or tertiary stages to remove persistent contaminants. The process can also occur in the presence of UV light (known as photo-Fenton) to increase the production of hydroxyl radicals and make the process more efficient [2,3]. The UV light helps speed up the reduction of ferric ions (Fe³⁺) to Fe²⁺, thereby enhancing the production of hydroxyl radicals [2,23]. Moreover, Fenton reagents can also be created using electrochemical mechanisms, making the process more sustainable and efficient [23]. The choice of Fenton-related AOPs or other AOPs discussed in this essay may be based on the overall water pH levels and prevailing conditions [3,23]. Studies have shown that Fenton-related AOPs are more suitable for destroying micropollutants in acidic conditions.

2.5. Other AOPs

Apart from the AOPs discussed above, other less common AOPs include WAO, electron beam irradiation, and ultrasonic irradiation. WAO oxidizes pollutants using O₂ at significantly higher temperatures and pressure [24]. The method is mostly used in sewage sludge to reduce the concentration of pollutants and make it easier to decompose using biological processes [2,24]. Electron beam irradiation relies on electrons to activate water molecules and generate highly reactive radicals needed to decompose stubborn contaminants. The process is often preferred in cases where there is a need to clean large volumes of wastewater without chemical compounds [25]. Ultrasonic irradiation creates hydroxyl radicals by passing ultrasonic waves into water, creating bubbles and intense heat, eventually breaking water molecules into reactive radicals [26]. The process can be used alongside other AOPs to enhance the generation of reactive radicals and overall efficiency.

All the core Advanced Oxidation Process (AOP) mechanisms highlighted in this study are summarized in

Table 1. Representative reactions of Advanced Oxidation Process (AOP) mechanisms.

AOP Method	Representative Reaction
Ozonation	O3 + H2O → 2 ·OH + O2
Fenton Reaction	Fe2+ + H2O2 → Fe3+ + ·OH + OH−
Photo-Fenton Reaction	Fe3+ + H2O + hv → Fe2+ + ·OH + H+
UV/H2O2 Process	H2O2 + hv → 2 ·OH
Photocatalysis (TiO2-based)	TiO2 + hv → e− + h+ h+ + H2O → ·OH e− + O2 → ·O2−

as follows.

3. Methods

The research design for this study was a systematic literature review conducted to showcase the sustainable benefits of AOPs for refraction wastewater treatment. This approach was chosen for its capability to summarize existing evidence, minimize overlap of papers, and to highlight gaps or underdeveloped areas in the literature, which may inform future research as well as policy [27]. Systematic reviews, an increasingly important tool in environmental and technology-related research to support evidence-based decision-making [28], provide organization and reproducibility in the evaluation of large bodies of evidence compared to experimental approaches (such as randomized controlled trials (RCTs)).

The systematic literature review was conducted in five sequential steps, in accordance with best practice for review methodology:

• How a Research Question is Framed:

Key Question: What are the sustainable advantages of AOPs in refractory wastewater treatment? This question, for example, defined the domains of the study, keywords, and search strategies. A clearly articulated research question is vital in systematic reviews to establish the pertinence and consistency of selected evidence sources [27].

2.

Identifying Relevant Sources:

The literature search was performed using the main academic databases, such as Scopus, Springer, ScienceDirect, and MDPI. The review emphasized peer-reviewed studies published between 2010 and 2025 to make sure the findings reflected current technological advances and environmental priorities. The time window restriction ensured that work carried out before AOPs were available as a technology [27] was not included.

3.

Usefulness of Studies:

All sources identified were assessed against the following quality criteria: research design, methodological robustness, feasibility and clarity of research questions, sample size, data analysis techniques, and potential bias. This qualitative appraisal process guaranteed that only trustable and scientifically robust studies were selected. The selection process was structured according to the PRISMA model (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) to facilitate informative reporting, minimize bias, and ensure reproducibility [29].

4.

Summarizing the Evidence:

A summary and synthesis of the chosen studies is presented in

Table 2. Inter-rater agreement.

Article Groups	Rater 1	Rater 2	Rater 3	Rater 4	Agreement
1	4	6	3	7	1
2	5	5	2	5	1
3	7	4	4	3	1
4	6	3	4	3	1
5	2	4	7	2	1
Total					5/5

, illustrating major findings, research methodologies, and noted benefits of AOPs in wastewater treatment. Topics included efficiency, environmental impact, energy use, and biological fit. The summary table facilitated comparisons across studies and contributed to the evaluation of the consistency and divergence of findings, as well as gaps that remain in the literature.

5.

Interpreting the Findings

The last step required a critical appraisal of the condensed evidence, particularly relative to selection and publication bias, data heterogeneity, and the degree of confidence in the evidence-based conclusions. The researcher was careful to disseminate results that were both correct and actionable for those working in the industry as well as for policy. It is recommended that systematic reviews interpret the strength of the existing body of evidence, and whether it is robust enough to support proposed interventions or whether further studies are warranted [30].

3.1. Framing the Question

Framing the research question was one of the most significant steps in this study because it determined the overall direction that the research took, especially in selecting and evaluating relevant sources. The study by Matafonova and Batoev [29] argues that questions for review should be framed in a structured way to ensure clarity and eliminate potential ambiguity. Once the questions are developed, modifications may occur in cases where the researcher wants to include different or alternative ways of analyzing data. For instance, if there is a need to change the research protocols, interventions, or study outcomes, the researcher may change the questions to reflect the required changes. This did not occur in this case because the research design focused on gathering data or evidence from past sources obtained from reputed journals and databases. A few government publications, book chapters, and technical reports were included in the review and are cited across other sections of the study.

The review question asked was, ‘What are the sustainable advantages associated with AOPs in refractory wastewater treatment?’ This question examines the advantages of sustainability associated with the AOPs when applied to the treatment of refractory wastewater. This question significantly contributes to this study’s originality because this area has received minimal research compared to other aspects of AOPs. This study provides evidence (highly evaluated evidence) that decision-makers can use to support the integration of AOPs in their wastewater treatment plants. This review question guided the search and selection of relevant sources to provide the required evidence. Additionally, the review question helped in developing appropriate inclusion and exclusion criteria to determine the quality of sources needed for inclusion.

3.2. Identifying Sources

The search for sources was extensive due to limited research on the study’s topic. The researcher conducted electronic searches through the Scopus and Web of Science databases to identify appropriate sources for the study. Digital sources were preferred in this case because they are easier to access and can assist reviewers in verifying information before publication. The electronic search involved keying in free texts obtained from the review question. The texts that were keyed in the search boxes include “sustainability advantages associated with the application of advanced oxidation processes in treating refractory wastewater”. The Boolean operators used in these databases also helped combine the keywords to broaden the search and produce relevant sources. The researcher also expanded the search by extracting more sources from the listed references in the selected sources—a method known as snowball sampling [30]. Snowball sampling helped obtain sources that did not provide direct answers to the research question but were relevant to the study.

However, before the selection began, and based on the review question, four-factor inclusion criteria were developed to guide the search and identification of appropriate studies. The factors included language, format, timeframe, and type of research. For language, this study only included articles published in the English language. Regarding timeframe, this study included articles published between 2010 and 2025, a period of 15 years, to enhance relevance and accuracy of the findings. The 2010–2025 period was chosen because it provides more recent evidence and updates relating to various aspects of AOPs, including efficiency, catalysts, and energy consumption. No geographical location was specified in this study because data could be obtained from studies conducted in Europe, the United States of America, China, and other Asian countries. Regarding the format, this study focused on peer-reviewed journal articles because these undergo vigorous valuation by experts in the field [30]. For types of research, the study included case studies, quantitative and qualitative studies, longitudinal studies, reviews, and empirical papers.

3.3. Selecting and Evaluating Sources

The PRISMA model helped in selecting and evaluating sources to ensure they met the inclusion criteria described above. The four main stages included identification, screening, eligibility, and inclusion. The PRISMA checklist document is provided in the Supplementary Materials (see Table S1), and the flowchart presented in Figure 1 provides a summary of the steps and results, including the number of articles included in the study. Once the sources were identified, they underwent vigorous screening to ensure they met the eligibility or inclusion criteria. Questions that were asked during the screening included the following:

• Is the article relevant to the review question?
• Is the article published between 2010 and 2025?
• Does the article explore sustainability advantages associated with the application of AOPs in treating refractory wastewater?
• Is the source a government document, journal, website, policy paper, or book?
• Does the article’s affiliation or funding indicate any significant bias regarding the findings?

These questions were based on the inclusion criteria and helped in screening each article to ensure they were eligible for inclusion. The researcher worked with several reviewers who were trained in how to review the articles based on the review questions. The researcher also conducted a pilot test with the help of reviewers to check the clarity of the questions and ensure every item in the inclusion criteria was met. Moreover, the pilot testing helped in improving the inter-rater reliability as well as Cohen’s kappa. In this case, Cohen’s kappa was used to measure the agreement between raters to determine if they effectively collaborated on identifying specific articles to be included in the study. These responses (re inter-rater agreement) are shown in Table 2.

A total of 350 articles were obtained from the databases after searching. After screening based on the inclusion criteria, 78 articles were removed because they were too general and not relevant to the research questions. Another 32 articles were removed because their publication dates fell outside the inclusion criteria (between 2010 and 2025). The remaining 240 articles were subjected to abstract analysis to determine if they were eligible for review. The abstract analysis helped in narrowing the list to 125 articles. These articles were then subjected to full-text analysis to determine the sustainable advantages associated with the use of AOPs in treating refractory wastewater.

Based on the review question and the inclusion criteria, the reviewers went through each article, examining the content, background, methodology, findings, and conclusions. Only articles that met the eligibility criteria were selected for review. The reviewers also had to agree on the level and quality of evidence before developing the final list. The strength of the evidence was ranked as poor, fair, moderate, good, and very good. From a list of 125 articles, the reviewers settled on 35 for inclusion based on their relevance and high quality of evidence. The selected articles had detailed findings and addressed all the aspects of the review question. The remaining articles were also used in other sections of this study to provide detailed background and discussion. Figure 1 below shows the PRISMA model as it was applied in this study.

The researcher classified the retrieved articles based on their titles, type of document, authors and date, and findings. This is shown further in the detailed data review table presented in the results section. With the help of four reviewers, the researcher was able to conduct a vigorous content analysis, focusing on the quality of the findings, consistency, and reliability. The four reviewers also helped in reducing potential biases in the review process, including selection bias. The reviewer examined the results of each article and agreed on whether it should be included or excluded depending on the relevance to the review question. The researcher then conducted the overall analysis and presented the findings as shown in the results table (see Table 2).

3.4. Analyzing Data

The analysis focused on the quality of the findings and themes related to the sustainable advantages associated with the use of AOPs in the treatment of refractory wastewater. Some of the main themes that emerged during this analysis included efficiency, operational costs, and environmental impacts. The analysis also examined the reliability of the findings, employing Cohen’s kappa. As 35 articles were included for review and subsequent analysis, the reviewers settled on number 10 as the consensus percent. They were classified into five categories as detailed in the inter-rater agreement table above. The reviewers then decided what rating values the two selected had in agreement and measured this against the total ratings. The results confirmed 5 out of 5 agreements between the raters, indicating 100 percent agreement. The data provided in this study are very reliable and can probably be used in a general way for improving wastewater plants.

3.5. Data Synthesis Methods

The data were synthesized using themes obtained from the articles. Based on the review question, the researcher focused on themes regarding the sustainable advantages that wastewater treatment facilities may gain from the application of AOPs. Some of the key themes that were found across the 35 articles included efficiency, flexibility, operational costs, rapid reaction rates, and minimal sludge production. Thematic analysis, according to Macías-Quiroga et al. [30], assists researchers in gaining a deeper understanding of the findings and drawing logical conclusions. The researcher conducted thematic analysis in three steps, including textual coding, development of descriptive themes, and analyzing each theme based on the review question. Textual codes were derived from the patterns of findings and wordings found across the 35 articles. Most of the codes were developed during the full-text analysis of the articles. The next step involved developing descriptive themes based on the textual codes. The final stage involved analyzing each theme based on the study topic and review questions.

3.6. Potential Limitations

The researcher was aware of potential biases that may emerge when using a systematic review of literature as a study design. Some of the key potential biases that can be examined include selection bias, publication bias, and study design bias. Publication bias was not significant for this study because it mainly occurs when publishers select articles whose findings are significant while ignoring articles with less significant results [29]. The broader approach that was adopted in this study helped in identifying a wide variety of articles, most of which were eliminated based on the inclusion and exclusion criteria. The risk of publication bias was reduced by expanding the search base to collect a wide variety of articles relevant to the topic. Selection bias was further reduced with the help of reviewers who examined each article, evaluated the findings, and agreed on whether it should be included or not based on the content quality and relevance. Moreover, grading articles based on the quality of evidence (poor, fair, moderate, good, and very good) helped in selecting the most appropriate articles based on facts, rather than the researcher’s expectations.

4. Results

Implementation of the PRISMA technique and ranking of the articles based on their quality and relevance helped in identifying 36 articles, as shown in

Table 3. Selected literature.

Title	Type of Document	Authors and Date	Findings
UV-based advanced oxidation processes for the treatment of odor compounds: Efficiency and by-product formation.	Journal Article	Zoschke et al., 2012 [31]	Indicates how UV-based AOPs remove bad odor compounds by oxidizing the organic compounds associated with the property. This helps in cleaning water from factories and municipal facilities by removing bad smells and colors.
Solar advanced oxidation processes as disinfection tertiary treatments for real wastewater: Implications for water reclamation.	Journal Article	Barcelo et al., 2013 [32]	Highlights how UV-based AOPs can be used in the tertiary treatment of wastewater as a disinfectant to kill harmful bacteria, fungi, and other contaminants. UV-based AOPs contain highly reactive species that destroy or inactivate microbes by destroying their deoxyribonucleic acid (DNA).
Application of AOPs and ozonation for elimination of micropollutants in municipal wastewater treatment plant effluents.	Journal Article	Rodriguez et al., 2013 [33]	Indicates how municipal facilities use AOPs and ozonation to destroy micropollutants, reduce the volume of sludge, and make more organic compounds available for further microbial degradation.
Advanced oxidation processes for wastewater treatment in the pulp and paper industry: A review.	Journal Article	Covinich et al., 2014 [34]	Explains why AOPs are chosen for refractory wastewater from pulp and paper-making factories because they react with a wide spectrum of contaminants. These factories also use AOPs to clean their effluents and make them less toxic before discharge.
An overview on the advanced oxidation processes applied for the treatment of water pollutants defined in the recently launched Directive 2013/39/EU.	Journal Article	Ribeiro et al., 2015 [35]	Recommends the use of AOPs in treating micropollutants that cannot be destroyed using microbial processes and other conventional mechanisms.
Removal of endocrine disruptors from urban wastewater by advanced oxidation processes (AOPs): A review.	Journal Article	Cesaro and Belgiorno, 2016 [36]	Indicates how urban wastewater facilities use AOPs to remove endocrine disruptors through oxidation and provide safer water for domestic or industrial use. AOPs are also more efficient and enable urban wastewater treatment facilities to minimize their operational costs.
Slaughterhouse wastewater treatment using an advanced oxidation process: An optimization study.	Journal Article	Davarnejad and Nasiri, 2016 [37]	Wastewater from slaughterhouses can be highly toxic and difficult to process using traditional wastewater treatment facilities. This study shows how AOPs break down complex organic compounds into smaller and less harmful compounds that can be processed further using biological mechanisms.
Potential use of solar photocatalytic oxidation in removing emerging pharmaceuticals from wastewater: A pilot plant study.	Journal Article	Almomani et al., 2018 [38]	Solar photocatalytic oxidation is more effective in producing more hydroxyl radicals than using standalone AOPs such as O3 or UV radiation. Photocatalytic oxidation also produces radicals that can destroy a wide spectrum of micropollutants.
Evaluation of advanced oxidation processes for water and wastewater treatment—A critical review.	Journal Article	Miklos et al., 2018 [39]	The researchers evaluated the effectiveness of AOPs in destroying stubborn organic compounds while comparing the outcomes with the conventional mechanisms. The results showed that AOPs are more effective, flexible, and consume relatively less energy.
A review of the existing and emerging technologies in the combination of AOPs and biological processes in industrial textile wastewater treatment.	Journal Article	Paździor et al., 2018 [40]	Indicates how combining AOPs with biological processes enhances targeting and ensures that even the most stubborn organic compounds from textile wastewater are reduced to smaller compounds for further degradation. Biological processes remove less complex organic compounds to ensure the final product is clean and safe.
Advanced oxidation processes for the removal of natural organic matter from drinking water sources: A comprehensive review.	Journal Article	Sillanpää et al., 2018 [41]	AOPs assist in removing natural organic matter through oxidation. AOPs also mineralize the by-products into carbon dioxide and water, significantly eliminating potential sludge.
Wastewater treatment by advanced oxidation process and their worldwide research trends.	Journal Article	Garrido-Cradenas et al., 2019 [42]	Examines worldwide research trends aimed at improving AOPs by making them more efficient and effective. The trends include catalyst development, hybrid systems (combining AOPs with other technologies), and advanced control or monitoring systems.
Mixed industrial wastewater treatment by combined electrochemical advanced oxidation and biological process.	Journal Article	Popat et al., 2019 [43]	Finding that electrochemical AOPs are more suitable for removing organic compounds from mixed industrial wastewater based on their stronger and more effective oxidative capacity.
Evaluation of advanced oxidation processes (AOPs) integrated with membrane bioreactor (MBR) for real textile wastewater treatment.	Journal Article	Sathya et al., 2019 [44]	Combining AOPs with other technologies such as MBR enhances targeting, removes all stubborn organics, and is more effective than standalone AOPs.
Enhanced treatment of pharmaceutical wastewater by combining three-dimensional electrochemical process with ozonation to in situ regenerate granular activated particle electrodes.	Journal Article	Zhan et al., 2019 [45]	Due to its complex organic compounds that cannot be removed using conventional biological processes, this study found that AOPs are more suitable for the treatment of pharmaceutical wastewater due to their strong oxidation effects. The process becomes even more efficient when applied in a three-dimensional process, including ozonation and in situ regenerating granular activated particle electrodes.
Treatment of dyeing wastewater by combined sulfate radical-based electrochemical advanced oxidation and electrocoagulation processes.	Journal Article	Chanikya et al., 2020 [46]	Wastewater containing dye can be difficult to treat due to complex organic compounds. This study recommends sulfate-radical-based AOPs for removing complex organics.
Photocatalysts in advanced oxidation processes for wastewater treatment.	Book Chapter	Fosso-Kankeu et al., 2020 [47]	Recommends photocatalysts in AOPs due to their stronger hydroxyl radicals that can react to a wide range of organic compounds. Photocatalysts, however, may contain by-products that require further processing to enhance environmental safety.
Textile wastewater treatment using advanced oxidation process.	Journal Article	Hutagalung et al., 2020 [48]	Recommends using AOPs in the treatment of textile wastewater because they are more efficient, demand less energy, and produce limited sludge.
A critical review on ibuprofen removal from synthetic waters, natural waters, and real wastewaters by advanced oxidation processes.	Journal Article	Brillas, 2021 [49]	Recommends AOPs for removing organic compounds from pharmaceutical wastewater due to their complex organic structures. Only AOPs can reduce them to simpler compounds for further processing.
Advanced oxidation processes coupled with nanomaterials for water treatment.	Journal Article	Cardoso et al., 2021 [50]	Combining AOPs with nanomaterials enhances the production of hydroxyl radicals, and supports advanced monitoring and control of all parameters, both effluent and influent.
Degradation of roxarsone in UV-based advanced oxidation processes: A comparative study.	Journal Article	Chen, Li, and Qian, 2021 [51]	Describes the degradation of roxarsone using UV-based AOPs as highly effective and more efficient than the conventional wastewater treatment processes. UV-based AOPs also produce stronger reactive species when combined with catalysts such as TiO2.
Treatment of laundry wastewater by solar photo-Fenton process at pilot plant scale.	Journal Article	García et al., 2021 [52]	Recommends using AOPs such as Fenton processes to treat wastewater from laundry activities. Solar photo-Fenton is more effective in producing hydroxyl radicals and can remove a wide range of organic compounds from wastewater, making the process more efficient and economical.
Advanced oxidation processes: A promising route for abatement of emerging contaminants in water.	Journal Article	Kusuma et al., 2021 [53]	Recommends using AOPs to remove emerging contaminants since they can react to and oxidize nearly all organic compounds.
Critical review of advanced oxidation processes in organic wastewater treatment.	Journal Article	Ma et al., 2021 [54]	AOPs are the most promising and efficient oxidation technology for treating organic wastewater. Some of the key challenges found include high initial costs and changing regulatory frameworks.
A review of integrated advanced oxidation processes for organic pollutant removal.	Journal Article	Nidheesh et al., 2021 [55]	AOPs possess a greater capacity to remove a wide variety of pollutants and make wastewater more biodegradable. Potential drawbacks include the high cost of operations resulting from energy demand and chemicals.
Effect of residual H2O2 on the removal of advanced oxidation byproducts by two types of granular activated carbon	Journal Article	Tang et al., 2021 [56]	Recommends the use of H2O2 for optimal production of hydroxyl radicals needed for removing complex organic compounds.
Pre-oxidation of spent lettuce wash water by continuous advanced oxidation process to reduce chlorine demand and cross-contamination of pathogens during post-harvest washing.	Journal Article	Wang et al., 2021 [57]	Pre-oxidation of wastewater using AOPs generally leads to a significant decline in the demand for chlorine throughout the process. AOPs destroy organic microbes in wastewater, leading to lower demand for disinfectants such as chlorine.
Toxicity changes of wastewater during various advanced oxidation processes treatment: An overview.	Journal Article	Wang and Wang, 2021 [58]	By measuring toxicity across the wastewater treatment journey, the level of toxicity significantly declines as AOPs destroy organic compounds in the wastewater. However, the types of AOPs determine the overall level of toxicity.
Evaluation of the advanced oxidation process integrated with microfiltration for reverse osmosis to treat semiconductor wastewater.	Journal Article	An et al., 2022 [59]	AOPs are more effective in destroying and removing organic compounds than biological processes, such as microfiltration using reverse osmosis.
Advances and trends in advanced oxidation processes and their applications. In Advanced Industrial Wastewater Treatment and Reclamation of Water.	Journal Article	Gautam et al., 2022 [60]	Recommends using AOPs for reclaiming industrial wastewater and addressing scarcity challenges affecting urban areas. The researcher also identifies key trends such as hybrid systems and the use of catalysts and how they may shape the future applications of AOPs in wastewater management.
Treatment of salon wastewater by peroxydisulfate-based advanced oxidation process (PDS-AOP) under solar light. Synergy through integrated technologies.	Journal Article	Maifadi et al., 2022 [61]	Due to the complex organics found in personal care products, findings recommend using AOPs for treating wastewater from salons. Specifically, the study found that peroxydisulfate-based advanced oxidation processes (PDS-AOP) are more effective in this task than other AOPs.
Key points of advanced oxidation processes (AOPs) for wastewater, organic pollutants and pharmaceutical waste treatment: A mini-review.	Journal Article	Pandis et al., 2022 [62]	This review found significant evidence supporting the application of AOPs in the treatment of wastewater from pharmaceutical companies or industries.
Advanced oxidation processes (AOPs)-based wastewater treatment—unexpected nitration side reactions—a serious environmental issue: A review.	Journal Article	Rayaroth et al., 2022 [63]	Although AOPs are highly reactive to a wide variety of organic compounds, not much was known about potential unexpected nitration side reactions until this study. The study recommends optimal controls to prevent unexpected reactions that can damage the expected outcomes.
Integrated system of anoxic/activated sludge and ultrafiltration membrane for zero liquid discharge of pharmaceutical industrial wastewater treatment.	Journal Article	Ali et al., 2023 [64]	AOPs are used in sludge treatment to reduce the volume and make it more available for further biodegradation. AOPs also reduce the toxicity of the sludge and make it less harmful to discharge without damaging surrounding ecosystems.
Challenges and emerging trends in advanced oxidation technologies and integration of advanced oxidation processes with biological processes for wastewater treatment.	Journal Article	Gopalakrishnan et al., 2023 [65]	Key challenges were identified affecting AOPs, including energy demand, higher initial costs, and chemical use. Key trends that will address some of these challenges include the development of better catalysts, advanced monitoring systems, and integration of AOPs with other technologies.

below. The researcher also reviewed the articles based on their content, validity, bias, and application in the wastewater treatment industry.

A wide variety of AOPs outperformed conventional treatment methods in the degradation of refractory organic pollutants across the studies covered in this review [66]. Overall, Fenton and photo-Fenton processes showed a COD removal efficiency of >95–97%, followed by UV/H₂O₂ and ozonation systems with 85–96% efficiency for diverse types of pharmaceutical and industrial effluents [67]. On the contrary, traditional biological protocols like activated sludge processes and membrane bioreactors (MBRs) had relatively insignificant removal efficiencies, around 40% to 85%, which is heavily contingent on the type of contaminant and concentration of pollutants [66]. Compounds such as bisphenol A (BPA) and naproxen (NPX) were only partially removed from wastewater in traditional systems with removal efficiencies frequently lower than 60% [68].

Energy-wise, although AOPs—especially ozonation and UV-based processes—need an electricity supply for the formation of radicals, they are usually less energy-consuming than electrochemical treatment systems and have shorter reaction times [9]. Furthermore, AOPs often reduce the overall energy use of the treatment chain by limiting sludge production and reducing the need for extensive post-treatment, which is often the case with many of the conventional route strategies [9].

From an operational cost perspective, AOPs require higher initial capital costs as they involve UV reactors, ozone generators, or photocatalytic materials [69]. Nonetheless, some studies have shown lower long-term operating costs due to AOPs reducing the proportion of chemical coagulants, secondary sludge handling, and COD penalties. In combined application with current treatment trains, regulatory compliance and contaminant removal standards that are being defined for water reuse scenarios, AOPs can reduce operating expenses by as much as 20–30% when used over a 10-year operational horizon [70].

Moving on, these quantitative comparisons illustrate the technical and economic benefits of AOPs and the position of these systems as a sustainable alternative or addition to traditional wastewater treatment systems.

5. Discussion

This study examined the sustainable advantages associated with the use of AOPs in the treatment of refractory wastewater. The study defines refractory wastewater as a type of effluent containing stubborn organic and inorganic contaminants from industrial processes, including the manufacturing of pharmaceuticals, paper, pulp, and personal care products [2,31]. Unlike other types of wastewaters, refractory organics are resistant to microbial degradation, have a complex molecular structure, and could be toxic if left untreated [31]. Since refractory waste is resistant to microbial degraders, such as bacteria, researchers and scientists should develop more innovative approaches to solving the problem. AOPs have emerged as the most effective solution for treating refractory wastewater [32]. This section discusses key application areas, sustainable advantages, process control and optimization, and critical considerations.

5.1. Mechanisms of AOPs

The study found that the most common AOPs include ozonation, photocatalysis, UV irradiation, and Fenton processes. However, at the heart of these AOPs is the generation of highly reactive agents known as hydroxyl radicals [33]. Currently, hydroxyl radicals are the strongest oxidants known to scientists and researchers. Unlike conventional microbial degraders, hydroxyl radicals attack resistant pollutants using various mechanisms, including transfer of electrons, hydrogen abstraction, and addition of radicals [19,34]. All these mechanisms have proven more effective in removing even some of the most resistant contaminants from various industrial processes [35]. Production of hydroxyl radicals is the most crucial aspect of the AOPs [5,30,35]. The primary goal is to produce the most reactive oxidant that can break down pollutants to their smallest particles or mineralize them into carbon dioxide or water.

The study found that hydroxyl radicals can be produced through O₃, H₂O₂, UV radiation, Fenton reactions, and electrochemical methods. Under certain conditions, the indirect oxidation of O₃, usually in the presence of water, generates two hydroxyl radicals and O₂ atoms [36] (Figure 2). Further oxidation of O₃ through a method known as the peroxone (O₃/H₂O₂) system has proven more effective in generating more hydroxyl radicals and a stronger decomposition capacity [37]. Apart from ozonation, hydroxyl radicals can also be produced through UV radiation, where UV light plays a crucial role in decomposing H₂O₂ or O₃ into highly reactive hydroxyl radicals, a process known as photolysis [38] (Figure 3). The addition of catalysts such as TiO₂ has also proven very effective in generating stronger oxidants to enhance the decomposition of organic pollutants [39] (Figure 4). To optimize the oxidation processes, it may be necessary to combine these oxidants in a controlled process to obtain the best outcomes.

Unlike various microbial degradation processes, hydroxyl radicals have a rapid reaction rate enhanced by their powerful oxidative nature. This helps in breaking down complex organic pollutants into smaller and less harmful compounds [40]. The breakdown enables further degradation using biological processes. However, the biggest advantage that comes with the hydroxyl radicals is their capacity to react with diverse organic compounds [29,31,41]. The strong oxidative power enables these radicals to destroy almost every known or emergent recalcitrant pollutant [42]. With such strong oxidative potential, AOPs may continue to transform wastewater treatment mechanisms for years to come. Moreover, AOPs have minimal by-products or sludge compared to other biological degradation approaches [43]. The final stages in AOPs often involve the elimination of potential by-products (if not mineralized) to ensure the final product is safe for consumption.

5.2. Applications of AOPs

The strong oxidative potential of AOPs makes them suitable tools in four critical application areas. The first application area is the reduction in organic content in the refractory wastewater. Industries such as textiles and pharmaceuticals produce wastewater containing complex organic compounds [19,27,41,43]. AOPs degrade these recalcitrant compounds by eliminating or reducing the organic content. This can be tested using the COD levels [44]. A significant reduction in the COD levels indicates that the wastewater contains minimal or no organic content. The overarching question, however, is the extent to which AOPs can eliminate organic compounds [45]. The timing is sometimes a challenge, especially in facilities with inadequate control mechanisms. The study by Zhan et al., 2019 [45] argues that sufficient controls should enable wastewater treatment facilities to achieve optimal results without using excessive energy and other critical resources.

Recent studies have also proven the viability of using AOPs in sludge treatment and bioavailability enhancement. Since AOPs are known destroyers of POPs, their application in sludge treatment has shown significant results [46]. Municipal or industrial wastewater facilities sometimes produce huge amounts of sludge that can be destructive to surrounding ecosystems if discharged without treatment [47]. Sludge may contain harmful chemicals, plastics, and other non-biodegradables that should be reduced before discharge. AOPs are specifically needed for reducing the volume and toxicity of the sludge, making it simpler, less harmful, and environmentally friendly [37,47,48]. Some of the treated sludge can be used in surrounding farms to support agriculture [48]. AOPs also assist in reducing the cost of sludge treatment, enabling municipal wastewater treatment facilities to minimize their overall operational costs.

Breaking down refractory organics enhances their availability for biological processes, including aerobic, anaerobic, and anoxic treatment. AOPs have become critical tools for breaking down recalcitrant compounds into simpler and less harmful compounds [49]. These compounds are then subjected to further treatment using biological processes. A study by Cardoso et al. [50] found a growing pattern where municipal wastewater facilities integrate AOPs into their regular treatment facilities. In the case of recalcitrant compounds, AOPs can be integrated into the primary stages of wastewater treatment to enhance the bioavailability of the organic compounds [3,4,5,48]. Some facilities integrate AOPs in the tertiary stages to convert by-products into less toxic substances, such as water or carbon dioxide [49]. The flexibility of the AOPs makes them largely available for various wastewater treatment needs across the stages. According to Cardoso et al. [50], AOPs can be integrated at any stage of wastewater treatment without the need for redesigning the facility.

The use of AOPs in areas with water scarcity provides a sustainable solution to water challenges. According to Chen et al. [51], AOPs can be used in treating reclaimed water to make it safer for domestic and industrial purposes. However, reclaimed water can sometimes be aesthetically displeasing due to bad color and strong odor [52]. Most wastewater reclaimed from industrial facilities may come with a strong smell that makes it difficult to use on farms or reuse in factories [52]. AOPs destroy the bad smell and odors by attacking the organic compounds associated with these properties [53]. Once the organic compounds are removed, the remaining clean water can be used for irrigation in the farms or rechanneled back to the factories for reuse in cooling machines or cleaning purposes [54]. AOPs support the sustainable circulation of water without depleting or putting excessive pressure on natural sources.

AOPs offer considerable flexibility for integration into both municipal and industrial wastewater treatment infrastructures without necessitating significant structural overhauls. Their modular nature allows them to be implemented as either pre-treatment, intermediate, or tertiary polishing steps, depending on site-specific contamination profiles. In municipal systems, AOPs can be appended after secondary treatment units to target micropollutants that evade biological degradation. Similarly, in industrial plants, AOPs are particularly suited to augment chemical or biological stages to enhance removal of persistent organics. Hybrid configurations—such as AOP-biological treatment or AOP-filtration systems—can optimize treatment by first transforming refractory pollutants into more biodegradable compounds, thereby enhancing downstream microbial degradation. Furthermore, the compact footprint and scalability of technologies like UV/H₂O₂ and photocatalysis make them ideal for decentralized treatment facilities or retrofitting in constrained urban environments. As equipment costs decline and energy-efficient designs emerge, modular AOP systems are becoming increasingly viable as drop-in enhancements to improve water quality and ensure compliance with evolving discharge standards.

AOPs offer great flexibility and can be integrated into both municipal and industrial wastewaters treatment configurations without requiring major change [32]. Their modular nature means they can be applied as pre-treatment, intermediate, or tertiary polishing steps based on site-specific contamination profiles. In municipal systems, AOPs can be added to post-secondary treatment units to destroy micropollutants that resist biological degradation [71]. In industrial plants, too, AOPs are uniquely positioned to supplement chemical or biological stages to improve removal of recalcitrant organics [9]. Hybrid configurations, like AOP-biological treatment or AOP-filtration systems, can be optimal for treating more resistant pollutants by initially altering them into more degradable compounds and subsequently increase their degradability by bacteria. In addition, the small footprint and scalability of technologies such as UV/H₂O₂ and photocatalysis also lend themselves to decentralized treatment works or retrofit into space-constrained urban settings. With decreasing equipment costs and new designs for energy-efficient modular AOP, there are feasible solutions available to be monolithically implemented for drop-in improvements on existing waters to meet the requirements of emerging discharge standards [69].

5.3. Comparative Analysis of AOPs

While AOPs are not the only oxidizing agents that can be used in reducing organic compounds, their efficiency and effectiveness make them a leading choice in refractory wastewater treatment. AOPs can react with a broad spectrum of organic compounds, including those considered more resistant and toxic [54,55,56]. Efficiency refers to how fast AOPs remove or reduce organic compounds at minimal energy consumption [57]. COD is one of the factors used in measuring the efficiency of AOPs. Compared to other oxidizing agents, AOPs have COD levels above 96 percent, making them more efficient in reducing organic compounds [3,4,5,19,58,59]. Fenton reactions, for instance, have demonstrated the ability to reduce COD by more than 97 percent [59]. High-efficiency levels enable AOPs to reduce organic compounds at minimal energy consumption.

The varying degrees of success continue to raise concerns among facilities using AOPs to remove refractory organics. According to Gautam et al. [60], the success rate of AOPs largely depends on specific contaminants present in wastewater. AOPs can either be used alone or in combination to achieve optimal outcomes [61,62,63]. For instance, sulfate radicals or UV radiation can be used alongside H₂O₂ to produce stronger and more reactive hydroxyl radicals. According to Ali et al. [64], wastewater treatment facilities should determine the types of contaminants present in wastewater to help identify more appropriate AOPs to achieve the best outcomes. In cases where combinations are required, a suitable calculation should be conducted to determine the amount of AOPs needed to remove specific contaminants [65,72,73,74]. Since some of the AOPs are still in their developmental stages, more research is needed to enhance efficiency and effectiveness, especially when using more than one AOP to enhance the production of radicals.

Comparative studies in recent years have shown that AOPs outperform conventional processes in terms of the degradation of these organic pollutants. Fenton and photo-Fenton processes had COD removal efficiencies between 95 and 97 percent, and UV/H₂O₂ and ozonation were able to achieve removal efficiencies in the broad intervals of 85 and 96 percent for pharmaceuticals and industrial effluents [75]. In sharp contrast, biological treatments, such as activated sludge and membrane bioreactors, achieved 40–85% efficiency, highly dependent on contaminant type and concentration. The specific energy consumption of AOPs (particularly UV-based and ozonation systems) is accompanied by electricity usage for radical generation; however, the preferential lower reaction time and lower sediment generation of AOPs call for a less resource-consuming effluent separation, resulting in a relatively small footprint for reactors [76]. On an operational level, AOPs require larger capital investments (specialized equipment, e.g., UV reactors and ozone generators), but studies suggest that long-term operating expenses can be decreased by 20–30%, particularly in hybrid systems, owing to reduced use of chemicals, sludge handling, and regulatory penalties [69].

Despite being touted as the most cost-effective wastewater treatment mechanism, the implementation of AOPs is still subject to varying costs. It can be difficult for wastewater facilities to determine the cost of operations when using AOPs, especially in cases where scalability may be needed as the process develops [77,78]. According to Ike et al. [79], major cost determinants when using AOPs include the types of reagents needed, energy demands, and process scalability. Some AOPs, such as UV radiation or ozonation, often require limited reagents to produce hydroxyl radicals [79]. However, strong energy demand may escalate costs in cases where more hydroxyl radicals are needed to remove complex refractory organics. Rueda-Márquez, Pintado-Herrera, Martín-Díaz, Acevedo-Merino, and Manzano [80] believe that AOPs’ effectiveness and overall efficiency depend on their capacity to remove organic compounds within a short duration. This not only translates to lower costs but also limited energy demand, saving significant energy while generating limited carbon emissions.

A cost analysis is often needed to determine immediate and long-term expenditures. The initial cost for O₃ or UV-based AOPs is often high due to higher equipment expenditures [31,81,82,83,84]. The installation cost also includes technical labor requirements that may be difficult to access in some geographical areas. Although AOPs often attract higher installation or upfront expenses, they become more economical over time due to relatively lower chemical consumption [85]. Most traditional wastewater treatment mechanisms have become more expensive over time due to higher maintenance costs and relatively higher chemical demand [86,87,88]. AOPs rely mostly on natural elements, such as O₃ or UV light, to initialize the decomposition of H₂O₂ into hydroxyl radicals [89]. After making the initial investments, the maintenance cost becomes less expensive as the process depends heavily on nature and certain pH conditions for the reactions.

Most critically, innovations behind AOPs have been aimed at achieving a balance between treatment efficiency and ecological stewardship. The environmental effects of AOPs remain a significant consideration among researchers and even regulators [90]. Although AOPs are designed to destroy recalcitrant organics, they often produce certain by-products that can be harmful to the environment. A study by Mahdad, Younesi, Bahramifar, and Hadavifar [91] found that the ecological viability of AOPs largely depends on their by-product profiles and the degradability of these compounds. The reduction of complex organics may generate by-products that require further processing before discharge [92]. Some AOPs, however, mineralize the by-products into carbon dioxide and water, reducing potential environmental concerns. Before selecting the most appropriate AOP, it may be necessary to examine its potential by-product profile and degradation mechanisms [93]. This helps in optimizing not just the outcomes but also reducing harmful ecological footprints.

AOPs that use naturally occurring elements and fewer chemical additives are preferred due to their minimal environmental impact. UV radiation is one of the AOPs with minimal environmental effects because it depends on the decomposing capacity of the UV light needed to facilitate the production of hydroxyl radicals [94]. However, environmental concerns may emerge when catalysts such as TiO₂ are used to optimize the production of hydroxyl radicals [95]. While catalysts boost the production of reactive radicals, they are treated as chemical additives and may become a significant environmental concern due to their complex by-product profiles [96,97]. Electrochemical AOPs are also considered problematic due to their complex by-products. Despite these environmental concerns, AOPs remain more effective in conserving the environment due to minimal sludge production [77,97,98,99,100,101]. Even in cases where sludge is produced, AOPs help break complex organics into simpler and less harmful compounds that can be discharged without destroying the surrounding ecosystem.

While AOPs receive a great deal of acclaim for their degradation efficiencies and low sludge production, large-scale implementations could lead to significant environmental tradeoffs that must be addressed. Concerns revolve around the actual high energy consumption for ozone and UV-based AOPs, which may counteract their environmental benefits if originated from fossil energy [102]. When operated in regions that do not have access to clean energy, the carbon footprint of these systems further increases. In addition, catalyst materials like TiO₂, iron salts, or new nanomaterials, effective for hydroxyl generation, may also involve ecotoxicological risks or produce solid waste that needs to be adequately managed or regenerated to prevent secondary pollution [103]. To alleviate this environmental burden, catalyst reuse and regeneration strategies, such as photocatalyst immobilization, magnetic separation, or use of green synthesis methods, are actively explored [104]. Also, inappropriate process parameter control can result in the generation of intermediate or unknown transformation products, some of which could be more persistent or toxic than the parent compounds. Hence, LCAs considering AOPs over the complete life cycle and ecotoxicological evaluation are required to confirm that the deployment of AOPs does not negatively affect long-term sustainability goals, which are largely associated with SDG 6 [11].

5.4. Standardization and Quality Controls

The process of producing hydroxyl radicals and using them to destroy refractory organics requires precise controls to enhance efficiency and effectiveness. The control and optimization processes are centered around key parameters, such as reaction conditions and dosage control [51,101,105,106]. According to Karunakaran and Senthilvelan [107], reaction conditions provide optimal temperatures and pH levels to enhance the production of hydroxyl radicals. Conditions such as pH, ligand concentration, Fe²⁺, and H₂O₂ have to be controlled to provide maximum OH·. During Fenton reactions, hydroxyl radical production is best between the pH values of 2.7 and 3.5 [3,4,5,15,108,109]. Under these reactions, pH values above 3.5 lead to slow and minimal production of hydroxyl radicals [110]. Continuous monitoring of the pH values and providing regular feedback help in controlling the processes and ensuring optimal outcomes.

Dosage control refers to the number of oxidants or reagents that must be fine-tuned to enhance efficacy and economic viability. While O₃ and UV light are available in nature, they have to be controlled during reactions to avoid unwanted outcomes [111]. H₂O₂ is another key reagent that must be provided in the recommended dosage to enhance the efficacy of the hydroxyl radicals [112]. If the reagents are provided at less than the recommended dosage, the level of hydroxyl radicals generated may not be sufficient to reduce or destroy the targeted refractory organics [113,114,115]. There have also been cases where non-targeted contaminants in water end up consuming the reactive species, detracting from the entire process and delaying the desired outcomes [116]. Providing the recommended dosage ensures there are sufficient hydroxyl radicals to destroy even the non-targeted contaminants.

Data analysis, feedback loops, and continuous monitoring provide better opportunities for controlling the reaction processes using real-time observations and actions. Most facilities using AOPs also rely on advanced sensor technologies to collect and analyze real-time data, enabling swift responses to changing conditions [117]. Some facilities have also installed artificial intelligence (AI) tools, such as machine learning devices, to study the influent and effluent parameters [118]. This enables AOP systems to react to potential changes in the wastewater stream and adapt to treatment parameters in real-time [119,120,121]. The use of AI tools such as machine learning also helps in setting optimal conditions to produce hydroxyl radicals, including setting the required pH range [122]. A proper control system generally prevents waste and makes AOPs more effective and sustainable.

Since environmental regulations keep changing to reflect emerging threats and requirements, AOP systems need to undergo regular verification to enhance compliance. The authors of [123] found that regulatory frameworks are mostly centered around sludge production and treatment residue. Standardization and quality controls have helped in making AOPs more reliable in reducing toxicities that may be discharged into surrounding ecosystems [124]. Even factories not associated with wastewater treatment rely on AOPs to treat their effluent to remove potentially toxic substances before discharge [125]. Continuous research and development have also helped in developing better ways of sludge treatment to make it less toxic and even productive when used as fertilizer on farms [126]. Unlike traditional wastewater treatment mechanisms, AOPs are more proactive and capable of adapting to changing conditions and demands.

While technically superior in terms of refractory pollutant removal, the economic viability of AOPs at full-scale remains a crucial driver of adoption. However, AOP infrastructure capital investments (e.g., cost of UV reactors, ozone generators, advanced catalysts, etc.) are often much larger than that of common biological or physicochemical treatment systems [10]. In addition, operational costs can differ greatly according to the specific AOP implemented, energy source, and wastewater matrix. UV/H₂O₂ and ozonation are considered energy-intensive but can lead to long-term savings due to reduced use of chemical coagulants, lower sludge disposal costs, and decreased penalties associated with non-compliance [127]. Recent comparative cost analyses indicate that the adoption of AOPs into hybrid treatment systems can achieve total operating cost reductions of 20–30% for 10 years, and even more significantly at facilities configured for the treatment of high-strength industrial effluents. Furthermore, increasing availability of renewable energy sources, lowering equipment costs, and fiscal support or tax credits related to clean water technologies in many countries are opening new financial opportunities for adoption [9]. As such, assessing the life cycle cost for AOP implementation, considering environmental and regulatory advantages, will be crucial for understanding its overall economic value compared to traditional methods.

5.5. Comparison with Other AOPs: Efficiency, Cost, and Energy Consumption

AOPs have shown higher operational efficiency and lower energy consumption and environmental impact when compared to other advanced treatment processes, such as MBRs, electrocoagulation, and hybrid biological treatment [123]. While MBRs are proven technologies for wastewater treatment, their applications have been subject to various limitations [123]. For instance, the efficiency and effectiveness of MBRs depend on their filtrates’ pore size distribution. Since it may be difficult to control the pore size distribution, as noted by [123], most conventional facilities using MBRs are continuously replacing them with the track-etch membrane technology to address such weaknesses. Comparatively, the effectiveness and efficiency of AOPs largely depend on the hydroxyl radicals produced to address the targeted organics [124]. Studies indicate that AOPs are more efficient and cost-effective in degrading targeted organics than MBRs and other advanced treatment processes, such as electro-coagulants [125]. Moreover, challenges such as membrane fouling make MBRs less suitable for persistent organic compounds that are more likely to be adsorbed or embedded within the membrane matrix, significantly impairing the treatment process.

While AOPs have shown significant success rates in removing emerging organic compounds, other advanced treatment processes, such as MBRs, have not shown a similar or better success rate. In a study conducted by [123], the effectiveness of MBRs was tested using emerging organic contaminants, such as NPX, BPA, and sulfamethoxazole (SMX), in aquatic environments. Even after using the track-etch membranes, the BPA was removed at a rate slightly higher than 90 percent [123]. However, the NPX and SMX were removed at a rate slightly higher than 40 percent [123]. Comparatively, AOPs have a significantly higher removal success rate than MBRs. The study by Kitanou et al. [126] found that OH produced using UV light successfully eliminated BPA at a rate higher than 96.8 percent but less than 99 percent [126]. These studies indicate that AOPs have a higher success rate in removing emerging organics than the other advanced wastewater treatment options, such as MBRs.

Regarding environmental impact, AOPs have shown greater efficacy in eliminating sludge and other potentially harmful by-products. AOPs are often used alongside MBRs to complement the process by removing stubborn organics [124]. The high efficiency rate makes AOPs more suitable for protecting the environment against carbon emissions. AOPs consume less energy and take a shorter duration to eliminate POPs [123,125]. Electrochemical reactions, such as electro-coagulant processes, may consume less energy but are more likely to leave behind potentially harmful by-products [126]. AOPs address this challenge by mineralizing some by-products into water and carbon dioxide, making them more effective for environmental protection [125]. By reducing sludge into smaller and less toxic compounds, AOPs significantly reduce the toxicants that may pollute nearby rivers or landfills. The environmental effect of AOPs is largely determined by their reduction of stubborn organics, minimal energy consumption, and elimination of pollutants.

However, the cost-effective aspect of AOPs largely depends on various factors, including operational costs, type of AOP technology, and contaminant type [124]. AOPs are generally considered more cost-effective when treating wastewater containing POPs, such as BPA from pharmaceuticals [123]. AOPs can be used at the beginning or end of the wastewater treatment process to eliminate the toxic organics [124,126]. AOPs also differ significantly based on their overall costs associated with the technology used. Electro-Fenton AOPs are generally more cost-effective because they consume less energy and take a relatively shorter time to eliminate POPs [125]. Electro-Fenton AOPs also mineralize by-products into water and carbon dioxide, making them more environmentally viable than other AOPs [126]. However, the sulphate-based AOPs may require higher installation cost, even though they have shown higher capabilities in removing a wide spectrum of stubborn organics.

A closer inspection of both AOPs and traditional methods shows certain performance advantages which support their advantages towards sustainability. For example, COD removal efficiencies over 95–97% are achieved by Fenton and photo-Fenton processes, while UV/H₂O₂ and ozonation systems show removal efficiencies in the range of 85–96%, depending on effluent type [124,125,126]. In comparison, traditional biological processes, like activated sludge and MBRs, report lower and more inconsistent removal efficiencies of between 40 and 85%, with serious capacity issues in fully treating micropollutants such as BPA and SMX, which are typically removed in less than 60% of cases [128,129].

Under optimized conditions, AOPs would be energy efficient as well. Ozonation and UV-based AOPs require initial energy input for the generation of radicals but ultimately consume less energy owing to shorter reaction times and a reduced formation of sludge that can be costly in terms of downstream processing. Specifically, the electro-Fenton processes are characterized by low power demands in comparison with other electrochemical unit systems [130].

Regarding economics, while AOPs may present considerable capital investment, as they require UV reactors, ozone generators, and catalysts, several case studies reveal the opportunity for a significant reduction in operational costs of up to 20–30% over a period of 10 years. This is primarily attributed to reduced chemical consumption, negligible sludge management, and prevention of regulatory SAN penalties for COD exceedances [69].

Overall, these comparison results strongly suggest the need for wider applications of AOPs, particularly in industrial or municipal applications focused on persistent organics. Such quantitative metrics not only showcase the technical benefits but also substantially augment the sustainability profile of AOPs concerning the objectives of SDG 6 [11].

Although AOPs can demonstrate significant potential at the laboratory and pilot scales, their application in full-scale industrial wastewater treatment is still limited owing to various technical and economic barriers. A major concern related to the continuous use of energy input and existing infrastructure is the high operational costs associated with UV- and ozone-based systems, such as for ozone generators or UV reactors. Furthermore, the efficiency of AOPs can be highly dependent on the quality of the influent used as well as on the influent pH and the presence of radical scavengers, which, in turn, require real-time monitoring and cultivated control systems that are not always readily available in high-throughput infrastructures [71]. Another major challenge involves the possible formation of toxic or unknown transformation by-products, especially when AOP’s are applied with little control or post-treatment monitoring. This combined with a lack of standardized regulatory frameworks by region leaves little for industries to justify the large-scale investment being made [69]. But while issues remain, new solutions, such as hybrid systems (AOPs combined with biological treatment), renewable energy sources, and improvements in catalyst engineering, have the potential to address them. Additionally, increasing regulatory requirements for meeting quality standards for effluent discharge may further drive AOP adoption, assuming that economic viability and long-term sustainability are demonstrated at commercial scales [69].

5.6. Barriers to Large-Scale Implementation and Policy Implications

Although AOPs have shown promising performance and sustainability potential [71], the large-scale implementation of AOP–based systems in wastewater treatment facilities is still limited due to technical and economic barriers [70]. The main bottleneck here is the capital cost (including UV reactors, ozone generators, and unique photocatalytic materials) of infrastructure. For example, without access to renewable energy sources, higher energy consumption for some AOP configurations could result in a sizeable increase in operational costs. In addition, since the efficiency of the process is highly dependent on temperature, pH, reagent dosage, and wastewater matrix composition [131], it is critical to use advanced control systems and real-time monitoring to ensure process conditions remain stable and hydroxyl radical generation is optimized. The absence of universally accepted guidelines for AOP deployment creates uncertainty in terms of regulation and compliance pathways, particularly in developing countries [131]. The systems used to manage environmental regulations span large ranges across regions, and to date, AOPs have not been formally included as part of national water reuse frameworks in some jurisdictions. Therefore, the harmonization of regulatory standards and the integration of AOPs into wider water management policies are key focus areas for their adoption [131]. These challenges serve to highlight a pathway of interdisciplinary innovation, cost-benefit assessments, and policy alignment that can help realize the full transition of AOPs from laboratory to full-scale industrial implementations.

Though AOPs usually operate based on radical oxidative degradation for resistant contaminants by highly active species like ·OH and others, combining them with biological treatment technologies has been shown to greatly improve the overall treatment efficiency [9,132]. In hybrid AOP-biological systems, AOPs act as a pre-treatment step that decreases the toxicity and degrades complex organic pollutants into more biodegradation intermediates, which sets a better covalent environmental condition for downstream microbial degradation. The energy efficiency of such hybrid systems is critically based not only on AOP performance but also on the resilience and composition of the microbial communities mediating the biological phase. Indeed, some microbial populations have shown a higher ability to degrade AOP-transformed intermediates, whereas for others, the presence of residual oxidants or by-products has been noted to inhibit their metabolism [9,132]. Changes in microbial community structure—because of temperature, pH, nutrient availability, or influent composition—can therefore affect treatment outcomes. In this regard, optimizing such hybrid systems is reliant not just on the chemical control of AOP parameters, but also on a close ecological understanding of microbial dynamics. Additional investigations of microbial interactions with AOPs, such as metagenomic profiling and the design of adaptive bioreactors, may assist with the increased predictability and efficiency of these systems in a variety of wastewater matrices [9,132].

5.7. Critical Considerations

One of the key findings of this study is the ongoing research on various aspects of AOPs. The current body of literature may not be sufficient to address all the demands that AOPs are expected to meet [93,133]. AOPs remain an evolving technology with significant promise, especially in areas regarding refractory wastewater management [2,4,5,15,133]. Although AOPs were first discovered in the 1980s, their advancements have been subject to the development of modern technologies that have helped in controlling the reactions and improving the production of hydroxyl radicals [134]. Since the 1980s, a significant percentage of AOPs have been studied and used in the treatment of municipal wastewater and industrial effluents [106,112,133,135]. Future studies and applications are more likely to address challenges regarding the properties of contaminants and optimal operating conditions.

Future innovations and research regarding AOPs may focus on developing better catalysts, enhancing efficiency, and on better combination methods. Catalysts such as TiO₂ have shown a significant impact on enhancing the production of hydroxyl radicals [110]. According to Shet and Shetty [136], the use of TiO₂ enhances the production of hydroxyl radicals by more than 90 percent. Future innovations may also seek new ways of combining AOPs with biological processes to enhance the targeting and destruction of refractory pollutants [135]. Furthermore, combining AOPs with other biological processes helps in eliminating potentially harmful by-products and protecting surrounding ecosystems [4,5,43,117]. Studies indicate that using contemporary chemicals in most industrial processes generates emergent pollutants that are more complex and toxic [134,136]. Future innovations should address developing more reactive hydroxyl carbons to address a broader spectrum of contaminants.

Scaling up pilot studies is also required to convert more AOP concepts into real-world applications. For instance, one of the AOP projects still in its development stages is that of hybrid systems. According to Abid et al. [137], these hybrid systems combine AOPs with other technologies to create more effective wastewater treatment mechanisms. Membrane filtration is an example of a hybrid system still in its developmental stages [137]. Unlike standalone AOPs, membrane filtration has better synergistic advantages and is more capable of removing more stubborn pollutants with minimal environmental concerns [117,134]. Pilot studies should be ramped up to ensure innovative approaches such as membrane filtration are converted to real-world applications to enhance wastewater treatment and address water scarcity issues [37,39,137]. The combination of AOPs with other technologies is also needed to enhance targeting and make the process more efficient and environmentally sustainable.

Apart from scaling up pilot tests, continuous research and development are needed in areas such as advanced monitoring systems. Deng and Zhao, 2015 [134] argue that AOPs are only as effective as the control processes that enhance the production of reactive species and enhance the removal of stubborn pollutants. Advanced monitoring systems are designed to provide real-time data regarding the processing parameters and enhance timely interventions [5,16,17,115,134]. Advanced monitoring helps in optimizing performance and enhancing consistency in the treatment outcomes. Research into greener AOPs also seeks to replace fossil fuels with renewable energy sources to minimize carbon emissions associated with wastewater treatment [126]. Solar-powered processes may help in replacing fossil fuels and providing more sustainable energy to meet the growing demands [94,106,137]. The future of AOPs depends on scaled-up studies and financial support to convert more projects into real-world applications.

5.8. Applications and Policy Recommendations

Wastewater treatment plants can implement AOPs in their new or existing facilities as either primary or tertiary treatment mechanisms. As a primary treatment mechanism, AOPs can help in reducing POPs to make them more biologically available for further degradation [126,138]. For instance, wastewater from pharmaceuticals and paper manufacturing facilities may require AOPs as a primary treatment mechanism to reduce the stubborn organics into biologically degradable compounds [139]. Infrastructural requirements may include additional machines based on the type of contaminants and AOP technology selected for a particular wastewater treatment facility. AOPs can also be integrated in the tertiary stages for sludge treatment or elimination of persistent organic contaminants that have not been removed through biological processes.

Based on their cost and environmental advantages, governments can incentivize the adoption of AOPs by funding the development phases or installation. Governments can subsidize or waive taxes associated with AOPs to reduce their costs and make them more affordable and accessible by most municipal or private wastewater treatment plants [133,140]. The subsidies will help minimize the overall costs associated with the AOPs, including installation expenses. Governments should also consider supporting case studies or pilot projects where AOPs or advanced technologies such as the development of better catalysts are being investigated to improve wastewater treatment [124,136,141]. The pilot studies may include real-world implementation of AOPs to demonstrate their economic and environmental capabilities in wastewater treatment [137,141]. The government should take a leading role in promoting AOPs because it may lower the cost of wastewater treatment facilities funded by taxpayers.

Although this review highlights the potential of AOPs for eco-sustainable wastewater treatment, to enable their transfer to practical applications, targeted experimental and technological innovations need to be implemented [142,143]. The next research stage is pilot-scale demonstration projects with relevant operational conditions in actual wastewater matrices—specifically, for industrial effluents with compound pollutant loads. Hybrids incorporating AOPs and MBRs, electrocoagulation or anaerobic digestion could help in assessing synergies and cost-performance optimizations from hybrid systems. In addition, real-time monitoring sensors and artificial intelligence will be used to build smart control systems that dynamically optimize parameters like reagent dosing, energy consumption, and radical generation [142,143]. The development of an effective catalyst is a major hurdle; hence, it is crucial to research cheaper, durable, and regenerable photocatalysts, like doped TiO₂ or magnetic composites, for long-term reductions in cost and environmental footprint. The data can also inform the development of standardized performance metrics, life cycle assessments, and stakeholder engagement models that should be included in a future roadmap to inform regulatory acceptance of AOPs and guide policymakers with respect to scaling up adoption of AOPs in different application settings [142,143].

6. Conclusions

This study was conceived to thoroughly investigate the potential of AOPs as an efficient and sustainable technology for the treatment of refractory wastewater—wastewater characterized by organic bodies resistant to biodegradation. As global water scarcity continues to escalate and impact more than 4 billion people, and as freshwater ecosystems come under ever increasing stress, AOPs offer an important technology to increase the sustainability of water reuse, help reduce environmental loads and limit the overexploitation of natural water bodies. From this perspective, the implementation of AOPs takes a direct approach to the goals of Sustainable Development Goal 6 to improve water quality, encouraging safe reuse, and reducing hazardous chemical discharge.

In the end, the results of this study validate that AOPs are a critical stream for changing complex organic spoil features into biodegradable shape via ·OH, which originated through ozonation, UV radiation, Fenton reactions, and photocatalysis. In the domain of AOPs, TiO₂-doped photocatalytic systems have shown increased radical production, contributing to the oxidative potential required to mineralize pollutants into harmless final products, including CO₂ and H₂O; these mechanisms enable AOPs to mitigate key aspects of toxicity and environmental destruction and thus to offer a more cost-effective and energy-efficient option than traditional approaches.

Although this study is at laboratory scale, it indicates that AOPs can be inserted within conventional existing waste-water treatment (primary, secondary, tertiary treatment) facilities for removal of refractory pollutants, improving effluent biodegradability and diminishing industrial waste-water odor and coloration. AOPs are especially relevant for global application, especially in rapidly urbanizing or water-stressed areas, given their flexibility and compatibility with both centralized and decentralized treatment infrastructures.

A number of steps need to be taken to scale AOPs to full scale beyond laboratory or pilot applications. These include:

• Demonstration approaches at pilot scale to validate the efficiencies in real wastewater environment and fluctuating contaminant loads.
• Assessments of economic feasibility, including adjustments for measuring life-cycle costs and affordability for operators, both municipal and industrial.
• The design of cheaper, less toxic catalysts with greater stability and lower energy demands.
• Enhancing sustainability and reducing operational expenses through renewable energy source integration.
• The integration of intelligent monitoring systems like real-time feedback control to maintain compliance, streamline processes, and save energy.
• Policy incentives and regulatory structures to facilitate investment in sustainable treatment technologies as well as mechanisms to support knowledge transfer between regions.

In conclusion, AOPs represent a strategic enabler of sustainable wastewater treatment and circular water reuse systems. They provide a practical option to cut down on pollution from industrial and municipal waste discharges and to improve access to safe water resources. Although there is strong evidence supporting the technical efficiency of these solutions, large-scale implementation of natural carbon removal solutions will require continued innovation from multiple disciplines, more coordinated policy support, and a focus on research translation to more scalable, context-sensitive practice. These are essential to progressing the global sustainability agenda, especially in the context of Sustainable Development Goal 6 and integrated water resource management.