Challenges and Emerging Trends in Advanced Oxidation Technologies and Integration of Advanced Oxidation Processes with Biological Processes for Wastewater Treatment

Abstract

One of the biggest problems the world is currently experiencing is wastewater treatment. Numerous pollutants are released into water bodies by enormous amounts of effluents from varied sources. This paper provides a concise summary of the state of the art in AOPs, biological treatment, and their efficient application for the degradation of the numerous recalcitrant organic contaminants found in wastewater. The employment of a photoreactor is necessary for the efficient execution of the majority of photocatalytic processes. This review examines the effectiveness of several reactor configurations with varying geometries. Integrating different AOPs and AOPs with biological approaches for degrading pollutants in wastewater was also discussed. It is important to emphasize that an integrated AOP/biological system produces by-products that are not hazardous, uses little energy, and completely degrades pollutants. The review also outlines the challenges and issues of wastewater treatment for an environmentally and economically feasible process.

1. Introduction

One of the essential needs of all living things, including humans, is safe and uncontaminated drinking water. However, these days, finding it has become a big issue. The water used by humans for most activities produces wastewater. Wastewater accounts for over 80% of the water provided for domestic use [1]. About 5–10 billion tons of industrial waste are generated each year and discharged untreated into the environment [2]. If released directly into the environment without treatment, the wastewater generated from various industries can cause significant environmental problems [3]. Therefore, removing emerging pollutants is essential to making the water safe for drinking, and the release of wastewater should not cause any adverse effect on the environment. Moreover, recycling and reusing treated wastewater is necessary to reduce freshwater consumption. There are physical, chemical, and biological treatment methods; these are the most commonly employed wastewater treatment technologies. In the case of physical treatment, the removal of organics from the wastewater is challenging. The existing biological and chemical treatment methods include ozonation, electrochemical treatment, photocatalysis, aerobic treatment, activated sludge, anaerobic treatment, coagulation–flocculation treatment, etc. [4]. Aerobic biological activities require a lot of energy and generate a significant amount of biomass. Anaerobic biological processes are susceptible to shock loading, which requires further treatment of the wastewater produced before the final release. Although biological processes are efficient and cost-effective, they primarily need a vast area and have a significant energy demand (for aeration) and a substantial quantity of produced sludge [5]. Chemical treatments can quickly oxidize and totally breakdown the organic contaminants, making them an effective wastewater treatment approach [6]. As a result, the development of advanced oxidation processes (AOPs), which have an excellent potential for ultimately destroying a variety of resistant pollutants, has shown to be a promising alternative [7].

It involves the generation of powerful oxidizing radical groups, such as hydroxyl radicals, which function as oxidizing agents and mineralize organic chemical substances into CO₂ and H₂O [7]. This is possible because hydroxyl radicals (OH^●), after fluorine radicals, have the highest oxidation potential (E0: 2.8 eV vs. normal hydrogen electrode (NHE)). The most potent oxidant, fluorine, which has an oxidation potential of 3.06 V, cannot be utilized to treat wastewater due to its toxicity [8]. Ozone is an extremely potent oxidant with a redox potential of 2.08 eV that may directly oxidize microorganisms and a variety of organic substances. It can also cause secondary reactions by generating hydroxyl radicals that can subsequently react with the microcontaminants. However, this process has some drawbacks, including the poor solubility of ozone in water, the high energy requirement, and the generation of reaction products that could be even more hazardous than the parent compounds [9]. Other frequently found oxidants, such as hypochlorite and hydrogen peroxide, are less reactive because they have paired electrons in their chemical structures. Hydrogen peroxide (H₂O₂) is an environmentally friendly oxidant with high oxidation potentials across the entire pH range. These properties give it exceptional bleaching and antiseptic capabilities that have led to many applications in wastewater treatment industries [10] (Tong et al., 2022). Thus the oxidation potential of an oxidant is used to measure the capacity of an oxidant to initiate chemical processes [11].

Table 1. The oxidation potential of various oxidative species.

S. No.	Oxidation Species Oxidation Potential	eV
1	Fluorine	3.06
2	Hypochlorous acid	1.49
3	Chlorine	1.36
4	Hydrogen peroxide	1.77
5	Ozone	2.07
6	Perhydroxyl radical	1.70
7	Hydroxyl radical	2.80
8	Nascent oxygen	2.42

shows the oxidation potential of various oxidative species [12].

AOPs such as photocatalysis and photo-Fenton have been widely considered to be very effective in removing persistent organic pollutants. It has been proved that AOPs can be utilized as a pretreatment to convert contaminants into shorter-chain compounds [13]. Mineralization by AOPs becomes exceedingly expensive because the oxidation intermediates generated during chemical oxidation become more difficult to completely degrade. The hydroxyl radicals, however, transform the resistant substances into compounds that are more readily biodegradable, which can then be further mineralized by microorganisms [14]. A single advanced oxidation processes such as the solar photo-Fenton and solar photocatalytic process have inherent challenges and limitations with respect to efficiency and economics because the solar photo-Fenton process is pH dependent and the quantum yield is less in the solar photocatalytic process. Considering the above drawbacks, there is a need for the combination of the two processes rather than the individual process and for the optimization of the combined treatment system to achieve the required target within a short reaction time. However, in general, AOPs are expensive, and one of the shortcomings that prevent the wide application of this treatment process in developing countries lies in its high operational cost due to its high energy consumption and chemical reagents. In order to reduce the energy cost, renewable solar energy is used for the treatment process. Combining several AOPs can greatly increase the oxidation efficiency of the pollutants compared to an individual treatment method because of the synergistic effect of different compounds [15]. To improve the overall treatment efficiency of the process and to reduce the chemical cost, the advanced oxidation processes are coupled with the biological treatment process [16].

This review paper briefly summarizes the different types of AOPs, the combination of two or more AOPs, and the integration of AOPs with the biological processes for the removal of toxic contaminants from wastewater. Moreover, this review outlines the challenges and issues of wastewater treatment technologies and the economic aspects of the treatment processes.

2. Mechanism of Advanced Oxidation Processes

There are two types of AOPs: homogeneous processes and heterogeneous processes. The homogeneous processes in the AOP are defined by the chemical alterations resulting solely from the interactions between the chemical reagents and the target substances [17]. In heterogeneous processes, the reactants and products adsorb and desorb on the catalyst’s active sites. As the reaction occurs, the desorption of products and the absorption of new species takes place on the active sites; thereby, the efficiency is affected by the surface characteristics and the pore structure of the catalyst [18]. The different methods for the production of hydroxyl radicals take place via several combinations, such as the Fenton process (Fe²⁺/H₂O₂), the solar photo-Fenton process (solar/Fe²⁺/H₂O₂), solar photocatalysis (solar/TiO₂/H₂O₂), peroxone (Ozone/H₂O₂), the combination of peroxone with ultraviolet (Ozone/H₂O₂), Ozone/UV, and photolysis (H₂O₂/UV). The oxidation by-product formation mechanisms in the various AOPs are discussed.

2.1. Solar Photo-Fenton Process

Among the AOPs, the solar photo-Fenton process is particularly interesting for treating a large variety of hazardous pollutants and is one of the most environmentally benign treatment systems [19]. Due to the potential utilization of sunlight, this technique has drawn more attention. As a result, it may be considered to be an affordable AOP as sunlight is always available in a tropical country such as India [20,21]. Figure 1 shows the schematic diagram of the SPF.

As shown in Equation (1), catalysis by ferrous iron breaks down hydrogen peroxide in an acidic medium to produce highly reactive species such as OH^● radicals without requiring high pressure or temperature. When solar irradiation is used, Fe³⁺ hydroxy complexes (Fe(OH)²⁺) and Fe³⁺ organic complexes (FeOOCR²⁺) can perform the photo-reduction in Equation (2) and the photodecarboxylation in Equation (3), enabling the regeneration of iron and generating extra OH^● radicals [22]. The hydroxyl radicals break down the complex organic structures into simpler compounds, and the partial oxidation of the non-biodegradable organics contributes to the biodegradability of the wastewater.

Fe²⁺ + H₂O₂ → Fe³⁺ + OH^● + HO

(1)

[Fe(OH)]²⁺ + hν → Fe²⁺ + OH^● + H⁺

(2)

Fe(OOCR)²⁺ + hν → Fe²⁺ + CO₂ + R^•

(3)

In the solar photo-Fenton process, the significant characteristic of iron is that it undergoes cyclic oxidation and reduction. Table 2 shows the treatment of wastewater by various solar photo-Fenton processes.

Xu et al. (2007) [23] used the solar photo-Fenton technique to treat a pulp and paper mill’s bleaching wastewater. According to the reports, the TOC removal was usually faster for 15 min using a cost-effective source of solar light irradiation. After that, there was a slow increase for 3 h. Sheik et al. (2008) [24] explored the photocatalytic oxidation of organic contaminants such as p-nitroaniline, p-aminophenols, and acetanilide. It was found that the organic compounds were entirely oxidized and degraded into CO₂ and H₂O. Karimi et al. (2011) [25] focused on the discoloration of wastewater from a paper mill by the photo-Fenton process. The research revealed that after 15 min of photo-Fenton treatment, the soda effluent had dropped nearly 65% of its original color.

Lucas et al. (2012) [26] reported experimentally that about 90% of DOC degradation was attained with 5 mg/L of Fe²⁺ and 50 mM of H₂O₂ for the treatment of pulp and paper mill wastewater. The same DOC degradation can be achieved by solar photo-Fenton using less H₂O₂ and a shorter time. Bernabeu et al. (2012) [27] employed the photo-Fenton process to degrade emerging pollutants. According to the findings, emerging contaminants at high concentrations are removed at an acidic pH rather than a neutral pH. Trovo et al. (2012) [28] evaluated the photodegradation of paracetamol using ferrous sulphate and potassium ferrioxalate under artificial solar light. In contrast to FeO_x, FeSO₄ decreased Fe³⁺ to Fe²⁺, enhancing the decomposition by generating higher concentrations of hydroxylated intermediates. Garcia and Buitron (2013) [22] proposed the photo-Fenton process and obtained 97% decolorization of each azo dye and the development of chemometric tools, a strategy that reduced the peroxide from 33% to 65%. Turbay et al. (2013) [29] examined the photo-Fenton reaction-based tetracycline antibiotic degradation under varying hydrogen peroxide doses. The outcome demonstrated the improved effectiveness of the treatment and a more effective use of the hydroxyl radicals generated in the reaction medium.

Rocha et al. (2014) [30] treated extracted petroleum wastewater utilizing sunlight as the irradiation source. After 7 h of exposure to sunlight, the experimental results showed a reduction of about 92.7% and 96.2% of the polycyclic aromatic hydrocarbons and the aromaticity, respectively. Rodriguez et al. (2014) [31] explored the potential of the photo-Fenton process to decolorize and mineralize simulated wastewaters from wool dyeing tanks. The results indicated a high level of efficiency in decreasing TOC, COD, and BOD₅. Velegraki and Mantzavinos (2015) [32] studied the solar photo-Fenton technique for the treatment of winery effluent and achieved mineralization at a low catalyst dose (Fe²⁺ = 5 mg/L) and relied on an oxidant consumption of 500 mg/L. Chueca et al. (2015) [33] assessed the efficacy of a mild solar photo-Fenton system for disinfecting the actual effluents containing fecal bacteria. Despite the complexity and variety of the pollutants in the effluents, the tests conducted on the real effluents yielded highly encouraging results. Soares et al. (2015) [34] treated the textile dye wastewater using ferric organic ligands with the solar photo-Fenton process. It was reported that 87% of the mineralization was achieved at neutral pH values with biodegradability enhancement. Guzman et al. (2016) [35] assessed solar photo-Fenton treatment and various ozone-based processes for the pretreatment of synthetic samples of citrus wastewater. According to the findings, the photo-Fenton method removes COD and DOC at rates of 76.9% and 53.3%, respectively, which are higher than those of ozone-based systems.

Benitez and Penuela (2018) [36] examined the complete removal of benzophenone and the biodegradability increase using the solar photo-Fenton process under simulated solar irradiation. Costa et al. (2018) [37] treated biodiesel effluent, employing the solar photo-Fenton approach to decrease COD by adding oxalate at 50 kJ m⁻² of accumulated UVA radiation. About 72% of COD and 76% of BOD removal were achieved using 1 mmol L⁻¹ of ferrioxalate. Garcia et al. (2021) [38] investigated an advanced method for post-treating laundry effluent using the solar photo-Fenton process. It was found that a high removal efficiency was achieved, reaching 89% and 96%, respectively, in just 8 min. Arka et al. (2021) [39] investigated the solar photo-Fenton method for the treatment of institutional wastewater. According to the study, the solar UV/Fe²⁺/H₂O₂ process was very efficient in treating the wastewater from institutions, achieving a greater pollutant removal rate. The removal of six representative pharmaceuticals (PHCs) in raw hospital wastewater was investigated by Lumbaque et al. (2021) [40]. It was observed how the molar ratios (1:1 and 1:2) affected the breakdown of PHCs and the production of their transformation products during the treatments. The primary findings showed that PHC degradation is greatly favored in Fe³⁺:EDDS (1:2) due to the overall H₂O₂ consumption and the enhanced iron stability, with degradation efficiencies of above 77% in most PHCs.

Lin and Lin (2021) [41] used a solar photo-Fenton technique to remove cytostatic medicines. In comparison to three other Fe(III)-ligand complexes, the addition of EDDS caused the degradation of cyclophosphamide and 5-fluorouracil to occur more quickly. Gualda-Alonso et al. (2022) [42] carried out a continuous flow solar photo-Fenton process, employing a 100 m² raceway pond reactor (RPR), for the elimination of emerging contaminants. The RPR was run in continuous flow mode with 0.1 mM FeSO₄ and 1.47 mM H₂O₂ at an acidic pH and 1 h of hydraulic residence time. The liquid depth was fixed at 10 cm for winter and 18 cm for summer, removing >85% of the CEC in both cases. Pandey et al. (2023) [43] treated contaminated water by solar photo-Fenton using Fe-rich catalysts and obtained 82% COD removal.

The discussions mentioned above provide a general overview of the refractory compounds degraded by the photo-Fenton process and the effectiveness of the mineralization of these chemical pollutants under observed optimal conditions.

Table 2. Treatment of wastewaters by various solar photo-Fenton processes.

S. No.	Wastewater	Process	Operational Conditions	Degradation Efficiency	Reference
1	Pulp and paper mill	Solar photo-Fenton	Fe(II) concentration from 31 to 310 mg L−1 (initial pH 3.0, 30 °C), initial H2O2 concentration from 0.5 to 3 Dth	TOC—82% Time—120 min	Xu et al., 2007 [23]
2	p-nitroaniline	Solar photo-Fenton			Sheik et al. (2008) [24]
3	Soda pulping effluent	Fenton and photo-Fenton	FeSO4—1 mM H2O2—570 μL	Color—65%	Karimi et al. (2011) [25]
4	Pulp and paper mill	Solar photo-Fenton	5 mg/L of Fe2+ and H2O2 of 50 mM	90% of DOC mineralization	Lucas et al. (2012) [26]
5	Azo dye, Acid Blue 161	Solar photo-Fenton	H2O2/Fe2+ = 12, pH = 2.5 to 4.0	Degradation 40 %	Trovo et al., 2016 [28]
6	Sulfonated azo dyes	Solar photo-Fenton	Fe2+ = 5 to 38 mg/L, H2O2 = 98 to 828 mg/L	Color—97%	Garcia and Buitron (2013) [22]
7	Petroleum extraction	Photo-Fenton	485.3 mmol/L of H2O2	Hydrocarbons and aromaticity of about 92.7% and 96.2%	Rocha et al. (2014) [30]
8	Winery effluent	Photo-Fenton	(Fe2+ = 5 mg/L)	-	Velegraki and Mantzavinos (2015) [32]
9	Citrus wastewater	Photo-Fenton	H2O2 = 1017 mg/L, pH = 7	COD—77%	Guzman et al., 2016 [35]
10	Biodiesel effluent	Solar photo-Fenton	1 mmol L−1 of ferrioxalate	72% of COD and 76% of BOD	Costa et al. (2018) [37]
11	Laundry effluent	Solar photo-Fenton	H2O2 (50–400 mg/L), Fe2+ (2.75–10 mg/L)	SDS removal—96%	Garcia et al. (2021) [38]
12	Industrial effluent	Solar photo-Fenton	(H2O2)—0.25 to 1.25 g/L, Fe2+—0.005 to 0.12 g/L, pH (2 to 10), time (30 to 180 min)	color (91%), turbidity (90%), and COD—86%)	Arka et al. (2021) [39]
13	Hospital wastewater	Solar photo-Fenton	Fe3+:EDDS = 1:2, H2O2 (230 mg L−1)	Degradation = 77%	Lumbaque et al., 2021 [40]
14	Cytostatic drugs	Solar photo-Fenton	pH 3.0–8.5, 0.1 mM Fe(III)-EDDS, 1 mM H2O2		Lin and Lin, 2021 [41]
15	Emerging contaminants	Solar photo-Fenton	0.1 mM FeSO4 and 1.47 mM H2O2 at an acidic pH and 1 h of hydraulic residence time	>85%	Gualda-Alonso et al., 2022 [42]
16	Paraquat (PQ)-contaminated water	Photo-Fenton	Fe-containing industrial waste	COD—82%	Pandey et al., 2023 [43]

Table 2. Treatment of wastewaters by various solar photo-Fenton processes.

S. No.	Wastewater	Process	Operational Conditions	Degradation Efficiency	Reference
1	Pulp and paper mill	Solar photo-Fenton	Fe(II) concentration from 31 to 310 mg L−1 (initial pH 3.0, 30 °C), initial H2O2 concentration from 0.5 to 3 Dth	TOC—82% Time—120 min	Xu et al., 2007 [23]
2	p-nitroaniline	Solar photo-Fenton			Sheik et al. (2008) [24]
3	Soda pulping effluent	Fenton and photo-Fenton	FeSO4—1 mM H2O2—570 μL	Color—65%	Karimi et al. (2011) [25]
4	Pulp and paper mill	Solar photo-Fenton	5 mg/L of Fe2+ and H2O2 of 50 mM	90% of DOC mineralization	Lucas et al. (2012) [26]
5	Azo dye, Acid Blue 161	Solar photo-Fenton	H2O2/Fe2+ = 12, pH = 2.5 to 4.0	Degradation 40 %	Trovo et al., 2016 [28]
6	Sulfonated azo dyes	Solar photo-Fenton	Fe2+ = 5 to 38 mg/L, H2O2 = 98 to 828 mg/L	Color—97%	Garcia and Buitron (2013) [22]
7	Petroleum extraction	Photo-Fenton	485.3 mmol/L of H2O2	Hydrocarbons and aromaticity of about 92.7% and 96.2%	Rocha et al. (2014) [30]
8	Winery effluent	Photo-Fenton	(Fe2+ = 5 mg/L)	-	Velegraki and Mantzavinos (2015) [32]
9	Citrus wastewater	Photo-Fenton	H2O2 = 1017 mg/L, pH = 7	COD—77%	Guzman et al., 2016 [35]
10	Biodiesel effluent	Solar photo-Fenton	1 mmol L−1 of ferrioxalate	72% of COD and 76% of BOD	Costa et al. (2018) [37]
11	Laundry effluent	Solar photo-Fenton	H2O2 (50–400 mg/L), Fe2+ (2.75–10 mg/L)	SDS removal—96%	Garcia et al. (2021) [38]
12	Industrial effluent	Solar photo-Fenton	(H2O2)—0.25 to 1.25 g/L, Fe2+—0.005 to 0.12 g/L, pH (2 to 10), time (30 to 180 min)	color (91%), turbidity (90%), and COD—86%)	Arka et al. (2021) [39]
13	Hospital wastewater	Solar photo-Fenton	Fe3+:EDDS = 1:2, H2O2 (230 mg L−1)	Degradation = 77%	Lumbaque et al., 2021 [40]
14	Cytostatic drugs	Solar photo-Fenton	pH 3.0–8.5, 0.1 mM Fe(III)-EDDS, 1 mM H2O2		Lin and Lin, 2021 [41]
15	Emerging contaminants	Solar photo-Fenton	0.1 mM FeSO4 and 1.47 mM H2O2 at an acidic pH and 1 h of hydraulic residence time	>85%	Gualda-Alonso et al., 2022 [42]
16	Paraquat (PQ)-contaminated water	Photo-Fenton	Fe-containing industrial waste	COD—82%	Pandey et al., 2023 [43]

2.2. Solar Photocatalytic Process

Solar photocatalysis has been widely used to degrade organic compounds. According to numerous studies, many organic contaminants have been totally oxidized in irradiated semiconductor solutions [44]. Photocatalysts remove pollutants from wastewater primarily through hydroxyl radical (OH^●) attacks by transforming them into harmless compounds such as water and CO₂ [45]. The photocatalyst that has been researched the most is TiO₂. However, its usage is restricted to ultraviolet light because of the wide energy band gap (3.2 eV) [46]. When exposed to ultraviolet light, a semiconductor catalyst such as TiO₂ or another transition metal oxide is initiated by the photon absorption with enough energy to be equal to or greater than the catalyst’s bandgap energy, stimulating an electron to move from the valence band to the conduction band, creating a hole in the valence band while using just 4% of the solar radiation [47,48,49].

TiO₂ photocatalysis uses UV-A radiation (λ < 387 nm) in the presence of oxygen and water to produce hydroxyl radicals for the excitation of the photocatalyst [50]. The overall rates of pollutant degradation are significantly influenced by the surface area of the TiO₂ catalysts that contains active sites. Thus, increasing the TiO₂-specific surface area effectively increases the pollutant adsorption capacity on the catalyst. As a result, a larger surface area with a more significant number of active sites will lead to a faster and more extensive reaction [51].

The activation Equation (4) can be written as

TiO₂ + hν → h⁺ + e⁻

(4)

It is possible to express the oxidation and reduction reactions as given in Equations (5) and (6)

h⁺ + OH⁻ → -OH^●

(5)

e⁻ + O_2ads⁻ → O_2ads⁻

(6)

Another critical factor in the process of photocatalytic oxidation is the pH level. In alkaline media, the reaction of OH^● with holes on the TiO₂ surface causes the formation of hydroxyl radicals. Thus, hydroxyl radicals accelerate photocatalytic oxidation at higher pH solutions and vice versa [52]. Figure 2 shows the schematic diagram of the SPC.

Adish and Kanmani (2010) [53] used a single baffle reactor to photo-catalyze H₂O₂ to treat phenolic wastewater. According to reports, the solar/Fe²⁺/H₂O₂ process degrades phenol two to three times more quickly than the solar/TiO₂ process. Kumar et al. (2011) [54] studied the degradation of pulp and paper mill wastewater by a photocatalytic process. The findings showed that the increase in the BOD/COD ratio after photocatalytic oxidation was 0.09 for the primary wastewater with the removal of the COD (57.9%), BOD (42.9%), and color (89.2%). Dimitrakopoulou et al. (2012) [55] exploited eight different TiO₂ catalysts for the degradation of amoxicillin, and it was reported that Degussa P25 was the most active of the catalysts, with 93% mineralization after 25 and 95 min of reaction time at the amoxicillin concentrations of 10 mg/L and 250 mg/L of titania. Singh et al. (2013) [52] examined the photocatalytic oxidation of the organic content in synthetic wastewater at the pH values 2, 4, 6, 8, and 10 and at normal pH. The outcomes demonstrated clearly that an 86% reduction in COD was achieved at a normal pH level (pH = 6.8), and it was reported that an acidic pH was unfavorable for the reduction of organic content. Nagpure et al. (2013) [56] studied the photocatalytic decomposition of paper mill wastewater with ultraviolet and catalysts such as TiO₂. The degradation efficiency was observed as 91.34% using TiO₂ as a photocatalyst, with an optimum catalyst dose of 80 mg/50 mL for an irradiation time of 8 h under continuous stirring with maximum intensity. Ruiz et al. (2013) [57] studied the photodegradation of trimethoprim by various nanoparticles deposited on TiO₂. It was reported that the degradation of trimethoprim was only 50% of the organic matter mineralization with pure TiO₂-P25.

Shao et al. (2013) [58] produced mesoporous TiO₂ nanotubes with a large diameter for the photodecomposition of effluent from paper manufacturing. The result indicated that after 12 h of photodegradation, the chemical oxygen demand and chroma percent degradations of the paper manufacturing wastewater were approximately 73% and 99.5%, respectively. Metribuzin, a common herbicide, was transformed and mineralized in extensive detail by TiO₂-driven photocatalysis under simulated sunlight, as described by Antonopoulou et al. (2014) [59]. The results pointed out that the complete transformation of metribuzin was attained within 40 min at a metribuzin concentration of 10 mg/L, a TiO₂ concentration of 100 mg/L, and I = 750 W/m², whereas 80% mineralization was achieved within 300 min of irradiation.

Thomas et al. (2014) [60] compared the efficiency of TiO₂ and few-layer graphene nanocomposites for degrading rhodamine B under solar radiation. The titania incorporation on the few-layer graphene improved titania’s visible light photocatalytic activity, decreased electron–hole recombination, and increased electron–hole mobility. Fernandez et al. (2015) [50] investigated the photocatalytic treatment of polluted wastewater. The effects of many factors, including the type of microorganism, the water temperature, the dissolved oxygen concentration, and the water matrix composition were studied, and the results showed that this process has great potential for the chemical reduction and disinfection of pathogens. Murgolo et al. (2015) [61] investigated a novel photocatalyst based on nanosized TiO₂ upon radiation by both UV and solar-simulated light, resulting in degradation efficiencies ranging from 9 to 87% and 9 to 96%, respectively.

Solano et al. (2018) [62] evaluated the removal efficiency of hexavalent chromium (Cr⁶⁺) and divalent zinc (Zn²⁺) using heterogeneous photocatalysis with TiO₂. The result revealed that this process was efficient for the total removal of a Cr⁶⁺ concentration of 5 mg/L and was less effective for higher concentrations (15, 25 ppm); however, this technology was ineffective for Zn²⁺ removal. Yang and Yang (2018) [63] addressed the decomposition of rhodamine B using photocatalyst TiO₂. The result indicated that the catalyst reuse was found to be efficient up to 8 times with the same degradation efficiency. Aljouboury et al., 2021 [64] studied the ideal conditions for natural petroleum wastewater, utilizing TiO₂/ZnO/Fenton/solar and TiO₂/ZnO/air/solar. According to the reports, the TiO₂/ZnO/air/solar process achieved the highest treatment effectiveness with the COD removal of 74% and the TOC removal of 99% under ideal circumstances.

Kader et al. (2022) [65] investigated the synthesis of TiO₂-based photocatalysts, the production of TiO₂-based immobilized borosilicate glass reactors, and the use of the reactors to the treatment of methyl orange dye under UV light. The results revealed that the photocatalyst effectiveness dropped with the increase in pH and the initial dye concentrations. Total organic halogen was utilized as an analytical tool by Abusallout and Hua (2022) [66] to assess the effectiveness of solar TiO₂ photocatalysis in dehalogenating disinfection by-products in the water. Compared to pH 5, the TOX dehalogenation was improved at pH 9, and the addition of hydrogen peroxide slightly improved the TOX elimination. The findings demonstrated that compared to the anatase and rutile TiO₂ particles, the mixed-phase TiO₂ (Aeroxide P25) was significantly more efficient at removing TOX.

To immobilize TiO₂ for the removal of paraben from wastewater, Martins et al., 2022 [67] tested the suitability of utilizing polymeric supports. According to the results, polydimethylsiloxane is a suitable material to support TiO₂ for wastewater treatment by solar photocatalytic oxidation. Using two nanocomposites (TiO₂ and TiO₂ (Ag) doped), Behera et al. (2022) [68] compared the photocatalytic activity on the breakdown of phenol from water. The nanocomposites were created using the UV photo-reduction method with silver loadings of 0.25, 0.5, 0.75, and 1% (w/w). At pH 7, about 98% of the phenol was degraded in 180 min with 0.5 g L⁻¹ photo-catalyst TiO₂ (Ag-1.0) EY. Rueda-Marquez et al., 2020 [69] examined the solar photocatalytic process using immobilized TiO₂ for treating urban wastewater and obtained a maximum removal of greater than 40% of the pharmaceutically active compounds. Gai et al., 2021 [70] synthesized potassium and iodide TiO₂ and observed the enhancement of photocatalytic activity under simulated sunlight. Giang et al., 2023 [71] synthesized sulfur-doped TiO₂ for the degradation of crystal violet. The findings revealed the potential uses of the material in environmental treatments. Rapti et al., 2022 [72] treated hospital wastewater using TiO₂ with the catalyst loading between 150 mg/L and 200 mg/L and attained the degradation efficiency of 73%.

Although numerous research studies on the photocatalytic degradation of wastewater pollutants using TiO₂ as a semiconductor have been reported, its industrial application has some constraints, including the recombination of the photogenerated electron–hole pair, the lack of and the inability to produce a catalyst with high photon efficiency and low cost that can absorb a broader range of the solar spectrum, and the surface structure [47]. Moreover, the aggregation of the TiO₂ catalyst reduces the surface area and hence the effectiveness of the catalyst [73]. Because of the large amounts of chemicals required for proper performance, photocatalysis is only sometimes considered a viable treatment to be used alone; however, their combination with biological processes may improve the overall process efficiency, increasing their viability.

Table 3. Treatment of wastewater by various solar photocatalytic processes.

S. No.	Wastewater	Process	Operational Conditions	Degradation Efficiency	Reference
1	Pulp and paper mill	Solar photocatalytic (TiO2)	-	COD—57.9%, BOD—42.9%, color—89.2%	Kumar et al. (2011) [54]
2	Amoxicillin	Solar photocatalytic (TiO2)	TiO2 catalysts 100 to 750 mg/L	93% mineralization	Dimitrakopoulou et al. (2012) [55]
3	Synthetic wastewater	Photocatalytic oxidation	pH values 2, 4, 6, 8, 10	COD—86%	Singh et al. (2013) [52]
4	Trimethoprim	Photocatalytic oxidation	-	50% mineralization	Ruiz et al. (2013) [57]
5	Pulp and paper mill	Photocatalytic oxidation	-	COD—73%, chroma percent—95%	Shao et al. (2013) [58]
6	Metribuzin	Photocatalytic oxidation	TiO2—100 mg/L and I = 750 W/m2	80% mineralization Reaction time—300 min	Antonopoulou et al. (2014) [59]
7	Organic pollutants	Nanosized TiO2 supported on single-wall carbon nanotubes	-	UV—9 to 87%, simulated solar light—9 to 96%	Murgolo et al. (2015) [61]
8	Hexavalent chromium	Solar photocatalytic (TiO2)	pH = 2, 6 and 10, TiO2—2.0, 2.5, and 3.0 g/L	Complete removal for 5 mg/L Cr6+	Solano et al. (2018) [62]
9	Rhodamine B	Solar photocatalytic (TiO2)	Film thickness = 361 nm Specific surface area of 47.72 m2 g−1	98.33% in 30 min	Yang and Yang (2018) [63]
10	Urban wastewater	Immobilized TiO2		High removal (>40%)	Rueda-Marquez et al., 2020 [69]
11	Petroleum wastewater	TiO2/ZnO/Fenton/Solar and TiO2/ZnO/Air/Solar	ZnO dosage of 54 g/L and TiO2 dosage 50 g/L	COD—74%, TOC—99%	Aljouboury et al., 2021 [64]
12	Organic pollutant	Solar TiO2 photocatalysis		Perfect acid and alkali resistance	Gai et al., 2021 [70]
13	Methyl orange dye	TiO2-based photocatalysts	pH—7.0, reaction time—5.5 h of UV irradiation, photocatalyst (0.12 g)	Degradation—75.8%	Kader et al. (2022) [65]
14	Dehalogenation disinfection by-products	Solar TiO2 photocatalysis	pH 9, TiO2—100 mg/L	Effective removal of TOX	Abusallout and Hua (2022) [66]
15	Pharmaceutical residues	Solar TiO2 photocatalysis	TiO2—200 and 300 mg L−1	Higher than 56%	Rapti et al. (2022) [72]
16	Crystal violet	Titanium dioxide/graphene aerogel-doped sulfur		Enhanced photo-degradation	Giang et al., 2023 [71]

shows the treatment of wastewater by various solar photocatalytic processes.

3. Biological Treatment Processes

Biological treatment is an essential component of any wastewater treatment plant that handles wastewater with soluble organic pollutants from municipalities, industries, or a mix of the two. Aerobic microorganisms require oxygen and organic substances to function and develop. The organic matter in the wastewater provides nutrients, and the air is typically pumped into the treatment tank to supply oxygen. Aerobic digestion produces carbon dioxide, energy, and metabolized solids, which precipitate as its by-products. Bacteria and other microorganisms that survive without oxygen are known as anaerobic microbes. Compared to aerobic bacteria, these microbes degrade organic pollutants more slowly. Anaerobic microbes produce methane, carbon dioxide, and more anaerobic bacteria.

Vohra et al. (2005) [74] examined the ability of white rot fungi to degrade lignin. According to the studies, the wastewater treatment processes eliminated 71% of the lignin and 48% of the COD. Chelliapan et al. (2012) [75] studied the degradation of the paper mill wastewater by an anaerobic process. The results showed that 98% COD removal was attained with an organic loading rate of 1.560 kg/m³.d. Pleurotus sajor caju and P. ostreatus were used by Belem et al. (2008) [76] to remediate the pulp and paper mill waste. After 14 days, no changes were seen for any of the effluent compositions or inoculation species, and the biological treatment reduced COD by 65–67%. For the treatment of tannery effluent, Durai et al. (2011) [77] investigated the effectiveness of a bench-size aerobic sequencing batch reactor. The optimized factors were the initial substrate concentration, the hydraulic retention period, and the organic loading rate. Based on the findings, it was determined that the highest removals of COD and color were 79% and 51%, respectively.

Ventura et al. (2011) [78] treated real domestic wastewater with a pilot-scale anaerobic-anoxic-oxic process, with a submerged membrane in the oxic tank and a thermophilic aerobic digestion reactor. The removal of MLSS, TCOD, BOD, TN, TP, and E. coli by the A₂O-MBR-TAD process was reported to be nearly 99%, 96%, 70%, 83%, and 99%, respectively. Sludge generation was reported to be reduced to almost zero by adjusting the flow rate of the MBR’s waste stream. In a sequencing batch reactor with bacterial consortia (Klebsiella sp., Alcaligens sp. and Cronobacter sp.), Kumar et al. (2014) [79] explored the treatment of wastewater from pulp and paper mills. According to the experimental findings, the demands for chemical and biological oxygen were reduced by 72.3% and 91.1%, respectively. Chen et al. (2018) [80] investigated the wastewater treatment of the pulping industry using fly ash in SBR. According to the results, the addition of fly ash in the bioreactors enhanced the settling and flocculation affinity of the activated sludge without affecting the efficiency and stability of the biological process.

Rhodococcus opacus was studied by Goswami et al. (2018) [81] in a ternary substrate system to determine whether it could simultaneously biodegrade the polycyclic aromatic hydrocarbons (PAHs) and accumulate lipid. It was found that the individual effect of the PAH content was more important, and the initial concentration and aromatic complexity of the PAHs were the key factors affecting the PAH biodegradation effectiveness in the combination. Su et al., 2021 [82] explored the efficiency of the combined Fenton process with an aerated biological filter for the treatment of organic compounds collected from the secondary treatment of an industrial park wastewater treatment plant. It was shown that the combined process resulted in a synergistic effect with more than 70% mineralization at reduced dosages of Fenton’s reagent.

Although these methods are economically viable, the disadvantage of aerobic digestion is that it produces significant amounts of biosolids or sludge that must be disposed of. When the nutrient-rich sludge is improperly released into ponds or rivers, it can lead to eutrophication, which destroys fish and other aquatic species. Methanogenic archea are bacteria with a slow growth rate, which is a drawback of anaerobic processes. If the influent is strong in sulfur, this process emits volatile organic acids that cause odors. Removing the ammonia-nitrogen is ineffective, and the pH must be regulated together with the monitoring of the volatile fatty acids. Additionally, biological systems seldom achieve their maximum efficiency due to the intermittent load and the lack of an efficient monitoring parameter for the live biomass. Refractory substances are not effectively removed, even if the traditional biological processes are often effective at degrading the contaminants found in wastewater [83]. Therefore, a low-cost alternative treatment is required to eliminate the persistent organic compounds in wastewater.

4. Combination of Different Advanced Oxidation Processes

As wastewater is reused, the research has recently concentrated on integrated treatments for the breakdown of complex organic contaminants. Depending on the chemical composition of the wastewater, different AOPs can be used alone or in combination because various reagent systems are used to produce the radical species. Combining the two other AOP processes is an attractive option, and the different combinations of AOPs result in synergistic effects that enhance the removal efficiency. Additionally, when several advanced oxidation processes are utilized in combination systems, the amount of reagent consumed is reduced, which results in a decrease in the production of inhibitory by-products. Table 4 shows the treatment of wastewater by integrated AOPs.

The following is some of the literature based on the combined advanced oxidation processes.

Berberidou et al. (2007) [84] used ultrasound irradiation in combination with heterogeneous (TiO₂) and homogeneous photocatalysis to investigate the decomposition of Malachite green in the water. Due to increased reactive radical formation and a potential increase in the catalyst’s active surface area caused by ultrasound, it was concluded that TiO₂ sono-photocatalysis was more rapid than the individual process. Zhang and Pagilla (2010) [85] addressed the treatment of pesticide wastewater by combining nanofiltration (NF) and photo-Fenton’s reagent and evaluated the economic analysis. It was reported that enhancing NF could minimize the treatment cost. Na et al. (2012) [86] examined the degradation of diethylpthalate by combining sono-photolysis and sono-photocatalysis to confirm the synergistic degradation of diethylpthalate. With sono-photolysis and sono-photocatalysis, diethylpthalate was shown to degrade and mineralize significantly with synergistic effects of 1.29 and 1.95, respectively. Jadhav et al. (2013) [87] utilized hydrodynamic cavitation (HC) and a combination of HC and H₂O₂ to investigate the degradation of imidacloprid in aqueous solution. The rate of degradation of imidacloprid was significantly accelerated by HC + H₂O₂, resulting in complete degradation in 45 min with a synergistic coefficient of 22.79. Basturk and Karatas (2014) [88] studied the decolorization of CI Reactive blue 181 by the sono-Fenton and Fenton processes. It was reported that the color removal of 88% was attained for the Fenton process under optimum conditions, whereas 93.5% color removal was attained for the sono-Fenton process in optimum conditions. Thus, it was noticed that high decolorization was obtained by the sono-Fenton process.

Gogate and Patil (2014) [89] investigated the degradation of triazophos by hydrodynamic cavitation and advanced oxidation processes. Under optimum operational conditions, hydrodynamic cavitation (HC) alone was reported to degrade triazophos by about 50% and 80% by integrating HC and Fenton’s reagent. In contrast, the combination of HC and ozonation resulted in complete degradation. Qiu et al. (2014) [90] investigated the combined electrolysis and ozonation methods for treating wastewater contaminated with nitrophenol contaminants. The outcomes showed that the combined procedure had a significant synergy for deterioration. Giannakis et al. (2015) [91] examined the efficiency of the coupling of sonication and photo-Fenton for the inactivation of bacteria in the secondary treated effluent. The combined approach increased the effectiveness of removal and suggested feasible, affordable solutions for implementing this method. Zhan et al. (2019) [92] explored 3D electrolysis, ozonation, and the combined process (3D/O₃) for pharmaceutical wastewater treatment. The findings revealed that ozonation and a 3D electrochemical method synergized the breakdown of hazardous organics in pharmaceutical effluent. Chanikya et al., 2020 [93] compared the efficacy of the combined sulphate radical-based electrochemical advanced oxidation process (EAOP) + electrocoagulation (EC) and EC+EAOP methods for the treatment of textile wastewater. The iron plate served as the cathode and the Pt/Ti plate as the anode. The results showed that a higher COD reduction of 93.5% was observed with EAOP, followed by EC with less specific energy consumption and sludge production.

Bui and Minh (2021) [94] investigated the treatment effectiveness of red wastewater by examining COD reduction with several types of advanced oxidation technologies and their combinations. The findings showed that Fenton/TiO₂/O₃/UV was the most efficient approach for treating red wastewater among the investigated procedures. Under the ideal operating parameters, it allowed a reduction of more than 99% of COD during 30 h of treatment. Titchou et al. (2021) [95] examined the combinations of AOPs. The findings showed a synergistic effect on the degradation of organic compounds by combining UV or ultrasonic radiation with Fenton sulfate-based oxidation and photocatalysis, resulting in complete removal efficiency. Dogan et al. (2021) [96] evaluated the combination of Fenton and enhanced photo-Fenton UF system performances using a membrane oxidation reactor (MOR). This study shows the MOR performance synergy allows effluents to be discharged directly into the sewage system after pH correction with complete treatment.

Dindas et al., 2020 [97] treated pharmaceutical wastewater with the combination of electrocoagulation, electro-Fenton, and photocatalytic oxidation. The result reported that the sequential process removed 64% of TOC and 70.2% of COD. Maifadi et al., 2022 [98] treated salon wastewater with the integrated AOP/membrane process and obtained TOC removal of 89.74%. An et al., 2022 [99] investigated the integration of AOP with microfiltration for treating semiconductor wastewater. The result showed the highest permeability and low energy consumption of the membrane used in the microfiltration. Doltade et al., 2022 [100] studied the integrated oxidation process for treating dye wastewater. The findings revealed that about 91% of COD removal was obtained when ozone and H₂O₂ were used as oxidizing agents.

The above kinds of literature provide the efficiencies of various combinations of AOPs. When used individually, the solar photo-Fenton and photocatalytic processes produce difficulties with restricted applicability and increased treatment costs. The two advanced oxidation processes are thus integrated to increase the degradation efficiency and generate various synergistic reactions by generating hydroxyl radicals under mild experimental conditions to widen the practical applications of industrial and environmental interest [89]. Therefore, integrating several modern oxidation systems is conceptually advantageous and enables the reduction in the scope of the individual processes without affecting the overall degradation rates, while also lowering investment and operating costs [101]. Figure 3 depicts a schematic illustration of the combined solar photo-Fenton and photocatalytic processes.

Table 4. Treatment of wastewater by integrated AOPs.

S. No.	Wastewater	Process	Operational Conditions	Degradation Efficiency	Reference
1	Malachite green	Ultrasound irradiation+ TiO2+ solar photo-Fenton	Ultrasound power—75–135 W TiO2—0.1–0.5 g L−1, Fe3+—7–20 mg L−1	Complete malachite green degradation	Berberidou et al., 2007 [84]
2	Pesticide wastewater	Nanofiltration and solar photo-Fenton	pH 3, H2O2 = 1:100 and H2O2:Fe(II) = 40:1	Maximum removal efficiency	Zhang and Padilla (2010) [85]
3	Diethyl phthalate	Sonolytic, photolytic, and sonophotolytic processes	Ultrasound (283 kHz)	Synergistic effects of 1.68 and 1.23	Na et al. (2012) [86]
4	Imidacloprid	Hydrodynamic cavitation (HC) + Fenton, HC + photo-Fenton, Hc + photolytic, HC + photocatalytic	Imidacloprid:H2O2 as 1:40 and the molar ratio of FeSO4·7H2O:H2O2 as 1:40	Synergetic index of 3.636 (HC) + Fenton and 2.912 HC + photo-Fenton	Jadhav et al. (2013) [87]
5	CI Reactive Blue 181	Fenton and sono-Fenton process	Fenton process— [Fe2+] = 30 mg/L, [H2O2] = 50 mg/L and pH = 3 sono-Fenton [Fe2+] = 10 mg/L, [H2O2] = 40 mg/L and pH = 3	Color removal Fenton—88% Sono-Fenton—93.5%	Basturk and Karatas (2014) [88]
6	Triazophos	Hydrodynamic cavitation and advanced oxidation processes	Inlet pressure (1–8 bar) and initial pH (2.5–8)	80% degradation	Gogate and Patil (2014) [89]
7	Nitrophenol	Ozonation and electrolysis	Reaction time—60 min	TOC—91%	Qiu et al. (2014) [90]
8	Synthetic wastewater	Sonication, mild photo-Fenton treatment	Ultrasound 20 W recirculating flow rate: 4.39 L/h Iron 1 ppm treated volume: 500 mL H2O2 10 ppm	Maximum E.coli inactivation	Giannakis et al. (2015) [91]
9	Pharmaceutical	3D electrolysis, ozonation	-	TOC abatement to ~71%	Zhan et al. (2019) [92]
10	Pharmaceutical	Electrocoagulation + electro-Fenton, electrocoagulation + photocatalytic oxidation	1 h, 5 mA/cm2 pulsed current density, Fe:H2O2—1:10 4 h PcO using 1.5 g/L TiO2 and 10 mM H2O2	64.0 % TOC, 70.2 % COD, 97.8 % BOD	Dindas et al., 2020 [97]
11	Red wastewater	advanced oxidation processes	-	COD > 99%	Bui and Minh, 2021 [94]
12	Organic pollutants	Ozonation, Fenton, sulfate-based processes, photocatalysis, and sonolysis	-	Removal efficiency > 90%	Titchou et al., 2021 [95]
13	Pulp and paper mill wastewater	Hybrid Fenton and photo-Fenton	-	Fenton—64.0–74.9%	Dogan et al., 2021 [96]
14	Salon wastewater	Peroxydisulfate-based AOPs	pH 2–6, reaction time—7 h	TOC—89.74 %	Maifadi et al., 2022 [98]
15	Semiconductor wastewater	Ozone + microfiltration	pH—6.8–8.0	Maximum permeability, low energy consumption	An et al., 2022 [99]
16	Industrial wastewater	Ozone + H2O2	Treatment time—60 min, stirring at 240 RPM	COD—91%	Doltade et al., 2022 [100]

Table 4. Treatment of wastewater by integrated AOPs.

S. No.	Wastewater	Process	Operational Conditions	Degradation Efficiency	Reference
1	Malachite green	Ultrasound irradiation+ TiO2+ solar photo-Fenton	Ultrasound power—75–135 W TiO2—0.1–0.5 g L−1, Fe3+—7–20 mg L−1	Complete malachite green degradation	Berberidou et al., 2007 [84]
2	Pesticide wastewater	Nanofiltration and solar photo-Fenton	pH 3, H2O2 = 1:100 and H2O2:Fe(II) = 40:1	Maximum removal efficiency	Zhang and Padilla (2010) [85]
3	Diethyl phthalate	Sonolytic, photolytic, and sonophotolytic processes	Ultrasound (283 kHz)	Synergistic effects of 1.68 and 1.23	Na et al. (2012) [86]
4	Imidacloprid	Hydrodynamic cavitation (HC) + Fenton, HC + photo-Fenton, Hc + photolytic, HC + photocatalytic	Imidacloprid:H2O2 as 1:40 and the molar ratio of FeSO4·7H2O:H2O2 as 1:40	Synergetic index of 3.636 (HC) + Fenton and 2.912 HC + photo-Fenton	Jadhav et al. (2013) [87]
5	CI Reactive Blue 181	Fenton and sono-Fenton process	Fenton process— [Fe2+] = 30 mg/L, [H2O2] = 50 mg/L and pH = 3 sono-Fenton [Fe2+] = 10 mg/L, [H2O2] = 40 mg/L and pH = 3	Color removal Fenton—88% Sono-Fenton—93.5%	Basturk and Karatas (2014) [88]
6	Triazophos	Hydrodynamic cavitation and advanced oxidation processes	Inlet pressure (1–8 bar) and initial pH (2.5–8)	80% degradation	Gogate and Patil (2014) [89]
7	Nitrophenol	Ozonation and electrolysis	Reaction time—60 min	TOC—91%	Qiu et al. (2014) [90]
8	Synthetic wastewater	Sonication, mild photo-Fenton treatment	Ultrasound 20 W recirculating flow rate: 4.39 L/h Iron 1 ppm treated volume: 500 mL H2O2 10 ppm	Maximum E.coli inactivation	Giannakis et al. (2015) [91]
9	Pharmaceutical	3D electrolysis, ozonation	-	TOC abatement to ~71%	Zhan et al. (2019) [92]
10	Pharmaceutical	Electrocoagulation + electro-Fenton, electrocoagulation + photocatalytic oxidation	1 h, 5 mA/cm2 pulsed current density, Fe:H2O2—1:10 4 h PcO using 1.5 g/L TiO2 and 10 mM H2O2	64.0 % TOC, 70.2 % COD, 97.8 % BOD	Dindas et al., 2020 [97]
11	Red wastewater	advanced oxidation processes	-	COD > 99%	Bui and Minh, 2021 [94]
12	Organic pollutants	Ozonation, Fenton, sulfate-based processes, photocatalysis, and sonolysis	-	Removal efficiency > 90%	Titchou et al., 2021 [95]
13	Pulp and paper mill wastewater	Hybrid Fenton and photo-Fenton	-	Fenton—64.0–74.9%	Dogan et al., 2021 [96]
14	Salon wastewater	Peroxydisulfate-based AOPs	pH 2–6, reaction time—7 h	TOC—89.74 %	Maifadi et al., 2022 [98]
15	Semiconductor wastewater	Ozone + microfiltration	pH—6.8–8.0	Maximum permeability, low energy consumption	An et al., 2022 [99]
16	Industrial wastewater	Ozone + H2O2	Treatment time—60 min, stirring at 240 RPM	COD—91%	Doltade et al., 2022 [100]

5. Coupling of the Advanced Oxidation Process and the Biological Process

Recently, the combination of advanced oxidation and biological processes has been effectively used as a viable approach to treating recalcitrant wastewater. The implementation of biological treatment is both economical and environmentally friendly. However, it cannot sufficiently remove the organic materials in high concentration levels, and it usually requires a longer time to degrade the pollutant, and the efficiency of the treatment is affected by the formation of toxic by-products [102]. Consequently, chemical pretreatment is necessary to transform the toxic compounds into less harmful and more biodegradable molecules when direct biological treatment is not viable [103]. Thus, the AOP is used as a pretreatment to improve biodegradability and make the wastewater compatible with further biological treatment. Table 5 shows the treatment of wastewater by the combined AOPs and biological processes.

The following is some of the literature based on the coupled AOP–biological process.

By combining activated sludge and ozonation processes, Assalin et al. (2009) [4] investigated wastewater treatment from pulp and paper mills. The wastewater was acclimatized for 39 days for the growth of microorganisms using biomass from domestic sewage. The wastewater was subjected to ozonation at pH 8.3 and 10 for 60 min. From kraft E1 effluent at pH 10, the combined process eliminated 80% of COD, TOC, and color. The feasibility of completely removing the phenol was 75%. It was determined that it was only the use of integrated processes that resulted in less consumption of ozone and that they were, hence, economical. To decontaminate wastewater contaminated with commercial pesticides, Zapata et al. (2010) [104] suggested a design plan for a solar photo-Fenton/biological system that performed in tandem. The combined system had an efficiency of 84%, with 49% attributable to the biological stage and 35% to the photo-Fenton treatment.

Dhir et al. (2011) [105] examined the coupled biological and photochemical processes for treating pulp and paper mill wastewater. The results revealed that about 81% and 93% of degradation efficiency was obtained for C and E1 wastewater, respectively, for the combined AOP and biological process under operating conditions. Ruas et al. (2012) [106] explored the removal of refractory organic compounds from kraft bleaching wastewater using advanced oxidation techniques that combined hydrogen peroxide with UV radiation. A horizontal flow anaerobic immobilized biomass reactor was used for anaerobic pretreatment. The wastewater was treated in the reactor for 19 h of hydraulic retention time. The predicted removal efficiency for COD was 61%, TOC 69%, BOD₅ 90%, and AOX 55%. The H₂O₂/UV process as a post-treatment resulted in the removal efficiency of COD in the range from 0 to 11%, UV₂₅₄ in the range from 16% to 35%, lignin from 0 to 29%, and AOX from 23 to 54%.

Botia et al. (2012) [107] investigated using a biological–photocatalytic coupled system to treat effluent from pulp bleaching. The biological pretreatment system was injected with the fungus Trametus pubescens immobilized on polyurethane foam. According to the findings, it was found that about 96% of the initial total organic carbon was degraded completely. To treat wastewater containing pesticides from the washing of phytopharmaceutical plastic containers, Moreira et al. (2012) [108] treated the wastewater containing pesticides using the combined biological treatment and AOPs. The biological treatment was conducted before the AOPs because of the high biodegradability of the material. This resulted in reductions in COD, DOC, and BOD₅, and this process was followed by the photo-Fenton reaction, which resulted in 86% mineralization. The COD concentration was brought down below the detection threshold by this integrated treatment method.

A multistage treatment system was developed by Silva et al. (2013) [109] to treat landfill leachate using biological oxidation, the solar photo-Fenton process, and, again, the activated sludge biological oxidation. The first biological oxidation process eliminated 95% of the nitrogen and 39% of the DOC. The experiment without sludge removal resulted in a 30% increased H₂O₂ and UV energy consumption during the photo-Fenton reaction. The overall treatment process results in high efficiency in removing identified organic trace contaminants. Merayo et al. (2013) [110] treated the pulp and paper mill wastewater by ozonation and solar photocatalysis with a combination of biological processes. As a pre- or post-treatment for the biological process, it was revealed that the photocatalytic process did not significantly reduce COD; however, the use of ozonation significantly increased the COD removal to 90% for the combined treatment. Souza et al. (2013) [111] studied the treatment of winery wastewater utilizing biological oxidation and the solar photo-Fenton process combined with the biological approach. The results showed that it takes roughly 10 and 6 days, respectively, to attain the COD value utilizing simply the biological system or the combined system.

Marsolek et al. (2014) [112] investigated the AOP–biological process that was sequentially coupled for wastewater treatment. By adjusting the wavelength of light, the catalyst, and the treatment time in a photocatalytic reactor, four different effluents were produced from 2,4,5-trichlorophenol, revealing that photocatalytic pretreatment conditions can be controlled to attain a range of treatment objectives. Blanco et al. (2014) [113] studied textile wastewater degradation by integrating an aerobic sequencing batch reactor and photo-Fenton oxidation. It was found that 97% COD and 95% TOC reductions were achieved by the combined SBR and photo-Fenton process under the optimum conditions. After the combined treatment, it was determined that the resulting water was subjected to reverse osmosis to achieve the required water characteristics for 100% internal reuse.

Mendez et al. (2015) [114] explored heterogeneous photodegradation, Fenton, biological techniques, and their combinations for the degradation of phenol, formaldehyde, and phenol–formaldehyde mixtures from water. The results revealed that the combined process was found to be optimal for wastewater treatment. Amoxicillin and cloxacillin-containing antibiotic wastewater were subjected to combined photo-Fenton–SBR treatment in a study by Elmolla and Chaudhuri (2012) [115]. The photo-treated wastewater was subjected to the sequencing batch reactor, which operated under different hydraulic retention times. It was observed that about 89% of the COD was removed under the operating conditions at 90 min of reaction time and 12 h of HRT.

Sathya et al. (2019) [116] treated textile wastewater with the integration of a membrane bioreactor with ozone and achieved the maximum removal efficiency of 94% color and 93% COD removal. Liu et al. (2022) [117] treated Congo red wastewater by integrated photocatalysis and biodegradation. The result clearly indicated that about 76% of the COD removal was obtained for the degradation of Congo red. Faggiano et al. (2023) [118] treated the olive mill wastewater with a moving bed biofilm reactor and a photo-Fenton-like process. It was observed that about 91% of the COD removal was achieved with MBBR, but the phenol removal was about 57%. However, with the photo-Fenton-like process, the phenol removal was found to be 99%. Liu et al. (2023) [119] explored the integration of photocatalysis with MBBR for treating emerging organic pollutants and attained a COD removal of 80.7%. Ali et al. (2023) [120] studied the efficiency of treating pharmaceutical wastewater containing acetylsalicylic acid, and it was observed that by using this integrated system, 99.7% of the acetylsalicylic acid was removed with a high quality of treated water.

Table 5. Treatment of wastewater by combined AOP and biological processes.

S. No	Wastewater	Process	Operational Conditions	Degradation Efficiency	Reference
1	Industrial park wastewater	Fenton oxidation + biological aerated filter	2.0 mM FeII, 10 mM H2O2, pH 3.0 and 3.0 h of the reaction time	EfOM (>70%)	Su et al., 2021 [82]
2	Dyeing wastewater	Electrochemical advanced oxidation process (EAOP) and electrocoagulation (EC)	-	COD reduction of 93.5%	Chanikya et al., 2021 [93]
3	Mixed industrial wastewater	Electrochemical advanced oxidation processes (EAOPs) and biological treatment	pH-3, effective area-25 cm2, electrode spacing-1 cm while voltage10 V, persulphate dosage-200 mg L−1, and catalyst dosage-10 mg L−1	94% COD removal	Popat et al., 2019 [121]
4	Organic pollutants	AOP + membrane process	-	Removal efficiency > 90%	Titchou et al., 2021 [95]
5	Hospital	AOP + adsorption	H2O2 of 100 mg L−1 and multiple additions of Fe2+ of 5 mg L−1 at times of t = 0, 5, and 10 min	98–100% removal rate	Della-Flora et al., 2021 [122]
6	Citrus wastewater	ozone-based processes	150 min. at 1.9gO3/L, 1017 mg/L of H2O2, UV radiation and pH ~ 7	77% of chemical oxygen demand and 53% of dissolved organic carbon	Guzman et al., 2016 [35]
7	CI Reactive Blue 181	Fenton and sono-Fenton	[Fe2+] = 30 mg/L, [H2O2] = 50 mg/L and pH = 3 [Fe2+] = 10 mg/L, [H2O2] = 40 mg/L and pH = 3	Colour removals Fenton—88% sono-Fenton 93.5%	Basturk and Karatas (2014) [88]
8	Synthetic wastewater	Sonication, mild photo-Fenton treatment	Ultrasound 20 W Recirculating Flow rate: 4.39 L/h Iron 1 ppm Treated Volume: 500 mL H2O2 10 ppm	Maximum E. coli inactivation	Giannakis et al. (2015) [91]
9	Pharmaceutical	3D electrolysis ozonation	-	TOC abatement to ∼71%	Zhan et al. (2019) [92]
10	Paper mill	Activated sludge-ozonation	pH 8.3	COD—75.5%, TOC- 59.1%, colour—77%, Total phenols—52.3%	Assalin et al. (2009) [4]
11	Pulp and paper mill	Biological and photochemical	3 g L−1 TiO2, pH 6.0 and 0.01 mol L−1 NaOCl	81% and 93% in C and E1 effluent	Dhir et al. (2011) [105]
12	Kraft bleaching effluent	Anaerobic immobilised biomass reactor and hydrogen peroxide with UV radiation	19 h of hydraulic retention time (HRT)	COD (61 ± 3%), TOC (69 ± 9%), BOD5 (90 ± 5%) and AOX (55 ± 14%)	Ruas et al. (2012) [106]
13	Textile wastewater	Membrane bioreactor+ ozonation and photocatalysis	Photocatalyst-500 mg/L, ozone-5 g/h	Color—94%, COD—93%	Sathya et al., 2019 [116]
14	Congo red wastewater	Photocatalysis + biodegradation		COD—94.3%	Liu et al., 2022 [117]
15	Olive mill wastewater	MBBR + Photo-Fenton	MBBR—24h, Photo-Fenton—3 h	BOD—97%, COD—91%, Total phenols—57% Photo-Fenton, total phenols—99%	Faggiano et al., 2023 [118]
16	Synthetic wastewater	Photocatalysis and moving bed biofilm reactor	Hydraulic retention time of 24 h	COD—67.7–80.7 %	Liu et al., 2023 [119]
17	Pharmaceutical wastewater	Anoxic/activated sludge and ultrafiltration membrane	pH 6–7 and F/M of 0.85 at HRT of 10 h	Removal efficiency—90%	Ali et al., 2023 [120]

Table 5. Treatment of wastewater by combined AOP and biological processes.

S. No	Wastewater	Process	Operational Conditions	Degradation Efficiency	Reference
1	Industrial park wastewater	Fenton oxidation + biological aerated filter	2.0 mM FeII, 10 mM H2O2, pH 3.0 and 3.0 h of the reaction time	EfOM (>70%)	Su et al., 2021 [82]
2	Dyeing wastewater	Electrochemical advanced oxidation process (EAOP) and electrocoagulation (EC)	-	COD reduction of 93.5%	Chanikya et al., 2021 [93]
3	Mixed industrial wastewater	Electrochemical advanced oxidation processes (EAOPs) and biological treatment	pH-3, effective area-25 cm2, electrode spacing-1 cm while voltage10 V, persulphate dosage-200 mg L−1, and catalyst dosage-10 mg L−1	94% COD removal	Popat et al., 2019 [121]
4	Organic pollutants	AOP + membrane process	-	Removal efficiency > 90%	Titchou et al., 2021 [95]
5	Hospital	AOP + adsorption	H2O2 of 100 mg L−1 and multiple additions of Fe2+ of 5 mg L−1 at times of t = 0, 5, and 10 min	98–100% removal rate	Della-Flora et al., 2021 [122]
6	Citrus wastewater	ozone-based processes	150 min. at 1.9gO3/L, 1017 mg/L of H2O2, UV radiation and pH ~ 7	77% of chemical oxygen demand and 53% of dissolved organic carbon	Guzman et al., 2016 [35]
7	CI Reactive Blue 181	Fenton and sono-Fenton	[Fe2+] = 30 mg/L, [H2O2] = 50 mg/L and pH = 3 [Fe2+] = 10 mg/L, [H2O2] = 40 mg/L and pH = 3	Colour removals Fenton—88% sono-Fenton 93.5%	Basturk and Karatas (2014) [88]
8	Synthetic wastewater	Sonication, mild photo-Fenton treatment	Ultrasound 20 W Recirculating Flow rate: 4.39 L/h Iron 1 ppm Treated Volume: 500 mL H2O2 10 ppm	Maximum E. coli inactivation	Giannakis et al. (2015) [91]
9	Pharmaceutical	3D electrolysis ozonation	-	TOC abatement to ∼71%	Zhan et al. (2019) [92]
10	Paper mill	Activated sludge-ozonation	pH 8.3	COD—75.5%, TOC- 59.1%, colour—77%, Total phenols—52.3%	Assalin et al. (2009) [4]
11	Pulp and paper mill	Biological and photochemical	3 g L−1 TiO2, pH 6.0 and 0.01 mol L−1 NaOCl	81% and 93% in C and E1 effluent	Dhir et al. (2011) [105]
12	Kraft bleaching effluent	Anaerobic immobilised biomass reactor and hydrogen peroxide with UV radiation	19 h of hydraulic retention time (HRT)	COD (61 ± 3%), TOC (69 ± 9%), BOD5 (90 ± 5%) and AOX (55 ± 14%)	Ruas et al. (2012) [106]
13	Textile wastewater	Membrane bioreactor+ ozonation and photocatalysis	Photocatalyst-500 mg/L, ozone-5 g/h	Color—94%, COD—93%	Sathya et al., 2019 [116]
14	Congo red wastewater	Photocatalysis + biodegradation		COD—94.3%	Liu et al., 2022 [117]
15	Olive mill wastewater	MBBR + Photo-Fenton	MBBR—24h, Photo-Fenton—3 h	BOD—97%, COD—91%, Total phenols—57% Photo-Fenton, total phenols—99%	Faggiano et al., 2023 [118]
16	Synthetic wastewater	Photocatalysis and moving bed biofilm reactor	Hydraulic retention time of 24 h	COD—67.7–80.7 %	Liu et al., 2023 [119]
17	Pharmaceutical wastewater	Anoxic/activated sludge and ultrafiltration membrane	pH 6–7 and F/M of 0.85 at HRT of 10 h	Removal efficiency—90%	Ali et al., 2023 [120]

As a result of the reports in the literature that biological wastewater treatment is ineffective at successfully removing recalcitrant and hazardous contaminants, a new hybrid technique that combines advanced oxidation processes and biological processes has been developed. AOPs are the only effective treatment options for wastewater containing recalcitrant organic compounds, although they frequently involve substantial capital and operating costs. By combining the economic and environmental benefits of biological and AOP treatments, it would be possible to remove hazardous substances from wastewater practically. This can be achieved by applying AOP as a pretreatment to the wastewater containing recalcitrant organic compounds and optimizing the reagent consumption within the short reaction time, resulting in the generation of fully biodegradable intermediates. Thus, the pretreated wastewater can be subjected to biological treatment for the complete removal of organic matter.

6. Solar Photocatalytic Reactors

Solar energy is a practical and efficient solution to address the energy crisis and promote environmental protection. The sun is our planet’s most plentiful source of energy, and during clear weather, when the sun is near its zenith, the amount of solar radiation that reaches the earth’s surface equals about 1 kW/m². Direct radiation, also known as beam radiation, originates from the sun’s disc directly, while diffuse radiation travels to the earth after being dispersed by the atmosphere in all directions. Thus, the total amount of solar radiation equals the sum of the two elements (direct and diffuse radiation). Solar energy is focused on collectors that gather and concentrate the solar radiation onto a smaller receiving surface to reach high temperatures.

Therefore, a photoreactor that can effectively interact with the photocatalyst and solar photons is required for the industrial use of a photocatalytic process. The photons from solar irradiation are used to degrade wastewater in solar photocatalytic reactors. More and more research is focusing on solar photoreactors using sunlight as an economical alternative to expensive UV radiation [123]. To degrade organic compounds, many solar photocatalytic reactor designs have been developed. These reactor systems are categorized as concentrating and non-concentrating reactor systems based on how well they absorb sunlight [124]. Curved mirrors are used in concentrating reactors to focus solar radiation onto a receiver, which is then heated by the radiation. A fluid that travels through the receiver receives the heat that was absorbed. The usage of these promotes the enhancement of their efficiency, which is dependent on direct-beam irradiation. The term “parabolic trough” refers to a solar thermal collector with a parabola form that is straight in one dimension and curved in the other two. It is lined with a polished metal mirror. The focal line, where things are placed that is meant to be heated, is where the sunlight’s energy is focused after entering the mirror parallel to its plane of symmetry.

The direct ultraviolet photons of the solar spectrum are focused using a parabolic trough reactor [125]. Singh et al. (2013) [52] examined the photocatalytic oxidation of synthetic wastewater containing high COD using a parabolic trough reactor. TiO₂ has been used as a photocatalyst. It was reported that a maximum COD reduction of 86% was obtained. The drawbacks of parabolic reactors include the fact that they can only receive direct radiation and have a significant cost associated with the tracking system and an inadequate quantum yield when using TiO₂. Non-concentrating reactors are simple in design and static without any solar tracking device. They are usually a flat plate with a determined tilt. The main advantage of a non-concentrating reactor is the reduced cost. The following is some of the literature on different types of reactors. Non-concentrating reactors absorb direct and diffuse solar radiation with negligible optical losses, whereas in a concentrating reactor system, the sunlight is concentrated, thus decreasing the size of the reactor compared to that of the non-concentrating reactor. The literature studies on various types of reactors used for wastewater treatment are listed in Table 6.

6.1. Thin Film Fixed-Bed Reactor

The wastewater runs over the sloped P₂₅ TiO₂ DEGUSSA-coated plate that makes up the thin film fixed-bed reactor (TFFBR). The pump that is found within the reactor can regulate the flow rate. The key benefits of this TFFBR are (i) its good optical efficiency, (ii) its uncomplicated design, and (iii) the inexpensive capital cost. Kernani et al. (2014) [126] studied the treatment of landfill containing recalcitrant organic compounds using a thin film fixed-bed reactor. It was reported that about 92% COD removal was achieved at a pH of 5.

6.2. Compound Parabolic Reactor

A large-scale solar thermal energy collection is made possible by the compound parabolic concentrator (CPC). Without the need for a sophisticated sun-tracking mechanism, a CPC is made up of two parabolas with a compound rotation [102]. The compound parabolic concentrator has been extensively interpreted as a good option for solar photochemical applications. With the use of a pilot CPC, Velegraki and Mantzavinos (2015) [32] successfully mineralized and detoxified winery wastewater by using the sunlight-driven Fenton process.

6.3. Double-Skin Sheet Reactor

A flat box with two parallel UV translucent Plexiglas panes spaced a few centimeters apart makes up the double-skin sheet reactor. Channels are created inside the reactor by Plexiglas strips wrapped around the double skin. During insolation, this channel system is used to pump the suspension of the photocatalyst [127]. Dillert et al., 1999 [125] treated various types of biologically pretreated industrial wastewater in a double-skin sheet reactor. The disadvantage of this reactor is that it can function in slurry mode and has low optical efficiency.

6.4. Shallow Pond Reactor

Using direct and diffuse radiation, the shallow pond reactor may operate on sunny and gloomy days, and it is a non-concentrating reactor [128]. Toor et al., 2006 [129] investigated the decomposition of diazo Direct Yellow 12 using the semiconductor TiO₂ as a catalyst in a non-concentrated shallow pond slurry-type reactor under UV light. It was reported that about 94% COD reduction was achieved after 2.5 h, and complete decolorization was achieved after 1.5 h by UV-vis analysis.

6.5. Pebble Bed Reactor

The innovative pebble bed reactor utilizes white pebbles coated with TiO₂ and a horizontal or slope solar trough collector to degrade wastewater. The pebbles are dispersed in a triangular arrangement to aid in the interaction between the liquid and the photocatalyst. The outcome revealed that under sunlight, 72% colour and 3–35% TOC removal was accomplished [130].

6.6. Fountain Reactor

The fountain photocatalytic reactor is made up of a flattened water bell exposed to sunlight or UV lamps. Water is pumped through a specially made nozzle to create a thin film, which emerges from the nozzle in the form of a smooth, roughly horizontal, and radially widening water fountain [131].

In the above discussions, a reactor using immobilized TiO₂ with a laminar flow, such as a thin film fixed-bed, pebble bed, or fountain reactor, only has a limited ability to degrade substances. Similarly, a shallow tank reactor with immobilized TiO₂ had the highest throughput while only achieving partial degrading efficiency [128]. Despite several types of research on the efficient use of various reactors, large-scale reactor applications are missing due to opacity, light scattering, and depth of radiation penetration.

Table 6. Various types of solar photocatalytic reactors used for the degradation of wastewater.

S. No	Types of Reactor	Pollutant/ Wastewaters	Reference
1	Raceway pond reactor	Micropollutants	Arzate et al., 2017 [132]
2	Raceway pond reactor	Chlorpyrifos	Amiri et al., 2018 [133]
3	Parabolic trough collector	Phenol	Abid et al., 2017 [134]
4	Water bell reactor	Industrial wastewater	Maksoud et al., 2018 [128]
5	Compound parabolic reactor	Pharmaceuticals	Almomani et al., 2018 [135]
6	Solar pond reactors	Reactive orange dye	Chavaco et al., 2017 [136]
7	Batch stirred and fluidized bed reactor	Phenol	Shet and Shetty, 2016 [137]
8	Compound parabolic collector	Winery effluents	Velegraki and Mantzavinos, 2015 [32]
9	Thin film fixed-bed reactor	Amaranth	Sudrajat and Babel, 2016 [138]
10	Photocatalytic solar tower reactor	Volatile organic compounds	Negishi and Sano, 2014 [139]
11	Water falling DBD reactor	Mesotrione	Jovic et al., 2014 [140]
12	Thin film cascade reactor	Benzoic acid	Chan et al., 2003 [141]
13	Thin film fixed-bed reactor	Pesticides	Shankar et al., 2004 [142]
14	Water fountain photocatalytic reactors	Textile dyeing wastewaters	Kanmani et al., 2003 [124]
15	Thin film multi-tubular photoreactor	Methyl orange	Adams et al., 2013 [143]
16	Double-skin sheet reactor	Industrial wastewater	Dillert et al., 1999 [125]
17	Shallow pond reactor	Direct Yellow 12 dye	Toor et al., 2006 [129]
18	Falling film reactor	Gelatin industry	Velmurugan et al., 2016 [144]
19	Fixed-bed reactor	Methyl orange	Khalilian et al., 2015 [145]
20	Continuous stirred tank reactor	2,4-dinitrophenol	Miyawaki et al., 2016 [146]
21	Thin film fixed-bed reactor	Landfill water	Kernani et al., 2014 [126]
22	Pebble bed reactor	Textile wastewater	Rao et al., 2012 [130]
23	Fountain reactor	Water purification	Puma and Yue, 2001 [131]
24	Compound parabolic reactor	Industrial wastewater	Nair et al., 2016 [147]
25	Annular photoreactor	Pharmaceutical	Sarkar et al., 2017 [148]
26	Raceway pond	Landfill leachate	Spineillo et al., 2023 [149]

Table 6. Various types of solar photocatalytic reactors used for the degradation of wastewater.

S. No	Types of Reactor	Pollutant/ Wastewaters	Reference
1	Raceway pond reactor	Micropollutants	Arzate et al., 2017 [132]
2	Raceway pond reactor	Chlorpyrifos	Amiri et al., 2018 [133]
3	Parabolic trough collector	Phenol	Abid et al., 2017 [134]
4	Water bell reactor	Industrial wastewater	Maksoud et al., 2018 [128]
5	Compound parabolic reactor	Pharmaceuticals	Almomani et al., 2018 [135]
6	Solar pond reactors	Reactive orange dye	Chavaco et al., 2017 [136]
7	Batch stirred and fluidized bed reactor	Phenol	Shet and Shetty, 2016 [137]
8	Compound parabolic collector	Winery effluents	Velegraki and Mantzavinos, 2015 [32]
9	Thin film fixed-bed reactor	Amaranth	Sudrajat and Babel, 2016 [138]
10	Photocatalytic solar tower reactor	Volatile organic compounds	Negishi and Sano, 2014 [139]
11	Water falling DBD reactor	Mesotrione	Jovic et al., 2014 [140]
12	Thin film cascade reactor	Benzoic acid	Chan et al., 2003 [141]
13	Thin film fixed-bed reactor	Pesticides	Shankar et al., 2004 [142]
14	Water fountain photocatalytic reactors	Textile dyeing wastewaters	Kanmani et al., 2003 [124]
15	Thin film multi-tubular photoreactor	Methyl orange	Adams et al., 2013 [143]
16	Double-skin sheet reactor	Industrial wastewater	Dillert et al., 1999 [125]
17	Shallow pond reactor	Direct Yellow 12 dye	Toor et al., 2006 [129]
18	Falling film reactor	Gelatin industry	Velmurugan et al., 2016 [144]
19	Fixed-bed reactor	Methyl orange	Khalilian et al., 2015 [145]
20	Continuous stirred tank reactor	2,4-dinitrophenol	Miyawaki et al., 2016 [146]
21	Thin film fixed-bed reactor	Landfill water	Kernani et al., 2014 [126]
22	Pebble bed reactor	Textile wastewater	Rao et al., 2012 [130]
23	Fountain reactor	Water purification	Puma and Yue, 2001 [131]
24	Compound parabolic reactor	Industrial wastewater	Nair et al., 2016 [147]
25	Annular photoreactor	Pharmaceutical	Sarkar et al., 2017 [148]
26	Raceway pond	Landfill leachate	Spineillo et al., 2023 [149]

7. Economic Analysis of Advanced Oxidation Processes

Cost is a major consideration whenever new technologies are being developed. It shows whether any new, generally accepted commercial techniques or technologies are significantly lowering processing costs and whether they are commercially feasible or not [150]. The primary variables affecting solar detoxification systems are the installed costs, operation costs, and treatment costs. The installed cost comprises all expenses related to the design, installation, and startup of the facility. The operational cost is the second deciding aspect in the cost of treatment after the total installed cost. The total costs of operating and maintaining the facility, including all labor and material expenditures (such as those associated with procurement, maintenance, and supervision), chemical supplies, and electricity costs, must be included in the operating cost. The cost comparison is an important component in comparing AOPs, depending on their intended application, such as a pre- or partial treatment to attain total mineralization (>99%) [151].

By using an integrated photo-Fenton and electrocoagulation process, [152] Modenes et al. (2012) conducted a cost study for tannery effluent while only taking into account the direct cost elements. According to [21], who handled the wastewater from polyester dyeing, it costs too much to use artificial radiation in the photo-Fenton process (17.4, 9.6, and 2.9 EUR/m³ for acrylic, cotton, and polyester effluents, respectively). Using (simulated) solar light makes it possible to achieve high levels of color removal and a considerable decrease in COD and DOC removal at a reduced total cost of a factor of approximately three. According to the published research, the solar photo-Fenton process will cost roughly 2.11 EUR/m³ to run for five years with a treatment capacity of 50 m³/d (USD 2.2155) [153]. Alalm et al. (2014) [154] explored the solar photo-Fenton process using a compound parabolic collector reactor for extracting phenol from an aqueous solution. The costs connected with a large-scale reactor were examined. It was observed that the overall cost of the best economic scenario with the greatest phenol degradation was 2.54 EUR/m³. Torres et al., 2015 [155] studied the economic assessment of landfill leachate by the solar photo–Fenton process with the design flow rate of 40 m³/d, and the estimated treatment cost was 43 EUR/m³ (USD 45.15). Selvabharathi et al. (2016) [156] carried out a cost analysis by analyzing the degradation of tannery wastewater by the combined homogeneous and heterogeneous process and estimated the annual treatment cost as USD 19.35.

According to research by Aljouboury et al. [64] from 2021, the best economic condition with the greatest phenol degradation costs was found to be 2.54 EUR/m³. Furthermore, the TiO₂/ZnO/air process was estimated to cost 9 USD/m³, while the TiO₂/ZnO/Fenton method was estimated to be about 8 USD/m³. (Mousset et al., 2021) [151] evaluated the operating costs for each AOP by normalizing them based on the treated volume and mass of TOC removed and taking into account sludge treatment and chemical and power consumption. It was clear that due to its electrocatalytic behavior, electro-Fenton was the most cost-effective AOP (108–125 EUR/m³), regardless of the mineralization target (50%, 75%, or 99%). Benis et al. (2021) [157] treated laundry wastewater by the combined biological and physicochemical process and reported that the operating cost analysis for the combined process was 0.65 EUR/m³. It has been estimated that the operating costs and investment involved in the photocatalytic treatment of wastewater are lower than other wastewater treatment technologies such as ozone treatment, the chlorination of water, and the wet oxidation process of water, incineration, and membrane separation [150]. One of the main drawbacks of the use of AOPs for the treatment of new compounds is their high cost. Hybrid techniques involving biological treatment and advanced oxidation processes were created to make them more energetically effective and applicable. AOPs have been recommended as a supplement to biological treatment for the removal of evolving compounds because they offer considerable advantages over using biological approaches alone. One of the ways used to reduce costs, increase permeate quality, and improve treatment efficiency is the combination of AOPs and biological treatments.

Thus, the literature above discussed the cost of treatment for various advanced oxidation processes. It was observed that the treatment cost of the individual process is more compared to the combined process, and therefore, the integration of AOPs with a biological treatment may be a suitable solution to reduce the operating cost and make the process more profitable.

8. Wastewater Treatment: Challenges and Opportunities

The discharge of harmful chemicals into water sources, which disrupts the ecosystem’s ability to operate, is one of the most important problems people now confront. Currently, the treatment of wastewater is a difficult endeavor that depends not only on regional and socioeconomic factors but also on laws for controlling the waste. As each treatment has unique advantages and difficulties in terms of operational complexity, ecological consequences, sludge generation, viability, applicability, and cost-efficiency, the optimal technique has not yet been determined [158]. Solar photocatalysis is a powerful and environmentally friendly energy source for degrading vulnerable organic pollutants in water sources. Even though these single AOPs are quite effective at treating wastewater, they have certain limitations such as the possibility of the formation of secondary pollutants, the necessity of expensive catalysts and electrode materials, and high costs.

The integration of AOPs and the coupling of AOPs with traditional technologies have advanced in several areas, including economic efficiency, environmental friendliness, and operability [159]. A key challenge in the homogeneous Fenton method is that it has a significant issue with iron-containing sludge, which needs additional treatment. The fact that the reaction occurs at low pH levels is another disadvantage of Fenton’s method. Although maintaining a low pH is not a concerning issue, a scaled-up process will require low pH values to be taken into consideration when designing the materials for reactors and pipelines. Though much of the literature reported the application of the solar photo-Fenton process for the degradation of wastewater containing different complex organic compounds with high efficiency, the Fenton process is still not considered to be the ultimate treatment technique because it works at very low pH. Therefore, this process is pH-dependent and strict pH control is required, thus avoiding any precipitation of inactive iron oxyhydroxides and the inevitability of its post-treatment expense when neutralizing the treated water before disposal. However, an important drawback to industrial applications of the photo-Fenton process is the consumption of oxidizing reagents and the chemicals required for increasing the alkalinity of the wastewater during pH adjustment, which inhibits its wide application, potentially resulting in higher costs.

Finding an appropriate catalyst (second phase) to treat wastewater is the most difficult aspect of heterogenous AOPs. However, the photocatalysis process has certain noticeable drawbacks, such as the high costs associated with removing the catalyst after use and the challenge of achieving radiation uniformly throughout the entire catalyst surface at a greater scale [158]. The removal of recalcitrant organic compounds is accomplished through the integration of two or more treatment approaches. In terms of energy-saving and treatment effectiveness, this method is reliable and stable. Numerous studies have shown that these hybrid approaches improve water contaminant separation efficiency while consuming less energy [160]. Wastewater treatment methods that are integrated have demonstrated tremendous promise for the mitigation of environmental pollution issues and the improvement of the availability of potable water. As a result, in addition to enhancing individual processes, future studies will focus on developing possible integrated treatment processes.

To increase the use of AOPs and AOP–biological processes in wastewater treatment, a techno-economic comparison with other technologies is being conducted [161]. When compared to traditional wastewater and water treatment systems, combination processes are more efficient. However, for each water and wastewater sample, a different type of pretreatment and combination may be required. However, several restrictions have prevented integrated wastewater/water systems from being widely used on an industrial scale. Large-scale integrated wastewater and water treatment processes have not been used due to the lack of technological maturity and feasible data, the difficulty of managing integrated systems, and the activity reduction in treatment materials such as photocatalysts and adsorbents over time [160].

9. Conclusions

This review focuses on individual AOPs, integrated AOPs, and AOPs + biological processes for treating various industrial wastewaters. In this study, a methodical way to evaluate different AOPs was provided. Advanced oxidation processes provide effective methods for removing organic pollutants from water. Compared to conventional wastewater treatment systems, hybrid and integrated procedures are more efficient. Combining AOPs with a biological treatment may be an ideal approach for reducing operating costs and increasing the profitability of the treatment process. AOPs are often more expensive and require more reagents and energy sources, which limits their industrial applicability. AOP efficiency can be increased by adopting specific reactor configuration. A solar photoreactor is required for the photochemical treatment of wastewater using a light source. Due to the high operational costs of UV-based AOPs, solar irradiation may provide a long-term solution to these constraints. Therefore, it is necessary to assure that the new technology will be cost-competitive by including all the expenditures in the economic strategy.