Photocatalytic Ozonation for Remediation of Contaminants in Aquaculture Effluent: A Review

1. Introduction

Natural water regularly gets contaminated from anthropogenic sources. These materials originate from various sources, such as domestic, storm, urban runoff, agricultural and aquaculture runoff, as well as industrial discharge. They can exist as a mixture within sewer inflow or infiltration. The contaminants found in natural water are referred to as dissolved organic matters (DOM) and dissolved particulate matters (DPM), which are commonly used as indicators for monitoring wastewater quality [1].

Several research groups worldwide have studied the effectiveness of using a combination of photolysis, ozone, oxygen, and hydrogen peroxide, as well as various homogeneous and heterogeneous catalysts and photocatalysts in both light and dark conditions, to treat contaminated water and wastewater. These techniques were investigated to evaluate their capacity to decompose pollutants and assess the treatment efficiency. This study aims to provide an analysis of various technologies used for treating aquaculture wastewater using advanced oxidation processes (AOPs) focusing on photocatalysis and ozonation. It examines their advantages and disadvantages, as well as their respective treatment efficacies. Several findings for the disintegration of new pollutants including antibiotics that may be found in water are reviewed as well.

1.1. Aquaculture Effluent

Aquaculture is directly affected by numerous laws such as land, water environmental and even trade laws. Some laws may be specific to certain region or countries. All these laws are there to safeguard public domain, thus preventing the waste created by aquaculture farms from becoming a public burden [2]. Fish cultivation is one of the aquaculture frameworks with the significance of financial matters. Because of fish discharge and feed extras, this causes excess of phosphorus and nitrogen in the water which can corrupt neighboring water bodies especially during high tide, seepage and flooding. However, the repercussions are low when compared to those from industrial and household wastewater [3]. Therefore, aquaculture wastewater treatment is essential to avert more damage to the aquatic and adjacent terrestrial environment.

Omofunmi et al., [4] investigating the effect of aquaculture effluent produced from catfish farming (Clarias gariepinus), noted that chemical, physical, and biological criteria were important aspects in water quality analysis. Physicochemical parameters, such as alkalinity (CaCO₃), ammoniacal nitrogen (NH₃), biological oxygen demand (BOD), chemical oxygen demand (COD), pH, temperature, total solids (TS), total nitrogen (TN), total phosphorus (TP), and suspended solids (SS), are the common measurements noted in water quality index (WQI) (DOE, 2020) in order to evaluate the quality of the given water sample. The water quality index, introduced in 1965, was adapted by Asian, African, and European countries and fitted to the requirements according to national and international organizations, with several improvements and modifications to suit the individual countries [5].

However, the specific values assigned to each surface water category can differ based on the type and suitability within a particular region, mainly due to the lack of generally recognized composite indicator for water quality. In Malaysia, for example, the water quality analysis focuses on six parameters, namely BOD, COD, SS, NH₃, and pH. These parameters are utilized to assess and monitor water quality in the country. Table 1.0 shows some examples of this indexing.

River water used for fish farming will contain high nitrogen and phosphorus concentrations leading to eutrophication. When eutrophication occurs, the water usually contains higher concentrations of BOD, SS, TN, and TP [10].

Antibiotics, feed-derived waste, and hormones are other compounds that are commonly detected in aquaculture wastewater. Feed-derived waste compromises phosphorus and nitrogen-based nutrients, which forms part of the suspended solid. These solids typically include 7%–32% of the TN and 30%–84% of TP in wastewater. The remainder are released in the dissolved fraction from the farm [11]. An innovative treatment method is certainly needed to address the shortcomings of conventional methods.

1.2. Aquaculture Effluent Sources and Components

According to Turcios and Papenbrock [12], solid waste comprises a huge proportion of total waste which typically originates through feed spillage alongside fish feces varying between 5 and 50mg/l inside flow-through farms. The main sources of solid waste are unattached feed and feces, as well as dead fish, from cultivated fish processing. Furthermore, solid waste is usually name as suspended solids alongside other solid components. Suspended solids are the most challenging to eradicate from culture systems as they consist mainly of fine particles suspended in the water. The larger solids can be separated from the culture column in a short time because of their larger sizes. Aerobic bacterial activities acting on solid waste present inside the aquaculture system will cause a higher demand for COD and BOD in the culture column. Only 30 per cent of the feed would be solid waste if aquafeeds were properly processed, efficiently fed, and used in the right sizes. By comparisons, the recirculated aquaculture systems can be more efficient in removing waste than flow-through systems [13].

Ongoing aquaculture methods normally impose strict limits upon the usage of chemicals in fish farms but some chemicals such as medicinal products, disinfectants, and antiviral products are commonly used. Drugs, including antibiotics, are utilized as prophylaxis and for curative objectives. Other medication such as anesthetics, ectoparasiticides and vaccines are used for the prevention and control of parasites and microbial infections. Lime is used for acidity treatment during the preparation of the tank, and similarly used are other substances deemed safe for fish. While said substances are vital for the cultivation of fish, they can still have possible adverse environmental effects as pond water is released and flows into natural water bodies as effluents [14].

The release of pathogens from aquaculture and wastewater discharges are another problem that can have negative effects on natural water sources. These discharges can introduce additional pathogenic loads to the environment, potentially causing stress and mortality. In semi-intensive, open flow pond settings commonly found in aquaculture, the effluent from aquaculture ponds is a characteristic feature. Furthermore, the use of organic fertilizers within aquaculture systems, which is practiced in many countries, may contribute to increased pathogen levels. For example, in certain regions, the utilization of four specific organic fertilizers—bovine blood, goats, swine, and poultry manure—has been associated with higher concentrations of fecal streptococci [15].

1.3. Environmental Impacts of Aquaculture

The global demand for cheap and sustainable protein sources has resulted in aquaculture is gaining worldwide prominence [16]. According to Muir and Young (1998), the worldwide dispersion of the aquaculture industry is impacted by numerous sectors such as geography, market requirements, structure availability, human capital, technical proficiency as well as institutional framework.

Despite its positive impacts on food security and economic growth, the ecological effects of aquaculture should not be disregarded. This due to the fact that farming of aquatic species necessitates the introduction of exogenous substances into culture water and the utilization of land for fish cultivation. These deliberately introduced elements serve various purposes, primarily to maintain fish health. Nonetheless, they are usually incompletely assimilated and remain as contaminants in the water. Additionally, the metabolic waste products of fish lead to the generation of waste in aquaculture water [13]. Table 1.6 below shows the possible environmental impacts of aquaculture.

Table 1. 1 Possible environmental Impacts of aquaculture.

Culture Method	Common species cultivated	Impact upon the environment
		Extensive
Seaweed culture		May colonize formerly pristine reefs, suffer bad weather losses.
Coastal bivalve culture	Mussels, oyster, clams, cockles	Public health hazards and consumer resistance Rough weather losses. Seed limitations;
Coastal fishponds	Mullets, milkfish, shrimps, tilapia	The demolishment of ecosystems, especially mangroves. Increasingly non-competitive with more intensive systems. Unsustainable with excessive population growth;
Pen and cage cultivation in eutrophic waters and/or dense benthos	carps, catfish, milkfish tilapias	Exclusion of traditional fishermen. Navigational risk. Management challenges. Wood demand.
		Semi-Intensive
Fresh- and brackishwater pond	shrimps and prawns, carps, catfish, milkfish, mullets, tilapias	Freshwater: health hazards for farm workers from infections transmitted through water. Brackishwater: soil and aquifers acidification, salinization. Market rivalry particularly for export output produce, availability and cost of feed and fertilizer.
Integrated agriculture-aquaculture	rice-fish; livestock/poultry-fish; vegetables - fish and all combinations of these	Health hazards for farm workers from infections transmitted through water. Potential consumer resistance to excreta-fed produce. Competing from other consumers of inputs such as livestock excreta and cereal brans. Hazardous compounds and pesticides in livestock feeds may accumulate in pond sediments and fish.
Sewage-fish culture		Health hazards for farm workers and consumers.
Cage and pen culture, especially in eutrophic waters or on rich benthos	carps, catfish, milkfish, tilapia	Exclusion of traditional fishermen. Navigational risk. Management challenges. Wood demand.
		Intensive
Freshwater, brackishwater and marine ponds	shrimps; fish, especially carnivores, catfish, snakeheads, groupers, sea bass	Effluents/drainage with high BOD and suspended solids levels. Market competition particularly for export products.
Freshwater, brackishwater and marine cage and pen culture	finfish, especially carnivores -groupers, sea bass, etc. - but also some omnivores such as common carp	Anoxic sediments accumulate underneath cages due to fecal and waste feed build-up; Market competition particularly for export products. Wood demand and other supplies.
Other	raceways, silos, tanks	Effluents/drainage high in BOD and suspended solids. Numerous location-specific issues.

Source: Modified from [18].

Table 1. 1 Possible environmental Impacts of aquaculture.

Culture Method	Common species cultivated	Impact upon the environment
		Extensive
Seaweed culture		May colonize formerly pristine reefs, suffer bad weather losses.
Coastal bivalve culture	Mussels, oyster, clams, cockles	Public health hazards and consumer resistance Rough weather losses. Seed limitations;
Coastal fishponds	Mullets, milkfish, shrimps, tilapia	The demolishment of ecosystems, especially mangroves. Increasingly non-competitive with more intensive systems. Unsustainable with excessive population growth;
Pen and cage cultivation in eutrophic waters and/or dense benthos	carps, catfish, milkfish tilapias	Exclusion of traditional fishermen. Navigational risk. Management challenges. Wood demand.
		Semi-Intensive
Fresh- and brackishwater pond	shrimps and prawns, carps, catfish, milkfish, mullets, tilapias	Freshwater: health hazards for farm workers from infections transmitted through water. Brackishwater: soil and aquifers acidification, salinization. Market rivalry particularly for export output produce, availability and cost of feed and fertilizer.
Integrated agriculture-aquaculture	rice-fish; livestock/poultry-fish; vegetables - fish and all combinations of these	Health hazards for farm workers from infections transmitted through water. Potential consumer resistance to excreta-fed produce. Competing from other consumers of inputs such as livestock excreta and cereal brans. Hazardous compounds and pesticides in livestock feeds may accumulate in pond sediments and fish.
Sewage-fish culture		Health hazards for farm workers and consumers.
Cage and pen culture, especially in eutrophic waters or on rich benthos	carps, catfish, milkfish, tilapia	Exclusion of traditional fishermen. Navigational risk. Management challenges. Wood demand.
		Intensive
Freshwater, brackishwater and marine ponds	shrimps; fish, especially carnivores, catfish, snakeheads, groupers, sea bass	Effluents/drainage with high BOD and suspended solids levels. Market competition particularly for export products.
Freshwater, brackishwater and marine cage and pen culture	finfish, especially carnivores -groupers, sea bass, etc. - but also some omnivores such as common carp	Anoxic sediments accumulate underneath cages due to fecal and waste feed build-up; Market competition particularly for export products. Wood demand and other supplies.
Other	raceways, silos, tanks	Effluents/drainage high in BOD and suspended solids. Numerous location-specific issues.

Source: Modified from [18].

2. Antibiotics

The government has fostered the growth of the aquaculture industry in Malaysia by providing diverse and extensive assistance and intensives, to farmers in this industry, indicating the growing interest of this industry in the country and in neighboring countries. However, the increasing of production has a correlation to the increased chances of epizootic infections. These widespread infections have been a staid hindrance in the growth of aquaculture, specifically in the shrimp and marine fish farming. Fish infections in the Malaysian aquaculture have been recorded since the mid of 1980s [19]. While several parasitic, bacterial, viral, and fungal pathogens are commonly found in the Malaysian aquaculture sector have been identified by Shariff et al. [20].

Antimicrobials are also utilized in livestock and aquaculture sectors. Their application can be classified as therapeutic, prophylactic, or metaphylactic. Therapeutic usage is the to the treatment of existing infections. Metaphylactic refers to a group-medication practice that aims to treat sick animals and at the same time treating others to prevent sickness. Prophylaxis is the use of antimicrobials in individuals or groups to avoid the occurrence of infections [21]. In aquaculture, antibiotics at therapeutic doses are oftentimes administered for short period orally to groups of fish that share tanks or cages. All permissible medications used in aquaculture must be licensed by the government authority. These regulatory authorities may define guidelines for antibiotic usage, including authorized modes of distribution, routes of delivery, dose forms, withdrawal periods, tolerances, and use by species, including dose rates and restrictions [21].

Fish disease is commonly caused by the existence of pathogens and unfavorable environmental circumstances which induce infection to animals. Many diseases in aquaculture are caused by inadequate sanitation and water quality. Farmers prefers to find a fast-problem-solving solution for the illness concern that they deal with. Both criteria have promoted the use of chemotherapeutics in managing illness. The term chemotherapeutant refers to any medication, drug or chemical utilized in preventing or treating diseases. This terms addresses chemical used to enhance the raising environment. However, it excludes dietary supplements such as vitamins and immunological boosters which undoubtedly played a significant role in disease management.

The 4 major categories of chemotherapeutants are topical disinfectants, antibiotics, probiotics, and anesthetics. Topical disinfectants have a broad spectrum which are commonly used to eradicate external bacteria, fungus, and protozoans. The third category consists of probiotics or bacterial concentrates which are utilized to increase microbial breakdown of organic accretions in the pond, hence lowering the biochemical oxygen demand and the potential of anaerobiosis. The last group, anesthetics, contains chemicals used to sedate fish during shipping and handling. The uncontrolled use of these chemotherapeutants might cause major difficulties.

2.1. Type of Antibiotics in Aquaculture

Parameters such as the type, quantity of antibiotics and frequency of usage used in aquaculture are determined by the type of species farmed, the farming environment, the production technology used, farming practices, expert support from veterinarian/fish health specialist, and food safety regulations applicable in target markets. A wide range of antibiotics are used to treat fish and shrimp infections. However, many of these cannot be used on a big scale due to legal constraints. Almost all antibiotics being use are generics sourced from China and Thailand. Commercial preparations (non-generics) from Japan, Europe, and North America are often labelled adequately, including composition information and application precautions. The subsequent subsections will discuss about selected antibiotics commonly used in aquaculture in Malaysia.

2.1.1. Tetracyclines

Tetracyclines, a class of antibiotics discovered in the 1940s and are a family of antibiotics that inhibit protein synthesis by inhibiting the attachment of aminoacyl-tRNA to the ribosomal acceptor (A) site. The use of tetracycline in Malaysia is very common because it belongs to a group of broad-spectrum which makes it more potent against a broad spectrum of Gram-negative and Gram-positive bacteria and because of their minimal expense compared to other antibiotics. There are almost 20 types of tetracycline antibiotics available but chlortetracycline, oxytetracycline and doxycycline are among the commonly used in veterinary medication [22].

Notwithstanding remedial purposes, in numerous different nations, antibiotic medications are regularly added into animal feed at subtherapeutic portions as development advertisers for swine, poultry and aquaculture. For a period of time, the utilization of antibiotics as development advertisers have been connected with advantageous perspectives (particularly expansion in the effectiveness of supplement take-up and commercial income for ranchers), however, there is research information that aligns the way this activity causes bacterial resistance, hypersensitivity responses in people and animals, some alteration in natural microflora and bacterial population amid some other negative impacts [23,24].

Tetracycline has a few therapeutic signs in managing disease problems in food-producing animals and pets. Therapeutic signs in animals include dermal, respiratory diseases, soft tissue diseases, peritonitis, metritis, and other intestinal diseases just as the treatment infection for bees and aquatic species. To treat and prevent diseases in food-producing animals, antibiotics were given through drinking water or food for easier administration [25]. Even though tetracycline brings a huge concern related to the increases in bacterial resistance [26] but the use of tetracycline is still allowed in many countries as a growth promoter except for European countries and the USA [27]. Tetracycline is effective in against Mycoplasma, Chlamydia, Pasteurella, Clostridium, Ornithobacterium rhinotracheale, and a few protozoans. Table 2.0 shows the applications of tetracycline for different species of food-producing animals [28].

2.1.2. Sulfonamides

Sulfonamides are synthetic antibiotics which have been frequently used to treat bacterial and protozoan infections in people, domestic animals, and aquaculture species since their introduction to clinical practice in 1935 [29,30]. Sulfonamides are a class of antimicrobial medicines that inhibit the folic acid pathways of susceptible microorganisms. Sulfonamides are now widely pushed by regulatory agencies worldwide, owing to their progressively dwindling usage in human medicine [31]. There are many types of sulfonamides such as sulfamethazine, sulfamerazine and sulfathiazole. Other than that, sulfonamides such as Romet-30 and Tribessen, were both permitted by the US FDA to be used in fish farming. Table 2.1 shows the application of sulfonamides in fish farming [20].

Table 2. 1 Application of Sulfonamides in fish farming.

Trade Name	Pharmacologically active substances	Indications for use (Advantages)	Disadvantages
Sulfonamides
Dimeton	sulfamonomethoxine & sulfadimethoxine	Control bacterial infections including Vibrio sp. Cure fin rot in seabass and grouper bred in floated cages	Limited Efficiency High Cost They create resistance in target infections and are unable to be utilized for lengthy periods of time.
Potentiated Sulfonamides
Romet-30	Sulfadimethoxine potentiated with ormetoprim	Control bacterial infections including Vibrio sp. Cure fin rot in seabass and grouper bred in floated cages More effective Less amenable to inducing resistance	-
Tribressen	sulfadiazine potentiated with trimethroprim

Table 2. 1 Application of Sulfonamides in fish farming.

Trade Name	Pharmacologically active substances	Indications for use (Advantages)	Disadvantages
Sulfonamides
Dimeton	sulfamonomethoxine & sulfadimethoxine	Control bacterial infections including Vibrio sp. Cure fin rot in seabass and grouper bred in floated cages	Limited Efficiency High Cost They create resistance in target infections and are unable to be utilized for lengthy periods of time.
Potentiated Sulfonamides
Romet-30	Sulfadimethoxine potentiated with ormetoprim	Control bacterial infections including Vibrio sp. Cure fin rot in seabass and grouper bred in floated cages More effective Less amenable to inducing resistance	-
Tribressen	sulfadiazine potentiated with trimethroprim

2.1.3. Nitrofurans

Nitrofurans are synthetic chemotherapeutic drugs with a vast antibacterial spectrum. They are effective against both gram-positive and gram-negative bacteria, including Salmonella and Giardia spp, trichomonads, amebae, and several coccidial species. Meanwhile, when contrasted to different antibiotics, their efficacy is nothing worth mentioning. Nitrofurans tends to disrupt numerous microbial enzyme systems, especially those involved in carbohydrate metabolism, also hindered the initiation of translation. Their primary action is bacteriostatic, but at high doses, they are also bactericidal. They are significantly more active in acidic conditions. Resistant mutations are uncommon, and clinical resistance develops at a slower pace. Nitrofurans are completely cross-resistance among themselves, but not with any other antibacterial agents [32]

Nitrofurans, such as furoxone, nitrofurazone (Furazolidone) and nifurpirinol (Furanace) were formerly considered to be promising for aquaculture. Furanace has subsequently been completely prohibited due to its carcinogenic potential, while the usage of the other two has been severely restricted. In Malaysia, all the three antibiotics are still available for use by the aquaculture sector, although they are not widely employed. Nitrofurans were frequently utilized as feed additives for growth stimulation, and mainly used for livestock such as poultry, swine, and cattle, aquaculture and bee colonies in the prophylactic and therapeutic treatment of bacterial and protozoan infections such as gastrointestinal enteritis caused by Escherichia coli and Salmonella spp. fowl cholera and coccidiosis black heads [33].

2.1.4. Chloramphenicol

Chloramphenicol (CAP) is a broad-spectrum antibiotic commonly utilized in animal farming and aquaculture sector. Chloramphenicol (CAP) derived naturally from Streptomyces venezuelae Ehrlich or produced synthetically, has a solubility of 2.5 g L−1 and a pKa of 9.5[34,35]. Despite being banned in various nations throughout the world, it is still being used in several underdeveloped countries because of its efficacy in human medicine even though it brings harmful effects on the surrounding aquatic environment [36]. Nonetheless, the unauthorized use of restricted antimicrobial drugs like chloramphenicol in aquaculture has become a serious problem in terms of consumer safety and the development of drug-resistant strains in bacteria. Furthermore, even at low doses, chloramphenicol can induce permanent aplastic anaemia in humans, providing a risk to workers who handle the items. Regardless, it is still accessible in Malaysia [20].

2.1.5. Oxolinic Acid

Oxalinic acid is a synthetic antimicrobial drug that is identical in structure to the naturally occurring nalidixic acid but is more active against both Gram-negative and Gram-positive bacteria. Oxalinic acid also has demonstrated action against a wide range of Gram-negative microbes, being highly active against proteus species but less active, or inactive, against pseudomonas [37]. This antimicrobial drug is an older member of a category of synthetic antibacterial drugs often known as quinolones. Despite the fact that the latest additions to the quinolones drug family out-perform oxalinic acid in terms of both bactericidal activity and bioavailability, its relatively low cost, low mammalian and fish contamination, and satisfactory performance render it a useful and widely used drug, especially in the aquaculture industry. [38].

Oxolinic acid is also utilized in veterinary medicine to treat problems arising from Gram-negative infections. Subsequently, the drug’s primary application is in the aquaculture industry, where, in addition to its Gram-negative action, its broad spectrum of activity against fungi, protozoans, and helminths has resulted in its beneficial application in fish farming in lots of nations, both as a prophylactic and chemotherapeutic agent. The price of the oxolinic acid is higher than other antibiotics currently accessible in the market [20]

2.2. Effect of Antibiotic Residue towards Environment

The most frequent method for administering antibiotics to aquatic animals is to combine them with specially prepared formulated feed. Fish and aquatic animals do not adequately metabolize antibiotics and will excrete them completely wasted into the environment. It is believed that 75% of the antibiotics provided to fish are eliminated and dissolves in water [39]. Unregulated use of antibiotics in the aquaculture industry to produce seafoods such as farm-raised fish and shrimps could potentially cause human health hazards and food safety risks that are mainly disregarded in most developing nations throughout world. The use of antibiotics in food-producing animals leads to the accumulation of drug residues in the edible tissues of the treated animal. Antibiotics administered according to label guidelines should not leave residues at slaughter.

Antibiotic residues in aquaculture products may cause bacterial resistance and toxicity to consumers, resulting in illness or death. For instance, chloramphenicol residues increase the chance of developing cancer. It can induce aplastic anemia, a disorder in which the bone marrow stops producing red and white blood cells which can be irreversible and deadly even at extremely low quantities. Other hazardous consequences include immunopathological and carcinogenicity effects by sulfamethazine, oxytetracycline, and furazolidone; mutagenicity and nephropathy from gentamicin; and allergy by penicillin. The prevalence of antibiotic residues in domestic animal products and their associated consumer health hazards have been reported, with minimal emphasis focused on the aquaculture sector. [21].

Regardless of the method or purpose of administration, accumulation of antibiotics as residues in tissues before being entirely metabolized or eliminated from the body. The residues in fish or other animal tissues are most found when animals are taken for human food while still undergoing treatment or immediately after medication before the withdrawal period expires [40]. The intake of such goods may cause several health concerns in people [30,41].

Chloramphenicol, dimetridazole, ipronidazole, nitroimidazoles, furazolidone, nitrofurazone, and fluoroquinolones are restricted for usage off-label in food-producing animals [43]. International standards established by the FAO/WHO Codex Alimentarius Commission mandate food safety activities and monitoring. Maximum residual limits (MRLs) of authorized veterinary medications in food consumption are established with permissible levels of parent pharmaceuticals and/or metabolites in food products of treated animals that are safe for people (National Council regulation EEC/2377/90). Despite attempts to unify maximum residue limits globally under the supervision of the World Trade Organization (WTO) and the Codex Alimentarius, MRLs remains differ from one nation to another. Furthermore, MRLs in a certain animal product may vary from one nation to another based on the local food safety regulatory bodies and medication consumption trends, and most emerging countries have yet to define their own MRLs. Drug manufacturers establish withdrawal periods during which treated animal products are not permitted to enter the food system. When veterinarians authorize extra-label use of antibiotics, the withdrawal period is expected to be changed correctly, with most times being expanded to decrease the possibilities of residue buildup in animal tissues [44].

3. Eliminating Pollutants by Application of AOPs

Advanced oxidation processes (AOPs) have received attention as the technology for the treatment of water and wastewater. AOPs were originally proposed for the treatment of drinking water in the 1980s and they were later employed for the treatment of various kinds of wastewaters [45]. When it comes to the complexes aromatic structure and resistant nature of dyes and compounds, conventional biological and chemical oxidation path are inefficient in degrading these compounds. AOPs are regarded as an extremely attractive technology in terms of water treatments for eliminating organic contaminants that are classified as non-biodegradable and destroying pathogenic microorganisms that unable to be approached with conventional technology [46]. AOPs are primarily employed to eliminate organic pollutants in water and sewage [45].

Advanced oxidation processes (AOPs) utilize reactive oxygen species (ROS), particularly hydroxyl radicals (·OH), to degrade organic contaminants in wastewater. These hydroxyl radicals have a high oxidation potential and are capable to react with a various organic compound, making them non-selective. The result is the breakdown of the contaminants into harmless by-products such as carbon dioxide and water.

AOPs, or Advanced Oxidation Processes, are oxidation techniques characterized by the generation of reactive oxygen species, particularly hydroxyl radicals, in sufficient quantities to facilitate the production of treated effluents. Hydroxyl radicals are non-selective and have high redox potential (2.8 eV) [47]. Carbon radicals (R· or R·−HO), are produced by their reaction with organic pollutants that can be converted to organic peroxyl radicals. As a result of their reactivity, the radicals generated in AOPs undergo further reactions, accompanied by the formation of other reactive species. These reactions lead to the chemical breakdown and disintegration of pollutants present in the water and in certain circumstances even complete mineralization of the target water pollutants occurs.

The implementation of advanced oxidation processes (AOPs) became necessary for wastewater treatment whenever a requirement needed to remove micro-pollutants from water sources. Conventional techniques like membrane bioreactors were discovered to be insufficient in this regard, as the continuous use of these bioreactors resulted in the accumulation of particles from previous processes, leading to membrane clogging and hindrance in the separation of micro-pollutants [48].

Although conventional methods have been effective in treating wastewater to a certain extent by removing many pollutants, the increasing water scarcity has created a need for water reuse. This requires more thorough filtration of wastewater to achieve higher standards of water quality ideal for domestic and industrial application. The primary and secondary treatments alone have not been sufficient in raising the standard of reusable water, as they struggle to remove organic contaminants, toxic substances, and nutrients present in low concentrations. As a result, advanced treatment methods have been explored and identified to complement the secondary treatment and effectively remove these residual contaminants [49,50]. When AOP is implemented in tertiary treatment, hydroxyl radicals are generated in situ because of their short lifetime by a range of procedures, including a combination of oxidizing agents, ultrasound or irradiation, and catalysts [51]. Titanium dioxide (TiO2) is the most commonly used catalyst. An alternative to it, new systems were established which utilize immobilized catalysis [52].

AOPs consist of various combinations of ozone, sonolysis, hydrogen peroxide, fenton, sulfate radicals, UV radiation, and photocatalytic tehniques that are efficient in oxidizing various contaminants in water, air, and contaminated soil. One of the advantages of AOPs when contrasted with traditional treatment is that during mineralization the biodegradability of the wastewater accelerates when utilizing AOP as the treatment. Moreover, AOPs detoxify wastewater by degrading and eliminating organic pollutants into a less toxic compounds [53].

AOPs can be grouped based on homogeneity and heterogeneity as well as the presence of the irradiation. There are several established ways of AOPs for wastewater treatment process, as the basic chemical oxidation process is insufficient for extremely polluted wastewater [54]. Works of past research demonstrated that hydroxyl radicals with high oxidizing potential will be produced via different types of AOPs for degradation of dyes such as photocatalysis photolysis, (UV/H2O2), photolysis, (UV/O3), photo-Fenton, electrochemical oxidation, ozonation and sonolysis [55].

AOPs have several advantages, including quick reaction rates, non-selective oxidation, which allows several contaminants to be treated at the simultaneously, and the ability to lower pollutant toxicity. AOPs can complete mineralization, but it may be expensive, thus biological approaches are better for the last step of dye treatment [56].

3.1. Eliminating Pollutants by Application of Photolysis and Ozonation (Photo-Ozone hybrid)

Ozone, a potent oxidant is capable of degrading organic contaminants through 2 pathways: (1) direct electrophilic attack by molecular ozone; (2) indirect attack by hydroxyl radical generated by the ozone decomposition reaction. It was noticed that elevating the ozone concentration improves the degradation rate of some pollutants while having no noticeable influence on the degradation of some other pollutants. The results might be because ozone molecules react directly with contaminants in four categories: (i) the oxidation-reduction reaction between O3 and HO−2 (or O∙−2) are electron transfer process. (ii) Ozone reacts with pollutant through cycloaddition process by forming a five-member ring ozonide structure. (iii) Ozone is an electrophilic agent that attacks the nucleophilic position of the organic compounds and other groups in the aromatic molecule, such as -OH−, -NO−2 and –Cl resulting in a substantial effect on the reactivity of the aromatic ring. (iv) Ozone has nucleophilic properties, and nucleophilic reactions happen with molecules, especially when the chemical has carbonyl, double, or triple nitrogen carbon bonds [57].

Advanced oxidation processes (AOPs), such as photocatalysis and ozonation, hold significant promise for water purification. Photocatalysis is an innovative technology that harnesses the ability of semiconductors to generate hydroxyl radicals or other active agents when illuminated with the appropriate wavelength of light. This process facilitates the degradation of organic pollutants and the disinfection of water through mineralization. On the other hand, ozonation provides several advantages in wastewater treatment plants, including the activation of sludge reactions and the elimination of persistent organic contaminants from wastewater. According to Semblante et al. (2017), ozonation leads to sludge solubilization and then reduces biomass yield.

Ozonation is utilized in the upkeep of aquaria in aquaculture and in water treatment plants for drinking water purification due to its sterilizing and purifying properties. Table 3 shows the articles published on the treatment of wastewater using photolysis and ozonation treatment processes.

4-chloro-2-aminophenol (4C2AP) is a highly hazardous organic compound that is used typically for the manufacturers of dyes and pharmaceuticals. Barik and Gogate [59], did research on the degrading of 4C2AP by combining ultrasound, photolysis and ozonation processes. It was found that US + UV + O₃ combination process was the most promising technique producing maximal degradation of 4C2AP in both ultrasonic horn which was a full removal and bath (89.9%) with a synergistic index of 1.98 and 1.29 respectively. This work has clearly shown that combined processes are able to remove toxic pollutants.

Ozonation and photocatalysis generate ROS by different means and can be use in a hybrid mode either via simultaneous or sequential mode to effectively degrade recalcitrant compounds [68]. Antibiotics such as sulfamethoxazole, tetracycline, ciprofloxacin, and trimethoprim are the antibiotics commonly used in aquaculture. Lu et al. [62], conducted a study on the degradation of antibiotics commonly found in aquaculture effluent using simultaneous catalytic ozonation and photocatalysis. They employed MgMnO3- as a bifunctional catalyst. The research revealed that the combined process generated a significantly higher amount of hydroxyl radicals compared to individual ozonation or photocatalysis. This synergy between the two processes contributed to the high efficiency of mineralization observed in this combined approach.

Similarly, Asgari et al. [67], investigated the photocatalytic ozonation of ciprofloxacin, an antibiotic, using UV A radiation. The research demonstrated that the process achieved a remarkable degradation rate of approximately 98.5% for ciprofloxacin. Photocatalytic ozonation demonstrated superior performance when compared to other treatment methods in terms of ciprofloxacin degradation

3.2. Mechanism of Photocatalytic Ozonation

The combination of UV radiation and ozonation was discovered to be an efficient catalytic system for degrading persistent contaminants in wastewater. This process begins with the photolysis of ozone, which then followed by the generation of hydroxyl (OH) radicals through the reaction of atomic oxygen (O) with water. The synergistic effect of atomic oxygen (O) and UV radiation improves the decomposition of ozone and promotes the direct and indirect production of hydroxyl radicals [69]

The hydroxyl radicals produced in an aqueous solution attack the aromatic ring of the dye molecule, producing the formation of smaller aliphatic molecules such as organic acid, aldehydes, ketones. Ultraviolet radiation enhances the degradation of resistant dyes by producing OH• radicals therefore boosting the degradation process [70]. Eq. (1) to (5) illustrate the production of hydroxyl radical

O₃ + uv → O₂ + O∙

(1)

O∙ + H₂ O → 2OH∙

(2)

2O∙ + H₂ → OH∙ + OH∙ → H₂ O₂

(3)

The indirect production of OH• by the following reaction is also possible.

O₃ + H₂O → O₂ + H₂O₂

(4)

H₂O₂ → 2 OH∙

(5)

Combination of O₃ with UV improves the degradation of azo dye at all pH [71]. According to Wu [72], the O₃/UV process had a higher decolorization rate constant that the O₃ system.

According to Mehrjouei, Müller, and Möller [73], photocatalytic ozonation process consume less ozone and are more economical than ozonation and photocatalysis. Titanium dioxide is a typical photocatalyst, yet it is photoactive in UV radiation, unlike WO₃, Fe₂O₃, In₂O₃, and BiVO₄ functioned as visible-light-responsive photocatalysts and WO₃ is an efficient catalyst for the solar photocatalytic ozonation process because it works within the solar energy wavelength range.

The synergy between photocatalysis and ozonation in water treatment has been recorded to significantly enhance oxidation efficiencies in contrast to the individual oxidation efficiencies of photocatalysis and ozonation alone. This positive synergistic effect of photocatalytic ozonation leads to the formation of hydroxyl (OH•) radicals, which effectively enhance the mineralization of organic pollutants [74]. According to Quiñones et al. [75], solar photocatalytic ozonation was found to be a cost-effective method for treating municipal secondary effluent compared to the solar photo-Fenton system.

Additionally, another benefit of photocatalytic ozonation is its potential application in disinfecting municipal wastewater. Numerous studies have reported the inactivation of bacteria and observed no resurgence of E. coli after 24 and 48 hours of post-treatment in the absence of light. Nevertheless, certain studies have indicated that certain bacteria can recover and resume growth following their initial inactivation [76,77]. This could have occurred because the effected bacteria could exploit the available carbon and energy sources in the treated water as substrates, allowing them to metabolize and regenerate [78].

Figure 1 shows the mechanism of photocatalytic ozonation. When light falls on the surface of photocatalyst (T), the photogenerated holes and electrons travel to the surface of the photocatalyst, participating in the redox reactions with adsorbed species resulting in the formation of superoxide radical anion (O∙⁻²) and hydroxyl radical (OH•) respectively, as displayed by Equation below:

Photocatalyst + hv → Photocatalyst + h⁺ + 𝑒−

(6)

𝑇 + hv → h⁺ + 𝑒⁻

(7)

O₃ + e⁻ → O∙⁻

(8)

O∙⁻₃ + H⁺ → HO∙⁻

(9)

HO∙⁻₃ → O₂ + OH∙

(10)

O₃ + H₂O → H₂O₂

(11)

H₂O₂ → HO⁻₂ + H⁺

(12)

The proficient trapping of photogenerated electrons by ozone contributes to the synergistic performance of photocatalysis and ozonation.

e− + O₂ → O∙⁻

(13)

O∙⁻₂ + O₃ → O∙⁻₃ + O₂

(14)

TiO₂-based photocatalysts are emerging materials that display exceptional absorption characteristics concerning organic compounds found in wastewater, primarily attributed to their remarkable inherent qualities. Rivas, Beltrán, and Encinas [61], did research on the degradation of a mixture of 9 antibiotics using UV irradiation, ozone gas and TiO₂ photocatalyst. Research outcomes have indicated that the introduction of TiO₂ photocatalyst notably enhanced the mineralization degree.

Equation 15 shows the reaction mechanism of TiO₂ photocatalyst, under light irradiation (photons), electrons and positive holes are generated in the conduction and valence band of titanium dioxide according to equation (15)[79]. The holes can either react directly with the organic molecules (equation 19) or form hydroxyl radicals (equation 17) that subsequently oxidize organic molecules (equation 20) [80]. The electrons can also react with organic compounds to provide reduction products (equation 15-21). The role of oxygen is important because it can react with the photo-generated electrons,

TiO₂ + hv → e^- + h⁺

(15)

hv+ + OH^- → OH∙

(16)

hv+ + H₂O → OH∙ + H⁺

(17)

e^- + O → O₂^-∙

(18)

hv⁺ + Organic → Oxidation products

(19)

OH∙ + Organic → Degradation products

(20)

e^- + Organic → Reduction products

(21)

There exist certain constraints associated with TiO₂ photocatalysis when applied to organic pollutants in aquaculture wastewater. According to Kusiak-Nejman and Morawski [81], the surface of TiO₂ is the main site of photocatalytic ozonation, but electron transfer limitations can reduce TiO₂ efficiency as a photocatalyst. Additionally, the recombination of electron and hole charges within titania can potentially occur, resulting in the dampening of photocatalytic activity. Efficiency is limited by charge recombination after excitation. Modification of TiO₂ nanoparticles surface with metals and non-metals are promising steps that lead to increased photocatalytic efficiency [82].

Joseph et al. [83], on the research of photocatalytic chlorination of methylene blue using a newly synthesized titanium dioxide-silicon dioxide (TiO₂-SiO₂) photocatalyst. This is one of the modifications of titania photocatalysts in improving the degradation of persistent pollutants in wastewater. The catalyst was synthesized using a sol-gel method followed by UV-treatment. Research outcomes indicated that the enhanced removal of methylene blue under aqueous medium was identified because of synergistic effect between chlorination and photocatalysis activated in the presence of TiO2 -SiO2 photocatalyst.

4. Economic Aspect of Photo-Ozone Hybrid

The use of photocatalytic ozonation to remove biodegradable contaminants from water is not economically feasible because it is still one of the more expensive treatment technologies. Despite photocatalytic ozonation’s effectiveness and its synergistic effects on the breakdown of contaminants in water, economic factors should also be focused on. Contrasting photocatalysis in the presence of oxygen (which needed power for UV irradiation sources) to photocatalytic ozonation (which consumes extra electrical energy for ozone generation), and the ozonation process which involves the use of an oxygen tank.

By calculating and evaluating the specific energy consumption for each oxidation system, it is possible to estimate and determine the energy consumed during the process in relation to the amount of decomposed materials.

5. Conclusions

In summary, the present review elucidates the benefits of photocatalytic ozonation in aquaculture wastewater treatment. The option of using these AOP hybrid systems in a sequential or simultaneous mode enables these systems to be fitted into existing water treatments systems thereby minimizing any required investments for plant upgrades. Further improvement can be made by exploring suitable catalyst that can be activated by ozone gas or UV irradiation or even by visible light. This will improve the mineralization ability of these hybrid systems resulting in a total mineralization of numerous contaminants including resistant pollutants present in the wastewater. The insights obtained from the literature can guide future advancements in several areas, identifying potential areas for improvement and innovation to meet the demands of the Sustainable Development Goals and the Environment Social and Governance framework.