An affordable and sustainable tertiary treatment is imperative to solve the secondary contamination issues related to wastewater reuse. To decontaminate and disinfect the actual secondary treated wastewater, various types of advanced oxidation processes (AOPs) have been studied. The optimization of the oxidant and catalyst is carried out to identify the best-performing system. Under selected experimental conditions, UV/peroxymonosulfate (PMS), O3/PMS, UV/MnO2, O3/MnO2, UV/O3/H2O2, O3/MnO2/H2O2, UV/MnO2/H2O2, and UV/O3/MnO2 has been identified as an efficient treatment option for simultaneous decontamination (>90% COD removal) and disinfection (100% inactivation of the total viable count of bacteria). The techno-economic assessment revealed that UV/MnO2 (23.5 $kg−1 of COD) UV/O3/MnO2 (37.4$ kg−1 of COD), UV/H2O2/MnO2 (36.4 $kg−1 of COD), and O3/MnO2/H2O2 (32.5$ kg−1 of COD) are comparatively low-cost treatment processes. Overall, UV/MnO2, UV/H2O2/MnO2, and O3/MnO2/H2O2 are the three best treatments. Nevertheless, further investigation on by-product and catalyst toxicity/recovery is needed. The results showed that AOPs are a technologically feasible treatment for simultaneously removing persistent organic pollutants and pathogens from secondary treated wastewater.

• Simultaneous removal of pathogens and organic contaminants is essential to recycling wastewater.

• Advanced oxidation processes are cost-effective tertiary treatment.

• Photocatalysis offered complete removal of pathogens and ∼90% removal of chemical oxygen demand (COD) within a short reaction time.

• Inactivation of bacteria is better in the peroxymonosulfate system.

• Photocatalytic ozonation was a cost-effective treatment for COD removal.

### Graphical Abstract

Graphical Abstract
Graphical Abstract

Water is the source of all life and should be used sustainably. However, frequent changes in lifestyle, urbanization and improper treatment of wastewater increased water pollution exponentially (Diaz-Elsayed et al. 2019). In recent years, water reuse is a feasible and effective way to solve the water crisis sustainably (Yang et al. 2021). Globally, more than 14 billion m3 y−1 of wastewater has been reused and utilized in various applications including crop irrigation (Leonel & Tonetti 2021) and aquifer recharge (Pronk et al. 2021). A complex matrix of the wastewater restricts the efficiency of traditional wastewater treatment (Morris et al. 2021). However, reusing the partially treated/secondary biologically treated wastewater may cause diverse health and environmental issues (Aemig et al. 2021; Ngweme et al. 2021). Over a decade, the lesser efficiency of biological processes for treating persistent organic pollutants (POPs) (e.g., pharmaceuticals, personal care products, steroids, endocrine disruptors, flame retardants, and pesticides) and pathogens has been proven, which necessitated an effective tertiary treatment (Palatsi et al. 2021). Chlorination is the widely used low-cost tertiary treatment for pathogen removal. However, it is not fit for the removal of POPs. In addition, the development of harmful chlorinated by-products is the inevitable negative impact of chlorination (Kali et al. 2021). In such a regard, advanced oxidation processes (AOPs) (e.g., UV treatment, ozonation, photocatalysis) and adsorption are used to improve the quality of the treated wastewater for reuse (Rout et al. 2021). AOPs offer simultaneous decontamination and disinfection and can be considered a possible tertiary treatment (Pandis et al. 2022).

Various types of precursors such as irradiation (e.g., UV, visible, sunlight), oxidizing agents (e.g., hydrogen peroxide, ozone, persulfate/peroxymonosulfate), and catalysts (e.g., TiO2, Fe2O3, MnO2) are used to generate reactive radicals in AOP, which results in simultaneous disinfection and decontamination of wastewater (Garrido-Cardenas et al. 2020). The UV treatment offers enhanced disinfection when compared to traditional chlorination, while insignificant UV transmittance may develop the regrowth of bacteria through DNA repair, and it is a fundamental concern (Umar et al. 2019). Ozonation is the preferred tertiary treatment for upgrading the existing wastewater treatment process, and it often results in the complete elimination of pathogens. Contrastingly, the development of brominated by-products is also a concern in ozonation (Rodríguez-Mata et al. 2019; Rizzo et al. 2020). So, depending on the wastewater characteristics, regulatory requirements, sustainability, economic aspects, and technical feasibility, a single AOP or a combination of AOPs can be used as a tertiary treatment (Pandis et al. 2022). Photocatalysis is the frequently recommended tertiary treatment option for wastewater containing low-toxicity values, while the recycling of catalysts is the hurdle (Mahy et al. 2021; Vučić et al. 2021). Photocatalytic ozonation and photo-Fenton showed better performance for sewage water treatment (Mahy et al. 2021; Maniakova et al. 2021). Sulfate-radical-based AOPs are more effective for trace-level POPs and pathogens removal, with remarkable mineralization efficiency, than hydroxyl-radical-based AOPs (Scaria & Nidheesh 2022). However, their efficiency test on a pilot scale is limited (Scaria & Nidheesh 2022). It is noteworthy to mention that the concentration of POPs and pathogenic compounds are region-specific (Renganathan et al. 2021). So, further verification is obligatory to assess the efficiency of AOP as a tertiary treatment in municipal wastewater treatment plants of a specific region (Radwan et al. 2022).

To date, neither design criteria nor kinetic studies-related guidelines have been available to evaluate the efficiency of AOPs for the degradation of POPs, causing the usage of different experimental and analytical methods. However, governmental agencies have used chemical oxygen demand (COD) as a referential parameter to access the concentration of organic contaminants in the treated wastewater (CPCB 1986; APHA 2017). For instance, developed countries (e.g., South Korea) are utilizing COD and total organic carbon to frame the discharge standards for wastewater (Park et al. 2022). In addition, Rodríguez-Mata et al. (2019) proved the replacement of complex chromatographic techniques with COD to monitor the degradation efficiencies of different AOP processes against organic pollutants. Likewise, Martínez-Huitle et al. (2015) and Rajasekhar et al. (2020) showed the application of COD to access the performance efficiency of electrochemical oxidation for removing organic pollutants from wastewater. Therefore, COD-based approaches serve as simple, viable, and a cost-effective tool to map the efficiency of various AOP processes against POPs.

In the present study, efficiencies of different AOPs for treating secondary treated wastewater have been studied by monitoring the pH, electrical conductivity (EC), COD, biological oxygen demand (BOD), and total viable count (TVC) of bacteria. Additionally, the energy requirements of the processes have been calculated, and insights related to the techno-economic assessment are provided. To this end, this study may act as pioneer research to understand the efficacies of the different AOP processes for degrading POPs in secondary treated wastewater.

### Chemicals and reagents

Potassium iodide, manganese sulfate, ferric chloride, calcium chloride, sodium azide, potassium dichromate, manganese dioxide, sodium thiosulfate, phosfate buffer solution, mercuric sulfate, silver sulfate, ferrous ammonium sulfate, titanium dioxide, and sulfuric acid were purchased from Merck Chemicals. Hydrogen peroxide was obtained from Finar chemicals, and sodium persulfate, sodium peroxy monosulfate, ferric oxide, ferrous sulfate, and manganese dioxide were obtained from Sigma Aldrich. The purity of all the chemicals purchased was ≥ 99%. The chemicals were utilized without further purification.

### Sample collection

The secondary biologically treated water (SBTW) samples were collected individually from the sewage treatment plant (STP) located at Anna University, Chennai. The treatment scheme followed in the STP is a bar screen, activated sludge process followed by dual media (activated carbon) filters. Fresh effluents were collected daily from the outlet of the secondary clarifier (except in the case of STP maintenance and other issues) from March 2021 to January 2022. To ensure the sample quality, all the collected samples were stored at 4 °C.

### Experimental setup

The treatment of SBTW was studied using an AOP-based lab-scale reactor with a working volume of 2.2 L. The reactor (13.5 cm (D) × 28.5 cm (L)) was assembled in stainless steel (SS 316) with a UV lamp (12 W, 254 nm). The light source was immersed right in the middle of the solution, and it was covered with quartz sleeves (1 mm thickness) as shown in Figure S1.

The suspended solids present in the wastewater samples were removed through a sand-bed reactor, and the filtered samples were used in the AOP process. Different AOP experiments for the removal of POPs and disinfection of pathogens in the SBTW were investigated (Figure 1). All AOP experiments were carried out at neutral pH and under ambient conditions.
Figure 1

Schematic presenting the different AOP processes used in the simultaneous decontamination and disinfection of pathogens from SBTW.

Figure 1

Schematic presenting the different AOP processes used in the simultaneous decontamination and disinfection of pathogens from SBTW.

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#### Photolysis

The batch treatability studies were carried out by varying the UV dosage of 2.4, 4.8, 9.6, 14.4, and 28.8 mJ cm−2. The treated wastewater samples collected were subjected to physicochemical and biological analysis.

The UV/H2O2 experiments were studied by varying the concentration of H2O2 (30%) from 5, 10, 20, 40, and 100 mg L−1 in an AOP reactor with constant UV irradiation. To neutralize the residual H2O2, sodium sulfite is mixed with samples before physicochemical and biological analysis.

#### Photocatalysis

Photocatalysis experiments were carried at constant UV radiation (12 W at 254 nm) and varying the concentration of catalysts (Table S1). UV radiation (12 W at 254 nm), H2O2 dosage (10 mg L−1), and reaction time (10 min) were kept constant for UV/ Fe2+/H2O2, UV/Fe2O3/H2O2, UV/TiO2/H2O2, and UV/MnO2/H2O2 processes. The concentrations of catalyst have been varied and the treated solution was neutralized with sodium sulfite to quench the residual OH.

#### Ozonation and catalytic ozonation

Ozone (O3) was generated using pure oxygen in a Faraday ozone apparatus (A2G, India). The flow rates of oxygen and ozonation were 5 L min−1 and 0.354 g h−1 L−1, respectively. The O3 inflow was connected to the bottom of the reactor and in the course of the ozonation process, a UV lamp was removed from the reactor.

The ozonation experiments were carried out at a constant O3 dosage (13 mg min−1) and at a neutral pH and ambient temperature. Rest of the procedure was like the photolysis and photocatalytic experiments. The different concentrations of oxidant and catalysts used in the optimization studies are reported in Table S2.

#### Photolytic and photocatalytic ozonation

Photolytic ozonation experiment was conducted at constant UV irradiation (12 W at 254 nm) and O3 dosage (13 mg min−1). The treated samples were mixed with sodium sulfite to neutralize the residual O3 prior to the physicochemical and biological analysis.

Photocatalytic ozonation experiments (e.g., UV/H2O2/O3, UV/Fe2+/O3, UV/Fe2O3/O3, and UV/MnO2/O3) were conducted by maintaining constant UV irradiation (12 W at 254 nm) and O3 dosages (13 mg min−1) along with varying the oxidant or catalyst dosages (Table S3). The treated samples were collected and mixed with sodium sulfite prior to the physicochemical and biological investigations.

### Analytical methods

COD was analysed using open reflux method using I-Therm (AI-7782) (Method name: 5220 B) and BOD5 was analysed based on American Public Health Association (APHA) method 5210 B (APHA 2017)). Infra Digi portable meter was used to measure the pH and electrical conductivity of wastewater. Phenate method was used for nitrite analysis, using UV-Visible spectrophotometer at 543 nm (Method name: APHA 4500-Nitrite B, (APHA 2017)). Pour plate method is used to count the viable count of bacteria.

### Wastewater characterization

The physicochemical characteristics of the secondary treated municipal wastewater collected from the Anna university STP are listed in Table 1. The average pH, EC, COD, BOD, and bacterial count of the collected sample were 7.8, 1.22 mS cm−1, 43 mg L−1, 14.9 mg L−1, and 280,000 colony forming units (CFU) mL−1, respectively. The discharge standards for effluent in India for pH and COD are 6.5–9 and <50 mg L−1 (CPCB 1986; APHA 2017). The observed COD value was found to be within the discharge standard. On the other hand, the bacterial count of the collected wastewater was comparatively higher (800–280,000 CFU mL−1) than the effluent discharge value (<100 CFU mL−1 (CPCB 1986; APHA 2017)). Similar observation was also reported by Rajasekhar et al. (2020). This proves that the presence of higher concentrations of bacterial count in the SBTW indicates the potential toxicity of treated effluent. Thereby it is evident that in order to meet the effluent discharge standards, tertiary treatment is required.

Table 1

Physicochemical characteristics of the wastewater

ParametersMinMaxAverageSDEffluent discharge standardsa
pH 7.20 8.45 7.80 0.34 6.5–9
EC (mS cm−11.10 1.42 1.22 0.08 –
COD (mg L−124.0 67.0 43.0 9.67 <50
BOD (mg L−14.00 26.0 13.0 6.01
Bacterial Count (CFU mL−1800 1,600,000 160,000 280,000 <100
ParametersMinMaxAverageSDEffluent discharge standardsa
pH 7.20 8.45 7.80 0.34 6.5–9
EC (mS cm−11.10 1.42 1.22 0.08 –
COD (mg L−124.0 67.0 43.0 9.67 <50
BOD (mg L−14.00 26.0 13.0 6.01
Bacterial Count (CFU mL−1800 1,600,000 160,000 280,000 <100

aEffluent discharge standards (CPCB 1986; APHA 2017).

### Removal efficiency of AOP processes

#### Reactive species generation in diverse AOPs

AOP processes such as photolysis, photocatalysis, ozonation, catalytic, and photocatalytic ozonation offer effective removal of POPs and pathogens from wastewater (Arslan-Alaton et al. 2021). Furthermore, UV irradiation in the presence of generic oxidants such as hydrogen peroxide (H2O2), persulfate (PS: S2O82−), and peroxymonosulfate (PMS: HSO5) results in the generation of OH and sulfate radicals (SO4•−) Equations (1)(3), which are non-selective reactive species known to increase the mineralization efficiency of the AOP process (Li et al. 2017; Luo et al. 2017). Similarly, O3 is a strong oxidant and involves disinfection and oxidation of contaminants through direct (e.g., electrophilic, nucleophilic, and dipolar addition reactions) and indirect (e.g., production of OH radicals via O3) oxidation mechanisms (Equation (4)). The degradation efficiency of the ozonation is faster (e.g., 50% in 40 min) than UV process (e.g., 20% in 60 min), but it results in lower mineralization, which is a drawback that needs to be addressed (Prieto-Rodríguez et al. 2013). To increase the mineralization, TiO2 has been added owing to its ability to mineralize the recalcitrant pollutants.
(1)
(2)
(3)
(4)
POPs degradation by TiO2-based photocatalysis is accompanied by the generation of electron-hole (e and h+) pairs via the migration of electrons (e) from the valence band (VB) to the conduction band (CB) (Equation (5)) (Carra et al. 2016). The photogenerated h+ reacts with water and yields OH. The generation of OH in the MnO2 reaction is similar to that of TiO2 (Chiam et al. 2020). On the other hand, h+ also reacts with the organic matter (OM) present in the wastewater and generates OH (Equations (6) and (7)) (Carra et al. 2016). However, the complex nature of the wastewater reduces the availability of the h+ through quenching reactions, subsequently decreasing the reactive oxygen species (ROS) production (De la Cruz et al. 2012). It has been found that the co-existence of H2O2 and hγ in iron-catalyzed systems is prominent in reactive radical production (Equations (8)–(10)) (Carra et al. 2016). So, the presence of Fe2+ increases the POPs removal and reduces the residual H2O2.
(5)
(6)
(7)
(8)
(9)
(10)

The reaction speed of the AOP has a direct relation to the reactive radical production and quenching phenomenon of the OM/radical present in the wastewater. Therefore, comprehensive assessment of these technologies is essential to identify the efficacies of AOPs as a possible tertiary treatment for wastewater.

### Photolysis and photocatalysis

The photolytic and photocatalytic processes are based on direct photolysis of the POPs in the presence of UV radiation and catalytic material, respectively. The COD removal and inactivation of TVC of bacteria in photolysis were 20 and 80% in 60 min, respectively. To further enhance the removal efficiency, catalyst has been added. The photocatalysis with different oxidants H2O2, PS, PMS, Fe2+, Fe2O3, MnO2, and TiO2 was studied and the results were summarized in Table 2 and Figure 2. The optimization of the oxidant and catalyst for removing COD and TVC was conducted, and the results are shown in Figure S2 and Figure S3. From the optimization studies, the parameters showing the maximum removal for simultaneous decontamination and disinfection have been reported.
Table 2

Simultaneous removal of COD and pathogens by photolysis and photocatalysis

Treatment processCatalyst dosageReaction time (min)Initial COD (mg L−1)Final COD (mg L−1)COD removal (%)Initial TVC (CFU mL−1)Final TVC (CFU mL−1)TVC removal (%)
UV 12 W, 254 nm 60 55 44 20 2 × 103 10 × 103 80
UV/H2O2 40 mg L−1 40 55 19 65 2 × 103 3 × 101 98
UV/PS 3 mM L−1 60 52 18 65 5 × 105 4 × 101 99
UV/PMS 0.5 mM L−1 40 59 16 72 7 × 103 100
UV/Fe2+ 15 mg L−1 15 34 06 94 28 × 105 51 × 102 99
UV/H2O2/Fe2+ 5 mg L−1 10 34 00 100 28 × 105 5 × 102 100
UV/Fe2O3 75 mg L−1 20 32 00 100 12 × 103 27 × 101 98
UV/H2O2/Fe2O3 50 mg L−1 10 38 04 89 12 × 103 4 × 101 99
UV/MnO2 50 mg L−1 20 48 04 90 10 × 103 4 × 101 99
UV/H2O2/MnO2 50 mg L−1 10 28 00 100 25 × 103 12 × 101 99
UV/TiO2 10 mg L−1 30 44 04 90 17 × 103 108 × 101 93
UV/H2O2/TiO2 35 mg L−1 10 50 04 92 12 × 103 7 × 101 99
Treatment processCatalyst dosageReaction time (min)Initial COD (mg L−1)Final COD (mg L−1)COD removal (%)Initial TVC (CFU mL−1)Final TVC (CFU mL−1)TVC removal (%)
UV 12 W, 254 nm 60 55 44 20 2 × 103 10 × 103 80
UV/H2O2 40 mg L−1 40 55 19 65 2 × 103 3 × 101 98
UV/PS 3 mM L−1 60 52 18 65 5 × 105 4 × 101 99
UV/PMS 0.5 mM L−1 40 59 16 72 7 × 103 100
UV/Fe2+ 15 mg L−1 15 34 06 94 28 × 105 51 × 102 99
UV/H2O2/Fe2+ 5 mg L−1 10 34 00 100 28 × 105 5 × 102 100
UV/Fe2O3 75 mg L−1 20 32 00 100 12 × 103 27 × 101 98
UV/H2O2/Fe2O3 50 mg L−1 10 38 04 89 12 × 103 4 × 101 99
UV/MnO2 50 mg L−1 20 48 04 90 10 × 103 4 × 101 99
UV/H2O2/MnO2 50 mg L−1 10 28 00 100 25 × 103 12 × 101 99
UV/TiO2 10 mg L−1 30 44 04 90 17 × 103 108 × 101 93
UV/H2O2/TiO2 35 mg L−1 10 50 04 92 12 × 103 7 × 101 99
Figure 2

Efficiency of photolytic and photocatalytic processes for simultaneous decontamination and disinfection.

Figure 2

Efficiency of photolytic and photocatalytic processes for simultaneous decontamination and disinfection.

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Under batch optimization studies, the UV/H2O2 experiment showed COD removal and inactivation of TVC as 65 and 98% in 40 min with an H2O2 dosage of 40 mg L−1, respectively. The UV/PS experiment showed COD removal and inactivation of TVC of bacteria at 65 and 99% in 60 min of reaction time with a PS dosage of 3 mM L−1, respectively. The UV/PMS showed COD removal and inactivation of TVC as 72 and 100% in 40 min with 0.5 mM L−1 of PMS dosage, respectively. The higher removal of POPs and pathogens by PS and PMS systems is associated with their lower O-O bond energy (140 kJ mol−1) than H2O2 (213 kJ mol−1) which favored higher activation of PS/PMS by UV irradiation (Yang et al. 2019). Additionally, irradiation of UV is sufficient to break the O-O bond in the PS/PMS, and results in higher radical generation (Ghanbari & Moradi 2017). Contrastingly, usage of higher concentrations of PMS (>3 mM L−1) results in a gradual decrease in the pH of the solution (e.g., below 3); therefore, further steps to neutralize the pH of the treated solution are necessary.

UV/Fe2+ experiment showed the COD removal and inactivation of TVC as 94 and 99% in 15 min with Fe2+ dosage of 15 mg L−1, respectively. The UV irradiation minimizes the iron sludge generation via successful recycling of Fe2+ to Fe3+ and vice-versa (Zhang et al. 2019). The addition of H2O2 (10 mg L−1) into the UV/Fe2+ experiment improved the rate of reaction and showed 100% COD removal and 99.9% inactivation of TVC of bacteria in 10 min with Fe2+ dosage of 5 mg L−1, respectively. H2O2 supports more OH formation via oxidation of Fe2+ to Fe3+ (Equation (8)) (Zhang et al. 2019). Further reaction of H2O2 with Fe3+ results in Fe2+ generation (Equation (9)). More importantly, the reaction rate involved in the formation of OH via oxidation of Fe2+ was 6,000 times faster than the reduction of Fe2+ by H2O2 (Song et al. 2006). In this regard, adding H2O2 to the Fenton system improves the degradation of organic pollutants. The higher degradation potential of heterogeneous photo-Fenton in comparison with the persulfate system is due to the higher production of the OH, which is known to decrease the COD by means of multiple mechanisms (Cai et al. 2020; VM Starling et al. 2021). In contrary, SO4•− destroys the pollutants from SBTW with the aid of only electron transfer reactions (VM Starling et al. 2021). Moreover, the photo-Fenton process is more effective for treating the low concentrations of organic contaminants than persulfate-based processes (Zhang et al. 2019). Nevertheless, the turbidity of the solution in heterogeneous Fenton reaction is increased due to the formation of reddish-brown flocs (Figure S4). The existence of iron precipitates requires sedimentation or filtration before the discharge of the effluent. In such a regard, the heterogeneous photo-Fenton reaction is not reported as an effective treatment for POPs and pathogen removal (Prieto-Rodríguez et al. 2013; Hassanshahi & Karimi-Jashni 2018).

The UV/Fe2O3 experiment showed the COD removal and inactivation of TVC of bacteria as 100% and 97.8% in 20 min with a Fe2O3 dosage of 75 mg L−1, respectively. The addition of H2O2 (10 mg L−1) into UV/Fe2O3 decreased the COD removal efficiency. For example, the achieved COD removal and inactivation of TVC of bacteria were 89 and 99% in 10 min with Fe2O3 dosage of 50 mg L−1, respectively. Slightly decreased removal efficiency of UV/Fe2O3 in the presence of H2O2 is related to the higher ability of Fe2O3 to produce reactive radicals (e.g., O2•−) than the H2O2. Such a type of efficacy of Fe2O3 in the presence of UV irradiation was also confirmed by Li et al. (2014) and Wang et al. (2015). The UV/MnO2 experiment showed the COD removal and inactivation of TVC of bacteria as 90 and 99% in 20 min, respectively. The addition of H2O2 (10 mg L−1) into the UV/MnO2 experiment enhanced the rate of reaction and showed the COD removal and inactivation of TVC of bacteria as 100 and 99% in 10 min, respectively. Here, H2O2 retards the electron-hole recombination and results in the OH generation (Chiam et al. 2020). Further, H2O2 reacts with UV and generates OH radicals; these types of multiple sources for OH generation were responsible for the higher degradation of COD and pathogens. The UV/TiO2 experiment showed COD removal and inactivation of TVC of bacteria as 90 and 93% in 30 min with TiO2 dosage of 10 mg L−1, respectively. The addition of H2O2 (10 mg L−1) in UV/TiO2 enhanced the rate of reaction and showed the COD removal and inactivation of TVC of bacteria as 92 and 99% in 10 min with TiO2 dosage of 35 mg L−1, respectively. The higher removal of MnO2 than TiO2 originated from the narrow-band gap, which results in higher production of OH. This kind of higher removal efficiency of MnO2 over TiO2 was also reported by Chiam et al. (2020) and Rahmat et al. (2019). On the other hand, the cost of MnO2 is cheaper than TiO2, making it a more viable solution, as it would further reduce the operational expenditure (Sigma 2022).

Based on the POPs removal and disinfection of pathogens in wastewater, the best treatment processes were ranked in the order of UV/Fe2+/H2O2 > UV/Fe2O3 > UV/MnO2/H2O2 > UV/Fe2+ > UV/TiO2/H2O2 > UV/TiO2 > UV/MnO2 > UV/H2O2/Fe2O3 > UV/PMS > UV/PS > UV/H2O2.

### Ozonation and catalytic ozonation

The simultaneous destruction ability of the ozonation and catalytic ozonation was investigated and summarized in Table 3 and Figure 3. The optimization of the oxidant and catalyst for removing COD and TVC was conducted, and the results have been outlined in Figure S5 and Figure S6. From the optimization studies, the parameters showing the maximum removal for simultaneous decontamination and disinfection have been reported.
Table 3

Simultaneous removal of COD and pathogens by ozonation and catalytic ozonation

Treatment processCatalyst dosageReaction time (min)Initial COD (mg L−1)Final COD (mg L−1)COD removal (%)Initial TVC (CFU mL−1)Final TVC (CFU mL−1)TVC removal (%)
Ozone alone – 40 52 26 50 2 × 105 11 × 102 99
O3/H2O2 20 mg L−1 60 56 22 60 2 × 105 4 × 101 99
O3/PS 1 mM L−1 60 40 33 18 4 × 105 100
O3/PMS 3 mM L−1 60 61 17 72 1 × 105 100
O3/Fe2+ 2.5 mg L−1 60 36 77 10 × 103 21 × 102 83
O3/H2O2/Fe2+ 10 mg L−1 10 28 00 100 40 × 103 100
O3/Fe2O3 25 mg L−1 30 60 05 91 12 × 103 16 × 101 98
O3/H2O2/Fe2O3 50 mg L−1 10 38 04 89 25 × 102 57 × 101 77
O3/MnO2 50 mg L−1 15 40 04 90 81 × 103 39 × 101 99
O3/H2O2/MnO2 50 mg L−1 10 44 04 90 81 × 103 62 × 101 99
Treatment processCatalyst dosageReaction time (min)Initial COD (mg L−1)Final COD (mg L−1)COD removal (%)Initial TVC (CFU mL−1)Final TVC (CFU mL−1)TVC removal (%)
Ozone alone – 40 52 26 50 2 × 105 11 × 102 99
O3/H2O2 20 mg L−1 60 56 22 60 2 × 105 4 × 101 99
O3/PS 1 mM L−1 60 40 33 18 4 × 105 100
O3/PMS 3 mM L−1 60 61 17 72 1 × 105 100
O3/Fe2+ 2.5 mg L−1 60 36 77 10 × 103 21 × 102 83
O3/H2O2/Fe2+ 10 mg L−1 10 28 00 100 40 × 103 100
O3/Fe2O3 25 mg L−1 30 60 05 91 12 × 103 16 × 101 98
O3/H2O2/Fe2O3 50 mg L−1 10 38 04 89 25 × 102 57 × 101 77
O3/MnO2 50 mg L−1 15 40 04 90 81 × 103 39 × 101 99
O3/H2O2/MnO2 50 mg L−1 10 44 04 90 81 × 103 62 × 101 99
Figure 3

Ozonation and catalytic ozonation for simultaneous decontamination and disinfection.

Figure 3

Ozonation and catalytic ozonation for simultaneous decontamination and disinfection.

Close modal

The direct ozonation of SBTW showed the COD removal and inactivation of TVC of bacteria as 50 and 99% in 40 min with 13 mg L−1 of O3 dosage min−1. The selectivity of O3 limits its application for the abatement of POPs in SBTW owing to the absence of electron-rich functional groups (Xing et al. 2014). The addition of H2O2 increased the decomposition rate of O3 and increased the generation of OH. For instance, the reaction rate constant of the OH (i.e., 106–109 M−1s−1) is far higher than O3 (Wert et al. 2009). The O3/H2O2 experiment showed the COD removal and inactivation of TVC of bacteria as 60 and 99% in 60 min with an H2O2 dosage of 20 mg L−1, respectively. The performance efficacy of O3 and O3/H2O2 is not efficient for reducing the COD. In this regard, the use of oxidants (PS and PMS) with the ability to selectively oxidize the low level of POPs is conducted (Cheng et al. 2016). The O3/PMS showed 72% of COD removal and 100% disinfection of bacteria in 60 min with 3 mM L−1 of PMS dosage. Similarly, Yang et al. (2015) proved that the yields of OH (0.430 moles) and SO4•− (0.450 moles) were similar. The production of equimolar concentrations of the reactive species was responsible for the enhanced removal of COD and pathogens in the O3/PMS process (Deniere et al. 2022). A indistinguishable observation of pathogen removal was observed for the O3/PS process (100% TVC of bacteria in 60 min with PS dosage of 1 mM L−1). On the other hand, the achieved COD removal by O3/PS (18%) is comparatively lower than O3/PMS (72%). PS is known to develop weaker interaction with organic matter and the by-product formation potential of PS via hydroxylation of an aromatic ring is comparatively lower than PMS and H2O2 (Xie et al. 2015). A similar observation of reducing the degradation efficiency of p-cresol and bisphenol-A in the presence of dissolved organic matter in O3/PS was studied by Kow et al. (2021) and Ghazi et al. (2014), respectively. In O3/PS system the highly reactive OH was converted directly to a less reactive O2•−. This type of OH scavenging effect was evident in the wastewater samples containing aromatic organic pollutants (Ghazi et al. 2014).

The O3/Fe2+ batch experiments showed 77% COD removal and 83% inactivation of TVC of bacteria in 60 min with Fe2+ dosage of 2.5 mg L−1, respectively. The addition of H2O2 increased O3 dissociation and it showed 100% COD removal and 100% inactivation of TVC of bacteria in 10 min of reaction time, respectively. Adding Fe2+ increased the O3 dissociation via forming FeO2+. The presence of H2O2 increased the generation of hydroperoxyl anion (HO2) and synergistically increases the production of OH (Mansouri et al. 2015). The O3/Fe2O3 experiment showed 91% COD removal and 98% inactivation of TVC of bacteria in 30 min, respectively. Adding H2O2 (10 mg L−1) into O3/Fe2O3 experiment inhibited the rate of reaction and showed 89% COD removal and 77% inactivation of TVC of bacteria in 10 min. The reaction of Fe2O3 with O3 in the presence of H2O2 produces surface adsorbed OH (OHads), which further undergoes self-scavenging reaction by complexing with O3 and Fe3+ (Bing et al. 2015). It reduces the reactive species (OH) formation and decreases the removal efficiency. The O3/MnO2 experiment showed the maximum COD removal of 90% and inactivation of TVC of bacteria of 99% in 15 min with MnO2 dosage of 50 mg L−1, respectively. The MnO2 exhibits a net negative surface charge at pH 5–11 and can enhance electron transfer with O3, which increases the production of OH (Xing et al. 2016). The addition of H2O2 (10 mg L−1) into O3/MnO2 did not exhibit any impact on the rate of reaction and showed 90% COD removal and 99% inactivation of TVC of bacteria in 10 min of reaction time with MnO2 dosage of 50 mg L−1, respectively.

Based on the organic micro pollutants removal and disinfection of pathogens in wastewater, the best treatment processes were ranked in the following order: O3/H2O2/Fe2+ > O3/Fe2O3 > O3/MnO2 > O3/H2O2/MnO2 > O3/H2O2/Fe2O3 > O3/Fe2+ > O3/PMS > O3/H2O2 > O3 > O3/PS.

### Photolytic and photocatalytic ozonation

The simultaneous destruction ability of the ozonation and catalytic ozonation was investigated and compiled in Table 4 and Figure 4. From the optimization studies, the parameters showing the maximum removal for simultaneous decontamination and disinfection have been reported.
Table 4

Simultaneous removal of COD and pathogens by photolytic and photocatalytic ozonation

Treatment processDosageReaction time (min)Initial COD (mg L−1)Final COD (mg L−1)COD removal (%)Initial TVC (CFU mL−1)Final TVC (CFU mL−1)TVC removal (%)
UV/O3 12 W, 254 nm, 13 mg min−1 of O3 30 16 12 25 2 × 105 2 × 102 99.9
UV/O3/H2O2 20 mg L−1 30 36 04 88 10 × 103 00 100
UV/O3/Fe2+ 10 mg L−1 10 48 08 83 24 × 103 00 100
UV/O3/Fe2O3 25 mg L−1 10 48 06 87 24 × 103 00 100
UV/O3/MnO2 25 mg L−1 10 48 06 87 24 × 103 00 100
Treatment processDosageReaction time (min)Initial COD (mg L−1)Final COD (mg L−1)COD removal (%)Initial TVC (CFU mL−1)Final TVC (CFU mL−1)TVC removal (%)
UV/O3 12 W, 254 nm, 13 mg min−1 of O3 30 16 12 25 2 × 105 2 × 102 99.9
UV/O3/H2O2 20 mg L−1 30 36 04 88 10 × 103 00 100
UV/O3/Fe2+ 10 mg L−1 10 48 08 83 24 × 103 00 100
UV/O3/Fe2O3 25 mg L−1 10 48 06 87 24 × 103 00 100
UV/O3/MnO2 25 mg L−1 10 48 06 87 24 × 103 00 100
Figure 4

Efficiency of photolytic and photocatalytic ozonation for simultaneous decontamination and disinfection.

Figure 4

Efficiency of photolytic and photocatalytic ozonation for simultaneous decontamination and disinfection.

Close modal

The UV irradiation along with ozonation (UV/O3) showed the COD removal of 25% and 99.9% inactivation of TVC of bacteria, in 30 min. Mecha et al. (2016) observed similar efficacy (>100%) of UV/O3 process for degrading organic pollutants in primary and secondary wastewater. To increase the COD removal efficiency and inactivation of TVC of bacteria in minimal contact time, various oxidants with different dosages were studied (e.g., UV/H2O2/O3, UV/Fe2+/O3, UV/O3/Fe2O3, and UV/MnO2/O3). Under batch optimization studies, the UV/O3/H2O2 experiment showed 88% COD removal and 100% inactivation of TVC of bacteria in 30 min with an H2O2 dosage of 20 mg L−1. The UV/O3/Fe2+ batch experiments showed 83% COD removal and 100% inactivation of TVC of bacteria in 60 min, respectively. Espejo et al. (2015) achieved 88% of COD and 75–100% of pathogens removal using UV/O3/Fe2+. However, the presence of ferrioxalate compounds induced the toxicity of the treated effluent against Daphnia magna. Therefore, toxicity of the treated effluent after photocatalytic treatment should be conducted in future. The UV/O3/Fe2O3 experiment showed 87% COD removal and 100% inactivation of TVC of bacteria in 30 min of reaction time. The UV/O3/MnO2 experiments showed the maximum COD removal of 87 and 99% inactivation of TVC of bacteria in 15 min of reaction time with MnO2 dosage of 25 mg L−1. Among the studied treatment processes, the UV/O3/H2O2 and UV/O3/MnO2 showed comparatively better performance for the removal of organics and inactivation of microorganisms in the lowest reaction time (10 min).

### Techno-economic assessment

The techno-economic assessment is a fundamental tool to assess the feasibility of the studied processes on a practical scale. For the calculation of cost, the consumption of electricity for UV and O3 was taken and the prices for H2O2, MnO2, and PMS were considered. In addition to that, the electricity consumption of tertiary pumps was kept constant for all the processes. The cost of electricity per kWh was taken as $0.12 and the power consumption for generation of O3 was 148 W. The operational cost is a sum of electrical energy cost and chemical cost (Mohan & Gokul 2021). The power consumption is calculated using Equation (11). (11) where P and t are power consumption in watt and reaction time in min, respectively. Energy cost and chemical cost were calculated using Equations (12) and (13). (12) where E is the energy in kWh, EC is the electricity cost ($ per kWh), CODremoved is the maximum COD removal (mg L−1), and V is the volume of sample (L).
(13)

Table 5 shows the cost ($kg−1) for UV/PMS, UV/MnO2, O3/PMS, O3/MnO2, UV/ H2O2/O3, UV/MnO2/O3, UV/ H2O2/MnO2, O3/H2O2/MnO2. The addition of oxidants and catalysts reduced the reaction time and increased the efficiency of single UV and O3 processes. Furthermore, the addition of catalyst increased the operational cost because it included the reagent cost as well. The operational costs of UV, UV/PMS, UV/MnO2, and UV/MnO2/H2O2 were 56.6, 85.7, 23.5, and 36.4$ kg−1 of COD. Catalytic ozonation offered higher operational cost reduction than single ozonation (Table 5). The highest operational cost was observed for UV/PMS (85.7 $kg−1 of COD) and O3/PMS (631$ kg−1 of COD removed) due to the high chemical cost of PMS and O3. Lower cost was observed for UV/MnO2 (23.5 $kg−1 of COD removed), O3/MnO2/H2O2 (32.5$ kg−1 of COD), UV/MnO2/H2O2 (36.4  kg−1 of COD removed), and UV/O3/MnO2 (37.4 $kg−1 of COD). The presence of maximum removal with low cost by photocatalytic processes was also observed by Agarkoti et al. (2022) and Gmurek et al. (2015). The high cost associated with the ozonation process was also reported by Mohan & Gokul (2021). Table 5 Techno-economic analysis of proposed AOP processes as tertiary treatment step Treatment processReaction time (min)COD removal efficiency (%)Energy cost ($ kg1)Chemical cost ($kg1)Operational cost ($ kg1)
UV 60 20.0 56.6 – 56.6
UV/PMS 40 72.9 9.65 76.0 85.7
UV/MnO2 20 91.7 4.72 18.8 23.5
UV/MnO2/H2O2 10 100 3.71 3.10 36.4
O3 40 50.0 197 – 197
O3/PMS 60 72.1 175 457 631
O3 /MnO2 15 90.0 53.3 23.0 76.3
O3/H2O2/MnO2 10 87.5 30.5 2.07 32.5
UV/MnO2/O3 10 87.5 32.9 4.48 37.4
UV/H2O2/O3 30 88.9 130 10.4 140
Treatment processReaction time (min)COD removal efficiency (%)Energy cost ($kg1)Chemical cost ($ kg1)Operational cost ($kg1) UV 60 20.0 56.6 – 56.6 UV/PMS 40 72.9 9.65 76.0 85.7 UV/MnO2 20 91.7 4.72 18.8 23.5 UV/MnO2/H2O2 10 100 3.71 3.10 36.4 O3 40 50.0 197 – 197 O3/PMS 60 72.1 175 457 631 O3 /MnO2 15 90.0 53.3 23.0 76.3 O3/H2O2/MnO2 10 87.5 30.5 2.07 32.5 UV/MnO2/O3 10 87.5 32.9 4.48 37.4 UV/H2O2/O3 30 88.9 130 10.4 140 Globally, the reuse of used water is essential to tackle the upcoming water stress, necessitating reconciling the removal of organic contaminants and microbial communities from wastewater. In this regard, different combinations of photolysis, photocatalysis, ozonation, catalytic ozonation, and photocatalytic ozonation have been studied. Combined AOP processes have demonstrated better performance than single processes. For instance, UV/Fe2+/H2O2, UV/MnO2/H2O2, and O3/Fe2+/H2O2 removed 100% of COD and caused complete inactivation of the pathogen in 10 min. Contrastingly, UV and O3 showed 20 and 50% of the COD removal and 80 and 90% of inactivation of TVC of bacteria in 60 and 40 min, respectively. From this, the addition of the catalysts showed evident enhancement in the simultaneous removal of organic contaminants and pathogens. Specifically, Fe2O3 and MnO2 showed better performance than TiO2-based catalysts, and it is related to their ability to surface the effective production of the OH. In terms of oxidant, H2O2 and PMS showed similar enhancement in the organic contaminant removal, whereas the inactivation of the bacteria was better in PMS. Techno-economic analysis indicates that UV/O3/MnO2 (37.4$ per kg of COD), UV/MnO2/H2O2 (37.4 $per kg of COD), and O3/MnO2/H2O2 (32.5$ per kg of COD) are low-cost treatment options for the decontamination and disinfection of SBTW. PMS-based technologies were governed by the high chemical cost (76 \$ per kg of COD), and O3/PMS was a high-cost treatment followed by O3. Overall, UV/MnO2, UV/MnO2/H2O2, and O3/MnO2/H2O2 are the three best tertiary treatments for STPs. However, recovery and recyclability of the catalyst, by-product formation and their toxicity, and nanomaterial-related toxicity of the treated solution need to be investigated. On the other hand, this study proves the efficacy of the AOPs as a cost-effective tertiary treatment for existing sewage treatment plants.

The authors acknowledge the ﬁnancial support received from the Water and Effluent Treatment IC, Larsen and Toubro, Chennai. The ﬁrst author, P. Ganesh Kumar, also acknowledges the Water and Effluent Treatment IC, Larsen and Toubro, Chennai and Anna University, Chennai for providing the opportunity to work in this project.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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