In this study, both synthetic leachate (SL) and actual leachate samples were utilized to investigate the efficacy of the hybrid CF/(AOP_S)/EAAS method for leachate treatment.

The synthetic leachate (SL) was prepared in the laboratory by mixing metals and nutrients in distilled water according to the composition specified in Table S1. This approach allowed for better control of loading levels, assessment of treatment system performance, and evaluation of the initial leachate concentration’s impact on treatment efficacy.

Physicochemical analyses were conducted to compare the prepared synthetic leachate solution with actual leachate samples, as shown in Table 1. Actual leachate samples were collected from municipal waste disposal sites in Qaemshahr and compost factories in Behshahr, located in northern Iran. The samples were collected during both the dry and wet seasons to account for seasonal variations.

The collected actual leachate samples were filled in high-density polythene containers and stored in a refrigerator at 4 degrees Celsius [10, 27]. The samples were categorized into three groups: young, middle-aged, and elderly, based on their physicochemical characteristics and levels of chemical oxygen demand (COD) to biochemical oxygen demand in 5 days (BOD₅) ratio, as presented in Table S2.

This experimental design allowed for the assessment of the hybrid CF/(AOP_S)/EAAS method on diverse leachate types, representing different stages and variations in leachate composition and characteristics.

For the experiments, the following chemicals were used: Aluminium sulfate Al₂(SO₄)₃), Ferrous sulfate heptahydrate (FeSO₄·7H₂O), Ferric chloride (FeCl₃), Concentrated sulfuric acid(H₂SO₄), Sodium hydroxide (NaOH), Potassium hydrogen phthalate (KHP). These chemicals were purchased from Merck (Germany).

Additionally, the following chemicals were obtained from Sigma-Aldrich (USA): Potassium persulfate (K₂S₂O₈), Hydrogen peroxide (H₂O₂), Ammonium chloride (NH₄Cl) Sodium thiosulfate (Na₂S₂O₃). All the chemicals used were of analytical grade to ensure accuracy and reliability in the experimental results. The photolysis experiments employed two UVC lamps with nominal power ratings of 8 W and 15 W, which were sourced from Philips (China). These lamps were utilized to provide the necessary ultraviolet light for the photolysis reactions.

The physicochemical characteristics of the leachate were determined in accordance with the methods outlined in Standard Methods for the Examination of Water and Wastewater [28].

For the COD analysis of the leachate, the closed reflux method and colorimetric method No. 5220D were employed. The absorption of the vial contents at a wavelength of 600 nm was measured using the DR 2800-HACH spectrophotometer for colorimetry. The BOD concentration was determined using Method No. 5210B. The Total Kjeldahl Nitrogen (TKN) was measured using Method 4500-Norg C. Method No. 2540B was employed to determine the Total Suspended.

The pH parameters were determined using a HI2211pH/ORP meter. Turbidity was measured using a HACH-2100P instrument. Conductivity was measured using an AQUALYTIC-Sens Direct Con200 device. Coagulation studies on the leachate were conducted using jar test equipment, specifically the Jar Tester Phipps & Bird Stirrer Model-7790-402. Additionally, the leachate was analyzed for metal content using inductively coupled plasma mass spectrometry (ICP-MS, 7500 Agilent).

The experimental setup involved treating raw leachate in four stages for the study: coagulation-flocculation, sand filtration, AOPs, and extended aeration.

In order to investigate the effects of independent variables and effective factors, the one-factor-in-time method was employed. This method allows for the systematic evaluation of variables over a specific time period. To pretreat the leachate, a coagulation-flocculation process was implemented using a Jar-test Apparatus. This process helps to remove suspended solids and organic matter from the leachate. Next, sand filtration was carried out using a Polyvinyl chloride (PVC) column that was filled with 700 g of sand. Sand filtration aids in the removal of fine particles and further clarification of the leachate. The photo oxidation processes were conducted in the laboratory using a reactor with a volume of 2 L. The reactor was equipped with a Philips UV lamp. To address the high temperature generated by the UV lamp, the system was placed inside a 4 L cylinder with a larger diameter. This cylinder was filled with water to act as a temperature regulator. A water coolant was employed for this purpose. The sample temperature was carefully monitored and maintained at a constant level using a thermometer in the laboratory environment. Additionally, a continuous supply of air was introduced into the reactor chamber to ensure proper mixing of the reactor contents and provide the necessary oxygen for the photolysis process to proceed smoothly.

A laboratory-scale extended aeration-activated sludge treatment system has been constructed. The system consists of an aeration tank (AT) with a rectangular shape and an effective volume of 4 L. This tank is connected to a cone-shaped settling tank (ST) with a volume of 1 L. Air is injected into the leachate using a pump in the aeration tank (AT) and distributed evenly by two diffuser stones. To facilitate the feeding of recycled effluent, a baffle is created at the head of the AT, forming a small area. The schematic diagram of different stages of treatment in the batch flow leachate treatment system (BFLTS) can be seen in Fig. S1.

According to Fig. S2, it is observed that the COD and Turbidity removal efficiency follows the order of Alum < Ferrous sulphate < Ferric chloride. That means Alum had the lowest efficiency in removing COD and Turbidity, Ferrous sulphate had a higher efficiency than Alum, and Ferric chloride showed the highest efficiency among the coagulants tested. Iron sulfate (FeSO₄) at pH 9 and 1.5 g L⁻¹ coagulant dose showed 46% and 35% greater effectiveness in removing COD and turbidity compared to aluminum sulfate (Alum) at pH 7 and 2 g L⁻¹ dose. On the other hand, iron chloride (FeCl₃) at pH 6 and 0.8 g L⁻¹ dose exhibited an even higher effectiveness, with a 63% and 67% improvement in removing COD and turbidity compared to the same dose of aluminum sulfate (Alum) at pH 7.

With 0.8 g L⁻¹ of FeCl₃, ferric chloride outperformed other coagulants in leachate pretreatment, effectively removing the highest amounts of COD and turbidity at pH 6. As a result, ferric chloride (FeCl₃) was selected as the preferred coagulant for the coagulation-flocculation process. The removal efficiency (%) values for the parameters COD, BOD, TKN, and Turbidity, using iron chloride in the coagulation-flocculation process with three different concentrations of synthetic leachate samples, are depicted in Fig. 1 (a), (b), (c), and (d).

The study aimed to evaluate the efficiency of UV-activated PS/H₂O₂ and Heat-activated PS/H₂O₂ oxidation processes in treating pollutants found in the effluent resulting from the coagulation, flocculation, and sand filtration treatment of leachate.

The EAAS process was chosen to effectively treat organic matter in the leachate using optimal operating conditions in a Coagulation/Flocculation/Sand filtration/AOP effluent treatment. During a three-week adaptation period of activated sludge, the Mixed Liquor Suspended Solids (MLSS) concentration was maintained between 2900–3500 mg L⁻¹, and the COD removal remained constant at 75%. To achieve the highest COD and BOD removal, the ideal Hydraulic Retention Time (HRT) and Solid Retention Time (SRT) were examined. The input effluent had an average BOD₅ of 556 mg/L and COD of 339 mg/L.

In the biological stages of each process, a Hydraulic Retention Time (HRT) of 1 day and 2 days were implemented, along with 18 hours and 36 hours of air pumping into the reactor, maintaining a dissolved oxygen level of 4–5 mg L⁻¹. The study assessed the long-term effectiveness of the aeration-activated sludge process in removing COD and BOD. For the 1-day HRT, the removal rates were 84.2% for COD and 92% for BOD. With a 2-day HRT, the removal rates increased to 87.3% for COD and 94.7% for BOD. This indicates that the EAAS process, with optimized HRT and SRT, can effectively achieve significant reductions in COD and BOD levels. Fig. S5 illustrates the removal efficiency of four experimental variables of leachate under optimal conditions.

The four experimental variables of the Batch Flow Leachate Treatment System (BFLTS) were compared, and their findings are illustrated in Fig. S6. The results indicate that the UV-PS/H₂O₂ process outperformed both the C-F/SF and EAAS systems in terms of COD, TKN, and Heavy Metal removal, producing a significantly higher-quality effluent. Notably, the C-F/SF process exhibited superior turbidity removal compared to the other methods (Fig. S7). Furthermore, the BOD₅/COD ratios of the C-F/SF, UV-PS/H₂O₂, and EAAS processes were measured at 0.42, 0.59, and 0.27, respectively, highlighting the greater effectiveness of the EAAS system in achieving biological treatment of the effluent.

Table 2 provides a summary of the evaluation conducted on the overall performance of the Batch Flow Leachate Treatment System (BFLTS) for treating SL. It presents the impact of the system on the removal efficiency of four artificial leachate experimental variables for each treatment process, specifically under optimal conditions.

The concentration of COD in the final treated SL varied across different leachates. For SL1, the COD concentration was 191 mg/L, while for SL2 and SL3, it was 53 mg/L and 16 mg/L, respectively. Notably, the pollutant concentrations in the final effluent of SL2 and SL3 meet the maximum discharge standards implemented by most countries. Additionally, the effluent from SL1 is close to meeting the standards. This implies that all three effluents from the treatment system are suitable for discharge into surface water, absorption wells (in the case of SL3 and SL2), or even for agricultural and irrigation purposes (in the case of SL1). As for the removal of metals, the study found that, on average, 82%, 86%, and 58% of metals were successfully removed during the C-F/SF, UV-PS/H₂O₂, and EAAS processes, respectively. Among these processes, the UV-PS/H₂O₂ process exhibited the highest removal rates for metals, specifically 70% for arsenic and 80% for nickel. These findings are detailed in Table 3. It is crucial to monitor and mitigate heavy-metal contamination in water sources to safeguard human health and preserve the environment [31, 32].

Based on the results of the preliminary study with artificial leachate, the best treatment process and its related conditions for leachate treatment were identified. Then, the optimal treatment process was carried out using actual leachate and the results are presented in Table S3. In addition, the effluent from young, middle-aged, and elderly leachates displayed different physicochemical characteristics. For instance, the pH values were measured at 6.2, 6.8, and 7.5 respectively. Similarly, the electrical conductivity (EC) values recorded were 548, 520, and 435 μS/cm. additionally, the total suspended solids (TSS) were found to be 178, 164, and 143 equations should be numbered in Arabic numerals for the respective age groups. Notably, the final effluent exhibited a clear coloration in all leachate types. This suggests successful treatment in removing unwanted substances from the leachate.

The pretreatment of leachates using coagulation-flocculation is important to reduce suspended particles, organic content, color, and remove heavy metals. The dark color of leachate is primarily caused by humic chemicals and organic compounds present in it [33–35]. By implementing pretreatment, light penetration and chemical oxidation reactions can be improved, leading to higher treatment effectiveness. Several factors play a role in leachate treatment, including minerals, organic components, pH, stirring velocity, and reaction time [10, 36].

Ferric chloride (FeCl₃) is considered highly effective in removing pollutants from leachate. The coagulant demonstrates a higher removal rate at low pH values, although its efficiency decreases as the pH increases. On the other hand, ferrous sulphate exhibits a lower capacity for removing COD, which improves when the pH is adjusted to 9. It’s worth noting that increasing the dosage of the coagulant leads to lower removal rates and a higher color intensity in the solution. In fact, during the coagulation-flocculation process using FeCl₃, approximately 76.9% of the landfill leachate’s COD was successfully eliminated [37]. Similar investigations achieved removal percentages of 73%, 95%, and 97% for COD, TP, and TSS, respectively, using a dosage of 300 mg/L of FeCl₃.6H₂O [38]. Another study found that an optimal dosage of 2 mg/L ferric chloride at pH 11 resulted in a COD removal efficiency of 71% [39].

In confirmation of other studies conducted in this field, this research found that FeCl₃ is superior to the other two coagulants examined in terms of the removal of all monitored pollutants in the leachate [33, 40]. In investigating the effect of pH on COD removal, it was found that iron chloride coagulant shows a higher removal rate at low pH values while the removal rate decreases at higher pH values [40, 41]. The optimal pH range for this coagulant is found to be between 5 and 8, with a pH of 6 demonstrating the highest rate of removal. It is observed that at lower concentrations of coagulant, the effectiveness of COD removal by alum is reduced. However, at higher concentrations of coagulant, the removal of COD and turbidity increases as the concentration of aluminium hydroxide precipitation rises. This increment in concentration enhances the rate of interference and particle trapping. According to the information provided, it is likely that a concentration of 2 g L⁻¹ would yield the highest removal rate of contaminants.

It appears that ferrous sulphate initially had a low capacity for removing COD, but when 1.5 g L⁻¹ was added at pH 9, the removal improved and reached 35%. When conducting coagulation tests, it is important to note that exceeding the optimal dosage of coagulants leads to lower impurity removal rates and an increase in the color of the solution. For wastewater treatment using the flocculation-coagulation method with alum, the optimum conditions are a pH of 8, settling duration of 4 minutes, and mixing speed of 100 rpm. Under these conditions, there was a significant reduction in total suspended solids (TSS), turbidity, color, and COD of 70.7%, 92.2%, 88.4%, and 100% respectively [42].

In the study, the effect of AOPs in leachate treatment is explored. AOPs utilize diverse mechanisms depending on the specific oxidant and initiator employed. In AOPs for leachate treatment, UV rays can be used to activate hydrogen peroxide (H₂O₂), resulting in the generation of additional hydroxyl radical (OH^•) species. These highly reactive OH^• radicals play a key role in the further oxidation of organic compounds present in solid waste leachate [43, 44]. This process helps to effectively degrade and remove organic pollutants, contributing to the treatment and purification of leachate. The reaction depicted in Eq. (1) represents the fundamental process of AOPs.

The process by which hydroxyl radicals (OH^•) are produced involves the hemolytic cleavage of the oxygen-oxygen bonds in hydrogen peroxide (H₂O₂) through the absorption of UV light within the wavelength range of 200 to 300 nm as shown in Eq. 2 to Eq. 5 [45–47].

Advanced photocatalytic oxidation processes can release sulfate radicals that are capable of degrading organic matter. In the photolytic persulfate system, ultraviolet radiation triggers the photolysis of S₂O8²⁻, resulting in the production of SO4^{• −}. The sulfate radical acts as a reaction initiator in sulfate-radical-based oxidation processes. Additionally, the decomposition of S₂O8₂₋ occurs through R(^•OH) that results from the photolysis of H₂O₂, as the reactions progress [48, 49]. In the UV/SRAOP process, the PS anion is energized by UV radiation, leading to the separation of the peroxide bond and the production of two moles of SR. The photolysis of PS generates SR and ^•OH. The initiator reactions of this system are as shown in Eq. (6–8):

The persulfate oxidation using heat activation are as Eq. (9–10) [50]:

The combination of H₂O₂ with PS enhancement the biodegradability (BOD₅/COD) in leachate and can generate both hydroxyl and sulfate radicals according to reactions as Eq. (11–14):

Dissolved organic matter (DOM) is a sink for radicals and the relevant index scheme are as Eq. (15–16) [51]:

Sulfate radicals generated from advanced photocatalytic oxidation processes are highly effective in breaking down organic matter. When combined with H₂O₂, they further enhance the biodegradability (as indicated by the BOD₅/COD ratio) of leachate. Moreover, this combination generates both hydroxyl radicals and sulfate radicals, which are capable of degrading pollutants efficiently. It’s a powerful process for organic matter removal and wastewater treatment. The results indicate that sulfate radical-based advanced oxidation processes (SR-AOPs) are more desirable compared to hydroxyl radical-based advanced oxidation processes (HR-AOPs) because the former oxidizes very little ammonia. This is advantageous as excessive oxidation of ammonia can lead to undesirable byproducts or affect the overall treatment process. So, SR-AOPs appear to be a promising option with reduced ammonia oxidation, highlighting their potential in specific wastewater treatment scenarios [50, 52].

The less reactive H₂O₂ may not undergo complete photolysis and generate fewer ions during the optional reaction. The efficiency of this process is influenced by several factors, including the dosage of H₂O₂, the method of H₂O₂ injection, and the size of the reactor [56]. These factors need to be carefully considered and optimized to ensure the successful reduction of micropollutants in wastewater treatment applications [53]. However, The UV/H₂O₂ system is widely recognized as an effective AOPs employed in wastewater treatment plants for the removal of micropollutants. Using UV/PS as a treatment strategy instead of UV/H₂O₂ has shown a substantial increase in the rate of COD elimination. It is important to consider the type of AOP chosen, as well as the physical and chemical characteristics of the target pollutants and the operating conditions, as they all play a vital role in determining the treatment efficiency [5, 54]. UV lamp power plays a crucial role in photocatalytic processes. When the leachate absorbs a larger fraction of UV radiation, it enhances both the photocatalytic activity and photolysis of organic materials. A higher UV lamp power results in increased energy for the activation of photocatalysts, leading to more efficient degradation of organic pollutants [55]. At higher-intensity radiation, peroxides absorb more photons, resulting in the formation of additional electron pairs. This, in turn, enhances the efficiency of hydroxyl and sulfate radicals in the photo-catalytic mineralization and removal of COD. However, when the turbidity of the leachate increases, and the absorption of UV photons decreases, the impact of UV intensity on the activation of peroxides and the removal of organic substances lessens. It’s important to note that the UV irradiation process alone may not be sufficient for effective COD removal. Typically, the COD removal achieved through UV irradiation alone is only about 10–20%. So, to achieve higher removal efficiency, additional treatment methods or processes may be required in combination with UV irradiation [56].

It’s interesting to note that the production rates of hydroxyl radical (^•OH) and sulfate radical (SO4^{• −}) remain constant throughout the process, regardless of the initial concentrations of organic and inorganic contaminants. On the other hand, as the concentration of organic matter increases, UV penetration in the leachate decreases. This reduction in UV penetration helps prevent the activation of oxidants [57, 58].

The best UV exposure time was found to be 70 minutes, and it’s interesting to note that extended exposure times didn’t significantly impact the elimination of COD. However, after 90 minutes, the chemical oxidation process reached its peak effectiveness due to stimulated heat. By increasing the duration of ultraviolet radiation, more radicals such as ^•OH and SO4^•− were synthesized through the generation of additional electrons [59]. This led to enhanced pollutant removal. According to previous research [60], the maximum removal efficiency for COD and NH₃-N was achieved after 120 minutes of oxidation, using the PS/H₂O₂ oxidation process. The removal rates were 81% for COD and 83% for NH₃-N.

When the dosage of PS (persulfate) and hydrogen peroxide (H₂O₂) is increased, it leads to the generation of SO4^•− and ^•OH radicals, which in turn results in a higher removal rate of COD. However, there’s an important point to consider. Beyond the optimal level of PS concentration, the SO4^•− radical actually transforms into the PS radical, which has a lower oxidation-reduction potential. This transformation can impact the oxidation process [61, 62]. Additionally, the SO4^•− radical can also act as a scavenger radical, catalyzing the transformation of SO4^•− back into PS [63]. On the other hand, when K₂S₂O₈ (persulfate) and H₂O₂ are mixed together, they exhibit more effective oxidant properties compared to when they are used separately [60].

In the studied AOPs, it was observed that the removal efficiency of leachate pollutants was higher when the pH was maintained at around 7±0.2. Interestingly, in leachate with higher initial pH, the pH gradually decreased over time. In this specific study, the reaction time for UV-activated peroxides and heat-activated peroxides was set at 90 and 70 minutes, respectively, to achieve a neutral effluent. This approach aimed to maintain the pH at a desirable level during the treatment process. Similar findings have been reported in previous studies regarding leachate treatment using UV/PS (persulfate) processes. They have observed a reduction in pH over time and the attainment of a neutral effluent [37, 64, 65]. The reduction of pH by minimizing alkalinity levels helps to decrease the inhibition caused by CO₃ ²⁻ and HCO₃ ⁻ ions, ultimately enhancing the oxidation efficiency [50].

In thermal oxidation processes, the activation of peroxides with heat generates potent oxidizing agents. It was found that at pH 7 and a temperature of 65°C, the highest COD removal was achieved. However, as the temperature increased beyond that point, the removal rate declined. This decrease in efficiency could be attributed to the impact of temperature on the decay rate of H₂O₂ (hydrogen peroxide) and PS (persulfate) [50, 66, 67]. So, it seems that maintaining a pH of 7 and a temperature of 65°C offers optimal conditions for obtaining the maximum COD removal during thermal oxidation processes. Higher temperatures might lead to the accelerated decay of key oxidizing agents, potentially affecting the overall removal efficiency. Hence, careful control of pH and temperature is crucial in achieving effective oxidation results.

The biodegradability ratio (BOD₅/COD) serves as a measure of the biodegradability of organic matter, and it typically decreases over time [68]. However, advanced coagulation/flocculation and oxidation processes have been found to enhance the biodegradability (BOD₅/COD ratio) of treated leachate when compared to raw leachate. This has been supported by various studies [69–72]. In the case of leachate treatment, after undergoing coagulation-flocculation and UV-PS/H₂O₂ AOP processes, the biodegradability of leachate witnessed an increase of 0.15, 0.3, and 0.47, respectively, for leachates SL1, SL2, and SL3. It is worth noting that the increase in biodegradability was more pronounced for older leachate compared to medium and young leachate. Among the oxidation processes, persulfate/hydrogen peroxide proved to be more effective in enhancing the biodegradability of leachate, with the biodegradability ratio increasing from 0.09 to 0.17 during the oxidation processes [60]. Thus, these findings suggest that advanced treatment methods, such as coagulation-flocculation and UV-PS/H₂O₂ AOP, can significantly improve the biodegradability of leachate, particularly for older leachate. Ultimately, in AOPs, the biodegradability of the treated leachate is enhanced compared to the raw leachate [73].

The triple chemical oxidation processes exhibited the highest removal efficiency for various parameters such as COD, BOD, TKN, and turbidity in leachate treatment [74]. The synergistic effect of oxidants proved to be highly effective in purifying leachate. Among the triple processes, the UV-PS/H₂O₂ process demonstrated the highest level of purification by removing significant amounts of COD, BOD, TKN, and turbidity. Interestingly, when examining the effectiveness of persulfate (PS) without UV radiation in reducing leachate COD, only a meager 5% COD removal was achieved. This indicates that PS alone is not effective for reducing leachate COD [37]. However, when both hydrogen peroxide and persulfate reagents were utilized in combination, the performance and efficiency of leachate oxidation improved significantly. Under optimal conditions of the PS/H₂O₂ process, the removal efficiency reached 81% for COD and 83% for NH₃-N [60]. In summary, the triple chemical oxidation processes, particularly the UV-PS/H₂O₂ process, proved to be highly effective in enhancing leachate purification by removing COD, BOD, TKN, and turbidity. The combination of hydrogen peroxide and persulfate reagents significantly improved the performance and efficiency of leachate oxidation compared to using persulfate alone. This highlights the importance of synergistic effects in achieving successful leachate treatment. The UV-PS/H₂O₂ process has demonstrated exceptional removal efficiencies for COD and NH₃-N, underscoring its effectiveness as a treatment method for leachate. This process holds great promise in reducing environmental pollution and ensuring the protection of groundwater and surrounding ecosystems [75, 76].

According to the study on stabilized landfill leachate, the H₂O₂-PS-US-UV-A system exhibited the highest COD removal rate, achieving 89%. The PS process alone demonstrated an efficiency of 19%, while the PS-UV-A process showed slightly improved efficiency at 22%. The PS-US process reached 32% efficiency, and when combined in the PS-US-UV-A system, the COD removal efficiency increased to 35% [77]. In this research, various types of AOPs were utilized for organic material degradation. Among these, the AOP process involving UV-PS/H₂O₂ was identified as an efficient treatment method for Municipal Solid Waste Landfill (MSWL) leachate, as indicated in Table S4.

When aerobic biological treatment is used alone and independently for leachate treatment, its effectiveness is limited due to the high organic load and the presence of toxic substances and heavy metals in the leachate. These factors have an inhibitory effect on the growth and metabolism of the microbial population involved in the treatment process [78–80]. To treat pretreated leachate effluent, a highly effective method involving activated sludge with an extended aeration procedure has been developed. When the reactor is operated under 2-day hydraulic retention time (HRT) and 23-day solids retention time (SRT) conditions, high removal efficiencies of COD and BOD are achieved. Independent control of HRT and SRT proves valuable in maximizing wastewater sludge removal efficiency when using activated sludge and extended aeration. The implementation of extended aeration-activated sludge systems (ICEAS) has shown effectiveness in treating landfill leachate, with COD removal rates ranging from 97.03% to 98.87% [81], total nitrogen (TN) removal at 81.5% [82], and 77.1% [83].

For optimal results, a hydraulic retention time (HRT) of 2 days with 4 hours of air pumping into the reactor has been found to be more effective [83, 84]. In the effluent treatment of the leachate pretreatment system using the extended aeration process, impressive removal rates of COD (95.8%) and NH₃ (99.2%) were achieved with an HRT of 12 hours [85]. Furthermore, an extended aeration process for leachate treatment showed a maximum COD removal of 36%, which was attained with an optimal retention period of 7 days [86]. The EAAS (Extended Aeration Activated Sludge) process, in combination with C-F/SF and UV-PS/H₂O₂ processes, has been identified as an effective alternative for leachate treatment that meets the requirements for effluent discharge into receiving waters. In addition, several studies have highlighted that integrating biodegradation with UV/H₂O₂-based AOPs yields better performance in treating landfill leachates [87–89].