Evaluation of combined efficiency of conventional coagulation-flocculation process with advanced oxidation process (sulfate-hydroxyl radical) in leachate treatment
Article information
Abstract
This study evaluated a novel municipal solid waste leachate (MSWL) treatment system called the Batch flow leachate treatment system (BFLTS). This process uses a combination of coagulation/flocculation (C-F), advanced oxidation (sulfate-hydroxyl radical), and extended aeration of activated sludge (EAAS) to treat MSWL. The results indicated that the primary treatment phase using coagulation/flocculation with 0.8 g L−1 FeCl3 at pH 6 achieved 67% turbidity and 63% chemical oxygen demand (COD) reduction. The secondary treatment phase with the presence of both K2S2O8 and H2O2 peroxides was more efficient than single peroxide processes. While PS-based or H2O2-based single peroxide processes are less effective (UV-PS 65.7%, UV-H2O2 43.2%, Heat-PS 58.6%, Heat-H2O2 34.5%, and Heat-PS/H2O2 74.8%). The UV-PS/H2O2 system achieved the highest COD removal rate of 89.4%. In the third treatment phase, the efficient removal of COD and Biochemical oxygen demand (BOD) under optimal operating conditions was 87.3% and 94.7% respectively. Overall, the BFLTS treatment system has demonstrated high efficiency in removing COD, BOD, TSS, Turbidity, TKN, and Heavy metals by 99%, 98%, 97%, 89%, 86%, and 98%, respectively. This hybrid process has potential for reducing organic load in MSWL and can be used for various leachates.
Abstract
Graphical Abstract
1. Introduction
Highly concentrated, resistant, and toxic liquid leachate is a byproduct of physical, chemical, and biological changes in municipal solid waste [1, 2]. Leachate contains hazardous compounds, that pose risks to human health, including various types of cancer, genetic harm, and birth defects [3]. Emerging pollutants such as persistent organic pollutants (POPs), endocrine-disrupting compounds (EDCs), pharmaceuticals, and personal care products (PPCPs) have been identified in landfill leachates, posing a serious threat to the environment and human health [4].
To mitigate the risks of leachate pollution and protect the environment and public health, sustainable and effective leachate treatment techniques need to be developed [5, 6]. Biological and physicochemical methods are commonly employed for leachate treatment [7], but selecting the most effective method remains a significant challenge [8, 9]. Stabilized landfill leachate, due to its resistant organic matter, often presents challenges for biological treatment methods [10].
Coagulation and flocculation, due to their simplicity and cost-effectiveness, are widely used as pre-treatment and post-treatment methods for leachate [11, 12]. However, with increasingly stringent environmental standards, it becomes necessary to explore new and advanced approaches to bridge the existing gaps and meet the current standards [13].
Microbial fuel cells (MFCs) have shown promise in removing pollutants and organic matter, making them a potential sustainable technology for wastewater and leachate treatment [14]. Other microbial electrochemical technologies, such as microbial electrolysis cells (MECs), microbial desalination cells (MDCs), microbial reverse electrodialysis cells (MRCs), and microbial electrosynthesis (MES), have also been investigated for their role in supplying electrons to microorganisms and facilitating pollutant removal [15–20].
Advanced oxidation processes (AOPs) utilize hydroxyl radicals (•OH) or sulfate radicals (SO4•−) as primary oxidizing agents. In recent years, AOPs that employ the generation of sulfate radicals have gained considerable recognition as a highly promising method for the removal of persistent organic compounds. This is primarily attributed to the remarkable redox potential (2.5–3.1 V) demonstrated by sulfate radicals, along with their efficient operation across a wide pH range [21]. The activation of persulfate anions (S2O8 2−) through UV radiation, heat, transition metals, high pH, iron ions, sound waves, or peroxide leads to the generation of highly reactive sulfate radicals that facilitate pollutant degradation [22–24].
Considering the complexity, variety, and high concentration of leachate pollution, combining different leachate treatment techniques has shown to be effective [25]. The combination of biological processes with physicochemical methods has gained attention as an efficient approach [8]. AOPs based on sulfate and hydroxyl radicals demonstrate the potential for mineralizing macro pollutants in leachate, rendering them non-toxic and harmless [1, 26].
Despite numerous studies on leachate treatment methods, the focus has often been on individual methods rather than the combination of physicochemical methods, advanced chemical oxidation, and biological methods. Hence, this research aims to modify and enhance hybrid treatment methods to meet the effluent standards for all types of municipal waste leachates.
In this study, a novel hybrid CF/(AOPS)/EAAS method for leachate treatment is proposed and investigated, aiming to effectively treat various types of leachates.
2. Materials and Methods
2.1. Preparation of Leachate Samples
In this study, both synthetic leachate (SL) and actual leachate samples were utilized to investigate the efficacy of the hybrid CF/(AOPS)/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.
Physicochemical characteristics of synthetic leachate
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/(AOPS)/EAAS method on diverse leachate types, representing different stages and variations in leachate composition and characteristics.
2.2. Chemicals
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.
2.3. Analytical Determinations
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).
2.4. Experimental Setup
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.
2.5. Experimental Procedure
2.5.1. Coagulation/flocculation process
This study identified a chemical coagulant for leachate using coagulation/flocculation processes in the Jar Test apparatus. The best coagulants were selected based on raw leachate characteristics, COD removal efficiency, and turbidity removal efficiency. The pH of the leachate was adjusted and contact times were applied for coagulation, flocculation, and settling. The optimal pH and dosage of each coagulant were determined, and the effluent was filtered before AOPs.
2.5.2. Advanced oxidation processes (AOPs)
This process investigates the effects of UV-PS/H2O2 and Heat-PS/H2O2 chemical oxidation processes on leachate treatment. The experiments were conducted in a laboratory reactor with a UV lamp, using two UV-C lamps with different power ratings and wavelengths. The leachate was treated with stock solutions of hydrogen peroxide and potassium persulfate and then mixed with aeration at room temperature. Heat-PS/H2O2 treatment was performed at different temperatures and reaction times. The effects of PS dosage, H2O2, initial concentration of pretreated leachate, pH, reaction time, UV radiation, and temperature were also studied. The independent variables’ values and ranges were chosen based on previous research.
2.5.3. Extended aeration-activated sludge (EAAS)
The acclimatized activated sludge from the Behshahr compost leachate treatment plant was initially added to the aeration tank as an inoculum. This process helps to introduce a community of microorganisms that are adapted to the specific conditions of the treatment plant and facilitate the breakdown of organic matter in the leachate. The leachate from the leachate oxidation stage was transferred to the aeration tank, with a fixed working volume of 4 L. After introducing the leachate into the reactor, aeration was initiated and continued for approximately 18 hours. After the treatment process, a portion of the treated leachate was released or discharged. To ensure the optimal functioning of the system, the sludge retention time (SRT) was calculated, ranging from 12 to 23 days, depending on the dilution of the leachate. For the aeration process, a pump and pipe equipped with a diffuser were used. This setup ensures that air is uniformly introduced into the leachate from the bottom of the aeration tank (AT). The diffuser helps to disperse the air bubbles evenly throughout the tank, facilitating the transfer of oxygen to the microorganisms and enhancing their activity in breaking down the organic matter.
The pH of the pre-treated leachate ranged from 6.2 to 8.6. This pH range suggests that the leachate had a relatively neutral to slightly alkaline nature. The temperature of the leachate during this stage ranged between 21 and 25°C. After reaching a steady state, the performance of an extended aeration-activated sludge system was assessed. This evaluation aimed to determine the effectiveness and efficiency of the system in treating the desired leachate. Based on preliminary studies and a literature review, the values and range of parameters for the leachate aerobic treatment process were determined [29, 30].
3. Results
3.1. Coagulation/Flocculation Process
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).
Removal efficiency of four experimental variables of synthetic leachate with coagulation/flocculation pretreatment using FeCl3 (0.8 g L−1 and pH 6) and Adsorption onto sand filtration.
3.2. Advanced Oxidation Processes (AOPs)
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.
3.2.1. Effects of UV radiation
Fig. 2 presents the pollutant degradation and COD removal rates over time using one of three UV-activated peroxides: PS (persulfate), H2O2 (hydrogen peroxide), and PS/H2O2 (combination of persulfate and hydrogen peroxide). The experiments were conducted at UV radiation intensities of 8 and 15 W. The results of the studies indicate that higher radiation intensity (15 W) resulted in higher COD removal in all processes as the reaction time increased. The removal efficiency of COD showed the greatest effect for each process using single-peroxide or two-peroxide systems at 70 minutes, after which the removal efficiency did not increase significantly. Generally, higher UV power leads to higher COD removal.
The effect of UV light intensity and various processes on the photochemical oxidation Removal efficiency of COD of leachate.
At pH 7.1, under UV light at 15 W (wavelength 254 nm) and a reaction time of 70 minutes, the COD removal efficiencies were 65.7%, 43.2%, and 89.4% for UV-PS, UV-H2O2, and UV-PS/H2O2 processes, respectively, with a persulfate dose of 2.5 g L−1 and an H2O2 dose of 1.5 g L−1. For the UV-activated PS, UV-activated H2O2, and UV-activated PS/H2O2 processes under 8 W UV radiation while maintaining other conditions, the ranges of COD removal rates were 48.5%, 37.4%, and 64.3%, respectively.
3.2.2. Effects of peroxide dosage
The effectiveness of different operating conditions for oxidation processes with varying doses of H2O2 on COD removal is shown in Fig. 3. The H2O2 reagent demonstrated a performance of 43.2% as an oxidant under the optimal conditions of pH 7.1, UV (15W), 1.5 g L−1 H2O2, and a reaction time of 70 minutes in the UV-activated H2O2 process. The combination of hydrogen peroxide with persulfate exhibited high efficiency in the degradation of organic content. The synergistic process utilizing PS/H2O2 activated with UV showed superior efficiency (89.4%) in the removal of COD from leachate. A dose of 1.5 g L−1 was determined to be the most effective for UV-activated peroxides (H2O2, PS/H2O2).
The effect of peroxide dosage, (H2O2, K2S2O8) and various processes on leachate (SL2) COD removal efficiency.
In the Heat-activated peroxide process, it was observed that the rate of COD removal increased as the concentration of persulfate (PS) was raised, up to 3 g L−1, and hydrogen peroxide (H2O2) was increased up to 2 g L−1. However, further increases in peroxide concentrations (PS to 4 g L−1 and H2O2 to 3 g L−1) resulted in a decrease in the removal rate. The highest efficiency of COD removal was achieved with the binary and triple processes, using a dosage of 3 g L−1 PS and 2 g L−1 H2O2, with activation at 65°C and a reaction time of 90 minutes, all conducted at pH 7. Under these optimal conditions, the COD removal efficiencies for each process were as follows: Heat-PS/H2O2 > Heat-PS > Heat-H2O2, with values of 74.8%, 58.6%, and 34.5%, respectively.
In the study conducted on the pre-treatment of leachate solution, different doses of hydrogen peroxide (0.5, 1.5, 2, and 3 g L−1) were tested along with other optimal conditions (pH 7.1, UV (15 W), time 70 min, and 2.5 g L−1 PS in the double peroxide process). The results showed that the COD removal percentages for UV-H2O2 were approximately 19.2%, 43.2%, 38.3%, and 24.8% for the respective doses of 0.5, 1.5, 2, and 3 g L−1 of H2O2 (Fig. 3). On the other hand, the COD removal percentages for UV-PS/H2O2 were 46.2%, 89.4%, 68.4%, and 53.2% for the respective doses of 0.5, 1.5, 2, and 3 g L−1 of H2O2. It was observed that when the H2O2 dosage exceeded 1.5 g L−1, the removal efficiency of COD decreased in the UV-activated photocatalytic AOPs. Hence, the optimal dosage of 1.5 g L−1 of H2O2 was selected.
At a pH of 7.1 and room temperature of 25°C ± 2°C, different dosages of persulfate (1.5, 2.5, 3, and 4 g L−1) were used in combination with UV light. The COD removal efficiencies for each of the UV-PS and UV-PS/H2O2 (with 1.5 g L−1 H2O2 in a double peroxide process) processes were as follows: 39.4%, 65.7%, 55.6%, and 41.8% for UV-PS, and 42.3%, 89.4%, 66.3%, and 48.7% for UV-PS/H2O2, respectively. It was observed that all the UV-activated processes yielded higher COD removal efficiency when 2.5 g L−1 of persulfate was used.
3.2.3. Effects of pH
The initial pH of the leachate plays a crucial role in the generation of free radicals in sulfate-hydroxyl radical-based oxidation. Different pH levels were tested in three UV-activated AOPs to determine the optimal conditions for increasing the removal rate. Fig. 4 (a) and (b) shows that as the initial leachate pH increased from 4 to 7, there was an increase in COD removal efficiency for all processes. However, when the pH was increased to 9, there was a minor decrease in the efficiency of the photocatalytic process. Furthermore, the decreasing trend of COD removal efficiency increased when the pH was raised from 9 to 11, particularly in the triple process. As a result, the optimal pH for the UV/Heat-activated PS/H2O2 processes investigated in this study was determined to be 7.1. The highest degradation of organic matter was achieved using the UV-PS/H2O2 and Heat-PS/H2O2 photocatalytic processes, respectively. These synergistic treatments were found to be most effective when applied close to the pH of the leachate.
The effect of pH, and various processes ((a), Activated by UV (15W), PS 2.5 g L−1, H2O2 1.5 g L−1, Reaction time 70 min), (b), Activated by Heat (65°C), PS 3 g L−1, H2O2 2 g L−1, Reaction time 90 min) on leachate COD removal efficiency (SL2).
Overall, the study emphasizes the importance of pH control in UV-activated AOPs for the efficient removal of organic matter and highlights the optimal pH range to maximize COD removal efficiency.
3.2.4. Effect of reaction times
It is evident that when the irradiation time was increased from 20 to 70 minutes and from 20 to 90 minutes in the chemical oxidation reactions, the oxidants H2O2 and PS had a greater opportunity to be exposed to UV radiation or heat and become activated. The parameters of irradiation time and contact time were investigated and are presented in Fig. S3. After 70 minutes and 90 minutes, more effective COD removal was achieved, with rates of 89.4% and 74.8% for the UV-PS/H2O2 and Heat-PS/H2O2 processes, respectively, in the samples of pre-treated leachate (SL2).
3.2.5. Effect of temperature
The removal efficiency (%) of COD using one of the three heat-activated peroxides (PS, H2O2, PS/H2O2) over time is illustrated in Fig. S4. The experiments for leachate (SL2) treatment were carried out at temperatures of 35, 50, 65, and 80°C. Our test results showed a decrease in COD removal efficiency at higher temperatures (80°C). Conversely, the results in Fig. S4 indicated that these peroxides were not very effective at lower temperatures. The reaction rate with organic matter and the efficiency of COD removal by peroxides were significantly slow at 35°C. At 65°C, the examined processes (PS/H2O2 > PS > H2O2) exhibited the highest effectiveness in leachate treatment, with corresponding COD removal percentages of 74.8, 58.6, and 34.5% respectively.
The UV-PS/H2O2 process has been selected as the most effective method for removing COD from pre-treated leachate. The initial parameters of different SL types (SL1, SL2, SL3) were analyzed to determine the optimal oxidant and pH conditions. The results revealed that the COD removal efficiency in SL1, SL2, and SL3 was 87.5%, 89.4%, and 93.2% respectively. It is worth noting that the UV-activated PS/H2O2 process exhibited the highest removal efficiency specifically in SL1, as shown in Fig. 5 (a), (b), (c), and (d).
The removal efficiency of four experimental variables of coagulation/flocculation/sand filtration effluent with Advanced oxidation processes (sulfate-hydroxyl radical) using UV (2.5 g L−1 PS, 1.5 g L−1 H2O2, pH7, UV 15 W, Temperature (25±2°C), Time 70min).
3.3. Extended Aeration-Activated Sludge (EAAS) Stage
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.
3.4. Assessing the Overall Efficacy of BFLTS in MSWL Treatment
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.
Summary of the effect of BFLTS on the removal efficiency of four experimental variables types of synthetic leachate in each treatment process 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].
Heavy metal concentration in synthetic leachate (SL2) before and after treatment
3.5. Analysis of Actual Leachate Samples
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.
4. Discussion
4.1. Coagulation/Flocculation Process
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].
4.2. Advanced Oxidation Processes (AOPs)
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.
4.3. Extended Aeration Process
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].
5. Conclusions
The study introduces a new combined CF/(UV-PS/H2O2)/EAAS method for leachate treatment, capable of handling high COD levels of up to 32000 mg/L. The research demonstrates that the double peroxide process method holds more promise compared to other AOPs examined. The triple processes involved in this study greatly benefit from the synergistic effect of peroxide in leachate treatment. Systems such as UV-PS/H2O2 and Heat-PS/H2O2 exhibit a favorable synergistic effect, particularly under optimal conditions. The effectiveness of these systems is highly dependent on pH and the initial concentration of peroxides. However, the heat-PS/H2O2 system is found to be less efficient than the UV-PS/H2O2 system.
The study reports impressive removal percentages for various parameters in actual leachate, including 99.2% COD, 99% BOD, 81.8% TKN, 94.8% TSS, and 87.6% turbidity. The average performance of the BFLTS (flocculation-coagulation process, AOP process, and extended aeration process) system in treating synthetic and actual leachate did not show significant differences. Based on the findings of the study, the combination of the flocculation-coagulation process, AOPs, and extended aeration process presents a highly effective and alternative approach for treating leachate generated from municipal solid waste.
Supplementary Information
Acknowledgements
This research was done for a doctoral thesis that was approved by the Mazandaran University of Medical Sciences under the research ethics code IR.MAZUMS.REC.1401.377. The support received by the Health Sciences Research Center, the Student Research Committee, and the Deputy of Research and Technology at Mazandaran University of Medical Sciences is appreciatively acknowledged by the authors.
Notes
Conflict of Interest Statement
The authors declare that they have no competing financial interests or known personal relationships that could appear to influence the work reported in this article.
Author Contributions
M.A.Z (Professor) was responsible for research design, supervision, validation, article writing, and project management. A. A (Ph.D. Candidate) was responsible for the methodology and performing the experiments, data analysis, and writing the paper. Z.Y. (Professor) was a consultant in the design of the experiments and participated. E.B. (Assistant Professor) assisted in methodology and formal analysis.