Application of Cavitation Ozonation Process on Recalcitrant Organic Matter Degradation from Stabilized Landfill Leachate

Treatment of stabilized leachate as a by-product of municipal landfills has been a significant challenge as the leachate contains recalcitrant organic matter which has low biodegradability. In this study, the efficacy of the advanced oxidation process using cavitation–ozonation to remove recalcitrant organic matter in leachate samples from TPST Bantar Gebang was evaluated. Several operational and process parameters including pH, ozone flowrate, and contact time were varied to determine the best conditions for removing recalcitrant organic matter represented by Chemical Oxygen Demand (COD). This study determined the optimum operating conditions for the cavitation–ozonation process: pH 11, ozone discharge of 3 L/min, and contact time of 30 minutes. The result was a COD removal efficiency of 20.37%, an increase of 52.06% in the concentration of BOD5, and a 90% increase in the ratio of BOD5 to COD. This study has shown that cavitation–ozonation is an effective pre-treatment, as it increases the biodegradability of stabilized leachate and reduces the load on subsequent treatment processes.

Cavitation–ozonation; Leachate treatment; Recalcitrant organic matter

Various technologies have been implemented to remove recalcitrant organic compounds from stabilized leachate. Coagulation and chemical oxidation, chemical precipitation, activated carbon adsorption, and membrane processes are methods that have been recommended for treating stabilized leachate (Amokrane et al., 1997). Using FeCl₃ as the coagulant has been shown to remove about 82% to 85% of COD in leachate (Long et al., 2017). A three-step treatment process of aerobic activated sludge biological pre-oxidation (ASBO), coagulation/sedimentation, and photo-oxidation through a photo-Fenton (PF) reaction has yielded BOD concentrations less than 150 mg/L at the effluent (Silva et al., 2017). Nurrohman and Wardjito (2016) reported satisfactory results from combining physical and chemical treatments to remove low biodegradable contaminants using a flotation and coagulation process, whereas Lubis et al. (2019) removed more than 90% of COD in low biodegradable wastewater using electrocoagulation. Recently, there has been increased use of advanced oxidation processes (AOPs) to remove organic contaminants from wastewater. This is because the high oxidative power of ozone increases the biodegradability of the wastewater (Zhou et al., 2010, Gautam et al., 2019, Karamah et al., 2019). Pure ozone (O₃) and a combined gas (O₃/H₂O₂) were applied to increase the biodegradability of leachate. This increased the maximum BOD₅ by about 110% (Wang et al., 2004). Another study suggested that the AOP of raw leachate could remove 16% to 33% COD from raw leachate (Xu et al., 2018). The combination of photoelectron-oxidation (PEO) and NaClO enhanced Fe²⁺ coagulation was able to remove 75% to 85% of COD (Qiao et al., 2018). Removing up to 85% of COD was achieved by combining ozone with hydrogen peroxide or persulfate in the AOP process (Gautam et al., 2019).

There are critical benefits of applying AOP to treat landfill leachate. First, it does not generate harmful byproduct compounds into the environment. Second, the process is relatively rapid and more efficient in improving the biodegradability ofthe organic contaminant (Yasar et al., 2006). In this process, recalcitrant organic compounds are converted into stable organic compounds with a lower molecular weight. Third, AOP also oxidizes organic compounds into the most stable oxidation form, CO₂ and water, by complete mineralization. Thus, its biodegradability increases. Fourth, the remaining unreacted ozone is released into the air and decomposes into oxygen (Sharma et al., 2011).

Several studies have shown that leachate produced from TPST Bantar Gebang was contaminating the surrounding aquatic environment, and they suggested stricter operation of the landfill and more effective leachate treatment (Pujiindiyati and Sidauruk, 2015; Pujiindiyati et al., 2019). Unfortunately, despite its several benefits, there have been very few studies of the application of AOP to remove recalcitrant organic matter. Hence, this study aimed to evaluate the applicability of using the cavitation–ozonation process to treat leachate generated from landfills in Indonesia.

 The evaluation was based on sets of experiments using samples collected from TPST Bantar Gebang. Outcomes included removal of recalcitrant organic compounds and improvement of the biodegradability of organic compounds. This process was carried out under alkaline conditions to encourage the formation of hydroxyl radicals, a strong oxidizing agent.

In this study, the leachate from TPST Bantar Gebang was categorized as stabilized landfill leachate, characterized by a low BOD₅/COD ratio of 0.089 (< 0.1), pH 8.5 (> 7.5), COD concentration of 3186.32 mg/L (< 4000 mg/L), ammonia concentration of 576 mg/L (> 400 mg/L), and low metal concentration. The evaluation of the operational and process parameters of AOP in removing COD from leachate found that the optimal operating conditions for the cavitation–ozonation process were pH 11, ozone discharge of 3 L/min, and contact time of 30 min. Using those parameters, the efficiency of removing recalcitrant organic compounds (represented by COD) on the cavitation–ozonation process in optimal conditions was 20.37%, the increase in BOD₅ concentration was 52.06%, and the ratio of BOD₅/COD increased by 90% from 0.098 to 0.188. Finally, the analysis of the kinetics of the removal of recalcitrant organic compounds (represented by the COD parameter) on the cavitation–ozonation process was a second-order reaction. The reaction rate of COD removal (k) was 5×10^-8 M^-1 s^-1. This study has shown the efficacy of the cavitation–ozonation process as a pre-treatment for stabilized leachate before biological treatment, as it increases the BOD₅ concentration resulting in leachate with more biodegradability.

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