Application of fenton process in treating landfill leachate

Abstract

The composition of landfill leachate is complex due to the conditions within a landfill such as chemical and biological activities, moisture content and the degree of stabilization. Treatment methods must be matched to the actual characteristics of the leachate under examination. In particular, conventional biological methods, which are frequently used to treat landfill leachate, have not achieved the same level of success as they have in municipal wastewater treatment due to a great extent to the presence of nonbiodegradable and toxic organic compounds contained in landfill leachate. This research is undertaken to investigate various operating conditions for minimizing required Fenton reagents and sludge disposal and maximizing chemical oxygen demand (COD) as well as colour removal from the leachate. A number of combined processes were also developed that includes aeration followed by Fenton process, Fenton process followed by alum coagulation and alum coagulation followed by Fenton process in order to maximize the removal of organic compounds. Samples were collected from Matuail landfill site which is one of the major landfill sites of DCC located approximately 5 km southwest from the city centre. The ratios of BOD5 to COD of the samples indicate that leachate samples were in semi-aerobic intermediate phase and characterized as medium aged. For that the removal efficiency of Matuail landfill leachate through biological process will be fair. The leachate samples were made out of low bio-degradable compounds which indicate that these samples should be efficiently treated with chemical process like Fenton oxidation and coagulation process rather than biological process. With conventional extended aeration process which is widely practiced for the present treatment system of Matuail landfill site, the optimum removal of COD, BOD5, colour and ammonia were 36%, 95%, 20% and 95% respectively with an optimum detention time of 7 days. The residual COD content of both the samples was 6840 and 1720 mg/l respectively which exceeded the standard value of 200 mg/L (Bangladesh industrial effluent standards) and needs to be treated further. In Fenton process, considering both the removal efficiencies and sludge disposal, the optimum dosages of Fenton reagents were found as: Amount of H2O2 = 1000mg/l and Amount of Fe2+ = 1250mg/l. Most organic removal occurred in the first 30 min of Fenton oxidation. A pH of 5 for both Fenton oxidation and coagulation, a H2O2 to Fe2+ molar ratio of 1.3: 1 were selected to achieve optimum COD and colour removals. Under these optimal conditions the COD and colour removal efficiencies were found 80% and 97% respectively having sludge volume of 50%. The sludge production reduced significantly 16% for multiple dosages of Fenton reagents with higher removal of COD and colour. Again recycling of 40 ml Fenton sludge per liter of influent enhanced the COD removal efficiency up to 3% and reduced sludge volume production to 30%. COD removal efficiency decreased rapidly with the increase of initial COD strength. With the initial COD strength of Matuail landfill leachate below 3700 mg/l, the removal efficiency is satisfactory through individual Fenton treatment process with all optimum parameters. Otherwise additional pre or post treatment is necessary for agreeable residual COD concentration. In case of alum coagulation process, the maximum removal was observed at alum concentration of 15,000 mg/L and the optimum removals of colour and COD were found 66% and 34% respectively which were extremely low with respect to the Fenton treatment process. Following pretreatment with aeration, the landfill leachate was further treated using Fenton process and the results showed that residual COD met the Bangladesh standards for waste water discharge into inland surface water, with an efficiency of 89% COD removal and the colour was less than 120 Pt-Co units. In case of combined treatment process the sludge volume production was also reduced by 10%. Due to the application of alum coagulant as post treatment after Fenton oxidation, the COD removal was increased by 4% but at the same time the colour removal was reduced by 5% as well as the sludge production was amplified by 13%. Therefore this treatment option is conditionally satisfactory for higher removal of COD. Due to the combination treatment process of Fenton treatment followed by alum coagulant, the COD and colour removal were 54% and 13% respectively with optimum alum dose of 17,500 mg/l. Hence this combined treatment process is not an acceptable treatment option for the leachate of Matuail landfill site. In Fenton process, the sludge volume production is the major concern and this can be reduced by continuous addition of Fenton reagents and sludge recirculation during Fenton treatment. Therefore, it is suggested that not only biological aeration is enough to treat Matuail landfill leachate to the desired level but also further chemical treatment with Fenton reagent gives satisfactory results for treating the leachate.