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Effects of temperature on two-stage biostabilization of landfill leachate Zapf-Gilje, Reidar


Leachate is generated by seepage of water through sanitary landfills. Where conditions are unfavourable, leachate of high strength and large volumes may be produced, thus creating the potential for serious water contamination problems. This study investigated aerobic, two-stage biological treatment of high strength leachate, at different operating temperatures. Completely mixed batch reactors of 10 litre volume were employed and operated on a daily fill-and-draw basis. Nutrient loading was slightly in excess of BOD₅:N:P = 100:5:1. Two-stage biological treatment were performed at 23-25°C, 16°C and 9°C. At room temperature the first stage reactors were operated at a mean cell residence time (MCRT) of 6, 9 and 20 days and the polishing reactors at 6, 9, 10 and 20 days. At lower temperatures, the first stage reactors that were operated at 20 days MCRT and the 10 and 20 days MCRT polishing reactors were eliminated. The leachate employed was lysimeter generated and characterized by COD and BOD₅ concentrations of approximately 19,000 mg/L and 14,000 mg/L respectively. In addition, it contained a spectrum of heavy metals and inorganic solids. Organic removal by the first stage systems was exceptionally good. Better than 95% COD and 99% BOD₅ were removed under all conditions investigated. Removal of heavy metals was 90% or better for most of the 9 metals monitored. Nickel and magnesium experienced only an average of 50% reduction. For temperatures ranging from 9°C to 25°C the removal performance was not significantly affected. However, the effluent COD and BOD₅ were slightly higher at 9°C, especially for the reactor receiving the highest organic load of 3.2 kg COD/m³-day. As a result of the low residual concentration of organic material in the first stage effluent, the polishing reactors experienced washout at all temperatures but 9°C. At this temperature and for the range of MCRT's tested, the second stage digesters stabilized at low MLSS levels of 220-600 mg/L, removing about 45% of the residual COD and 80% of the residual BOD₅; in addition, manganese, iron and zinc were further reduced by 60-80%. Settling problems caused by bulking, deflocculation and probably some hindered settling were encountered throughout the experiment, especially at lower MCRT's and temperatures. Hence, in order to produce enough feed for the polishing reactor, all effluents were obtained by filtering, rather than settling. The filtered effluent satisfied the local pollution control objectives for most parameters under nearly all conditions tested; however, slightly higher concentrations are expected under field conditions due to solids lost in the effluent.

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