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Biological-chemical treatment of landfill leachate

University of British Columbia

Leachate is an effluent generated via the percolation of surface and groundwater through sanitary landfills. Depending on local conditions, large volumes of high strength and potentially toxic leachate can be produced resulting in significant deterioration of receiving water quality. Leachate collection and treatment systems are currently being developed. The purpose of this study was to determine the treatability of high-strength leachate using a series biological-chemical treatment system; that is, aerobic biological degradation in a fill and draw reactor followed by lime precipitation in a separate vessel. The system proved to be highly efficient in treating high-strength leachates both in terms of organics and heavy metals removal. Using a series of treatment combinations on a leachate with a COD of greater than 19,000 mg/ℓ and high metal concentrations, all the British Columbia 'AA' Level pollution control guidelines could be met with the exception of pH and manganese. Assessed on operating stability and treatment efficiency, the two most favourable treatment configurations were; an aerobic biological unit of 12 day mean cell residence time (MCRT), with an addition of 800 mg/ℓ Ca (OH)₂ for polishing, and a biological unit of 15 days MCRT, with an addition of 450 mg/ℓ Ca(OH)₂. The biotreatment system was very effective in the removal of organics. Soluble COD removals ranged from 97.2% to 98.6% over a temperature range of 5° to 24°C and mean cell residence times of 9 days to 25 days. Soluble BOD₅ removals ranged from 99.5% to 99.9%. Trace metals were also removed effectively. Metal removals were greater than 96% for Fe, Mn, and Zn, better than 80% for Ca, better than 70% for Pb, between 70 and 80% for Cr and Ni, and 40% for Magnesium. Temperature reductions did not significantly influence the bio-treatment removal efficiencies; however, some stress was noted in the biological systems at 5°C. Decreasing settleability and excess foaming was prevalent especially in the lower MCRT units. Unit failure was observed in the 6 day MCRT digester when the temperature was reduced from 9° to 5°C. Further tests on the cold temperature operation of biological units indicated that ambient temperature acclimatization may be required, prior to cold temperature operation. Lime precipitation performed well on most effluents over the range of dosages tested. Unfortunately, the dosages required were quite high, principly because of the very high sample alkalinities. For a lime dosage of 900 mg/ℓ Ca(OH)₂, at 25% reduction in COD was obtained on a sample with an initial COD of 551 mg/ℓ and suspended solids less than 25 mg/ℓ. Metal removals were substantially more impressive. At a pH of 11.5 (900 mg/ℓ Ca(OH)₂), typical reductions were >99% for Fe, 91% for Zn, 83% for Mn, 91% for Mg, and 73% for Ca. Other metals were initially at very low concentrations and as a result, were not monitored during the lime precipitation studies. The principle removal mechanisms for organic materials were adsorption and entrapment. A reasonable correlation was developed between initial COD and alkalinity, and the quantity of Ca(OH)₂ required to achieve a prescribed treatment level. Metals were removed by chemical precipitation, and to a small extent, adsorption and entrapment. The removal of metals was extremely dependent upon the solubilization pH of the respective metal hydroxides.

Text

2010-03-26

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http://hdl.handle.net/2429/22799

Civil Engineering

University of British Columbia

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