The treatment of leachates from domestic wastes in landfills—I: Aerobic biological treatment of a medium-strength leachate

A medium-strength leachate from domestic solid wastes in a landfill (COD 5000 mg l⁻¹, BOD 3000 mg l⁻¹ was treated using aerobic biological processes in continuous-flow, laboratory-scale reactors at low temperatures. Each unit was completely mixed, and mixed liquor was wasted such that solids retention time (SRT) was equal to the hydraulic retention period.At 10 C with addition of phosphate (COD:P less than 100:1) SRT values of 10 days were required to obtained well-clarified effluents, and high removal of BOD (>;98%) and COD (>;92%). Reduction of temperature to 5°C resulted in adverse effects on settling of sludges from units with SRT values of less than 10 days, but in other units good removal of organic materials could still be obtained. These units operated successfully with concentrations of mixed liquor volatile suspended solids (MLVSS) of 1450 mg l⁻¹, equivalent to a ratio of F/M of 0.21 kg BOD kg⁻¹ MLVSS day⁻¹ or less. Removal of ammoniacal nitrogen which took place (influent concentration 80 mg l⁻¹ as N) resulted from incorporation in biomass, and at SRT values of 10 days no nitrification took place at 5 or 10 C. Higher concentrations of ammonia in influent leachates resulted in ammonia in effluents when the ratio of BOD:N was less than about 100:3.6. Increasing the SRT of units to 20 days resulted in erratic conversion to nitrite, but reduced pH-values and possible simultaneous denitrification caused floating sludges and poorly-clarified effluents. Removal of ammonia is identified as a major problem when treating leachates, and further research is recommended.