A study was undertaken to examine the feasibility of biologically treating a combined waste stream of landfill leachate and domestic sewage. Two different aerobic, reactor feeding modes were employed in a fill-and-draw treatment system: once-daily and twice-daily feed, in equal time intervals. The ratios of sewage: leachate used were 80:20 and 60:40, by volume, while operating solids retention times (SRT) were 5, 10 and 20 days. Liquid temperatures were 22°C, 8°C and 5°C, on average, for the three different temperature profiles.

Results indicated that successful biotreatment of combined leachate and domestic sewage is possible, with both low effluent COD and suspended solids achievable under certain types of operating conditions. For example, the once-a-day system performed most efficiently at a 10-day SRT and 22°C temperature, for all organic loadings investigated, while the twice-a-day system performed best at the 20-day SRT and the highest organic loading. On the basis of specific oxygen uptake rate values, the twice-a-day system was found to be more efficient and faster at utilizing the available substrate. However, liquid temperatures down to 5°C resulted in reduced treatment efficiency and produced significant changes in the sludge settling characteristics. In some cases, enhanced filamentous growth prevailed, leading to extensive sludge bulking, while in other circumstances, filamentous growth was totally absent and pin-point flocs predominated. The sludge settling problems encountered had a direct effect on effluent quality.

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