Nitrification of anaerobically pretreated municipal landfill leachate at low temperature

Nitrification of anaerobically pretreated municipal landfill leachate (270–1000 mg COD l−1, 53–270 mg NH4–N l−1) was studied in laboratory-scale activated sludge reactors without (AS) and with (ASC) the addition of plastic carrier material. The reactors were run at 10°C (149 d), 7°C (21 d) and 5°C (16 d). With a loading rate of 0.027 g NH4–N g MLVSS−1 d−1 and a hydraulic retention time of ca. 3 d, at 10°C, complete nitrification was obtained in both reactors. At 7°C, and in loading rate of 0.023 g NH4–N g MLVSS−1 d−1, both reactors appeared to be overloaded and ammonium removal was only ca. 93%. At 5°C, with a lower loading rate of 0.010 g NH4–N g MLVSS−1 d−1, complete nitrification was obtained in the ASC reactor but in the AS reactor only 61% of ammonium was removed. Aerobic posttreatment produced effluent with 150–500 mg COD l−1, less than 7 mg BOD7 l−1, and, on average, less than 13 mg NH4–N l−1. In batch assays at 24 and 10°C, maximum nitrification rates were 0.13 and 0.03 g NO3–N g MLVSS−1 d−1, respectively, while ammonium removal rates were twice as high. The nitrification rates were higher with than without carrier material, suggesting that a large fraction of nitrifying micro-organisms was attached on the surface of the carrier material. The results show that nitrification of anaerobically treated leachate is feasible at temperatures down to 5–10°C. Benefits of the carrier reactor were evident only at low temperatures and under varying loading conditions.