Advanced nitrogen removal via nitrite from municipal landfill leachate using a two-stage UASB–A/O system

A system consisting of a two-stage up-flow anaerobic sludge blanket (UASB) reactor and an anoxic/aerobic (A/O) reactor was used to treat municipal landfill leachate. Denitrification took place in the first stage of the UASB reactor (UASB1). The chemical oxygen demand of the UASB1 effluent was further decreased in the second stage (UASB2). Nitrification was accomplished in the A/O reactor. When diluted with tap water at a ratio of 1:1, the ammonia nitrogen concentration of the influent leachate was approximately 1200 mg·L− 1, whereas that of the system effluent was approximately 8–11 mg·L− 1, and the corresponding removal efficiency is about 99.08%. Stable partial nitrification was achieved in the A/O reactor with 88.61%–91.58% of the nitrite accumulation ratio, even at comparatively low temperature (16 °C). The results demonstrate that free ammonia (FA) concentrations within a suitable range exhibit a positive effect on partial nitrification. In this experiment when FA was within the 1–30 mg·L− 1 range, partial nitrification could be achieved, whereas when FA exceeded 280 mg·L− 1, the nitrification process was entirely inhibited. Temperature was not the key factor leading to partial nitrification within the 16–29 °C range. The inhibitory influence of free nitrous acid (FNA) on nitrification was also minimal when pH was greater than 8.5. Thus, FA concentration was a major factor in achieving partial nitrification.

Graphical abstractA system comprising a two-stage up-flow anaerobic sludge blanket (UASB) reactor and an anoxic–aerobic (A/O) reactor was used to treat landfill leachate in the experiment. Part of the recycled effluent as well as the influent of the system was pumped into the first-stage UASB reactor (UASB1). The organic compounds in the raw landfill leachate were depleted through serving as the carbon source for denitrification of the recycled effluent in UASB1, while simultaneously, methanogenesis occurred. Some organic compounds in the UASB1 effluent were also depleted via methanogenesis in UASB2. Recycled sludge from the clarifying tank was then pumped into the anoxic zone, the first chamber of the A/O reactor, in which denitrification of NOx−-N (nitrite and nitrate nitrogen) took place. Nitrification of ammonia occurred in the aerobic zone of the A/O reactor. A diagram of the process is presented in Fig. 1. As a result, stable partial nitrification was obtained with 93% nitrite accumulation in the A/O reactor and 99% nitrogen removal efficiency even at low temperature in the whole system which was fully dependent on biological treatment in treating leachate. And these were little reported by other researchers.Figure optionsDownload full-size imageDownload as PowerPoint slide