Organic removal by denitritation and methanogenesis and nitrogen removal by nitritation from landfill leachate

A system consisting of a two-stage UASB and anoxic-oxic reactor was used to enhance COD and nitrogen removal from landfill leachate. To improve denitrification efficiency, the raw leachate with recycled final effluent was pumped into the first-stage UASB (UASB1) to carry out simultaneous denitrification and methanogenesis. The results over 180 d show that the maximum organic removal rate in UASB1 and UASB2 was 12.5 and 8.5 kg COD m(-3) d(-1) in the oxic zone of the A/O reactor, respectively. The COD and biochemical oxygen demand (BODS) removal efficiency of the system was 80-92% and about 99%, respectively. Without controlling temperature (17-30 degrees C) and dissolved oxygen (0.5-4.0mgL(-1)), the maximum NH4+-N removal rate was 0.68kg NH4+-N m(-3) d(-1), and about 99% of NH4-N removal was obtained by nearly complete nitritation. The 81-93% total nitrogen removal was obtained by complete denitrification in the UASBI and in the anoxic zone of the A/O reactor. Fluorescence in situ hybridization (FISH) analysis of the sludge samples from NO reactor showed that ammonia oxidizing bacteria (AOB) consisted 4% of the eubacterium, while nitrite oxidizing bacteria (NOB) counted less than 0.2% of that. The study shows that the main factors achieving and maintaining nitritation are a proper range of free ammonia concentration obtained by dilution recycled final effluent that inhibits NOB but not AOB; effective control on aeration time by indication of "ammonia valley" on pH profile; and highly efficient denitrification and its reproducing alkalinity to result in pH above 8.5. (c) 2007 Elsevier Ltd. All rights reserved.