Effect of oxygen concentration on biological nitrification and microbial kinetics in a cross-flow membrane bioreactor (MBR) and moving-bed biofilm reactor (MBBR) treating old landfill leachate

Ammonium nitrogen concentration in leachate from old Italian landfills ranges from 0.5 to as high as 3 g L-1. In this paper biological nitrogen removal from leachate has been achieved by partial nitrification to nitrite in a pure-oxygen membrane bioreactor (PO-MBR) and by subsequent denitrification in a moving-bed biofilm reactor (MBBR). When ammonium is biologically oxidized to nitrite, only 75% of the oxygen required for full nitrification is needed. Moreover, denitrification can be performed by saving 30-40% of the carbon required. The process was carried out by an MBR oxidation tank of 500 L equipped with an UF ceramic membrane followed by a 540-L post-denitrification tank filled with moving plastic support media. The best operational conditions to achieve partial nitrification were analyzed. TKN loading rate was variable from 50 to 120 g TKN (kg TSS day)(-1) with an influent ammonia concentration between 1000 and 1500 mg L-1. When DO concentration in the MBR was kept in the range 0.2-0.5 mg L-1, 90% oxidation of ammonia to nitrite was achieved, with stable inhibition of nitrite-oxidizing bacteria even at sludge retention time higher than 45 days.