The SHARON process was used to remove ammonium nitrogen from landfill leachate with moving bed biofilm reactor (MBBR).The start-up of biofilm reactor and the affecting factors on half-nitrosofication were investigated.Nitrosobacteria was selectively cultivated on the biofilm and short-cut nitrification was achieved in 4 weeks by controlling the following conditions as HRT=1 d

DO=0.5—1.0 mg·L-1

pH of about 7.5

T=30℃ and no sludge recycle.The experimental results showed that when the influent ammonium concentration was 500 mg·L-1

half-nitrosofication could be achieved at pH of 7.0 and DO of 1.5 mg·L-1 and when the influent ammonium concentration was 300 mg·L-1

half-nitrosofication could be achieved at pH of 7.0 and DO of 1.0 mg·L-1.The most probable number (MPN) measurement showed that the overwhelming quantity of nitrosobacteria immobilized on biofilm was the main reason for stable and high nitrite accumulation in the reactor.