Both “two-stage upflow anaerobic sludge blanket(UASB)+anoxic/oxic (A/O)” system and “one stage UASB+A/O” system were successively introduced to treat a leachate containing relatively low concentration of organics and high concentration of ammonia from a mature landfill site. The “two-stage UASB+A/O” system was used firstly. The denitrification of the return effluent was carried out in the first stage UASB (UASB1). The nitrification took place in the A/O reactor. The results showed that most biodegradable organic matter was removed in the UASB1

so “one stage UASB+A/O” system was used in the second phase experiment. The NH+4-N loading rate of A/O reactor and operation temperature were 0. 28—0. 60 kg NH+4-N·m-3·d-1 and 17—29℃

respectively. The COD removal efficiency and the final effluent COD were 50%—70%

1000—1500 mg·L-1

respectively. Short-cut nitrification with 90%—99% of nitrite accumulation efficiency took place in the system. The NH+4-N removal efficiency varied between 90% and 100%. When NH+4-N loading rate was less than 0. 45 kg NH+4-N·m-3·d-1

the NH+4-N removal efficiency was more than 98%

and effluent NH+4-N was below 15 mg·L-1. When the ratio of the feed COD to feed NH+4-N concentrations was 2—3

the total inorganic nitrogen (TIN) removal efficiency was 70%—80%. The sludge samples from A/O reactor were analyzed by using fluoresence in situ hybridization(FISH). The FISH analysis showed that ammonia oxidation bacteria (AOB) were 4% of the eubacteria

meanwhile nitrite oxidation bacteria (NOB) were less than 0. 2% of the eubacteria.