Short-cut removal of nitrogen from mature municipal landfill leachate at ambient temperature

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 NH₄⁴-N loading rate of A/O reactor and operation temperature were 0.28-0.60 kg NH₄⁺ -N·m^-3·d^-1 and 17-29°C, 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 NH₄⁺-N removal efficiency varied between 90% and 100%. When NH₄⁺-N loading rate was less than 0. 45 kg NH₄⁺/ -N·m^-3·d^-1, the NH₄⁺-N removal efficiency was more than 98%, and effluent NH₄⁺N was below 15 mg·L^-1. When the ratio of the feed COD to feed NH₄⁺-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.