Biological treatment of leachate from solid waste landfill sites-Alterations in the bacterial community during the denitrification process

Nitrite accumulation is frequently observed in leachate treatment plants. Generally, it has been attributed to unfavourable process conditions during the nitrification process. Production and utilization of nitrite during leachate treatment were investigated in an activated sludge bioreactor and an aerobic packed bed/anoxic activated sludge reactor. Under the conditions used, nitrite was accumulated by incomplete nitrification during the start-up period of the treatment only. On the other hand, high amounts of nitrite were built-up within the denitrification step. These high nitrite concentrations (more than 200 mg l ^-1 NO ^- ₂) were measured during the start-up period of the denitrification process and at TOC / N-ratios less than 1.5. Nitrite accumulation and nitrite utilization could be correlated with changes in the microbial community present. During the start-up period (about 20 days), most of the bacteria isolated from the denitrification tank were able to reduce both nitrate and nitrite. Later at a TOC / N-ratio of 3.5, a significant shift in the activated sludge biocommunity was observed. After 40 days, less than 0.1% of the isolated bacteria were capable to reduce nitrite. From the results obtained, it is concluded that the composition of the biocommunity plays a more important role in denitrification of leachate than previously considered.