Nitrogen loss in a nitrifying biofilm system

In a biological contactor that is part of the biological pretreatment of la ndfill leachate in Mechemich (Germany) nitrogen elimination of 60% or more was observed under low dissolved oxygen (DO) conditions. Ammonia was conver ted without accumulation of nitrite and with only little nitrate production . Interestingly, due to limited supply with organic substrate in the system this observation cannot simply be explained by a combination of convention al autotrophic nitrification and heterotrophic denitrification. In situ hyb ridization with 16S rRNA-targeted probes revealed the presence of large mic rocolonies of at least three different types of ammonia-oxidizing bacteria in those biofilm regions where extremely high nitrogen losses occurred. The se results were confirmed by comparative sequence analysis of biofilm-deriv ed amoA (encoding the active-site polypeptide of ammonia-monooxygenase) clo nes for molecular fine-scale analysis of the ammonia-oxidizing population. In batch tests inoculated with biofilm material nitrogen loss occurred with out dosage of organic substrate at a DO concentration of 1 mg/l. The simult aneous presence of ammonia and nitrite in the reactor induced the process o f complete nitrogen elimination. N-2 was identified to be the gaseous end p roduct of the reaction. These results indicate that under low DO concentrat ions autotrophic ammonia-oxidizers might be the causative agents of the obs erved nitrogen loss by performing aerobic/anoxic denitrification with nitri te as electron acceptor and ammonia (or perhaps hydroxylamin) as electron d onor. (C) 1999 IAWQ Published by Elsevier Science Ltd. All rights reserved.