NITROGEN REMOVAL IN INTERMITTENTLY AERATED BIOFILM AIRLIFT REACTOR

Denitrification was integrated in a nitrifying biofilm airlift suspens ion reactor by intermittent aeration. A total nitrogen-removal efficie ncy of 75% was reached at an aerobic ammonia load of 5 kg(N) . m(-3) . d(-1). Acetate was added to the reactor during the anoxic periods. Th e system was operated at sufficiently long anoxic hydraulic retention times to allow suspended growth of denitrifying heterotrophs. However, the heterotrophic biomass did not grow in suspension but grew as prot uberances and layers on the nitrifying biofilms. Nitrification and den itrification occurred via nitrite as an intermediate, possibly because denitrifiers outcompeted the nitrite oxidizers. Because of the establ ishment of the ''nitrite route,'' a low average COD/N ratio of 2.6 kg( COD).kg(N)(-1) was obtained. When denitrifying operation with acetate addition was started, initially almost all nitrogen removed was emitte d as nitrous oxide. Subsequently, the nitrous oxide emission strongly decreased to almost zero in a period of 30 days, while the nitrogen-re moval efficiency was maintained.