Simultaneous carbon and nitrogen removal in a mixed culture aerobic RBC biofilm

A mixed culture biofilm was developed with a sulfur oxidising, heterotrophic bacterium Thiosphaera pantotropha, autotrophic nitrifiers and other heterotrophs in a three stage rotating biological contactor (RBC). Specific benefits due to peculiar properties of T. pantotropha of simultaneous heterotrophic nitrification and aerobic denitrification were derived in the reactor investigated for a combined carbon and nitrogen removal from a synthetic domestic sewage. The first stage biofilm which contained T. pantotropha showed high COD and NH4+-N removal rates of 5.8–14.1 g COD/m3•d and 0.47–1.1 g N/m2•d for the corresponding loading rates of 6.9–20.7 g COD/m3•d and 0.69–2.09 g N/m2•d, respectively. Contrary to the conventional units designed for a concurrent carbon removal and nitrification, the nitrification rates increased linearly with an increase in organic loading rate before stabilising at about 1 g N/m2•d corresponding to a COD loading rate of about 15 g/m2•d and a nitrogen loading rate of 1.5 g N/m2•d showing a change in the order of reaction from first to zero. A simultaneous nitrogen removal of 20–68% was also obtained. The systemʹs performance indicated that a single stage aerobic biofilm can be developed to meet the increasingly stringent regulations on effluent nitrogen discharges affording several advantages over the conventional systems, e.g. low buffer requirements, no need for external carbon source for denitrification, etc., which may result in substantial reduction in the treatment cost.