Nitrogen removal using nitrifying biofilm growth and denitrifying suspended growth in a biofilm airlift suspension reactor coupled with a chemostat

A nitrifying biofilm airlift suspension reactor (with nitrifiers growing in biofilms) was coupled with a denitrifying chemostat (with denitrifiers growing in suspension) for integrated nitrogen removal. Initially the process was run in post-denitrification setup, during which nitrification and denitrification went via nitrate. Later recirculation between both reactors was applied (pre-denitrification setup), resulting in circulation of the suspended denitrifiers between the two reactors. The setup was suitable for nitrification and denitrification: 75% total nitrogen removal efficiency was obtained at an aerobic ammonia load of 5 kgN m−3 d−1. In the pre-denitrification setup nitrification and denitrification mainly occurred with nitrite as an intermediate. Probably the nitrite oxidisers and reducers competed for nitrite, which competition was won by the nitrite reducers. A low average COD/N-ratio of 3.0 gCOD gN−1 was obtained, in accordance with the “nitrite route”.