Engineering aspects and practical application of autotrophic nitrogen removal from nitrogen rich streams

The anaerobic ammonium oxidation (Anammox) process, discovered 20 years ago, is, in combination with partial nitritation, ideally suited to treat nitrogen rich waste water streams such as digester effluent. In this review the engineering aspects and the practical application of the process are reviewed. The conventional nitrification–denitrification and nitritation–denitritation are also discussed briefly. The environmental conditions affecting the nitrification process, free ammonia and nitrous acid concentration, temperature, pH and dissolved oxygen concentration, are discussed. These conditions can be controlled in such a way that the partial nitritation step produces an Anammox-suited influent. The Anammox reactor conditions should favour the growth of the Anammox organisms in view of their low growth rate and possible inhibition effects. Dissolved oxygen and nitrite concentrations should be kept as low as possible and biomass washout should be limited. If the partial nitritation process and the Anammox process are occuring in the same reactor, care should be taken to the dissolved oxygen concentration, the ammonium load and the nitrite concentration to obtain a sustainable co-existence between aerobic and anaerobic ammonium oxidizers. An overview is presented of the practical implementation of autotrophic nitrogen removal. The process can be accomplished in the same reactor (1-reactor system) or by using 2 separate reactors (2-reactor system). Typically the 1-reactor system is a biofilm or granular reactor where the ammonium oxidizers are active in the outer layers of the biofilm or granule, producing a suitable amount of nitrite for the Anammox organisms that are active in the inner layers. Transport of ammonium and the produced nitrite is governed by diffusion. Finally, the different nitrogen removal processes are compared in terms of operational conditions and a direction for future work is provided.