Novel microbial nitrogen transformation processes in constructed wetlands treating municipal sewage: a mini-review

Traditionally nitrogen transformation in constructed wetlands (CWs) has been attributed to the activities of aerobic autotrophic nitrifiers followed by anoxic heterotrophic denitrifiers. However, the nitrogen balances in such systems are far from being explained as a large fraction of the losses remain unaccounted for. The classical nitrification-denitrification theory has been successfully employed in certain unit processes by culturing fast-growing bacteria, but the CWs offer an ideal environment for slow-growing bacteria that may be beneficially exploited to achieve enhanced nitrogen removal by manipulating the environmental conditions in their favor. In the last three decades, many novel microorganisms have been isolated from CWs that have led to the discovery of some other routes that have made researchers believe could play a significant role in nitrogen transformation processes. The increased understanding of novel discerned pathways like anaerobic ammonium oxidation (ANAMMOX), heterotrophic nitrification and aerobic denitrification, which are mediated by specialized bacteria has indicated that these microorganisms could be enriched by applying selection pressures within CWs for achieving high rates of nitrogen removal. Understanding these novel nitrogen transformation processes along with the associated microbial population can provide new dimensions to the design of CWs for enhanced nitrogen removal.