Biological Nitrogen Removal from Domestic Wastewater

Nitrogen (N) removal from domestic wastewater is widely performed by nitrification coupled to heterotrophic denitrification. However, the organic carbon (C)-to-N ratio (C/N) in the wastewater is frequently too low to achieve proper N-removal efficiencies and external organic C sources are often necessary to support the process and meet the discharge standards for total N (TN), increasing the process cost.

The main cost, however, remains the high energy demand to provide the aeration necessary for nitrification. Alternatives have been developed to reduce the need for organic C source. The anaerobic/anoxic/aerobic process (A2/O), simultaneous nitrification/denitrification, shortcut nitrification, or nitrifier bioaugmentation are feasible solutions to upgrade wastewater treatment plants (WWTPs).

The combined and simultaneous use of autotrophic aerobic and anaerobic microbial ammonium (NH4+) conversions (i.e., anaerobic ammonium oxidation; anammox) provide the most cost-effective solutions, as they obviate the need for an organic C source and reduce the aeration costs by more than 40%, compared to the traditional nitrification/denitrification paradigm.

The first full-scale applications that take advantage of combined aerobic/anaerobic ammonium oxidation have been implemented to remove N from sludge digester effluent streams. In this way, the N load to the main liquid treatment train of the WWTP can be reduced by 10–30% with important cost savings.

A current challenge involves control of gaseous N-oxide emissions (NO and N2O) from N-removal processes. New technologies, such as microbial fuel cells (MFCs), are developed aiming at energy-balanced WWTPs. This article reviews the existing technologies for effective biological N removal, with attention to anaerobic processes..