Energy-efficient and carbon neutral anammox-based nitrogen removal by coupling with nitrate reduction pathways: A review

Sustainable development goal 6 emphasizes the efficient management of water resources, wastewater treatment, and reuse of treated wastewater. Removing nitrogen from wastewater was an economically expensive and energy-consuming wastewater treatment process. The discovery of anammox changes the paradigm of wastewater treatment. Nevertheless, coupling anammox with partial nitrification (PN-anammox) has been an immensely rewarding and scientifically supported side-stream wastewater treatment process. However, the PN-anammox process is inherited with severe issues of higher effluent nitrate and lower nitrogen removal efficiency at a lower temperature. Thus, it is evident that PN-anammox cannot meet the desired target without the involvement of others nitrogen cycle bacteria. The nitrate reduction pathways such as denitrifying anaerobic methane-oxidizing (DAMO) microbes, partial denitrification (PD), and dissimilatory nitrate reduction to ammonium (DNRA) seem the best alternative to reduce nitrate into nitrite or ammonium for anammox supplement. From the perspective of the environment, the coupling of anammox with PD, DAMO, and DNRA alleviates the use of organic matter, decreases greenhouse gas release, and reduces energy consumption. This review comprehensively discussed the importance and application perspective of anammox with diverse nitrate-reducing bacteria. Furthermore, research is still needed about DAMO-anammox and DNRA-anammox to achieve higher nitrogen removal efficiency. Future research should incorporate emerging pollutants removal in the anammox coupling process. This review will provide deep insight into the design of energy-efficient and carbon-neutral nitrogen removal from wastewater.