What innovative nitrite furnishing processes can be coupled with anammox for excellent nitrogen removal?

Anaerobic ammonium oxidation (anammox) is a promising and efficient biological nitrogen removal process, which can economically treat wastewater without organic supplement and aeration. However, to date, the insufficient nitrite in wastewater is the foremost problem encountered in anammox process. This restriction hinders full-scale application of anammox worldwide. Nitrite can be provided by innovative processes such as autotrophic denitrification and denitrifying anaerobic methane oxidation (DAMO). Therefore, the synergy between them and anammox is feasible means to solve the bottleneck. This review unravels the advantages and operating strategies of autotrophic denitrification, DAMO and their coupled processes with anammox. Besides, different electron donors such as reductive sulfur, iron, hydrogen, manganese and methane are analyzed in the two processes. The characteristics of microbial populations and their synergies in microbial communities are deciphered, and nitrogen removal contributions through different microbes are also highlighted. Still, nitrogen conversion pathways are clarified in the processes with different electron donors, and metabolic mechanisms of both autotrophic denitrifiers and DAMO microbes are revealed. Finally, the challenges to be focused in autotrophic denitrification and DAMO processes are proposed, and future perspectives on their coupled process with anammox are also put forward.