Fe(III)-induced ammonium production and its control strategy during nitrogen removal in denitrifying dependent anaerobic methane oxidation system

Denitrifying anaerobic methane oxidation (DAMO) process uses methane produced by anaerobic digestion to remove nitrogen, contributing to sustainable operation of wastewater treatment. Fe(III), which is widely present in wastewater, has been shown to affect nitrogen metabolites of DAMO archaea, but its long-term effects on nitrogen removal performance and microbial community structure of DAMO systems remain unclear. Here, we revealed that Fe(III) in the influent did not affect nitrate removal rates but could induce continuous ammonium production and emission in the membrane granular sludge reactor (MGSR), accounting for about 13.7 % of nitrate removed. Excess methane and no residual nitrate were necessary conditions for the Fe(III)-induced ammonium production. DAMO archaea as the microorganism responsible for ammonium production always dominated the microbial community, while the relative abundance of DAMO bacteria decreased and anaerobic ammonium oxidizing (anammox) bacteria began to increase during the long-term operation, resulting in versatile nitrogen removal. Technical analyses showed that the ammonium production could cause an increase of 41.7 % in methane consumption of DAMO unit for treating ammonium-containing influents, thereby reducing the techno-economics. Therefore, special attention should be paid to the possible Fe(III)-induced ammonium generation in DAMO-centric technologies and avoiding excess methane may be a practical strategy to prevent it from happening.