Fe(Ⅱ) improving sulfurized Anammox coupled with autotrophic denitrification performance: Based on interspecies and intracellular electron transfer

Insufficient nitrite supply and slow metabolism of Anammox bacteria (AnAOB) impeded the application of Anammox process in low level ammonia (LLA) (≤50 mg/L) wastewater. At the initial concentration of 50 mg/L NH4+-N and 75 mg/L NO3--N, Fe(Ⅱ) (10 mg/L) promoted the total nitrogen removal efficiency from 80.79 to 94.92 % by core-shell sulfurized AnAOB coupled with sulfur oxidizing bacteria (S0@AnAOB + SOB). AnAOB outcompeted SOB for nitrite, because the addition of Fe(Ⅱ) not only increased the nitrate reductase activity (37.54 %), but also enhanced the metabolism and electron capture ability of AnAOB, which was highly related with energy metabolic process: hydrazine dehydrogenase activity increased to 139.00 %. Particularly, Fe(Ⅱ) accelerated the interspecies electron transfer (INET) (from SOB to AnAOB) by stimulating the secretion of redox species and electron hopping in EPS. This study shed light on the mechanism of Fe(Ⅱ) promoting electron transfer in S0@AnAOB + SOB system, and provided basis for engineering practice.