Coexistence and competition of the nitrifying and anammox bacteria in SMBBR reactor with partial nitritation/anammox process treating synthetic low-strength ammonium wastewater

The nitrite-oxidizing bacteria (NOB) suppression largely restricts the implementation of partial-nitritation and anammox (PN/A) process for low-strength ammonium wastewater treatment. The main goal of the present study was to verify the feasibility of sequencing moving bed biofilm reactor (SMBBR) treating synthetic low-strength ammonium wastewater and the influence of operating conditions and microbial competitions on NOB suppression. The reactor was operated in sequencing batch reactor (SBR) mode with influent ammonium of 50 mg/L, in which the seeded biofilm harvested from an existing similar PN/A reactor treating high-ammonium synthetic wastewater. After increasing the dissolved oxygen (DO) from 0.50 to 1.25 mg/L-with hydraulic retention time (HRT) gradually decreasing from 15 to 5 h-stable nitrogen removal through PN/A was obtained. The long-term operation results showed nitrogen removal efficiencies of 66.23 +/- 7.78 % were stably obtained at a relatively low filling ratio of 25 %. The reactor operated in SBR mode allowed NOB suppression and low effluent ammonium due to the ammonium gradient created in cycles. Further, the activity of NOB was controlled at a low level at medium DO concentration. Both ammonium-oxidizing bacteria (AOB) competition acting as DO-cut and anaerobic ammonium-oxidizing bacteria (anammox, AMX) competition acting as nitrite-sink were found to be essential for NOB suppression, and DO-cut played a dominated role. A simple hypothetical microstructure of one-stage PN/A biofilm, assuming AOB layered in the surface of the biofilm while AMX and NOB in the interior, was proposed to define the mechanism of DO-cut and Nitrite-sink. The obtained results demonstrated SMBBR could be regarded as an alternative guideline for successful operation of mainstream PN/A as compared to the integrated fixed film activated sludge (IFAS) systems.