Free ammonia-free nitrous acid based partial nitrification in sequencing batch membrane aerated biofilm reactor

Membrane aerated biofilm reactor (MABR) has attracted a lot of attention as an energy-efficient integrated nitrogen removing technology in recent years. However, it is lacking of understanding to realize stable partial nitrification in MABR because of its unique oxygen transfer mode and biofilm structure. In this study, free ammonia (FA) and free nitrous acid (FNA) based control strategies for partial nitrification with low NH4+-N concentration were proposed in a MABR of sequencing batch mode. The MABR was operated for over 500 days under different influent NH4+-N concentrations. With the influent NH4+-N of around 200 mg-N/L, partial nitrification could be established with relatively low concentration of FA (0.4–2.2 mg-N/L) which suppressed nitrite oxidizing bacteria (NOB) on the biofilm. With lower influent NH4+-N concentration of around 100 mg-N/L, the FA concentration was lower and strengthened suppression strategies based on FNA were needed. With the final pH of operating cycles below 5.0, the FNA produced in the sequencing batch MABR could stabilize partial nitrification by eliminating NOB on the biofilm. Since the activity of ammonia oxidizing bacteria (AOB) was lower without the blow-off of dissolved carbon dioxide in the bubbleless MABR, longer hydraulic retention time was required to reach a low pH for high concentration of FNA to suppress NOB. After exposures to FNA, the relative abundance of Nitrospira was decreased by 94.6%, while the abundance of Nitrosospira increased greatly which became another dominant AOB genus in addition to Nitrosomonas.