Context. Mainstream nitritation in municipal wastewater treatment is challenging, due to the low bulk ammonium concentrations and low temperatures. It is especially challenging for biofilm processes, where NOB are more easily retained.
Gap. Hydroxylamine addition has been proposed as a solution because it is a natural AOB intermediate that stimulates AOB growth and inhibits NOB. It has been shown to effectively induce nitritation in suspended cultures, but little is known about how this method could work in biofilms.
Aim. The goal of this study was to explore the effectiveness of NH2OH addition for conventional and membrane aerated biofilm reactors (MABR), and for a range of biofilm thicknesses, NH2OH delivery modes, and NH2OH concentrations.
Methods. We used 1-D biofilm modeling, with an electron carrier model, to study the potential for using hydroxylamine (NH2OH) addition to achieve nitritiation in biofilms.
Findings. Results suggest that adding NH2OH intermittently can effectively promote nitritation. NOB were reduced to less than 0.1% of the total nitrifier population within several days in all scenarios. However, the required NH2OH concentration and time for NOB recovery varied greatly with the mode of delivery and type of biofilm. In MABR biofilms, nitritation could be maintained for over 100 days after NH2OH addition ceased. However, in conventional biofilms nitritation was lost after 10 days or less, due to NOB retention in the deeper, inactive zone. NH2OH supply from the biofilm base was most effective, especially in MABR biofilms, which often provide a niche for NOB at the base of the biofilm.
Utilization. We will discuss the mechanisms of AOB/NOB competition in biofilms, how NH2OH allows AOB to outcompete NOB, and the conditions under which NH2OH addition is most effective. This will guide future experiments using NH2OH addition in biofilms and will contribute to the understanding of how chemical addition influences biofilm microbial ecology in general.
