Ammonium concentration determines oxygen penetration depth to impact the suppression of nitrite-oxidizing bacteria inside partial nitritation and anammox biofilms

Biofilm-based Partial Nitritation and Anammox (PN/A) process has gained wide popularity in treating high ammonium (-1000 mg NH4+-N/L) sidestream wastewater while facing a significant challenge in terms of nitrite -oxidizing bacteria (NOB) suppression when applied for low-strength municipal wastewater treatment with low ammonium concentration (-50 mg N/L). Here, the significance of residual ammonium (NH4+) concentration to the mainstream PN/A biofilm system was systematically investigated through long-term and short-term exper-iments, in conjunction with analysis of the stratification of functional microorganisms and measurement of oxygen penetration. The experimental results collectively showed that a residual NH4+ concentration of -50 mg N/L was essential to stable NOB suppression, while a lower concentration of -5 mg N/L undesirably caused NOB proliferation. This result suggests that low NH4+ concentration in mainstream wastewater would lead to the intrinsic difficulty of NOB suppression in a biofilm PN/A process. Moreover, this study reveals that high NH4+ concentration promotes the activity of ammonia oxidizing bacteria (AOB) occupying the biofilm surface, thereby reducing oxygen penetration depth. Accordingly, NOB that inhabit a deep region of biofilm are suppressed by the limiting oxygen, while anammox bacteria are favoured instead. These findings provide important knowledge for the process control and expand the understanding of microbial interaction in mainstream PN/A biofilm systems.