Free ammonia shock treatment eliminates nitrite-oxidizing bacterial activity for mainstream biofilm nitritation process

Nitritation (NH₄⁺ → NO₂⁻) is a critical step to provide nitrite for the followed anammox in a two-stage nitrogen removal system. In the mainstream line of wastewater treatment plants, the nitritation has not been applied to date, because of the significant difficulties in the suppression of nitrite-oxidizing bacteria (NOB), especially in biofilm systems. This study aims to systematically assess the effect of free ammonia (FA) shock treatment on the mainstream biofilm nitritation process through an integration of laboratory reactor operation, microbial community analysis, incubation tests, and kinetic model evaluation. In a laboratory nitrifying moving bed biofilm reactor (MBBR) fed with domestic-strength synthetic wastewater, it was shown that with the exposure of carrier-biofilms containing ammonia-oxidizing bacteria (AOB) Nitrosomonas and NOB Nitrospira to a high-level of FA (1068 mg NH₃-N/L) over two days, a much higher residual AOB level was retained in the biofilm in comparison to NOB. The higher residual AOB on biofilm led to much faster recovery of AOB over NOB after the shock treatment, when normal operation resumed with the dissolved oxygen (DO) controlled at around 0.2 mg/L. The faster recovery of AOB than NOB subsequently gave rise to a stable, high nitrite accumulation ratio (nearly 100%) over a long period (two months). Collectively, these results suggest that FA shock treatment in conjunction with limited DO control is effective in eliminating NOB for mainstream biofilm nitritation process. The chemical cost would be marginal given the intermittent nature of the FA shock strategy and the readily available ammonium in the anaerobic sludge digestion liquor.