Effect of free ammonia inhibition on process recovery of partial nitritation in a membrane bioreactor

Long-term inhibitory effect of free ammonia (FA) on the catabolic and anabolic activities of an enriched Nitrosomonas culture was investigated. Ammonia oxidizing bacteria (AOB) was inhibited at FA concentration of 34.34 ± 6.85 mg NH3-N/L while threshold inhibitory FA concentration was likely 23.64 ± 4.51 mg NH3-N/L. Below the latter, cell-specific ammonia oxidation rate (AOR), cell number and extracellular polymeric substances (EPS) increased. The calculated yield showed AOB had tended to increase cell mass under FA inhibition. Nitrite concentration increased by 58.6% 12 days after FA decreased to 8.89 ± 0.95 mg NH3-N/L. However, AOB cells became more sensitive to free nitrous acid (FNA) after 70 days of FA inhibition. Long-term FA inhibition changed AOB cell morphology and this could have increased AOB vulnerability to FNA. This study indicated FA inhibited catabolic and anabolic activities, and that long-term FA inhibition had adverse impact on process recoverability and subsequent failure threshold. © 2019 Elsevier Ltd