Long-term domestication to Mn stresses alleviates the inhibition on anammox process.

Heavy metals such as Mn2+ are common contaminants in ammonium-rich wastewater. The information of Mn2+ effect on anammox process needs further investigation. The short- and long-term effects of Mn2+ on anammox were explored by anammox granular sludge. Batch tests showed that the half inhibition value (IC50 ) of Mn2+ was 4.83 mg/L. The anammox activity was severely inhibited in 0.5 hr under 15 mg/L Mn2+ . However, after long-term domestication by increasing the concentration of Mn2+ , both the low-load reactor (R1) and the high-load reactor (R2) performed well, achieving volumetric nitrogen removal rate of 6.36 kg/(m3 ·d) and 13.99 kg/(m3 ·d), respectively. The average ammonium and nitrite removal efficiency of both reactors under 200 mg/L Mn still maintained above 90%. The results from long-term reactors' operation showed that the serious inhibition effect indicated by the batch test was significantly exaggerated. The granules became dispersed after long-term operation in the high-load reactor (R2) which might be correlated to the high osmotic pressure caused by high Mn2+ load, and the mechanism needs to be investigated further. PRACTITIONER POINTS: The half inhibition value of Mn2+ on anammox sludge was 4.83 mg/L in batch experiment. 200 mg/L Mn2+ did not cause any inhibition on anammox process during long-term operation. Granular sludge is finer under high nitrogen loads with 200 mg/L Mn stress.