This study investigated the inhibitory effect of free nitrous acid (FNA) on the activity of ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) under anoxic conditions with different mixed liquid suspended solids (MLSS). Sequencing batch reactors were used to study the changes in the activity of AOB and NOB in nitrifying activated sludge based on four different MLSS (8398, 11254, 15998, and 19637 mg·L-1), after treatment, under anoxic conditions with FNA (at an initial concentration of 1.3 mg·L-1) for 48 h. The results showed that the pH increased by approximately 0.9, but the concentration of NO2--N did not decrease significantly. With over-aeration, the concentration of NH4+-N gradually degraded to 0 mg·L-1, and the removal rate of NH4+-N gradually increased to a maximum of 4.4-6.8 mg·(L·h)-1 which time used was shorter with the increase of the inhibition MLSS. The nitrite accumulation rate was more than 92% when the sludge concentration was 8398, 11254, 15998, and 19637 mg·L-1 and with over-aeration for 0-396 h, 0-396 h, 0-372 h, and 0-168 h, respectively. When aerated for 468 h, 468 h, 444 h, and 264 h, the NO2--N concentration and NAR decreased to 0, and NO3--N concentrations increased to their highest with the values of 42.6, 49.9, 42.9, and 47.9 mg·L-1 respectively.
Keywords: activity of nitrifying bacteria; anoxic; free nitrous acid; nitrite accumulation ratio; removal rate; sludge concentration.