Effect of free ammonia and free nitrous acid on nitrogen removal of heterotrophic nitrification and aerobic denitrification bacteria

Free ammonia (FA) and free nitrous acid (FNA) can inhibit nitrogen removal by nitrifying microorganisms. However, current studies on FA and FNA mainly focus on traditional nitrifying microorganisms, and little is known about the relationship between heterotrophic nitrification-aerobic denitrification (HN-AD) microorganisms and FA and FNA. This study selected Alcaligenes faecalis WT14, an HN-AD strain, as the research object. The effects of different FA and FNA concentrations on the nitrogen removal ability of strain WT14 were investigated under neutral (pH = 7) and alkaline (pH = 9) conditions. The results showed that FA and FNA affected the removal ability of strain WT14 to ammonia nitrogen (NH4+-N) and nitrite nitrogen (NO2−-N). The rapid reduction of NH4+-N removal efficiency was determined at different concentrations of 6.12 mg·L−1 (pH = 7) and 551.18 mg·L−1 (pH = 9) for FA, which was 0.483 mg·L−1 (pH = 7) and 0.005 mg·L−1 (pH = 9) for FNA, respectively. Strain WT14 preferred an alkaline environment for growth; the FA tolerance value was 1134.25 mg·L−1 in such an environment, which was 51.8 times that of a neutral environment. Furthermore, strain WT14 had a higher removal rate of NH4+-N and NO2−-N in an alkaline environment. Its highest removal rates of NH4+-N and NO2−-N were 15.03 mg·L−1·h−1 and 7.16 mg·L−1·h−1, respectively, which was much higher than the reported HN-AD microorganisms.