Roles of hydroxylamine and hydrazine in the in-situ recovery of one-stage partial nitritation-anammox process: Characteristics and mechanisms

Nitrate built-up is a serious operational difficulty in one-stage partial nitritation anammox (PN/A) process. To investigate an effective method for in-situ restoration, hydroxylamine (NH2OH) and hydrazine (N2H4) of 2 mgN/L were dosed in PN/A process with nitrate built-up in a comparative study. NH2OH treatment showed better performances on TN removal and nitrate reduction than N2H4 and blank control. Through 104 days' addition of NH2OH, MRNN (mole ratio of NO3−-N production to NH4+-N removal) was decreased from 70% to 19.91%; TN removal was increased from 0.01 to 0.18 kgN/(m3 d). After stopping the chemical addition, nitrate rebounded for N2H4 treatment, but the restoration effect was stable and persistent for NH2OH. NH2OH addition resulted in a low reductive potential (−250 mV) and exerted strong inhibitions on nitrite oxidizing bacteria activities. Additionally, rapid enhancement of ammonia oxidizing bacteria activities, functional gene (hao) and Nitrosomonas gave rise to the restoration of PN/A with NH2OH addition. [Display omitted] •NH2OH dosing showed better effects than N2H4 on in-situ restoration of PN/A process.•Nitrate rebounded in effluent after stopping N2H4 dosing.•NH2OH dosing caused a strong reductive potential (−250 mV) lasting for 210 min.•Rapid enhancement of AOB activities, hao and Nitrosomonas by NH2OH addition