In-situ restoring nitrogen removal for the combined partial nitritation-anammox process deteriorated by nitrate build-up

• The nitrogen removal of the CPNA process was deteriorated by nitrate build-up.
• The nitrate build-up could be in-situ solved by both NH2OH dosing and SRT control.
• The inhibition of NOB by NH2OH dosing was reversible.
• The evolution of functional bacteria confirmed this in-situ restoration strategy.

The nitrogen removal performance in the combined partial nitritation-anammox (CPNA) process was seriously deteriorated by the nitrate build-up. The purpose of this study was to develop and optimize an in-situ restoring strategy based on hydroxylamine (NH2OH) dosing and solids retention time (SRT) control for the deteriorated CPNA process. Results showed that the 0.55 kgN m−3 d−1 of nitrogen removal rate could be recovered by 20 mgNH2OH L−1 of hydroxylamine dosing and 40 days of SRT control, the nitrate concentration in effluent was decreased from the highest 548.4 mgN L−1 during deterioration to 65.1 mgN L−1 after restoration, and the ratio of NO3−-Nproduced/NH4+-Nconsumed in one SBR cycle was reduced from the highest 87.0% to 9.13% finally. The inhibition of nitrite-oxidizing bacteria (NOB) by NH2OH dosing alone was reversible because the nitrate build-up occurred again from 106.9 to 287.6 mgN L−1 within just 11 days after NH2OH dosing was stopped. The evolution of the anammox bacteria, ammonium-oxidizing bacteria (AOB) and NOB from quantitative PCR (qPCR) assays verified the changes of the nitrogen removal performance of the CPNA process and proved that this in-situ restoration strategy could successfully solve the problem of nitrate build-up in the CPNA process.