Nitrite survival and nitrous oxide production of denitrifying phosphorus removal sludges in long-term nitrite/nitrate-fed sequencing batch reactors

Abstract

Nitrite-based phosphorus (P) removal could be useful for innovative biological P removal systems where energy and carbon savings are a priority. However, using nitrite for denitrification may cause nitrous oxide (N2O) accumulation and emissions. A denitrifying nitrite-fed P removal system (SBRNO2−) was successfully set up in a sequencing batch reactor (SBR) and was run for 210 days. The maximum pulse addition of nitrite to SBRNO2− was 11 mg NO2−-N/L in the bulk, and a total of 34 mg NO2−-N/L of nitrite was added over three additions. Fluorescent in situ hybridization results indicated that the P-accumulating organisms (PAOs) abundance was 75 ± 1.1% in SBRNO2−, approximately 13.6% higher than that in a parallel P removal SBR using nitrate (SBRNO3−). Type II Accumulibacter (PAOII) (unable to use nitrate as an electron acceptor) was the main PAOs species in SBRNO2−, contributing 72% to total PAOs. Compared with SBRNO3−, SBRNO2− biomass had enhanced nitrite/free nitrous acid (FNA) endurance, as demonstrated by its higher nitrite denitrification and P uptake rates. N2O accumulated temporarily in SBRNO2− after each pulse of nitrite. Peak N2O concentrations in the bulk for SBRNO2− were generally 6–11 times higher than that in SBRNO3−; these accumulations were rapidly denitrified to nitrogen gases. N2O concentration increased rapidly in nitrate-cultivated biomass when 5 or 10 mg NO2−-N/L per pulse was added. Whereas, N2O accumulation did not occur in nitrite-cultivated biomass until up to 30 mg NO2−-N/L per pulse was added. Long-term acclimation to nitrite and pulse addition of nitrite in SBRNO2− reduced the risk of nitrite accumulation, and mitigated N2O accumulation and emissions from denitrifying P removal by nitrite.