Return sludge treatment with endogenous free nitrous acid limits nitrate production and N2O emission for mainstream partial nitritation/anammox

Growth of nitrite oxidizing bacteria (NOB) and emissions of nitrous oxide (N2O) pose hurdles to the development and sustainability of mainstream partial nitritation/anammox (PN/A). To overcome these, a novel concept was developed in this work, based on endogenous free nitrous acid (FNA) production, used for return sludge treatment. Two integrated-film activated sludge (IFAS) reactors were operated in parallel under mainstream conditions for 300 days. The biomass from one reactor was repeatedly exposed to free ammonia (FA) shocks (30 mg NH3-N/L for 1h). This had a limited impact on NOB, but inhibited anammox bacteria (AnAOB). The biomass from the second reactor was treated weekly with FNA shocks. The effects of varying combinations of FNA levels (0.5-3.0 mg HNO2-N/L) and contact times (4-24h) on NOB suppression and N2O emissions were tested. The process data in IFAS reactors, batch activity results and the microbiome analysis jointly demonstrated that when applying FNA shocks only to the flocs, aerobic ammonium-oxidizing bacteria (AerAOB) and NOB preferably grew in biofilms attached to the unexposed carriers; The FNA treatment of flocs and carriers had a negligible impact on AnAOB activity, and successfully favoured AerAOB over NOB activity. Our results also show alterations to the N2O emissions. With similar nitrite accumulation levels, treatment with higher FNA levels ( 2.0 mg HNO2-N/L) led to lower N2O emissions compared to the baseline and treatment with lower FNA levels. Weekly FNA shocks of 2.0 mg HNO2-N/L for 4 hours were successful for stable PN/A yielding nitrogen removal efficiencies ~65% at nitrogen removal rates ~130 mg N/L/d (20 °C). Furthermore, this was also the most effective approach with the lowest carbon footprint in a preliminary extrapolation of the proposed endogenous FNA technology.