Achieving complete nitrification below the washout SRT with hybrid membrane aerated biofilm reactor (MABR) treating municipal wastewater

This study analyzed the performances of a hybrid membrane aerated biofilm reactor (MABR) pilot plant in terms of nutrients removal of the attached growth and suspended biomass in comparison with a conventional activated sludge (CAS) system at different sludge retention time (SRT) (20−3) days. Overall, the MABR showed better performances than the CAS in terms of TSS (86% vs 79%), COD (89% vs 85%) and total nitrogen (80% vs 65%). The minimum SRT for achieving complete nitrification in the MABR was close to 3 days, corresponding to a SRT in the aerobic compartment of 1.9 days, whereas in the CAS it was equal to 8 days (aerobic SRT of 4.8 days). Nitrification rate in biofilm was on average equal to 0.40 gNH4-N h−1 (2.40 gNH4-N m−2d−1). Its contribution to the overall nitrification in the MABR plant was 25–30% on average, although it increased when the SRT was decreased. Particle size distribution and microscopic analyses showed particles of biofilm detached from the membrane of the MABR. The seeding effect allowed sustaining nitrification of the suspend biomass at very low SRT. The nitrification rate observed in the suspended biomass in the MABR slightly decreased from 3.42 mgNH4-N gVSS−1 h−1 to 2.87 mgNH4-N gVSS−1 h−1 when the SRT was decreased from 20 days and 3 days, whereas in the CAS it collapsed from 2.33 mgNH4-N gVSS−1 h−1 to 0.47 mgNH4-N gVSS−1 h−1, because of nitrifying washout. Moreover, the biofilm detachment involved a positive effect in settling properties of the suspended biomass.