Greenhouse gas emissions from membrane bioreactors: analysis of a two-year survey on MBRs

The study aims at evaluating the nitrous oxide (N2O) emission from membrane bioreactors (MBRs) for wastewater treatment. With this regards two-years of experimental activities were performed. More specifically, the study investigates the N2O emissions considering multiple influential factors: i. configurations (i.e., sequential batch- SB-MBR; pre-denitrification - DN-MBR; University Cape Town, UCT-MBR and UCT moving bed biofilm reactor, UCT-MB-MBR); ii. wastewater composition (municipal or industrial); iii. Plant operational conditions (i.e., sludge retention time, SRT, carbon-to-nitrogen ratio, C/N, hydraulic retention time, HRT); iv. and membrane modules. Among the overall analysed configurations the highest N2O emission occurred from the aerated reactors. The treatment of industrial wastewater, contaminated with salt and hydrocarbons, provided the highest N2O emission factor (EF): 16% of the influent nitrogen for the DN-MBR plant. The lowest N2O EF (0.5% of the influent nitrogen) was obtained in the UCT-MB-MBR plant likely due to an improvement in biological performances exerted by the co-presence of both suspended and attached biomass. The C/N ratio has been identified as a key factor affecting the N2O production. Indeed, in the UCT-MBR and UCT-MB-MBR configurations the decrease of the C/N ratio (from 10 to 2) promoted the increase of N2O emissions in both gaseous and dissolved phases, mainly due to a decreased nitrification/denitrification.