The influence of solid retention time on IFAS-MBR systems: Assessment of nitrous oxide emission

The aim of the present study was to investigate the nitrous oxide (N₂O) emissions from a moving bed based Integrated Fixed Film Activated Sludge (IFAS) - membrane bioreactor (MBR) pilot plant, designed according to the University of Cape Town (UCT) layout. The experimental campaign had a duration of 110 days and was characterized by three different sludge retention time (SRT) values (∞, 30 d and 15 d). Results highlighted that N₂O concentrations decreased when the biofilm concentrations increased within the aerobic reactor. Results have shown an increase of N₂O with the decrease of SRT. Specifically, an increase of N₂O-N emission factor occurred with the decrease of the SRT (0.13%, 0.21% and 0.76% of influent nitrogen for SRT = ∞, SRT = 30 d and SRT = 15 d, respectively). Moreover, the MBR tank resulted the key emission source (up to 70% of the total N₂O emission during SRT = ∞ period) whereas the highest N₂O production occurred in the anoxic reactor. Moreover, N₂O concentrations measured in the permeate flow were not negligible, thus highlighting its potential detrimental contribution for the receiving water body. The role of each plant reactor as N₂O-N producer/consumer varies with the SRT variation, indeed the aerobic reactor was a N₂O consumer at SRT = ∞ and a producer at SRT = 30 d.