Performance of a tertiary submerged membrane bioreactor operated at supra-critical fluxes

•Sustainability of an MBR operated at supra-critical fluxes.•Novel and promising operation mode for backwashing initiation.•Determination of residual fouling, reversible fouling rate and net permeate flux.•Applied for advanced treatment of secondary effluent from an WWTP.•Successfully operated without biomass purge resulting in a moderate MLSS.

A pilot-scale submerged aerobic membrane bioreactor (MBR) was run for over 3 months to assess the sustainability to operate at supra-critical fluxes. The MBR was applied as advanced treatment of secondary effluent from a conventional wastewater treatment plant. The system was successfully operated without biomass purge at hydraulic retention time (HRT) of 8.8 h, resulting in a moderate liquor suspended solid concentrations range (MLSS=4.1–7.1 g/l) in the bioreactor, according to the influent organic load fluctuations. Treatment performance was stable and achieved high conversion of ammonium to nitrate (96%) and dissolved organic carbon removal (53%). Short-term tests have been carried out according to a modified flux-step method to determine critical flux and evaluating optimum membrane cyclical aeration frequency. For the long-term tests, an alternative operation mode for backwashing initiation, based on a pre-selected transmembrane set-point, was applied. Under typical specific demand values (SADpnet=13.7–18.3 N m3/m3), continuous operation under different supra-critical filtration fluxes (J=60–80 l/h m2) and backwashing fluxes (40–80 l/h m2) can be maintained without any chemical cleaning. Analysis by means of sludge fractionation in lab-scale tests, at similar hydrodynamic conditions, indicated that the contribution of suspended solids to cake membrane fouling was estimated about 86–89%.