Application of a backwashing strategy based on transmembrane pressure set-point in a tertiary submerged membrane bioreactor

•Sustainability of a MBR operated at supra-critical fluxes.•Novel and promising operation mode for backwashing initiation.•Automatic adjust of the degree of membrane fouling allowed and MBR productivity through backwashing frequency.•Applied for advanced treatment of secondary effluent from a WWTP.•Successfully operated without biomass purge resulting in a moderate MLSS.

An alternative backwashing strategy to enhance water productivity in a tertiary submerged membrane bioreactor (MBR) was assayed. This strategy is based on automatically adjustment of the backwashing frequency as a function of the membrane fouling, which is expected to increase the net permeate flux produced. The effect of the key operational parameter (transmembrane pressure set-point, TMPsp) on membrane fouling and process productivity was evaluated on a pilot-scale tertiary MBR. The system was successfully operated for over 4 months with complete sludge retention achieving a high treatment performance with a moderate liquor suspended solid concentration, as a result of carbon substrate limited conditions. The analysis of the membrane fouling at supra-critical filtrate flux of 70 L/(h m2) with a specific aeration demand identical to that usual at full-scale (SADpnet=17 N m3/m3) showed that backwashing efficiency (described in terms of residual fouling resistance) was significantly affected by the selected TMPsp value. At high TMPsp, the efficiency decreased and chemical cleaning was necessary for membrane recovery. Nevertheless, moderate set-point values (30–40 kPa) provided high permeate net fluxes of 65–67 L/(h m2) for more than 2000 h of operation, while the reversible fouling rate was not considerably influenced by TMPsp. This was also confirmed by flux steps trials.