Fouling analysis of a tertiary submerged membrane bioreactor operated in dead-end mode at high-fluxes

Fouling deposition, consolidation and reversibility were assessed in a tertiary submerged membrane bioreactor. The unit was operated in dead-end mode (i.e. without air scouring during the filtration) and with an alternative physical cleaning strategy based on TMP set-point initiation. Several operating parameters such as filtration and backwashing fluxes, backwash duration and the aid of air scouring during backwashing were studied. The aim of the work was to optimise membrane productivity, specific aeration demand and operative time between chemical cleanings. The membrane bioreactor was operated for over 4 months with complete sludge retention, achieving a high treatment performance with moderate suspended solids concentration (MLSS=4-8 g/L). Flux-step trials pointed to the important role of the compression process and backwashing conditions in membrane fouling reversibility. During long-term tests, while reversible fouling was described by the compressible cake build-up model, residual fouling was modelled by a non-linear expression, assuming that fouling gradually reduces the available membrane area. Based on this approach, residual fouling consistently decreased with the backwashing flux applied. Analysis of the relative contribution of the different fouling layers revealed that the fraction formed by suspended solids was the main contributor. According to the residual fouling model, optimal membrane production can be established at a filtration flux of 40 L/h m2 and a backwashing flux of 27.5 L/h m2, due to the low specific energy demand, low membrane area requirement and reasonable operative time (top) between chemical cleanings.