Biofouling in an anaerobic membrane bioreactor treating municipal sewage

[Display omitted] ► Pilot-scale anaerobic membrane bioreactor met sewage reclamation criteria. ► Soluble products in reactor effluent had a bimodal molecular weight distribution. ► Biofouling layer was mainly comprised of cell-free organic matter (78%). ► The remnant fouling layer presented biologically-induced mineralization materials. ► The biomineralized structures would be the basis of irreversible membrane fouling. At present, considering the availability of reports on aerobic membrane reactor research and full scale experiences, there is a lack of understanding associated to anaerobic membrane bioreactor (AnMBR) applications for low-strength wastewater treatment. In this context, this research aims (1) to evaluate the performance of an AnMBR for municipal sewage treatment at ambient temperature and (2) to contribute to the understanding of AnMBR fouling by characterizing the cake layer with membrane autopsies. Raw sewage was fed at a hydraulic retention time (HRT) of 6 h into an up-flow anaerobic sludge blanket (UASB) reactor (0.849 m 3 volume) coupled to polyvinylidene fluoride (PVDF) external tubular ultrafiltration (UF) modules (100 kDa cut-off; total membrane area of 5.10 m 2). AnMBR permeate was a clear, suspended solids-free effluent with nearly 30 mg L −1 of chemical oxygen demand (removal of 93%) without fecal coliforms or parasite ova. Soluble constituents in the UASB effluent were grouped into two predominant fractions (bimodal distribution): higher (144 mg L −1) and lower (89 mg L −1) than membrane nominal cut-off, with an average effluent carbohydrate to protein (C/P) ratio of 0.75. Membrane autopsies were performed on two sections of UF unit, subsequent to a 55-h fouling build-up period (biofouled membrane – BFM – condition). Biofouling characteristics were compared with the fouling layer that remained after a partial (mild) cleaning procedure using chlorine (NaClO at 300 mg L −1, for 30 min). This cleaning practice accomplished a limited removal of fouling mass per unit area (13%). The remnant fouling layer apparently was in part formed by biologically-induced mineralization materials, synthesized in response to cleaning procedure. The resultant biomineralized deposits are an important structural component within the remnant cake layer and may be the basis of irreversible membrane fouling.