Membrane Bioreactor Effluent Water Quality and Technology – Organics, Nutrients and Microconstituents Removal

Membrane Bioreactor (MBR) technology has become well established worldwide over the last decade since it can produce consistent high quality effluent in a small footprint by using membranes for solids separation. Funded by the WateReuse Foundation, the primary focus of this study was to determine the extent to which superior solids separation and the flexibility to operate at higher solids retention time (SRT) coupled with lower hydraulic retention time (HRT) impact water quality parameters such as microbes, aggregate organics, nutrients, trace organic compounds, and trace metals. MBR water quality performance was assessed through integration of data obtained from peer-reviewed and grey literature, survey of MBR vendors and full-scale MBR installations, and development of an MBR predictive model for aggregate and microconstituents. About 80% of the surveyed facilities operated at flux of less than 15.5 gfd and mixed liquor suspended solids (MLSS) concentration of less than 12,000 mg/L. The MBR model demonstrated increase in total biodegradation of organic matter when SRT was increased from 2 to 15 days but no further biodegradation was observed with further increase in SRT. The literature demonstrated that denitrification efficiency increased as the COD/TKN ratio increased and a denitrification efficiency of >50% was achieved in every study where this ratio was at least 7. Since membranes provide better removal of solids, the MBR is expected to provide better removal of particle-associated metals and dissolved metals than the CAS process. The model outputs demonstrated little benefit from operating an MBR beyond an SRT of 20-30 days for biodegradable and volatile organic microconstituents while the compounds removed through adsorption showed best removal for SRT values lower than 10 days.