In drinking water treatment, ultrafiltration (UF) membrane systems are generally operated at 90 to 95% recovery with production losses resulting from waste residual streams such as backwash water and clean-in-place (CIP) liquid residuals. In drought-prone regions, it may be desirable to apply alternative UF plant design configurations to increase recovery rates and minimize water loss. This approach could consist of adding a secondary stage UF membrane for treatment of first stage UF residuals, or recycling the first stage UF waste residuals by blending a percentage of the backwash water with the raw water at the front of the treatment train. For small systems, the second option may present a more cost-effective solution. The overall objective of this research project was to investigate the potential impacts to UF permeate water quality and coagulation pretreatment efficacy in a bench-scale submerged UF membrane system operating with and without waste backwash water recycle. The results of the study showed that blending 10% waste backwash water with raw water did not negatively impact UF permeate water quality. The results also demonstrated that recycling waste backwash water prior to coagulation–UF treatment may improve organic removal and reductions in neat coagulant dosage may be possible to achieve specific DBP precursor removal targets.
Keywords: coagulation, filter backwash water, natural organic matter, ultrafiltration