Recycle of waste backwash water in ultrafiltration drinking water treatment processes

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.