Graywater treatment technologies and reuse of reclaimed water for toilet flushing

The reuse of wastewater is one effective approach to solving the problem of water resource scarcity. However, deterioration in the quality of reused water, such as increased odor and bacterial growth, restricts its reuse. The objectives of this study were to characterize graywater (GW) treatment technology and to verify the suitability of the reclaimed water for toilet flushing. A membrane bioreactor (MBR) and biological aerated filter (BAF) were used to treat GW in a 1-year laboratory-scale experiment. The optimal operational conditions of the MBR and BAF were as follows: hydraulic retention time = 2-3 h, dissolved oxygen = 4-7 mg/L, mixed liquor suspended solids = 3500-4500 mg/L, and contact reaction time = 1.96-5.89 h, dissolved oxygen = 3-5 mg/L, backwash cycle time = 24-48 h, respectively. The MBR treatment resulted in reductions in COD, NH₃-N, and turbidity of 60-90%, 80-90%, and 95-99%, respectively, whereas those of BAF treatment were 50-90%, 50-90%, and 80-90%, respectively. The BOD₅ values of MBR and BAF effluent were 1.2-4.5 mg/L and 2.5-7 mg/L, respectively. GW treated by both MBR and BAF met the standard for reusing water for toilet flushing. The effluent from MBR, BAF, and BAF + ultrafiltration treatment and purified mixed wastewater was used to simulate toilet flushing at 28 °C, with the addition of 5 mg/L NaClO to the reused water. The residual chlorine levels were 1.5, 0.6, 0.9, and 0.5 mg/L, respectively, after 15 days. No bacteria were detected in any of the reclaimed water after 15 days. The water quality of the effluent of MBR-treated GW was better than that of the mixed wastewater. The results show that it is viable to use GW purified by MBR for toilet flushing. This study provides a scientific basis for the popularization and application of reclaimed water for toilet flushing.