Risk assessment and classification of reusability of backwash water from drinking water treatment plants

Water shortages are a substantial challenge for human development worldwide. Drinking water treatment plants consume water resources during the water treatment process, producing wastewater with high potential for reuse. This study investigated the variations in water quality characteristics of backwash water from a water treatment plant using sand and activated carbon filters and conducted a risk assessment for its direct reuse. Significant differences in turbidity, dissolved organic carbon, total colony count, and aluminum and manganese concentrations were identified during the initial, intermediate, and final stages of backwashing using the two filter types. Compared to the raw intake water, backwash water from the sand filter had higher microbial risk when reused. Although the activated carbon filter was the main site for microbial activity, the biological and chemical risks were low in the reused backwash water from this filter type. To achieve low-risk reuse of backwash water, this study developed a new risk assessment method based on water quality fluctuations between raw and backwash water, enabling the classification of backwash water for reuse. This method calculated the changes in water quality indicators of raw and reused backwash water after mixing and compared them with historical water quality fluctuations in raw water. A 90 ​% assurance rate that the mixed-water quality met natural historical variations served as the criterion for determining risk-free reuse. According to this criterion, the reuse rates of backwash water from the sand filter at the initial and intermediate stages were <20 ​%, while those of backwash water from the sand filter at the final stage and from the carbon filter at all stages could reach 100 ​% (Considering the one-factor limitation, the number of colonies was taken as an example). This evaluation method can help drinking water plants to quickly assess the risk of directly reusing backwash water, achieving maximum safety without the need for major process adjustments. • Backwash water from sand and carbon filters was divided into detailed segments. • Different quality was found between different types and stages of backwash water. • A reuse calculation model was developed for backwash water based on segmentation. • The optimal reuse method was calculated based on the reuse calculation model.