Full‐Scale Assessment of waste filter backwash recycle

This article provides the results of studies of full-scale waste filter backwash water (FBW) recycle practices and water quality effects at six water treatment plants as well as recommendations for evaluating FBW recycle practices. Water quality parameters assessed included suspended solids, turbidity, particle counts, total and dissolved organic carbon, aluminum, and manganese. Results showed insignificant effects on plant influent water quality for recycle of clarified FBW and for a plant with continuous recycle. The FBW recycle quality was highly temporally variable for plants with only flow equalization (no solids removal), exhibiting significant peaks in solids levels that caused significant short-term increases in plant influent turbidity. However, the performance of downstream clarification (dissolved-air flotation or plate sedimentation) and filtration were not affected by FBW recycle for these plants. Evaluation of recycle effects requires knowledge of the site-specific details of plant operation (generation of recycle streams, flow control) and the chemistry of process waters (raw, recycle, and coagulated).