Comparison of process options for treatment of water treatment residual streams

Spent filter backwash water (SFBW) and clarifier sludge comprise the majority of the waste streams from conventional surface water treatment plants and collectively are referred to as composite residuals stream. Composite residuals streams can comprise up to 3-10% of the plant throughput and generally consist of concentrated metals (e.g., aluminum), colloidal material, natural organic matter (NOM), and pathogens (e.g., Giardia and Cryptosporidium). This research project evaluated the performance of four different treatment processes in terms of their capability of restoring this waste stream to a quality that was equal to or better than that of the source water quality. The unit operations evaluated were (i) gravity thickening, (ii) sedimentation with reflocculation, (iii) dissolved air flotation (DAF) with reflocculation, and (iv) ultrafiltration (UF). The water quality from the optimal trials met or exceeded the average raw water quality of the source water for all measured parameters with the exception of manganese. The optimal pH for sedimentation was determined to be 6.0 with no alum addition. The best coagulant dosage and recycle ratio for dissolved air flotation (DAF) were found to be 30 mg/L and 20%, respectively. The optimum settling time for thickening was determined to be 0.8-1.0 d, after which soluble metal concentrations began to increase because of re-solubilization. Ultrafiltration required no coagulant and yielded superior results to the other unit operations evaluated.