Polymer Dosing in Thickeners and Clarifiers – Case Study for Residuals Treatment at Large Surface Water Treatment Plant

Bench scale (jar tests) and full-scale testing of non-ionic polymer dosing at the gravity thickeners and backwash clarifiers was conducted at the Val Vista Water Treatment Plant Solids Handling Facility in Phoenix, Arizona. The purpose of the testing was to determine optimal polymer dosages for improving solids removal efficiencies under the given plant operating conditions. Other selected operational alternatives such as varying the backwash clarifier flocculator speeds and feeding thickened residuals to the backwash clarifiers influent were also investigated. Jar test parameteres were compared to full scale tests and testing performance guidelines for both backwash clarifiers and gravity thickeners were proposed for operators' quick reference. Bench and full scale test results indicated that the optimal polymer dosage for the gravity thickeners influent was approximately 0.5 pounds active polymer per dry ton solids with powdered activated carbon (PAC) present. Bench scale testing indicated that the optimal polymer dose for the backwash clarifiers influent was approximately 0.25 ppm of neat (as delivered) polymer with PAC present and approximately 0.50 ppm without PAC present. Full scale testing of polymer dosages in the backwash clarifiers indicated that the optimal dosage of polymer was 0.75 ppm without PAC present. Solids removal efficiencies at the backwash clarifiers increased with faster flocculator speeds. However, the jar test results indicated that at 100% speed (G=82/s) the solids removal efficiency increased only slightly compared to the 75% speed (G=53/s). Test results for addition of thickened residuals to the backwash clarifiers influent to improve solids removal efficiencies were inconclusive. Different solids monitoring techniques including turbidity, solids concentration and particle count were used.