Experiments were performed with anaerobically digested sludge and a plate and frame filter press to determine filter press performance over a range of polymer doses and under a variety of sludge and polymer mixing conditions. In addition, bench-scale polymer dosing and mixing experiments were conducted with the same sludge samples to determine the relationship between the bench-scale tests and actual plate and frame filter press performance. Dewatering rate was measured in bench- scale experiments with a Capillary Suction Time (CST) device and dewatering rate in pilot-scale experiments was quantified by measuring the volume of filtrate from the plate and frame press. The dimensionless quantity, Gt, was used to measure total mixing intensity input where G was the mean velocity gradient and t was the mixing time for the sludge and polymer.

The performance of the plate and frame filter press was optimized by predicting polymer dose with a bench-scale mixing device. For the plate and frame filter press used in this study, the polymer dose was selected by finding the optimum polymer dose for the bench-scale mixing system set at a Gt value of 30,000. The sludge and polymer were mixed with a Gt value of 5000 prior to introduction to the filter press. The polymer dosing and mixing scheme that was developed for the filter press in this study indicates that the filter press imparts some shear (G) on the sludge and that polymer must be provided to reagglomerate fractured sludge particles. An estimate of the Gt value for the filter press that was used in this study is 15,000 to 40,000.