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Effect of conditioning on the performance of a plate and frame filter press
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Experiments were performed on samples of alum, anaerobically digested, and aerobically digested sludges to determine optimum polymer dosages for various mixing intensities produced during conditioning by a high-stress mixing unit. Mean velocity gradient (G) values were established for each of the mixing speeds used ranging from 250 sec -1 to 4,000 sec-1. Using the optimum conditioning dosages determined each mixing speed, batches of optimally, conditioned sludge were introduced into a pilot scale plate and frame filter press. Filtrate volume per unit time and final cake solids were used to characterize the press performance. Results indicated that filter press, performance can be optimized by selecting mixing. speeds during conditioning that simulate the shear conditions produced in the filter press during operation. It was shown that press performance was substantially reduced using sludge conditioned at a mixing speed of 200 rpm, corresponding to a Gt equal to 17,000. Tests using mixing speeds of 400 rpm and 1800 rpm corresponding to Gt values of 32,000 and 230,000, respectively indicated better performance and, thus, the filter press is thought to generate Gt values within this range. A substantial increase in polymer requirements is shown for sludge conditioned at a mixing speed of 1800 rpm, and therefore, it was concluded that sludge conditioned at mixing speed of 400 rpm best characterized the filter press producing optimum operational conditions. It was also postulated that the filter press may not be characterized by a single Gt value, but, by a range of values dependent on press run time.
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