Declining-Rate Versus Constant-Rate Filtration

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Abstract
The study conclusively showed the benefits of the declining rate system. Both systems were operated at the average filtration rates at a municipal lime softening plant. The declining-rate filter system produced an average filtrate turbidity which was consistently and substantially better than the constant-rate filter at all three filtration rates studied. The average filter run length for the declining-rate system was substantially affected by the amount of turbulent piping head losses in the system and the maximum filtration rate allowed on the newly backwashed filter. When operated to the same head loss increase in the filter media, the constant-rate filter run length was substantially shorter than the run length obtained for the declining-rate filter system. The filtration rates in the individual filters of the declining-rate system remained constant between backwashings and declined in a stepwise fashion after each backwash was completed.

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