## Deepbed Filter Designs

Filters with deep beds of sand, diatomaceous earth, coke, charcoal, and other inexpensive packing materials have been successfully used in the filtration of potable water and can also be used in the small-scale treatment of dilute wastewater (see Figure 7.47.3). Their best applications are in polishing the filtrate from a continuous filter or the overflow from a primary or secondary settling tank. Without preseparation, the bed becomes loaded quickly. When the particles and bacteria in sizes smaller than the interstices

FIG. 7.47.11 Batch pressure filtration with variable pressure. Graphs 1 and 3 are generalized graphs of the original data taken during a run; graph 2 can be calculated from 1 and 3 or obtained from a flowmeter. The curved line on 2 is on or close to the characteristic curve of the pump. At any time, the following equation calculates the specific resistance a:

FIG. 7.47.11 Batch pressure filtration with variable pressure. Graphs 1 and 3 are generalized graphs of the original data taken during a run; graph 2 can be calculated from 1 and 3 or obtained from a flowmeter. The curved line on 2 is on or close to the characteristic curve of the pump. At any time, the following equation calculates the specific resistance a:

The variability or constancy of a indicates the compressibility or lack of it in the filter cake.

of the bed, plus suspended BOD, are removed from the liquid, exceptional clarity is obtained. The dissolved substances, including dissolved BOD, are not removed.

Environmental engineers can predict the variation of pressure loss and flow rate with time from small-scale tests if they determine K1 and K2 experimentally. The sand bed and waste material tested must be the same as that used in the process. Predicting the initial pressure-loss for various flow rates is possible.

The Ergun equation for the initial pressure loss in a deep-bed filter is particularly useful for granular materials because it covers both laminar and turbulent regimes as follows:

dp p

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