Gallons per day

Type of Sewage

per acre



Settled (overflow)


Biologically treated

Up to 500,000

granular material appropriate for the filtration process.

Backwashing is carried out in batches and continuously. The operator selects a backwashing velocity that fluidizes the granular bed without transporting it out of the equipment while the backwash removes the waste solids. The bed packing particles must have a higher settling velocity and usually a higher density than the particles or flocs of solid waste being removed.

Because of the variability of finely divided SS, buying filters cannot be done in the same way as pumps, motors, and instruments, i.e., on the basis of design specifications only. The buyer should participate in filtration tests long enough to obtain a statistical probability that the filter will work. Table 7.47.1 summarizes the types of filtration tasks and the equipment that best performs each one.

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