The filtration apparatus is a concrete box which contains sand (which does the filtering), gravel (which keeps the sand from getting out) and an underdrain (where the filtered water exits). After the filter is operated for a while, the sand becomes clogged with particles and must be backwashed. Flow through the filter is reversed and the sand and particles are suspended. The particles are lighter than the sand, so they rise up and are flushed from the system. When backwashing is complete, the sand settles down onto the gravel, flow is reversed and the process begins again.

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