Continuous deep-bed filters

All of the types of sand filter so far discussed are discontinuous in their operation, even if the cycle is quite long in the case of the slow gravity filter. The most recent development of the rapid sand filter has been to make it continuously operating, by
having the bed of sand or other materials move downwards through the filter (as in the DynaSand filter). The dirty solids normally fall into a conical hopper at the base of the filter, whence they are carried by a jet of air to a wash zone above the filter (as seen in Figure 3.87). Here they are washed clean of trapped solids and then returned to the top of the bed in the filter. There can now be no stratification of the solids by size or density, but the flow of water, counter-current to the movement of the solids, ensures good solids removal.

The same principle of the bed of solids moving continuously between filtration and cleaning zones has also been applied to the removal of dust from hot exhaust gas streams, the nature of the sand bed material being such as easily to resist the
high gas temperatures.

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