Deep-Bed Filters:

Deep-bed filters are commonly found just in makeup water systems, whereas they are used to filter water after it has been treated in a clarifier. Those are used to erase chlorine, organic matter, and extremely fine particulate matter.

A deep-bed filter is based on a support screen (decking), that is mounted a few inches above the bottom of the tank. A screen is perforated to permit water to flow through it. A coarse, aggregate layer of crushed rock or huge lumps of charcoal is placed on top of the screen, and the deep bed itself (2 to 4 feet of granular anthracite or charcoal) is placed on top of the aggregate. A filter is sized so in which there is 1 to 2 feet of "free board" above the deep bed.

Figure: Deep-Bed Filter

While the filter is within service, raw water is pumped in by a pipe which feeds a distribution pipe above the deep bed. Water is filtered as it percolates down by the granules.

(Charcoal granules will filter out organic matter, fine particulates, and chlorine, while anthracite granules erase just the particulates.) The water collects within the bottom of the tank, under the support screen, and leaves the filter by a pipe within the bottom of the filter vessel.

Deep-bed filters, like precoat filters, are cleaned through backwashing. Water is pumped by the distribution piping near the top of the filter. A flow rate of the water is kept high sufficient to lift the granulated charcoal or anthracite up into the free space. Water washes away the deposits which have accumulated. While the backwash cycle is finished, the flow is stopped, and the granules settle back down within the filter bed. The filter could then be put back within service.