Water-Supply Filter

an installation in water-treatment plants that is used to remove suspended substances from water by passing the water through a granular material, such as sand, pulverized anthracite, or pulverized marble. A water-supply filter is an open (gravity) tank, usually made of reinforced concrete, or a closed (pressurized) steel tank; a bed of filter material supported by a gravel bed overlies the tank’s drainage system. Water moves through the filter bed as a result of a pressure drop between the filter’s intake and outlet. To ensure the required degree of water clarification and to unclog the filter, the filter material is cleaned periodically by backwashing or some other means. The most important operational characteristic of a water-supply filter is the filtration rate, that is, the amount of water in cubic meters that passes through 1 m² of filter area per hour.

Depending on the amount and quality of the raw or discharge water, the required degree of purification, and other factors, the following main types of water-supply filters are used: slow sand filters, rapid sand filters (Figure 1), and ultrarapid sand filters. The filtration rates for the three types are 0.1–0.2 m³/hr, 5.5–12 m³/hr, and 25–100 m³/hr, respectively. Preliminary and coarsegrained filters are also employed. Slow sand filters are used mainly to purify slightly turbid water, usually without preliminary coagulation of the impurities. As a rule, rapid sand filters are used with preliminary coagulation and clarification in settling basins or in clarifiers containing a layer of floe. One type of rapid sand filter is the AKKh (Academy of Housing and Municipal Economy) double-flow filter, in which the water to be filtered is fed simultaneously from below and above, and the clarified water is discharged through a drainage system located within the filter bed. Ultrarapid sand, preliminary, and coarse-grained filters are used mainly for the partial clarification of water.

Figure 1. A single-bed central-channel rapid sand filter in section (A) and plan (B): (1) sand, (2) gravel, (3) drainage system consisting of perforated pipes, (4) wash-water gutters, (5) raw-water feed, (6) filtered-water outlet, (7) wash-water feed, (8) wash-water outlet, (9) central channel. The filter operates in two modes. During filtration, slide valves a and b are open while slide valves c and d are closed; during backwashing, a and b are closed while c and d are open.

The future development of water-supply filters is linked to the discovery of new and efficient filter materials, the development of efficient drainage and distribution systems, the optimization of water-treatment techniques, and the automation of water treatment.