Info

Rate of filtration =

minutes for water in filter to fall 1 ft Fill filter with water, shut off influent, and open drain.

Backwash time = 10 to 15 minutes minimum, until water entering through is clear

Normal washwater usage = 2 to 2.5% or less of water filtered

FIGURE 2.2 Slow sand filter for a small water supply. Minimum of two units. The difference in water level between the two glass tubes represents the frictional resistance to the flow of water through the filter. When this difference approaches the maximum head and the flow is inadequate, the filter needs cleaning. To clean, scrape the top 1 in. of sand bed off with a mason's trowel, wash in a pan or barrel, and replace clean sand on bed. Float control valve may be omitted where water on filter can be kept at a desirable level by gravity flow or an overflow valve. Add a meter, venturi, or other flow-measuring device on the inlet to the filter. Rate of flow can also be controlled by maintaining a constant head with a weighted float valve over an orifice or weir. See 1.18. Filtered water should be disinfected before use. Allow sufficient head room for cleaning the filter. Separation of the filter box from the filtered water storage clear well is usually required by health officials for public water supplies. Thoroughly ventilate filter box and water storage tank before entering and during occupancy.

FIGURE 2.2 Slow sand filter for a small water supply. Minimum of two units. The difference in water level between the two glass tubes represents the frictional resistance to the flow of water through the filter. When this difference approaches the maximum head and the flow is inadequate, the filter needs cleaning. To clean, scrape the top 1 in. of sand bed off with a mason's trowel, wash in a pan or barrel, and replace clean sand on bed. Float control valve may be omitted where water on filter can be kept at a desirable level by gravity flow or an overflow valve. Add a meter, venturi, or other flow-measuring device on the inlet to the filter. Rate of flow can also be controlled by maintaining a constant head with a weighted float valve over an orifice or weir. See 1.18. Filtered water should be disinfected before use. Allow sufficient head room for cleaning the filter. Separation of the filter box from the filtered water storage clear well is usually required by health officials for public water supplies. Thoroughly ventilate filter box and water storage tank before entering and during occupancy.

TABLE 2.2 Orifice Flow for Selected Orifice Diameters

Maximum Flow by Orifice Diameter" (gpd)6

TABLE 2.2 Orifice Flow for Selected Orifice Diameters

Maximum Flow by Orifice Diameter" (gpd)6

Maximum Head, (ft of water)

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