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(A) Size. The size shall be such to provide 100 square feet of filter area per person served or 200 square feet of filter area per bedroom. In systems serving an automatic washing machine the size shall be increased to provide 250 square feet of filter area per bedroom. It is recommended that the size be based on a filter 20 feet square for a two-bedroom home, with an additional ten feet of length added for each additional bedroom.
(B) Minimum filtering depth. The minimum filtering (sand) depth shall be 24 inches.
(C) Cover material and depth. The filter may be covered with a maximum depth of 12 inches of porous soil or medium to coarse gravel.
(D) Filter media. The filter media (sand) shall have an effective size of 0.6 to 0.1 millimeters and a uniformity coefficient of less than 3.5. It shall be washed and free from clay or silt.
(E) Distribution and collection lines. The distribution and collection lines should conform to the requirements for absorption lines as given in division (C) above in the seepage field section. The distribution lines should have a slope of two inches per 100 feet and should be spaced two feet apart, center-to-center. The collection lines should have a slope of six inches per 100 feet and one collection line should be provided for each ten feet of width or fraction thereof. The lower end of the distribution lines should be vented as required in division (G) below and the upper end of the collection lines should be completely plugged.
(F) Bedding material. The bedding material for the distribution and collection lines shall be placed as shown in Figure 8, and shall consist of washed gravel, washed crushed stone or slag. The coarse gravel may range in size from one-fourth inch to one and one-half inches in diameter. The collection lines should be laid directly on the cut, without placing gravel beneath the line.
(G) Venting. Vents should be placed on the downstream end of each of the distribution lines as shown in Figure 4. The vents should extend to, slightly above the ground surface and the outlet should be screened with one-fourth inch mesh screen.
(H) Drainage. Adequate drainage shall be provided to prevent ponding of surface water above the filter, and adequate drainage must also be available to ensure free drainage of the effluent from the filter.
(I) The effluent from a filter shall be chlorinated. Chlorine basins shall be sufficient to retain all liquid for 15 minutes and capacity of chlorinator shall be capable of feeding six parts per million (four ounces of 70% hypochlorite per 1,000 gallons of flow).
(1975 Code, § 38.38)