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"The design basis is 75 gal per person and 150 gal per bedroom.

^Includes provision for home garbage grinder and laundry machine. Larger than minimum size septic tank is strongly recommended. cSee detail drawings for construction specifications.

''Based on the results of soil percolation tests. Discharge all kitchen, bath, and laundry wastes through the septic tank, but exclude roof and footing drainage, surface and groundwater, and softener wastes.

"The design basis is 75 gal per person and 150 gal per bedroom.

^Includes provision for home garbage grinder and laundry machine. Larger than minimum size septic tank is strongly recommended. cSee detail drawings for construction specifications.

''Based on the results of soil percolation tests. Discharge all kitchen, bath, and laundry wastes through the septic tank, but exclude roof and footing drainage, surface and groundwater, and softener wastes.

FIGURE 3.3 Details for small septic tanks. Recommended construction for small septic tanks: Top—Reinforced concrete poured 3- to 4-in. thick with two 3/8-in. steel rods per ft, or equivalent, and a 20- x 20-in. manhole over inlet, or precast reinforced concrete 1-ft slabs with sealed joints. Bottom—Reinforced concrete 4in. thick with reinforcing as in "top" or plain poured concrete 6in. thick. Walls—Reinforced concrete poured 4in. thick with 3/8-in. steel rods on 6-in. centers both ways, or equivalent; plain poured concrete 6in. thick; 8-in. brick masonry with 1-in. cement plaster inside finish and block cells filled with mortar. Concrete mix—One bag of cement to 2.25 in. of sand to 3 ft3 of gravel with 5 gal of water for moist sand. Use 1:3 cement mortar for masonry and 1:2 mortar for plaster finish. Gravel or crushed stone and sand shall be clean, hard material. Gravel shall be 0.25 to 0.5 in. in size; sand from fine to 0.25 in. Bedding—At least 3 in of sand or pea gravel, leveled.

FIGURE 3.3 Details for small septic tanks. Recommended construction for small septic tanks: Top—Reinforced concrete poured 3- to 4-in. thick with two 3/8-in. steel rods per ft, or equivalent, and a 20- x 20-in. manhole over inlet, or precast reinforced concrete 1-ft slabs with sealed joints. Bottom—Reinforced concrete 4in. thick with reinforcing as in "top" or plain poured concrete 6in. thick. Walls—Reinforced concrete poured 4in. thick with 3/8-in. steel rods on 6-in. centers both ways, or equivalent; plain poured concrete 6in. thick; 8-in. brick masonry with 1-in. cement plaster inside finish and block cells filled with mortar. Concrete mix—One bag of cement to 2.25 in. of sand to 3 ft3 of gravel with 5 gal of water for moist sand. Use 1:3 cement mortar for masonry and 1:2 mortar for plaster finish. Gravel or crushed stone and sand shall be clean, hard material. Gravel shall be 0.25 to 0.5 in. in size; sand from fine to 0.25 in. Bedding—At least 3 in of sand or pea gravel, leveled.

FIGURE 3.4 Details for large septic tanks. (See Table 3.6.)

Care of Septic Tank and Subsurface Absorption Systems

A septic tank for a private home will generally require cleaning every 3 to 5 years, depending on occupancy, but in any case, it should be inspected once a year. If a garbage disposal unit is used, more frequent cleaning is needed. Septic tanks serving commercial operations should be inspected at least every 6 months. When the depth of settled sludge or floating scum approaches the depth given in Table 3.7, the tank needs cleaning.18 Sludge accumulation in a normal home septic tank has been estimated at 69 to 80 liters (18 to 21 gallons) per person per year.19 The appearance of particles or scum in the effluent from a septic tank going through a distribution box is also an indication of the need for cleaning. Routine inspection and cleaning will prevent solids from being carried over and clogging leaching systems.

Septic tanks are generally cleaned by septic tank-cleaning firms. Sludge sticking to the inside of a tank that has just been cleaned would have a seeding effect and assist in renewing the bacterial activity in the septic tank. The septic tank

TABLE 3.7 Allowable Sludge Accumulation (in.)

Tank Capacity (gal) Sludge Depth (in.)

Tank Liquid Depth (in.)

Tank Capacity (gal) Sludge Depth (in.)

Tank Liquid Depth (in.)

TABLE 3.7 Allowable Sludge Accumulation (in.)

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