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,oJ r SEWAGE TREATMENT - SMALL SYSTEMS-5 r;o. '0 : (,i f! 6.VIT. CLAY PIPE ~ . .c - ~ WALLS MAY BE OF BRICK, CONCRETE OR CONCRElE BLOCK ~ 6. VI T. CLAY PIPE SECTION . ~ PLAN DETA'L S 4.VIT.CLAY PER- FORATED PIPE DISTRIBUTOR BROKEN STON OR GRAVEL SECT\ON THRU SUBSURFACE T\LE SYSTEM (,oy be squore,circular or recta GIVEN: A proposed day school will have a total population of pupils on ochers equal to 250. The school will be equipped wi1tl showers. Several test holes have been dug at the site of the sewage treatment facilities and percolation tests indicate the the time for water to fall I inch equal to 5 minutes. TO FINO: The size of 0 septic tank to serve the school and the area of a subsurface tile system required. As an alternate to a subsurface tile system, a subsurface sand filter will be designed, assuming that a nearby stream is available for disposal of the filter effluent. SOLUTION: Total daily sewage flow = 250 " 20 gals. per day = 5000 gals. per day. Septic tank: The volume of a septic tank = 4875 gals. (see curve, page 19 -33 ) Assuming a tank water depth of 6 ft. .. tonk area will equal 7~~~ = 108 sq. feet. Assuming a ratio of lenoth to width of approx. 3 , tank win be 18 feet long by 6 feet wide, with depth of 6 feet . Subsurface tile system: Subsurface tile system required = 5000 gals. per day = 2083 sq. ft. 2.4 gals. per day per sq. ft. Assume width of trench = 24 inches (see table, page 19-34) Length of subsurface tile system = ~ == 1042 feet. Since length of tile is greater than 500 feet, use a dosing tank at end of septic took and provide 2 alternating siphons, each to dose t- of entire tile field. . Dosino tank: Assuming volume of dosing tank to fill tile 15% full and assumino 4 inch dia. tile, Volume of dosing tank = 0:75~ 1042 ~ 0.087 sq. feet (area of 4 inches) == 68 cu. feet. W;t:idth of 6 ft. (septic tonk width) and 0 s;phon drowdown of 16 In, . length of dosing tonk = = ~ = 8.5 fl '. . 5000 9015. per day Suhsurface sand fIlter: Area required ;;: I 15 I d ft = 4348 SQ, ft. _ . gas. per ay per SQ. .