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Septic Tank Sizing for the 2,000 Gallon Tank: V = 1,125 + 0.75Q. V = Net Volume of Tank, Q = Average Daily Flow (ADF). V = 1,125 + 0.75(600 gpd) = 1,575 gals. Detention Time - 2,000 gal tank: Each 2,000 gal tank for each residential structure: 2,000 gal total tank capacity x 0.70 (30% volume lost to sludge/scum accumulation) 1.5 (Peaking factor) x 600 gals/day (ADF) = 1.6 days. The minimum acceptable detention time is 1.0 day. Therefore, a 2,000 gal tank is adequate. MOUND SYSTEM SITE PREPARATION The following calculations are for the two separate mound systems to serve each residential structure exclusively. General construction and design criteria are also found in the California State Water Resources Control Board - Guidelines for the Design. Installation and Operations of Mound Sewage Disposal Systems, and as referenced, the EPA Design Manual - On Site Wastewater Treatment and Disposal Systems. The location of the two new mound systems is outlined on the Design Plans and are proposed to be adjacent to the east property line. At the start of mound construction, the native soil/sand media interface must be properly prepared. The native soil basal surface area must be stripped of all vegetation and then ripped preferably with full depth 8-inch farm implement shanks. Soil moisture content at the time of ripping is critical and should not be too wet or too dry. Ripping is to be north to south and only within the area of the sand/native soil interface. After ripping and before emplacement of the sand media, it is imperative that no equipment be driven over this area. DISCUSSION AND CALCULATIONS OF THE MOUND SYSTEM BASAL LOADING RATE The long-term acceptance rate (LTAR) is anticipated to be 0.1 gals/fe/day based upon the perc rate obtained (125 min/in). According to the referenced Section of the EHD OWTS Standards, the basal absorption area is to be 6,000 fe. The acceptance rate, translated to the ADF, is estimated to be: 0.1 gals/fe/day x 6,000 ft2 = 600 gpd. This ADF rate corresponds with an extremely high wastewater generation rate of 25 gals/hr/day, every day of the year. SAND MEDIA / GRAVEL BED SPECIFICATIONS AND LOADING RATE The filter media is a sand material that underlies the drain rock bed. The sand media should be tested by an engineering laboratory, or be certified by the sand supplier before emplacement. A sieve analysis (ASTM C-33 Specifications) should demonstrate the sand media material must not have more than 20% (by wt.) of material greater than 2 mm dia. and no more than 2% (by wt.) passing the No. 200 sieve (washed sand). Effective Diameter (d10 ) should be 0.20 to 0.30 mm with a Uniformity Coefficient (D60/D10 ) of approximately The sand media loading rate complying with the specifications above is 1.6 gpd/ft2. The gravel bed on top of the sand media shall also be tested for conformance with the following size distribution: 3/4 in. to 2.5 in. washed drain rock. A geotextile filter fabric must be placed over the top of the drain rock bed as noted on the Design Plans. 9 Chesney Consulting