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IV. MOUND SYSTEM DESIGN CRITERIA SSS§ 3.7. The design of the pressurized mound disposal system for this project is based upon criteria for efficient operation, disposal field longevity, and environmental protection. An overview, of the system operation is as follows: Influent waste from the two proposed residential structures will gravity through the structure's footing in a four-inch tightline into a two- compartment 2,000 gallon precast standard septic tank. One tank will be situated atop each respective building pad. Effluent from each respective tank will gravity down from the building pad and into a 1,200 gallon, two compartment pump chamber tank. The two pump chamber tanks are to be installed in reverse so that the larger front compartment is the actual pump (dose) chamber that houses the Filtered Pump Vault (Orenco P/N PVU57-1819) and an alternating duplex pump system for the effluent to be disposed of through the mound system and into the native soils. Regarding domestic wastewater volume generation, the USEPA (Ref. 11) references the average daily residential flow volumes are approximately 70 gallons per day per capita (gpdpc), and current practice is to assume that maximum occupancy is two people per bedroom. However, as the number of bedrooms increases within a structure, this flow volume calculation becomes exorbitant. For example, wastewater flow volumes for a 5 bedroom house: 70 gpdpc x 2 people per bedroom x 5 bedrooms = 700 gpd. This translates to 29 gallons of wastewater generated every hour of every day for any given year. Per the EHD Onsite Wastewater Treatment Systems Standards, Section 9.10.3, the design of a mound system is to conform to the EPA Design Manual - Onsite Wastewater Treatment and Disposal Systems. However, the EHD has promulgated the minimum size of the absorption area, which is to be 4,000 ft2 for a 3 bedroom structure and 1,000 ft2 for each bedroom above 3. Therefore, the absorption area for this project will be 6,000 ft2 for the primary residential structure and 6,000 ft2 for the accessory unit dwelling. A list of all parts and ancillary components to construct the mound systems, in addition to this report, will be provided to the contractor-of-record. A. SEPTIC TANKS AND PUMP CHAMBER (DOSING) TANKS Septic tanks store the accumulated sludge, scum and effluent (clear zone) volumes, which in turn affect the treatment of nitrogen within the tanks. The method to size the septic tanks is discussed below and excludes beneficial treatment of nitrogen that may occur in the pump chamber tank: For a pump out interval of every 3 years, tank volume V = 2.8 0 ave x Peaking Factor. The peaking factor is typically taken to be 1.5 based on a "peak day" since it is assumed the majority of flow will occur on any given day of the week. 8 Chesney Consulting