Septic Tank Design 101 What must first be determined is the waste stream source & strength - ie. laundromat, RV dump site, restaurant, residential home etc. Next, determine the design flow- Historical flow data is the pr method to determine the daily flow rate. In a new development where flow date is not available, design flow is commonly established by the number of bedrooms and/or people using the facility(s) .ie: With residential waste strength sewage assuming 30 ea. 3 bedroom homes @ 120 gallons per bedroom =360 gallons per home. (WA state standards) the design flow would be (10,800 gallons per day) Multiply your that design flow by 1.5-3x. Therefore the recommended tank size would be 16,200-32,400 gallons liquid capacity. Liquid capacity does not account for any required volume above the invert of the outlet. You may also need to consider state or local regulations for total volume requirements. Currently Washington State requires 20% reserve flow. Next we determine the tank configuration- First decide whether the tank will be single or dual compartment. If a dual compartment tank is selected the baffle wall will typically split the tank 2/3 & 1/3 (from the inlet side). Next the size of inlets, outlets and baffle tees (if applicable) are determined. The gravity line running to the septic tank usually dictates the plumbing size in the septic tank. Riser openings (typically 24”, 30” or 48” diameter) are usually spaced 10’ apart along the length of the tank shell. Effluent filters can be added to the outlet of the septic tank (sized per specific manufacturer’s recommendations) for filtering of solids. If there is a potential for standing groundwater; or if the installation will be in an area with a rainy climate and poor draining soils then anti-buoyancy measures must be implemented. The Designer must determine the “worst case scenario” ground water level. Once determined buoyancy calculation and anti-flotation measures can be implemented.
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