Tank, Trench, and Dose Sizing for a Four-Bedroom Home
A designer is preparing plans for a 4-bedroom home with a garbage disposal and a water softener. The local code assigns 120 gpd per bedroom, requires a 1,000-gal tank for up to three bedrooms plus 250 gal for each additional bedroom, and adds 250 gal when a garbage disposal is installed. The designer measured a soil loading rate of 0.45 gpd/ft² for the silty clay loam at the proposed trench bottoms. The plan shows six trenches, each 60 ft long and 3 ft wide, with 1.5-in laterals and 1/8-in orifices spaced 6 ft apart. The pump chamber has 150 gal of usable dose volume below the alarm float. The code requires a dose of at least five times the lateral pipe volume and a minimum 2-ft head at the distal orifice. The homeowner wants to keep the existing 1,250-gal tank to save money.
1. Calculate the minimum tank size and soil absorption area, and evaluate whether the six trenches are adequate.
Read the worked answer
Tank: 1,000 gal for up to three bedrooms plus 250 gal for the fourth bedroom equals 1,250 gal; add 250 gal for the garbage disposal, giving a 1,500-gal minimum. The existing 1,250-gal tank is undersized. Flow: 4 bedrooms × 120 gpd = 480 gpd. Required bottom area: 480 ÷ 0.45 = about 1,067 ft². With 3-ft-wide trenches, required linear length is 1,067 ÷ 3 = about 356 ft. Six 60-ft trenches provide 360 linear ft and 1,080 ft², which is adequate in area, provided the actual trench-bottom soil matches the tested rate and separation requirements are met.
2. Check the dose volume and identify any design deficiency.
Read the worked answer
Lateral pipe volume: 360 ft × approximately 0.092 gal/ft for 1.5-in pipe equals about 33 gal. Five times that is about 165 gal. The proposed usable dose volume is 150 gal, which is below the 165-gal minimum, so the design is deficient. The designer should increase usable pump-chamber volume to at least 165 gal with margin, reduce total lateral length, or use a smaller-diameter lateral if the hydraulics still work. The designer must also verify the 2-ft head at the distal orifice; the dose volume and orifice size together determine whether distribution is uniform.
3. What additional design checks should be made for the garbage disposal and water softener?
Read the worked answer
Confirm whether the local code permits water-softener brine discharge to the onsite system; many jurisdictions prohibit or limit it because brine can affect treatment and soil. If brine is allowed, the tank and soil sizing may need adjustment. For the garbage disposal, verify that the added 250 gal is the correct code allowance and whether additional biochemical oxygen demand or solids loading requires a larger tank or more frequent pumping. Specify pressure distribution with equal orifice size, level laterals, and a dose counter and alarm so the installed system can be verified against the design.