KMAC Engineering
N.Y.S. Licensed Professional Engineer
(K. Alfred Chu, PE)

Individual Residential Wastewater Treatment System

Design flows for various household plumbing fixtures:

Water saving fixtures (post 1991) less than 1.0 gal per flush water closets and 3.0 gpm faucets/showerheads - design flow = 90 GPD per bedroom.

Required separation distances from wastewater system components:

System Components........Wells...........Dwelling

Septic tank......................50ft.............10ft

Distribution box..............100ft............20ft

Absorption field..............100ft*...........20ft

*when sewage treatment systems are located in coarse gravel or upgrade and in the general path of drainage to a well, the closest part of the treatment system shall be at least 200 feet away from the well.

Minimum septic tank capacities:

1,2, or 3 bedrooms.........................1000 gallons

4 bedrooms....................................1250 gallons

Site requirements for design of individual wastewater treatment systems:

Absorption field system -

• 24"- 30" depth of percolation test hole.
• 4 feet minimum depth of in situ usable soil.
• 1-60 (minutes/inch) percolation rate.
• 0-15% allowable slope of site.

Required length of absorption trench (based upon 2 ft. wide trench):

e.g. A 4-bedroom home with 600 gal/day flow rate and 21-30 (min/inch) percolation rate requires 500 feet of trench.

Absorption trenches are constructed 18 to 30 inches below grade.

A minimum horizontal separation distance between parallel absorption trenches of 10', 9', 8' or 7' is provided when minimum vertical separation distance from the bottom of any absorption trench to high ground water, bedrock, or impermeable soil is 2', 3', 4' or 5', respectively.

Well

• 125 to 250 feet deep.
• recovery rate 3 to 5 gpm (gallon per minute).
• pressure switch cuts in at 20 psi and cuts out at 40psi (30/50; 40/60).
• pressure tank should withstand 100psi. (4.5 gallons for a small cottage - 2 people / 1 bath)
• pressure relief valve at 75 psi.
• pump capacity = GPD (gallon per day) / 2.