Septic tanks must:

1. (1) Have at least two compartments with the first compartment liquid volume equal to one-half to two-thirds of the total liquid volume. This standard may be met by one tank with two compartments or by two single compartment tanks in series.

2. (2) Have the following minimum liquid volumes:

   1. (a) For a single-family residence use Table VII:

      Table VII

      Required Minimum Liquid Volumes of Septic Tanks

      Number of Bedrooms	Required MinimumLiquid Tank Volume in Gallons
      ≤4	1,000
      Each additional bedroom	250

   2. (b) For OSS treating sewage from a residential source, other than one single-family residence, 250 gallons per bedroom with a minimum of 1,000 gallons;
   3. (c) For OSS treating sewage from a nonresidential source, three times the design flow.
3. (3) Comply with chapter 246-272C WAC.

[Statutory Authority: RCW 43.20.050(3), 43.20.065, chapters 70A.105 and 70A.110 RCW. WSR 24-06-046, § 246-272A-0232, filed 3/1/24, effective 4/1/25. Statutory Authority: RCW 43.20.050. WSR 05-15-119, § 246-272A-0232, filed 7/18/05, effective 7/1/07.]