(1) Septic tank compartments. Septic tanks must be designed and constructed with a minimum of two compartments. This standard may be met by one tank with two compartments or by two single compartment tanks in series.

(a) The capacity of the first compartment must accommodate at least one half but no more than two thirds of the total required liquid volume; and

(b) The capacity of the second compartment must accommodate the remaining total required liquid volume.

(2) Septic tank inlets. Septic tank inlets must meet the following:

(a) The inlet sanitary tee or baffle extends at least eight inches downward below the liquid level;

(b) The inlet sanitary tee or baffle extends above the liquid surface at least to the crown of the inlet pipe; and

(c) The invert of the inlet pipe is a minimum of two inches above the invert of the tank outlet.

(3) Septic tank outlets. Septic tank outlets must meet the following:

(a) The outlet sanitary tee or baffle extends below the liquid level at least thirty percent, but not more than forty percent of the liquid depth for tanks with straight vertical sides;

(b) The outlet sanitary tee or baffle extends below the liquid level at least twenty-five percent, but not more than thirty-five percent of the liquid depth in horizontal cylindrical tanks; and

(c) The outlet sanitary tee extends sufficiently to allow scum storage and venting, and to a point not less than one inch from the underside of the top of the tank. The outlet tee may extend into the riser for venting.

(4) Septic tank effluent screens or filters. Septic tanks must be designed and constructed to accommodate effluent screening devices or filters. The department and local health officers are encouraged to evaluate effluent screen or filter use on a case-by-case basis during the on-site sewage system design phase. Specific effluent screen or filter criteria or requirements, if any, are included under chapter 246-272A or 246-272B WAC.

(5) Septic tank intercompartmental wall fittings.

(a) The septic tank must have intercompartmental wall fittings that extend below the liquid level at least:

(i) Thirty percent, but not more than forty percent of the liquid depth for tanks with straight vertical sides; or

(ii) Twenty-five percent, but not more than thirty-five percent of the liquid depth in horizontal cylindrical tanks.

(b) Slots or ports may be used as intercompartmental fittings.

(i) The location of the slot or port must be at the same depth as the bottom of outlet tees or baffles; and

(ii) The opening must have a minimum area of twelve square inches with a minimum vertical dimension of three inches.

(6) Septic tank intercompartmental walls. The septic tank must have intercompartmental walls that:

(a) Restrict solids from moving from one compartment to the other except through the intercompartmental wall fittings; and

(b) Withstand pumping of the adjacent compartment without risking structural damage or functional failure.

(7) Septic tank scum storage. The septic tank must allow air space volume for scum storage of at least ten percent of the liquid volume of the tank. The department may approve an increase or decrease in the air space requirements according to the requirements under WAC 246-272C-0500.

(8) Septic tank length to width ratio.

(a) The length of a septic tank with a liquid capacity less than three thousand gallons must be a minimum of 1.25 times the width.

(b) The length of septic tanks with a liquid capacity greater than or equal to three thousand gallons must be a minimum of 1.5 times the width.

(9) Septic tank liquid capacity depth. Septic tanks must contain a liquid depth of not less than three feet.