Current through Register Vol. 24-18, September 15, 2024
(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.
Statutory Authority: RCW 43.20.050(2) and (3). 09-23-119,
§ 246-272C-0220, filed 11/18/09, effective
12/19/09.
