(1) The design engineer shall specify and the installer shall install only sewage tanks that comply with the requirements of chapter 246-272C WAC, On-site sewage system tanks, and this section.

(2) Sewage tanks must be tested for water tightness after installation at the project site, per requirements of WAC 246-272B-05200. The department shall not issue final approval for a LOSS with a sewage tank that does not pass the water tightness test.

(3) Sewage tanks used in proprietary treatment systems must be sized according to the manufacturer's specifications.

(4) Septic tanks must have:

(a) An effluent screen with a maximum mesh size of one-eighth inch for all applications;

(b) The following minimum liquid volumes:

(i) One thousand gallons per residence for LOSS treating sewage from a residential development where individual lots each have a tank;

(ii) Three times the daily design flow for all other LOSS.

(5) Where proprietary treatment is used, the department may approve other septic tank sizes if justified by the manufacturer.

(6) Sizing of a sewage tank used for hydraulic surge control or where batch treatment occurs must be justified and any effect on treatment must be addressed.

(7) The design engineer shall size the pump chamber so there is sufficient volume, at a minimum, for:

(a) Routine dosing;

(b) Pump submergence;

(c) Scum and sludge storage; and

(d) Emergency storage.

(8) Emergency storage must be provided in the pump chamber or in the LOSS at or before the point at which pumping will stop during a power outage, as follows:

(a) Twenty-four hours of reserve capacity for LOSS with design flow from 3,500 up to and including 14,500 gpd; and

(b) Twelve hours reserve capacity for LOSS with design flow above 14,500 gpd.

(9) No reduction in pump tank reserve volume is allowed with a back-up power generator.

(10) Grease interceptors must have:

(a) A minimum volume of one thousand gallons; or

(b) Two times the daily greywater design flow of the facility served, whichever is greater.