(1) On-site sewage systems may only be designed by professional engineers, licensed under chapter 18.43 RCW or on-site sewage treatment system designers, licensed under chapter 18.210 RCW, except:

(a) If at the discretion of the local health officer, a resident owner of a single-family residence not adjacent to a marine shoreline is allowed to design a system for that residence; or

(b) If the local health officer performs the soil and site evaluation, the health officer is allowed to design a system.

(2) The designer shall use the following criteria when developing a design for an OSS:

(a) All sewage from the building served is directed to the OSS;

(b) Sewage tanks have been reviewed and approved by the department;

(c) Drainage from the surface, footing drains, roof drains, subsurface stormwater infiltration systems, and other nonsewage drains is prevented from entering the OSS, the area where the OSS is located, and the reserve area;

(d) The OSS is designed to treat and disperse the sewage volume as follows:

(i) For single-family residences:

(A) The operating capacity is based on 45 gpd per capita with two people per bedroom.

(B) The minimum design flow per bedroom per day is the operating capacity of ninety gallons multiplied by 1.33. This results in a minimum design flow of one hundred twenty gallons per bedroom per day.

(C) A factor greater than 0.33 to account for surge capacity may be required by the local health officer.

(D) The local health officer may require an increase of the design flow for dwellings with anticipated greater flows, such as larger dwellings.

(E) The minimum design flow is two hundred forty gallons per day.

(ii) For other facilities, the design flows noted in "On-site Wastewater Treatment Systems Manual," USEPA, EPA-625/R-00/008, February 2002 (available upon request to the department) shall be used. Sewage flows from other sources of information may be used in determining system design flows if they incorporate both an operating capacity and a surge capacity.

(e) The OSS is designed to address sewage quality as follows:

(i) For all systems, the designer shall consider:

(A) CBOD5, TSS, and O&G;

(B) Other parameters that can adversely affect treatment anywhere along the treatment sequence. Examples include pH, temperature and dissolved oxygen;

(C) The sensitivity of the site where the OSS will be installed. Examples include areas where fecal coliform constituents can result in public health concerns, such as shellfish growing areas, designated swimming areas, and other areas identified by the local management plan required in WAC 246-272A-0015.

(D) Nitrogen contributions. Where nitrogen has been identified as a contaminant of concern by the local management plan required in WAC 246-272A-0015, it shall be addressed through lot size and/or treatment.

(ii) For OSS treating sewage from a nonresidential source, the designer shall provide the following information:

(A) Information to show the sewage is not industrial wastewater;

(B) Information regarding the sewage quality and identifying chemicals found in the sewage that are not found in sewage from a residential source; and

(C) A site-specific design providing the treatment level equal to that required of sewage from a residential source;

(f) The vertical separation to be used to establish the treatment levels and application rates. The selected vertical separation shall be used consistently throughout the design process.

(g) Treatment levels:

(i) Requirements for matching treatment component and method of distribution with soil conditions of the soil dispersal component are listed in Table VI. The treatment levels correspond with those established for treatment components under the product performance testing requirements in Table III of WAC 246-272A-0110. The method of distribution applies to the soil dispersal component.

(ii) Disinfection may not be used to achieve the fecal coliform requirements to meet:

(A) Treatment levels A or B in Type 1 soils; or

(B) Treatment level C.

(3) The coarsest textured soil within the vertical separation selected by the designer shall determine the minimum treatment level and method of distribution.

(4) The local health officer shall not approve designs for:

(a) Cesspools; or

(b) Seepage pits.

(5) The local health officer may approve a design for the reserve area different from the design approved for the initial OSS, if both designs meet the requirements of this chapter for new construction.