Section 24.17.140 Design.

24.17.140 Design.

(1) Only professional engineers, licensed under Chapter 18.43 RCW, or on-site sewage treatment system designers, licensed under Chapter 18.210 RCW, may design on-site sewage systems.

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

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

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

(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, the operating capacity is based on forty-five (45) gpd per capita with two (2) people per bedroom. The minimum design flow per bedroom per day is the operating capacity of ninety (90) gallons multiplied by one and one-third (1.33) to account for a thirty-three (33) percent surge capacity. This results in a minimum design flow of one hundred twenty (120) gallons per bedroom per day. The local health officer may require a factor greater than thirty-three (33) percent to account for surge capacity. The minimum design flow is two hundred forty (240) gallons per day. The minimum design flow of the OSS is two hundred forty (240) gallons per day. The health officer may require an increase of the design flow for dwellings with anticipated greater flows, such as larger dwellings; or

(ii) For single-family residences with one additional dwelling served by the same OSS:

(A) All requirements in subsection (2)(d)(i) of this section apply;

(B) The minimum design flow for one (1) additional dwelling is one hundred twenty (120) gallons per bedroom; and

(iii) For three (3) or more dwellings served by the same OSS:

(A) All requirements in subsection (2)(d)(i) of this section apply;

(B) The minimum design flow for the first dwelling is two hundred forty (240) gallons per day;

(D) The local health officer may require an increase of the design flow for dwellings with anticipated greater flows; and

(E) The local health officer shall require documentation including, but not limited to, an easement, covenant, contract, or other legal document authorizing access for construction, operation, maintenance, and repair; or

(iv) For other facilities, the design flows noted in the On-Site Wastewater Treatment Systems Manual, EPA 625/R-00/008, February 2002 must 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.

(v) All food establishments submitting designs for on-site systems are required to have grease traps or grease interceptors included in the design unless justified by the designer and approved by CCPH.

(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 Section 24.17.060;

(D) Nitrogen Contributions. Where nitrogen has been identified as a contaminant of concern by the local management plan required in Section 24.17.060, it shall be addressed through lot size and/or treatment.

(ii) When proposing the use of an OSS for nonresidential sewage, the designer shall provide to the health officer:

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

(B) Information to establish the sewage’s strength and identify chemicals found in the sewage that are not found in residential sewage; and

(iii) 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.

(iv) Treatment Levels.

(A) Requirements for matching the 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 WAC 246-272A-0110. The method of distribution applies to the soil dispersal component.

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

(I) BL1 or BL2 in Type 1 soils; or

(II) BLA.

Table VI—Treatment Component Performance Levels and Method of Distribution1
Vertical Separation in Inches	Soil Type
Vertical Separation in Inches	1	2	3—6
12 < 18	A & BL1—pressure with timed dosing	B & BL2—pressure with timed dosing	B & BL2—pressure with timed dosing
≥ 18 < 24	B & BL2—pressure with timed dosing	C & BL3—pressure with timed dosing	C & BL3—pressure with timed dosing
≥ 24 < 36	B & BL2—pressure with timed dosing	C & BL3—pressure with timed dosing	E—pressure with timed dosing
≥ 36 < 60	B & BL2—pressure with timed dosing	E—pressure	E—gravity
≥ 60	C & BL2—pressure	E—gravity	E—gravity

1 The treatment component performance levels correspond with those established for treatment components under the product testing requirements in WAC 246-272A-0110.

(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 health officer:

(a) Shall approve only OSS designs meeting the requirements of this chapter;

(b) Shall not approve designs for:

(i) Cesspools; or

(ii) Seepage pits.

(c) 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.

(5) Septic tanks shall:

(a) Have at least two (2) compartments with the first compartment liquid volume equal to one-half (1/2) to two-thirds (2/3) of the total required liquid volume. This standard may be met by one (1) tank with two (2) compartments or by two (2) single-compartment tanks in series. The health officer may consider variances from these requirements when requested.

(b) Have the following minimum liquid capacities:

(i) For a single-family residence, use Table VII—Required Minimum Liquid Volumes of Septic Tanks:

Table VII—Required Minimum Liquid Volumes of Septic Tanks
Number of Bedrooms	Required Minimum Liquid Tank Volume in Gallons
≤ 4	1,000
Each additional bedroom	250

(ii) For an OSS treating sewage from a residential source, other than one (1) single-family residence, two hundred fifty (250) gallons per bedroom with a minimum of one thousand (1,000) gallons;

(iii) For an OSS treating sewage from a nonresidential source, three (3) times the design flow.

(6) All pump chambers, except pump basins, must be designed to meet the following requirements:

(a) Have a minimum volume of one thousand (1,000) gallons;

(b) Provide an internal volume to account for the design flow, full-time pump submergence, space for sludge accumulation below the pump inlet and emergency storage volume of at least seventy-five (75) percent of the design flow;

(c) Follow any applicable DS&G or proprietary product design manual for all OSS components included in the pump chamber; and

(d) Comply with Chapter 246-272C WAC.

(e) For the purposes of this section, “pump basin” means a water-tight receptacle that contains a pump to convey sewage from a limited use area that is separate from the main wastewater sewer pipe leaving a structure, to the main treatment component of an OSS; typically much smaller than a pump chamber and separate from the main sewer pipe due to elevation restrictions. Pump basins are intended for limited, specialized uses, and not intended as replacement or substitute for a pump chamber. Pump basin must be in compliance with Chapter 246-272C WAC.

(7) Design Requirements—Soil Dispersal Components.

(a) All soil dispersal components, except one using a subsurface dripline product, shall be designed to meet the following requirements:

(i) Maximum hydraulic loading rates shall be based on the rates described in Table VIII.

Table VIII—Maximum Hydraulic Loading Rates
		Column A	Column B
Soil Type	Soil Textural Classification Description	Loading Rate for Residential Septic Tank Effluent Using Gravity or Pressure Distribution gal./sq. ft./day	Loading Rate for Residential Effluent Meeting Treatment Level C & BL3 or Higher Effluent Quality Using Pressure Distribution gal./sq. ft./day
1	Gravelly and very gravelly coarse sands, all extremely gravelly soils excluding those with soil types 5 & 6 as the nongravel portion, all soil types with greater than or equal to 90% rock fragments.	1.0	1.2
2	Coarse sands.	1.0	1.2
3	Medium sands, loamy coarse sands, loamy medium sands.	0.8	1.0
4	Fine sands, loamy fine sands, sandy loams, loams.	0.6	0.8
5	Very fine sands, loamy very fine sands; or silt loams, sandy clay loams, clay loams and silty clay loams with a moderate structure or strong structure (excluding a platy structure).	0.4	0.56
6	Other silt loams, sandy clay loams, clay loams, silty clay loams.	0.2	0.2
7	Sandy clay, clay, silty clay and strongly cemented firm soils, soil with a moderate or strong platy structure, any soil with a massive structure, any soil with appreciable amounts of expanding clays.	Unsuitable	Unsuitable

(ii) Calculation of the absorption area is based on:

(A) The design flow in subsection (2) of this section; and

(B) Loading rates equal to or less than those in Table VIII applied to the infiltrative surface of the soil dispersal component or the finest textured soil within the vertical separation selected by the designer, whichever has the finest texture. The installer shall submit documentation that fill material conforms to required sieve specifications.

(iii) Requirements for the method of distribution shall correspond to those in Table VI.

(iv) Soil dispersal components having daily design flow between one thousand (1,000) and three thousand five hundred (3,500) gallons of sewage per day shall:

(A) Only be located in soil types 1 through 5;

(B) Only be located on slopes of less than thirty (30) percent, or seventeen (17) degrees; and

(b) The local health officer may allow the maximum hydraulic loading rates in Table VIII of this section. Loading rates identified in Column B must not be combined with any dispersal component size reductions.

(c) All soil dispersal components using a subsurface dripline product must be designed to meet the following requirements:

(i) Calculation of the absorption area is based on:

(A) The design flow in WAC 246-272A-0230(2);

(B) Loading rates that are dependent on the soil type, other soil and site characteristics, and the spacing of dripline and emitters;

(ii) The dripline must be installed a minimum of six (6) inches into original, undisturbed soil;

(iii) Timed dosing; and

(iv) An OSS having daily design flows greater than one thousand (1,000) gallons of sewage per day:

(A) May only be located in soil types 1 through 5;

(B) May only be located on slopes of less than thirty (30) percent, or seventeen (17) degrees; and

(d) All SSASs shall meet the following requirements:

(i) The infiltrative surface may not be deeper than three (3) feet below the finished grade, except under special conditions approved by the health officer. The depth of such system shall not exceed ten (10) feet from the finished grade;

(ii) A minimum of six (6) inches of sidewall must be located in original, undisturbed soil;

(iii) SSAS beds are only designed in soil types 1, 2 and 3 or in fine sands with a width not exceeding ten (10) feet. Gravity beds must have a minimum of one (1) lateral for every three (3) feet in width;

(iv) A layer of between six (6) and twenty-four (24) inches of cover material;

(v) Individual SSAS laterals greater than one hundred (100) feet in length are to use pressure distribution;

(vi) Other features shall conform with the On-Site Wastewater Treatment Systems Manual, USEPA, EPA-625/R-00/008, February 2002, or later version except where modified by, or in conflict with, this section or local regulations.

(e) CCPH does not allow infiltrative surfaces to be deeper than three (3) feet below the finished grade.

(f) CCPH does not permit systems consisting solely of a septic tank and gravity SSAS in soil type 1.

(g) For an SSAS with drainrock and distribution pipe:

(i) A minimum of two (2) inches of drainrock is required above the distribution pipe; and

(ii) A minimum of six (6) inches of drainrock below the distribution pipe; and

(iii) The sidewall below the invert of the distribution pipe is located in original undisturbed soil.

(h) Both the primary and reserve areas must be sized at least one hundred (100) percent of the approved loading rates. The local health officer may require the sizing of the reserve area using the loading rate in Table VIII of this section. Column A must be used when sizing the reserve area, if the primary area is sized using Column B.

(i) When creating a new lot, all OSS components including the reserve drainfield area must be located on the same parcel as the facility the OSS is designed to serve.

(j) Designs must meet CCPH standards as defined in policies and procedures, and include:

(i) An arrow indicating north.

(ii) Buildings, when the location is known, shall be accurately shown. When a precise proposed building location is unknown, a prescribed building boundary line, or building envelope, that designates the area within which buildings will be placed, and within which OSS components may not be placed, shall be shown in lieu of precise building locations.

(iii) Septic tanks, pump chambers, sewage tanks, any existing OSS.

(iv) Vertical cross-section drawings showing the depth of the disposal component, the vertical separation, the depth of soil cover, and other OSS components constructed at the site.

(v) Pump specification and pump curve.

(vi) Elevations of pump and floats.

(vii) Sewer lines.

(viii) Initial system and reserve area.

(ix) Soil log locations.

(x) Wells, water lines, other utility lines and any abandoned wells.

(xi) Buildings, driveways and parking areas.

(xii) Surface water and/or known wetland buffers as shown by wetland delineation.

(xiii) Property lines, lines of easement and buffers.

(xiv) The location of existing and proposed encumbrances affecting system placement, including legal access documents if any component of the OSS is not on the lot where the sewage is generated.

(xv) Ditches, interceptor drains, footing drains, roof drain conveyance pipes.

(xvi) Location and percent of slopes such as cuts, banks or fills.

(xvii) General topography and/or slope of the site shown in sufficient detail that accurately represents simple and complex slope configurations.

(k) The OSS site must be staked for design review.

(8) Design Requirements—Facilitate Operation, Monitoring and Maintenance.

(a) The OSS must be designed to facilitate routine operation, monitoring and maintenance according to the following criteria:

(i) Requires risers to grade with secured water tight lids.

(ii) For all systems, service access and monitoring ports at finished grade are required for all system components. Specific component requirements include:

(A) The building sewer must have a cleanout with a screw cap for service access;

(B) Septic tanks must have secured service access manholes and monitoring ports for the inlet and outlet. Effluent filters must have access to finished grade;

(D) Other pretreatment units (such as aerobic treatment units and packed-bed filters) must have secured service access manholes and monitoring ports;

(E) Pump chambers, tanks and vaults must have secured service access manholes;

(F) Disinfection units must have service access and be installed to facilitate complete maintenance and cleaning, including an easy access, freefall sampling port; and

(G) Soil dispersal components shall have monitoring ports for both distribution devices and the infiltrative surface.

(iii) For systems using pumps, clearly accessible controls and warning devices are required including:

(A) All control panels and alarms must be located on the exterior of the facility;

(B) Process controls such as float- and pressure-activated pump on/off switches, pump-run timers and process flow controls;

(C) Diagnostic tools including dose cycle counters and hour meters on the sewage stream, or flow meters on either the water supply or sewage stream; and

(D) Audible and visual alarms designed to alert a resident of a malfunction. The alarm must be placed on a circuit independent of the pump circuit.

(iv) For conventional gravity systems with lift pumps, only audible alarms are required.

(v) If effluent filters are used, CCPH requires an extension handle for removal of outlet baffle filter not greater than eighteen (18) inches from the lid of the riser.

(b) All accesses must be designed to allow for monitoring and maintenance and shall be secured to minimize injury or unauthorized access in a manner approved by the health officer. (Sec. 15 of Ord. 2007-10-01; amended by Sec. 11 of Ord. 2021-11-05; amended by Sec. 12 of Ord. 2025-03-12)