WAC 246-272A-0234   Design requirements -- Soil dispersal components.  (1) All soil dispersal components, except one using a subsurface dripline product, shall be designed to meet the following requirements:

     (a) Maximum hydraulic loading rates shall be based on the rates described in Table VIII;


TABLE VIII

     Maximum Hydraulic Loading Rate
Soil Type Soil Textural Classification Description Loading Rate for Residential Effluent Using Gravity or Pressure Distribution

gal./sq. ft./day
1 Gravelly and very gravelly coarse sands, all extremely gravelly soils excluding soil types 5 & 6, all soil types with greater than or equal to 90% rock fragments. 1.0
2 Coarse sands. 1.0
3 Medium sands, loamy coarse sands, loamy medium sands. 0.8
4 Fine sands, loamy fine sands, sandy loams, loams. 0.6
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
6 Other silt loams, sandy clay loams, clay loams, silty clay loams. 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. Not suitable

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

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

     (ii) 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.

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

     (d) Soil dispersal components having daily design flow between one thousand and three thousand five hundred gallons of sewage per day shall:

     (i) Only be located in soil types 1-5;

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

     (iii) Have pressure distribution including time dosing.

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

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

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

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

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

     (c) Timed dosing; and

     (d) Soil dispersal components having daily design flows greater than one thousand gallons of sewage per day may:

     (i) Only be located in soil types 1-5;

     (ii) Only be located on slopes of less than thirty percent, or seventeen degrees.

     (3) All SSAS shall meet the following requirements:

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

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

     (c) Beds are only designed in soil types 1, 2, 3 or in fine sands with a width not exceeding ten feet;

     (d) Individual laterals greater than one hundred feet in length must use pressure distribution;

     (e) A layer of between six and twenty-four inches of cover material; and

     (f) Other features shall conform with the "On-site Wastewater Treatment Systems Manual," United States Environmental Protection Agency EPA-625/R-00/008 February 2002 (available upon request to the department) except where modified by, or in conflict with this section or local regulations.

     (4) For SSAS with drainrock and distribution pipe:

     (a) A minimum of two inches of drainrock is required above the distribution pipe;

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

     (5) The local health officer may allow the infiltrative surface area in a SSAS to include six inches of the SSAS sidewall height when meeting the required absorption area where total recharge by annual precipitation and irrigation is less than twelve inches per year.

     (6) The local health officer may permit systems consisting solely of a septic tank and a gravity SSAS in soil type 1 if all the following criteria are met:

     (a) The system serves a single-family residence;

     (b) The lot size is greater than two and one-half acres;

     (c) Annual precipitation in the region is less than twenty-five inches per year as described by "Washington Climate" published jointly by the Cooperative Extension Service, College of Agriculture, and Washington State University (available for inspection at Washington state libraries);

     (d) The system is located outside the twelve counties bordering Puget Sound; and

     (e) The geologic conditions beneath the dispersal component must satisfy the minimum unsaturated depth requirements to groundwater as determined by the local health officer. The method for determination is described by "Design Guideline for Gravity Systems in Soil Type 1" (available upon request to the department).

     (7) The local health officer may increase the loading rate in Table VIII up to a factor of two for soil types 1-4 and up to a factor of 1.5 for soil types 5 and 6 if a product tested to meet treatment level D is used. This reduction may not be combined with any other SSAS size reductions.

     (8)(a) The primary and reserve areas must be sized to at least one hundred percent of the loading rates listed in Table VIII.

     (b) However, the local health officer may allow a legal lot of record created prior to the effective date of this chapter that cannot meet this primary and reserve area requirement to be developed if all the following conditions are met:

     (i) The lot cannot meet the minimum primary and reserve area requirements due to the loading rates for medium sand, fine sand and very fine sand listed in Table VIII of this chapter;

     (ii) The primary and reserve areas are sufficient to allow installation of a SSAS using maximum loading rates of 1.0 gallons/square foot per day for medium sand, 0.8 gallons/square foot/day for fine sand, and 0.6 gallons/square foot/day for very fine sand; and

     (iii) A treatment product meeting at least Treatment Level D and pressure distribution with timed-dosing is used.



[Statutory Authority: RCW 43.20.050. 05-15-119, § 246-272A-0234, filed 7/18/05, effective 7/1/07.]