Greywater irrigation fields for Tier 2 and Tier 3 systems must be designed to meet the following requirements:
Irrigation area (square feet) | = | Greywater volume (gallons per week) divided by | ||
Evapotranspiration x Plant Factor x 0.62 |
Where: | |||
Evapotranspiration (ET) | = | The monthly average of May through September ET rates in inches divided by four, as determined by the Washington State University, State of Washington Irrigation Guide, 1985 (as amended 1990; 1992 for select western Washington crops), or weekly averages based on actual conditions; | |
Plant Factor | = | 0 to 0.3 for low water use plants; 0.4 to 0.6 for average water use plants; and 0.7 to 1.0 for high water use plants; | |
0.62 | = | The conversion factor (from inches of ET to gallons per week) |
Table II Soil Type Description and Maximum Hydraulic Loading Rate
Soil Type | Soil Textural Classification Description | Loading Rate for Greywater gal./sq. ft./day |
1 | Gravelly and very gravelly coarse sands, all extremely gravelly soils excluding soil types 5 and 6, all soil types with greater than or equal to 90% rock fragments. | Not suitable without augmentation 1.0 with augmentation |
2 | Coarse sands. | Not suitable without augmentation |
1.0 with augmentation | ||
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 |
Sieve Size | Percent Passing |
3/8 | 100 |
No. 4 | 95 - 100 |
No. 10 | 75 - 90 |
No. 40 | 25 - 40 |
No. 100 | 4 - 10 |
No. 200 | 2 - 5 |
Wash. Admin. Code § 246-274-415
Statutory Authority: RCW 90.46.015. 11-02-011, § 246-274-415, filed 12/28/10, effective 7/31/11.