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Effective April 28, 2015 Chapter Four Subsurface Sewage Treatment System Regulations SECTION 17 TYPE I SYSTEMS 17.1 Type I Systems. Systems designed according to this Section are considered Type I Systems, as defined in Section 2.2 (132). Systems in soils with a loading rate less than 0.45 gallons per day per square foot must not be used in a Type I system for new construction. 17.2 Trenches and Seepage Beds (1) Characteristics. To qualify as a trench or seepage bed system, the system must meet or exceed the following requirements: (A) Employ flow values in Section 12. (B) Meet or exceed applicable technical requirements of Section 13, Section 14, and Section 15. (C) Meet or exceed the requirements of Sections 16.2 and 16.3. (2) General. Seepage bed placement must be limited to areas having natural slopes of less than six (6) percent. Absorption areas for seepage beds must not be placed in soils with a loading rate of less than 0.45 gallons per day per square foot or as shown in Table VI or VIa. Seepage beds must not be located in floodplains. (3) Sizing of Trenches and Seepage Beds (A) The trench bottom absorption area is calculated by dividing the design flow by the appropriate soil loading rate in Table VI or Table VIa. (B) If gravity distribution is used in seepage beds, the seepage bed absorption area is calculated by dividing the design flow by the soil loading rate and then multiplying that value by 1.5. (C) If pressure distribution is used in seepage beds, the seepage bed absorption area is determined by dividing the design flow by the soil loading rate in Table VI or Table VIa. (D) The minimum sidewall absorption is six (6) inches. The bottom absorption area is allowed to be reduced, for gravity distribution trenches only, by the following: Sidewall Absorption- Sizing Inches Equivalent 12-17 20% 18-23 34% 24 40% (E) A 40 percent reduction is not allowed with a loading rate of 1.2 gallons per day per square foot. (4) Design and Construction of Trenches and Seepage Beds (A) Trenches must be no more than 36 inches wide. Any excavation wider than 36 inches is a seepage bed. A seepage bed must not be wider than 12 feet if gravity distribution is used and 25 feet if pressure distribution is used. Natural, undisturbed soil must exist between multiple trenches and seepage beds. Multiple seepage beds must be spaced at one-half the bed width. (B) A vertical inspection pipe at least four inches in diameter must be installed and secured in the distribution medium of every trench and seepage bed. The inspection pipe must be located at an end opposite from where the sewage tank effluent enters the medium. The inspection pipe must have three -eighths inch or larger perforations spaced vertically no more than six inches apart. At least two perforations must be located in the distribution medium. Perforations must not be located above the geotextile cover or wrap. The inspection pipe must extend to the bottom of the distribution medium, be secured, and be capped flush with or above finished grade. 67 Washington County Development Code