Septic Tanks Design And Construction Standards

01. Materials. New septic tanks will be constructed of concrete, or other materials approved by the Director. Steel tanks are unacceptable. (3-31-22) 02. Design. A professional engineer licensed by the state of Idaho must submit all septic tank designs to the Department for approval. If any design submitted for approval does not meet all requirements in Section 007, the engineer must demonstrate that any deviation is determined by sound engineering practice and meets the intent of the rules. (7-1-25) 03. Construction Requirements. All septic tanks will be water tight, constructed of sound, durable materials, and not subject to excessive corrosion, decay, frost damage or cracking. (7-1-25) 04. Concrete Septic Tanks. New concrete septic tanks will at a minimum meet the following requirements: (3-31-22) a. The walls and floor must be at least two and one-half (2 1/2) inches thick if adequately reinforced and at least six (6) inches thick if not reinforced. (3-31-22) b. The concrete lids or covers must be at least three (3) inches thick and adequately reinforced. (7-1-25) c. The floor and at least a six (6) inch vertical portion of the walls of a poured tank must be poured at the same time (monolithic pour). (3-31-22) d. The wall sections poured separately must have interlocking joints on joining edge. (7-1-25) e. All concrete outlet baffles must be finished with an asphalt or other protective coating. (3-31-22) 05. Horizontal Dimension Limit. No interior horizontal dimension of a septic tank or compartment may be less than two (2) feet. (3-31-22) 06. Liquid Depth. The liquid depth must be at least two and one-half (2 1/2) feet. (7-1-25) 07. Manufactured Tank Markings. Septic tanks manufactured in accordance with a specified design approved by the Director, will be legibly and indelibly marked with the manufacturer’s name or trademark, total liquid capacity, and must indicate the tank’s inlet and outlet. (7-1-25) 08. Minimum Tank Capacities. (3-31-22) a. Tanks serving single dwelling units. The minimum tank capacity is one thousand (1,000) gallons. For each bedroom over four (4) in a dwelling unit, add two hundred fifty (250) gallons. (7-1-25) b. Tanks serving all other flows. The minimum tank capacity per structure is one thousand (1,000) gallons or a volume equal to at least two (2) times the maximum daily flow, whichever is greater. (7-1-25) 09. Wastewater Flows from Various Establishments in Gallons per Day. ESTABLISHMENTS DWELLING UNIT IDAHO ADMINISTRATIVE CODE IDAPA 58.01.03 – Individual/Subsurface Sewage Department of Environmental Quality Disposal & Cleaning of Septic Tanks Rules Section 007 Page 13 Single Family Dwelling, Apartment, Mobile Home, 3 bedroom. Add/subtract 50 gallons per day/bedroom 250/Unit MULTIPLE RESIDENTIAL Hotel/Motel: With Private Baths Without Private Baths 60/Bedspace 40/Bedspace Overnight Accommodation: Central Toilet Central Toilet & Shower 25/Person 35/Person Rooming House/Bunk House Staff Resident Nonresident 40/Resident 40/Staff 15/Staff INSTITUTIONAL Church/Assembly Hall/Meeting House: With Kitchen 3/Seat 7/Seat Hospital: Kitchen only Laundry only 250/Bedspace 25/Bedspace 40/Bedspace Nursing Home/Rest Home 125/Bedspace Day School: Without Showers With Showers With Cafeteria, add Staff-Resident Nonresident 20/Student 25/Student 3/Student 40/Staff 20/Staff FOOD SERVICE Conventional Service: Toilet & Kitchen Wastes Kitchen Wastes 13/Meal 3.3/Meal Take Out or Single Service 2/Meal Drinking Establishment 2/Person COMMERCIAL AND INDUSTRIAL Laundry - Self Service 50/Wash Service Station 10/Vehicle Shopping Center (No food/laundry) 1/Pkg.Sp. Theaters 5/Seat ESTABLISHMENTS IDAHO ADMINISTRATIVE CODE IDAPA 58.01.03 – Individual/Subsurface Sewage Department of Environmental Quality Disposal & Cleaning of Septic Tanks Rules Section 007 Page 14 (7-1-25) 10. Total Volume. The total volume of a septic tank at a minimum must be one hundred fifteen percent (115%) of its liquid capacity. (7-1-25) 11. Inlets. (3-31-22) a. The inlet into the tank must be at least four (4) inches in diameter and enter the tank three (3) inches above the liquid level. (7-1-25) b. The inlet of the septic tank and each compartment must be submerged by means of a vented tee or baffle. (7-1-25) c. Vented tees or baffles must extend above the liquid level seven (7) inches or more but not closer than one (1) inch to the lid of the tank. (7-1-25) d. Tees must not extend horizontally into the tank beyond two (2) times the diameter of the inlet. (7-1-25) 12. Outlets. (3-31-22) a. The outlet of the tank must be at least four (4) inches in diameter. (7-1-25) Offices 20/Employee Factories: No Showers With Showers Add for Cafeteria 25/Employee 35/Employee 5/Employee Stores 2/Employee SEASONAL AND RECREATIONAL Swimming Pool: Toilet & Shower Wastes 10/Person Parks & Camps (Day Use): Toilet & Shower Wastes 15/Person Roadside Rest Area: Toilet & Shower Wastes Toilet Waste 10/Person 5/Person Designated Camp Area: Toilet & Shower Wastes Toilet Wastes 90/Space 65/Space Travel Trailer Park with Sewer & Water Hook-up 125/Space Seasonal/Construction Camp 50/Person Country Clubs Resident Member Add for Nonresident Member 100/Member 25/Person Public Restrooms: Toilet Wastes Toilet & Shower Wastes 5/Person 15/Person ESTABLISHMENTS IDAHO ADMINISTRATIVE CODE IDAPA 58.01.03 – Individual/Subsurface Sewage Department of Environmental Quality Disposal & Cleaning of Septic Tanks Rules Section 007 Page 15 b. The outlet of the septic tank and each compartment must be submerged by a vented tee or baffle. (7-1-25) c. Vented tees and baffles must extend above the liquid level seven (7) inches or more but not closer than one (1) inch to the inside lid of the tank. (7-1-25) d. Tees and baffles must extend below the liquid level to a depth where forty percent (40%) of the tank’s liquid volume is above the bottom of the tee or baffle. (7-1-25) e. Tees and baffles must not extend horizontally into the tank beyond two (2) times the diameter of the outlet. (7-1-25) 13. Scum Storage. A septic tank will provide an air space above the liquid level which will be equal to or greater than fifteen percent (15%) of the tank’s liquid capacity. (7-1-25) 14. Manholes. Manholes must extend to the finished grade. Access to each septic tank or compartment must be provided by a manhole twenty (20) inches in minimum dimension or a removable cover of equivalent size. Each manhole cover must be provided with a corrosion resistant strap or handle to facilitate removal. (7-1-25) 15. Inspection Ports. An inspection port measuring at least eight (8) inches in minimum dimension will be placed above each inlet and outlet. Manholes may be substituted for inspection ports. (7-1-25) 16. Split Flows. The wastewater from a single building sewer or sewer line must not be divided and discharged into more than one (1) septic tank or compartment. (7-1-25) 17. Multiple Tank or Compartment Capacity. Multiple or compartmented septic tanks connected in series may be used if the sum of their liquid capacities is at least equal to the minimum tank capacity in Subsection 007.07, and the initial tank or compartment has a liquid capacity of at least one-half (1/2) of the total liquid capacity of the septic tank facility. (7-1-25) 18. Minimum Separation Distances Between Septic Tanks and Features of Concern. (7-1-25) 19. Manufactured Tank Installation. If written installation instructions are provided by the manufacturer of a septic tank, the installer must follow those instructions relative to the stability and integrity of the tank unless otherwise specified in the installation permit. (7-1-25) 20. Inlet and Outlet Piping. Unless otherwise specified in the installation permit, piping material to Features of Concern Minimum Distance to Septic Tank in Feet Well or Spring or Suction Line Public Water Other 100 50 Water Distribution Line Public Water Other 25 10 Permanent or Intermittent Surface Water 50 Temporary Surface Water 25 Downslope Cut or Scarp 10 Dwelling Foundation or Building 5 Property Line 5 Seasonal High Water Level (Vertically from Top of Tank) 2 IDAHO ADMINISTRATIVE CODE IDAPA 58.01.03 – Individual/Subsurface Sewage Department of Environmental Quality Disposal & Cleaning of Septic Tanks Rules Section 008 Page 16 and from a septic tank or dosing chamber and to the drainfield must be approved by the Director and specified as follows. (7-1-25) a. ABS schedule forty (40) piping or material of equal or greater strength. (7-1-25) b. ASTM D-3034 or equivalent plastic pipe may be used to span the septic tank and dosing chamber if the excavation is compacted with fill material. (7-1-25) 21. Effluent Pipe Separation Distances. Effluent pipes have the same separation distance requirements as septic tanks unless otherwise approved by the Director. (7-1-25) 22. Septic Tank Abandonment. The property owner is responsible for septic tank abandonment and must use the following procedures. (7-1-25) a. Disconnect the inlet and outlet piping; (7-1-25) b. Pump the scum and septage with approved disposal; and (7-1-25) c. Fill the septic tank with earthen materials, physically destroy the septic tank, or remove the septic tank from the ground. (7-1-25)