340-073-0025 Tank Construction The following construction and manufacturing requirements apply to all septic tanks, holding tanks, dosing tanks, multiple-compartment combination septic and dosing tanks, and dosing septic tanks manufactured for use in Oregon unless specifically exempted by OAR chapter 340, divisions 071 or 073. (1) Compartments. Tanks may have single or multiple compartments. (a) Single-compartment tanks and multiple-compartment tanks must meet or exceed the minimum volume requirements described in OAR chapter 340, divisions 071 and 073. (b) Multiple-compartment tanks must comply with the following requirements: (A) The liquid capacity of the first compartment must be at least 2/3 of the total required liquid capacity, as measured from the invert elevation of the first compartment's outlet Tee fitting; (B) A compartment may not have an inside horizontal dimension of less than 24 inches. (c) The liquid depth of any compartment must be at least 30 inches. Liquid depths greater than 72 inches may not be considered in determining the working liquid capacity unless the tank has a capacity greater than 3,000 gallons. (2) Service access manhole. All tanks must have at least one service access manhole measuring at least 18 inches across its shortest dimension in each compartment. (3) Watertightness. After installation, all tanks must be watertight. The installer must test each tank for watertightness by filling the tank to a point at least 2 inches above the point of riser connection to the top of the tank. During the test there may be no more than a one gallon leakage over a 24 hour period. The tank manufacturer must deliver watertight tanks and should test each tank for watertightness before the tank is shipped from the manufacturing plant. (4) If the tank manufacturer does not fully assemble the tank, as with a two-piece concrete tank, the manufacturer must provide the bonding and sealing agents and an instruction manual for assembling the tank. (5) Structure: All tanks must be able to support an earth load of at least 300 pounds per square foot when the maximum coverage does not exceed 3 feet. Tanks installed with more than 3 feet of cover must be reinforced to support the additional load. Lateral load must be 62.4 pcf of equivalent fluid pressure (EFP). Tanks must be able to withstand long-term external hydrostatic loads in addition to soil loads. Internal hydrostatic pressures must be omitted to allow for septage pumping during critical groundwater conditions. A 2,500 pound wheel load concentrated over the critical elements of the tank shall also be considered. (6) Service access riser and cover. All tanks must be manufactured to accommodate installation of a watertight service access riser above one service access manhole. The riser must have a minimum nominal diameter of 20 inches when tank burial depths do not exceed 36 inches. Tanks designed for burial depths deeper than 36 inches must also be designed to accommodate installation of a 30-inch minimum diameter service access riser above each service access manhole. A gasketed riser cover must be provided and securely fastened or weighted to prevent unauthorized access. (7) Inlet and outlet Tee fittings. (a) The inlet and outlet Tee fittings must be of Schedule 40 PVC plastic, Schedule 40 ABS plastic, or other equally durable materials approved by DEQ with a minimum diameter of 4 inches. (b) The distance between the inlet and outlet Tee fittings in a single-compartment tank must at least equal the liquid depth of the tank. (c) The inlet and outlet Tee fittings in a single - compartment tank, where applicable, must be located at opposite ends of the tank. The inlet Tee fitting must be readily accessible by way of a watertight, 8-inch minimum diameter riser (with cover) and access hole positioned directly above the inlet Tee. The fittings must be attached in a watertight manner acceptable to DEQ. (d) The inlet fitting in all single-compartment tanks, except dosing tanks, and in each compartment of multiple-compartment tanks, must be a "sanitary tee" extending at least 6 inches above and at least 12 inches below the normal high and low liquid levels, respectively. (e) The outlet Tee fitting, holes, or ports provided in a vault or outlet effluent filter must be positioned to withdraw effluent horizontally from the clear zone at an elevation measured from the inside bottom of the tank to 65 to 75 percent of the lowest operating liquid depth. The net area of the ports must be at least 6 square inches. The outlet fitting in single-compartment tanks and in each compartment of multiple-compartment tanks must extend at least 6 inches above the highest normal liquid depth to provide scum storage. When the single-compartment tank is used as a holding tank, dosing septic tank, or dosing tank, the outlet Tee fitting must be provided with a watertight plug or omitted. The outlet Tee fitting may also be plugged or omitted in the last compartment of a multiple-compartment tank when a pump is placed in that last compartment. (f) Ventilation must be provided through the fittings by means of a 2-inch minimum space between the top of the inlet Tee fittings and the adjacent tank surfaces. (g) The invert of the inlet fitting must be at least 1 inch and preferably 3 inches above the invert of the outlet fitting or the highest normal liquid level. (h) A convenient means of monitoring sludge and scum accumulation must be provided, with access extending to ground level. (i) The tank manufacturer must provide with each Tee fitting an appropriate coupler that will provide a watertight connection between the fittings and the building and effluent sewer pipes. (8) At least 10% of the inside volume of a tank must be above the highest normal liquid level to provide scum storage and reserve. (9) Except as provided in OAR 340-073-0026, tanks shall be constructed of concrete, fiberglass, or other noncorrosive materials approved by DEQ: (a) Precast concrete tanks must have a minimum wall, compartment, and bottom thickness of 2-1/2 inches and must be adequately reinforced. The top must be at least 4 inches thick. (b) Cast-in-place tanks must be designed by a civil or structural engineer to the requirements of these rules, and the tank construction must be certified by the designer or qualified representative. A structural permit from the Building Codes Division or the municipality with jurisdiction (as defined in 455.010(4)) may be required when cast-in-place concrete tanks are used. (c) Tanks made of other corrosion resistant materials must be constructed to provide structural integrity to meet the requirements of sections (3), (4), and (5) of this rule. (10) All prefabricated tanks must be marked on the uppermost tank surface over the outlet with the liquid capacity of the tank, the burial depth limit, date of manufacture, and either the manufacturer's full business name or the number assigned by DEQ. (11) Each commercial manufacturer of prefabricated tanks must provide two complete sets of plans and specifications, prepared by a registered professional engineer licensed to practice in Oregon, to DEQ for review and approval. Plans submittal must include the structural analysis, calculation of total gallons, operating gallons, gallons per inch, and buoyancy, including predetermined countermeasures. (12) Each commercial manufacturer of pre-fabricated tanks must provide DEQ with written certification that tanks for use in onsite systems in the State of Oregon will comply with all requirements of this rule. (13) An installation manual, on waterproof paper or placed within a weather-resistant container, must be provided by the manufacturer with each tank distributed. The manual must describe proper installation of the tank, riser(s) and lid, pipe connections, watertight testing procedures, backfill, and any special precautions or limitations. [Publications: Publications referenced are available from the agency.] Statutory/Other Authority: ORS 454.625 & 468.020 Statutes/Other Implemented: ORS 454.615 History: DEQ 29-2025, amend filed 10/01/2025, effective 01/01/2026 DEQ 13-2019, amend filed 05/16/2019, effective 05/16/2019 DEQ 11-2004, f. 12-22-04, cert. ef. 3-1-05 DEQ 27-1994, f. 11-15-94, cert. ef. 4-1-95 DEQ 15-1986, f. & ef. 8-6-86 DEQ 5-1982, f. & ef. 3-9-82 DEQ 10-1981, f. & ef. 3-20-81