Or. Admin. R. 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 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 P.V.C. plastic, Schedule 40 ABS plastic, or other equally
durable materials approved by the department 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 the department.
(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 or siphon 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 the department:
(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 the
department.
(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 the department 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 the department
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.
Notes
Publications: Publications referenced are available from the agency.
Statutory/Other Authority: ORS 454.625 & 468.020
Statutes/Other Implemented: ORS 454.615
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