Current through Register Vol. 63, No. 3, March 1, 2024
(1) Pressurized distribution systems
receiving residential strength wastewater may be permitted on any site meeting
the requirements for installation of a standard onsite system and on other
sites where this method of effluent distribution is preferable and the site
conditions in this rule can be met.
(2) Except as allowed in OAR
340-071-0220(1)(d), pressurized distribution systems must be used where depth
to soil with rapid or very rapid permeability as defined in OAR
340-071-0100(148)(a) and (b) is less than 36 inches and the minimum separation
distance between the bottom of the absorption trench and such soil is less than
18 inches.
(3) Pressurized
distribution systems installed in soil with rapid or very rapid permeability as
defined in OAR 340-071-0100(148)(a) and (b) in areas with permanent water
tables may not discharge more than 450 gallons of effluent per 1/2 acre per day
except where:
(a) Groundwater is degraded and
designated as a non-developable resource by the Oregon Water Resources
Department; or
(b) A detailed
hydrogeological study discloses loading rates exceeding 450 gallons per 1/2
acre per day would not increase the nitrate-nitrogen concentration in the
groundwater beneath the site or at any down gradient location to above 5 mg/L.
(4) Materials and
construction.
(a) General.
(A) All materials used in pressurized systems
must be structurally sound, durable, and capable of withstanding normal
stresses incidental to installation and operation.
(B) Pump wiring must comply with applicable
building, electrical, or other codes. An electrical permit and inspection from
the Department of Consumer and Business Services, Building Codes Division, or
the municipality with jurisdiction, is required for pump wiring installation.
(C) A single compartment dosing
septic tank may not be used in a system with pressurized distribution laterals
unless the tank is partitioned with a flow-through below the tank's lowest
liquid level. The flow through port must be at 65 to 75 percent of the minimum
liquid level and be at least 4" in diameter.
(b) Pressurized distribution piping. Piping,
valves, and fittings for pressurized systems must meet the following minimum
requirements.
(A) All pressure transport,
manifold, lateral piping, and fittings must meet the requirements in OAR
340-073-0060(3).
(B) Pressure
transport piping must be uniformly supported along the trench bottom. The agent
may require the piping to be bedded in sand or other material approved by the
agent. A minimum 18 gauge, green-jacketed tracer wire or green color-coded
metallic locate tape must be placed above piping.
(C) Orifices must be located on top of the
pipe, except as noted in paragraph 4(b)(I) of this section.
(D) The ends of lateral piping must be
constructed with long sweep elbows or an equivalent method to bring the end of
the pipe to finished grade. The ends of the pipe must be provided with threaded
plugs, caps, or other devices acceptable to the agent to allow for access to
and flushing the lateral.
(E) All
joints in the manifold, lateral piping, and fittings must be solvent-welded
using the appropriate joint compound for the pipe material. Pressure transport
piping may be solvent-welded or rubber-ring jointed.
(F) A shut off valve must be placed on the
pressure transport pipe in or near the dosing tank when appropriate.
(G) A check valve must be placed between the
pump and the shut off valve when appropriate.
(H) All orifices must be covered by a
protective, durable, noncorrosive orifice shield designed to keep orifices from
being blocked by drain media or other system components. The shields or piping
must be removable for access to the orifices.
(I) The agent may specify alternate orifice
orientation and valve arrangements for conditions such as extended freezing
temperatures, temporary or seasonal use, or effluent characteristics.
(J) Where operating a pump could
result in siphonage of effluent to below the normal off level of the pump, an
anti-siphon measure in the form of a non-discharging valve designed for the
specific purpose must be used. The anti-siphon valve must be installed and
operated under manufacturer's specifications.
(c) Absorption trench sizing and
construction.
(A) A system using absorption
trenches must be designed and sized as OAR 340-071-0220(2) requires.
(B) Absorption trenches must be constructed
using the specifications for the standard disposal trench unless otherwise
authorized by the agent.
(C) The
trench must contain drain media at least 12 inches deep, with at least 6 inches
of media under the pressure distribution laterals and sufficient media above
the laterals to meet or cover the orifice shields to provide a smooth, even
cover.
(D) The top of the drain
media must be covered with filter fabric or other nondegradable material
permeable to fluids that will not allow passage of soil particles coarser than
very fine sand. In unstable soils, sidewall lining may be required.
(d) Seepage bed construction.
(A) Seepage beds may be used instead of
absorption trenches in soil as defined in OAR 340-071-0100(148)(b) if flows do
not exceed 600 gpd.
(B) The
effective seepage area must be based on the bottom area of the seepage bed. The
area must be at least 200 square feet per 150 gallons per day waste flow.
(C) Beds must be installed at
least 18 inches deep (12 inches with a capping fill) but not deeper than 36
inches into the natural soil. The seepage bed bottom must be level.
(D) The top of the drain media must be
covered with filter fabric or other nondegradable material that is permeable to
fluids but will not allow passage of soil particles coarser than very fine
sand.
(E) The bed must contain
drain media at least 12 inches deep with at least 6 inches of media under the
pressure distribution laterals and sufficient media above the laterals to meet
or cover the orifice shields to provide a smooth, even cover.
(F) Pressurized distribution piping must be
horizontally spaced not more than 4 feet apart and not more than 2 feet away
from the seepage bed sidewall. At least 2 parallel pressurized distribution
pipes must be placed in the seepage bed.
(G) A minimum of 10 feet of undisturbed earth
must be maintained between seepage beds.
(5) Hydraulic design criteria. Pressurized
distribution systems must be designed for appropriate head and capacity.
(a) Head calculations must include maximum
static lift, pipe friction, and orifice head requirements.
(A) Static lift where pumps are used must be
measured from the minimum dosing tank level to the level of the perforated
distribution piping.
(B) Pipe
friction must be based upon a Hazen Williams coefficient of smoothness of 150.
All pressure piping and fittings on laterals must have a minimum diameter of 2
inches unless submitted plans and specifications show a smaller diameter pipe
is adequate.
(C) A minimum head of
5 feet at the remotest orifice and no more than a 10 percent flow variation
between the nearest and remotest orifice in an individual unit are required.
(b) The capacity of a
pressurized distribution system refers to the rate of flow given in gallons per
minute (gpm).
(A) Lateral piping must have
discharge orifices drilled a minimum diameter of 1/8 inch and evenly spaced no
more than 24 inches apart in coarse textured soils or no more than 4 feet apart
in finer textured soils.
(B) The
system must be dosed at a rate not to exceed 20 percent of the projected daily
sewage flow.
(C) The effect of back
drainage of the total volume of effluent within the pressure distribution
system must be evaluated for its impact on the dosing tank and system
operation.
(6)
Service contracts. The owner of a pressurized distribution system must maintain
a contract, under OAR 340-071-0130(23), with a maintenance provider to serve,
maintain and adjust the onsite system. A service contract must be entered
before the system is installed and must be maintained until the system is
decommissioned.
Publications: Publications referenced are available from the
agency.