Current through Register Vol. 63, No. 3, March 1, 2024
(1) Criteria For standard subsurface systems.
Each site must meet all of the conditions in this section to be approved for a
standard subsurface system.
(a) Effective
soil depth must extend 30 inches or more below the ground surface as shown in
Table 3. [NOTE: All tables are found in OAR 340-071-0800.] A minimum 6-inch
separation must be maintained between the layer that limits effective soil
depth and the bottom of the absorption facility.
(b) Water table levels must be predicted
using standards in OAR 340-071-0130(23).
(A)
The permanent water table must be at least 4 feet below the bottom of the
absorption facility, except in defined geographic areas where DEQ has
determined through a groundwater study that less separation will not degrade
groundwater or threaten public health. In these exception areas, the permanent
water table must be at least 24 inches below the ground surface.
(B) A temporary water table must be 24 inches
or more below the ground surface. An absorption facility may not be installed
deeper than the top of the temporary water table.
(C) A groundwater interceptor may be used to
intercept or drain water from an absorption area on sites with adequate slope
to permit proper drainage. An agent may require a demonstration that the site
can be de-watered before issuing a site evaluation report approving the site.
Where required, groundwater interceptors are an integral part of the system but
do not need to meet setback requirements to property lines, wells, streams,
lakes, ponds, or other surface water bodies that are required for the
wastewater absorption area.
(c) Except as subsection (d) of this section
provides, soil with rapid or very rapid permeability must be 36 inches or more
below the ground surface. A minimum 18-inch separation must be maintained
between soil with rapid or very rapid permeability and the bottom of absorption
trenches.
(d) Sites may be approved
with no separation between the bottom of absorption trenches and soil with
rapid or very rapid permeability as defined in OAR 340-071-0100(148)(a) and (b)
and absorption trenches may be placed into such soil if any of the following
conditions occur.
(A) A confining layer
occurs between the bottom of absorption trenches and the groundwater table and
a minimum 6-inch separation is maintained between the bottom of absorption
trenches and the top of the confining layer.
(B) A layer of nongravelly (less than 15
percent gravel) soil with sandy loam or finer texture at least 18 inches thick
occurs between the bottom of the absorption trenches and the groundwater table.
(C) The projected daily sewage
flow does not exceed a loading rate of 450 gallons per acre per day.
(e) Slopes do not exceed 30
percent or the slope/effective soil depth relationship described in Table 3.
[NOTE: All tables are found in OAR 340-071-0800.]
(f) The site has not been filled or the soil
has not been modified in a way that would, in the agent's opinion, adversely
affect the system's functioning.
(g) The site is not on an unstable land form
that might adversely affect operation of the system.
(h) The site of the initial and replacement
absorption facility is not covered by asphalt or concrete or subject to
vehicular traffic, livestock, or other activity that would adversely affect the
soil.
(i) The site of the initial
and replacement absorption facility will not be subjected to excessive
saturation from artificial drainage of ground surfaces, driveways, roads, roof
drains, or other circumstances.
(j)
Setbacks in Table 1 except as modified by this subsection can be met.
(A) Surface waters setbacks. Setback from
streams or other surface waters must be measured from bank drop-off or mean
yearly high water mark, whichever provides the greatest separation
distance.
(B) Lots created before
May 1, 1973. For lots or parcels legally created before May 1, 1973, the agent
may approve installing a standard or alternative system with a setback from
surface waters of less than 100 feet but not less than 50 feet if all other
applicable provisions of this rule can be met.
(C) Water lines and sewer lines. Effluent
sewer and water line piping constructed of materials that are approved for use
within a building in the 2000 Edition of the Oregon State Plumbing Specialty
Code may be run in the same trench or may cross. Where the effluent sewer pipe
material is not approved for use in a building, it may not be run or laid in
the same trench as water pipe unless:
(i) The
bottom of the water pipe at all points is set at least 12 inches above the top
of the sewer pipe; and
(ii) The
water pipe is placed on a solid shelf excavated at one side of the common
trench with a minimum, clear, horizontal distance of at least 12 inches from
the sewer pipe.
(D)
Septic tank setbacks. The agent must encourage placing septic tanks and other
treatment units as close as feasible to the minimum separation from the
building foundation to minimize clogging the building sewer.
(E) Pressure transport pipe setback to well.
Notwithstanding the setback distance in Table 1, the agent may allow the
separation distance between a pressure transport pipe and a well to be less
than 50 feet but no less than 25 feet when [NOTE: All tables are found in OAR
340-071-0800.] :
(i) The pressure transport
pipe is PVC Sch. 40 or heavier pressure-rated piping meeting ASTM Specification
D-2241;
(ii) The pressure transport
pipe is placed within a larger diameter PVC or ABS Sch. 40 or heavier
encasement pipe, with the pipe ends located at least 50 feet away from the
well; and
(iii) All pipe joints in
the pressure transport pipe and encasement pipe are solvent-welded.
(2) Criteria
for sizing absorption fields. Absorption fields must be designed and sized
based on the criteria in this section.
(a)
Table 2, specifying quantities of sewage flows, or other information the agent
determines is reliable with the following exception. [NOTE: All tables are
found in OAR 340-071-0800.] A system must be sized on the basis of 300 gallons
sewage flow per day plus 75 gallons per day for the third bedroom when the
system:
(A) Is proposed to serve a single
family dwelling on a lot of record created before March 1, 1978, that is too
small to accommodate a system sized for a daily sewage flow of 450 gallons; or
(B) Serves specifically planned
developments with living units of three or fewer bedrooms and deed restrictions
prohibit an increase in the number of bedrooms.
(b) Table 4, specifying the minimum length of
absorption trenches based on soil texture and effective soil depth. [NOTE: All
tables are found in OAR 340-071-0800.]
(c) Table 5, specifying the minimum length of
absorption trenches based on soil texture and depth to temporary water. [NOTE:
All tables are found in OAR 340-071-0800.]
(d) Strength of the wastewater. If the
strength of the wastewater exceeds the maximum limits for residential strength
wastewater or the contents of the wastewater are atypical of residential
strength wastewater or pose a threat to groundwater, public health, or the
environment, the wastewater must be pretreated to acceptable levels before
being discharged into a standard or alternative system.
(3) Septic tank.
(a) Liquid capacity.
(A) The quantity of daily sewage flow
projected for a facility must be estimated from Table 2. The agent must
determine the projected daily sewage flow for establishments not listed in
Table 2. [NOTE: All tables are found in OAR 340-071-0800.]
(B) A septic tank that serves a commercial
facility must have a liquid capacity of at least two times the projected daily
sewage flow unless the agent authorizes otherwise. In all cases the capacity
must be at least 1,000 gallons.
(C)
The capacity of a septic tank that serves a single family dwelling must be
based on the number of bedrooms in the dwelling. For a dwelling with 4 or fewer
bedrooms, the tank capacity must be at least 1,000 gallons. Septic tank
capacity must be at least 1,500 gallons for dwellings with more than 4
bedrooms.
(D) The agent may require
a larger capacity than this subsection specifies as needed for special or
unique waste characteristics, such as flow patterns, volumes, waste strength,
or facility operation.
(b) Installation requirements.
(A) Septic tanks must be installed on a
level, stable base that will not settle.
(B) Septic tanks located in high groundwater
area must be weighted or provided with an antibuoyancy device to prevent
flotation under the manufacturer's instructions.
(C) Tanks must be installed with at least one
watertight riser extending to the ground surface or above. The riser must have
a minimum diameter of 20 inches when the soil cover above the tank does not
exceed 36 inches. The riser must have a minimum diameter of 30 inches when the
soil cover above the tank exceeds 36 inches or when the tank capacity exceeds
3,000 gallons. A gasketed cover must be provided and securely fastened or
weighted to prevent unauthorized access.
(D) Tanks must be installed in a location
that provides access for maintenance.
(E) Where practicable, the sewage flow from
an establishment must be consolidated into one septic tank.
(F) The agent may allow a removable plug to
be placed in the top of a septic tank inlet sanitary tee if the septic tank
discharges directly into a gravity-fed absorption facility.
(G) After installation all tanks must prove
watertightness under OAR 340-073-0025.
(H) Unless the agent allows otherwise, an
effluent filter meeting the requirements of OAR 340-073-0056 must be installed
at the septic tank outlet if a tank serves a commercial facility. A service
access riser and cover meeting the requirements of 340-071-0220(3)(b)(C) must
be placed above the effluent filter.
(c) Construction. Tank construction must
comply with minimum standards in OAR chapter 340, division 073, unless
otherwise DEQ authorizes otherwise in writing.
(d) Multi-compartment tank requirement.
(A) With the exception in paragraph (B) of
this subsection, if a sewage ejector pump precedes a septic tank, the tank must
have been manufactured as a multi-compartment tank under requirements in this
division and OAR chapter 340, division 073. An effluent filter must be
installed unless the agent allows other methods with equal or better
performance in preventing suspended solids from passing to the drainfield.
(B) If the sewage ejector pump
preceding the septic tank at a single family residence receives wastewater from
only a clothes washing machine and a sink, a single-compartment septic tank may
be used in lieu of a multi-compartment septic tank. The tank must meet the
minimum capacity requirement in subsection (a) of this section, and an effluent
filter must be installed in the tank's outlet tee fitting. Alternatively, the
agent may allow the filter to be placed in a separate vault and riser located
just outside the septic tank or may authorize other alternatives as
appropriate.
(4) Distribution techniques. Absorption
trenches must be constructed according to one of the methods in this section.
(a) Gravity-fed equal distribution (including
loop).
(A) Equal distribution must be used on
generally level ground. All trenches and piping must be level within a
tolerance of plus or minus 1 inch. All lateral piping must be at the same
elevation.
(B) A pressure-operated
hydrosplitter may be used to achieve equal distribution.
(C) To determine the total useable area of a
looped soil absorption facility, the agent must add the sum of the lengths of
the parallel absorption trenches and the lengths of up to two absorption
trenches intersecting the parallel trenches.
(b) Serial distribution. Serial distribution
is generally used on sloping ground. Each trench must be level within a
tolerance of plus or minus 1 inch. Serial distribution may be a combination of
equal distribution and serial distribution.
(c) Pressurized distribution systems.
Pressurized distribution must satisfy the requirements in OAR 340-071-0275.
(5) Distribution boxes
and drop boxes.
(a) Construction.
Distribution box and drop box construction must comply with standards in OAR
340-073-0035 and 340-073-0040.
(b)
Foundation. All distribution boxes and drop boxes must be bedded on a stable,
level base.
(c) In all gravity
distribution techniques, the connection of the effluent piping to the
distribution piping must include at least one distribution or drop box or other
device acceptable to the agent as a means for locating and monitoring the
absorption field.
(6)
Dosing tanks and dosing septic tanks.
(a)
Tank construction must comply with the standards in OAR chapter 340, division
73 unless DEQ authorizes otherwise in writing.
(b) The tank must be installed on a stable,
level base at a location that provides access for maintenance.
(c) The tank must be provided with at least
one watertight service access riser extending to the ground surface or above.
The riser must have a minimum diameter of 20 inches when the soil cover above
the tank does not exceed 36 inches. The riser must have a minimum diameter of
30 inches when the soil cover above the tank exceeds 36 inches. A gasketed
cover must be securely fastened or weighted to prevent unauthorized access.
(d) A tank located in a high
groundwater area must be weighted or provided with an antibuoyancy device to
prevent flotation under the tank manufacturer's instructions.
(7) Absorption trenches.
(a) Absorption trenches must be constructed
under the standards in this section unless otherwise authorized in this
division.
(A) Minimum bottom width of trench
- 24 inches.
(B) Minimum depth of
trench:
(i) Equal or looped distribution -18
inches.
(ii) Serial distribution -
24 inches.
(iii) Pressure
distribution - 18 inches.
(C) Maximum depth of trench - 36 inches.
(D) Maximum length of an
individual trench - 150 linear feet, unless the agent authorizes otherwise in
writing.
(E) Minimum distance of
undisturbed earth between trenches - 8 feet.
(b) The bottom of the trench must be level
within a tolerance of plus or minus 1 inch end to end and level from side to
side.
(c) When the sidewall within
a trench has been smeared or compacted, sidewalls must be raked to ensure
permeability.
(d) Trenches must be
constructed to prevent septic tank effluent from flowing backwards from the
distribution pipe to undermine the distribution box, the septic tank, or any
portion of the distribution unit.
(e) Drain media must extend the full width
and length of the trench to a depth of at least 12 inches with at least 6
inches of drain media under the distribution pipe and at least 2 inches over
the distribution pipe.
(f) Before
backfilling the trench, the drain media must be covered with filter fabric,
untreated building paper, or other material the agent approves.
(g) If trenches are installed in sandy loam
or coarser soils, filter fabric or other nondegradable material the agent
approves must be used to cover the drain media.
(8) Trench backfill.
(a) The installer must backfill the system.
Backfill must be carefully placed to prevent damage to the system.
(b) A minimum of 6 inches of backfill is
required. 12 inches is required in serial systems.
(c) Backfill must be free of large stones,
frozen clumps of earth, masonry, stumps, waste construction materials, or other
materials that could damage the system.
(9) Header pipe. Header pipe must be
watertight, have a minimum diameter of 3 inches, and be bedded on undisturbed
earth. Where distribution boxes or drop boxes are used, the header pipe between
the box and the distribution pipe must be at least 4 feet in length and be
installed level.
(10) Distribution
pipe.
(a) Distribution pipes must have a
minimum diameter of 3 inches.
(b)
Each disposal trench must have distribution piping that is centered in the
trench and laid level within a tolerance of plus or minus 1 inch.
(c) Distribution pipe must comply with
standards in OAR 340-073-0060(4).
(d) All perforated pipe must be installed
with centerline markings up.
(11) Effluent sewer. The effluent sewer must
extend at least 5 feet beyond the septic tank before connecting to the
distribution unit. It must be installed with a minimum fall of 4 inches per 100
feet and at least 2 inches of fall from one end of the pipe to the other. In
addition, there must be a minimum difference of 8 inches between the invert of
the septic tank outlet and either the invert of the header to the distribution
pipe of the highest lateral in a serial distribution field or the invert of the
header pipe to the distribution pipes of an equal distribution absorption
field. A minimum 18-gauge, green-jacketed tracer wire or green color-coded
metallic tape must be placed above the effluent sewer pipe.
(12) Curtain drain construction. Unless the
agent authorizes otherwise, curtain drains must comply with the following
requirements.
(a) Ground slope must be at
least 3 percent, or other landform features such as an escarpment must allow
for effective drainage.
(b) The
curtain drain must extend at least 6 inches into the layer that limits
effective soil depth or to a depth adequate to effectively dewater the site.
(c) Trench width must be a minimum
of 12 inches.
(d) Perforated pipe
must have a minimum diameter of 4 inches and must meet the requirements in OAR
340-073-0060(4).
(e) Perforated
pipe must be installed at least 2 inches above the bottom and along the full
length of the trench and must be covered by a minimum of 10 inches of drain
media.
(f) The curtain drain must
be filled with drain media to within 12 inches of the ground surface.
(g) Outlet pipe must be rigid,
smooth-wall, solid PVC pipe meeting or exceeding ASTM Standard D-3034 with a
minimum diameter of 4 inches. A flap gate or rodent guard must be installed.
(h) Filter fabric must be placed
over the drain media.
[NOTE; All tables are found in OAR
340-071-0800.]
Tables referenced are not included in rule text.
Click here for PDF
copy of table(s)
Statutory/Other Authority: ORS
454.625 &
468.020
Statutes/Other Implemented: ORS
454.615
& 468B.080