Or. Admin. Code § 340-071-0302 - Recirculating Gravel Filter (RGF)
(1)
Siting and absorption area construction criteria.
(a) RGFs approved for treatment standard 1
may be sited and sized as follows.
(A) In
areas with a temporary water table, as specifications for sand filters in areas
with temporary groundwater in OAR
340-071-0290 require.
(B) In areas with permanent groundwater,
where 4 feet of separation can be maintained between the bottom of the trench
and groundwater and the other criteria in OAR
340-071-0290 can be
met.
(C) On sites meeting criteria
for standard onsite systems in OAR
340-071-0220 or for pressurized
systems in OAR 340-071-0275.
(b) RGFs used in conjunction with approved
disinfection and approved nitrogen reduction processes and expected to meet
treatment standard 2 may be sited and sized as follows.
(A) On sites meeting the criteria for
treatment standard 1 in subsection (a) of this section.
(B) In areas with a permanent water table, as
specifications for sand filters in areas with a permanent water table in OAR
340-071-0290
require.
(c) Any type of
absorption area permitted for a sand filter system, including the gravel-less
absorption method, may be permitted for an RGF system.
(2) Design criteria.
(a) Filter design and dosing.
(A) The filter's basal or bottom area must be
sized based on a maximum organic load. For residential strength wastewater that
has been pretreated through a septic tank, the maximum hydraulic load allowable
is 5 gal/ft2/day.
(B) For BOD5
waste strengths stronger than residential strength wastewater but not exceeding
400 mg/L, the filter size must be increased proportionately.
(C) Higher strength wastewaters must be
pretreated or will require special consideration. In no case may the
concentration of greases and oil applied exceed 30 mg/L.
(b) Filter media.
(A) Where CBOD5 removal must be at least 85
percent based upon the raw sewage concentration applied to the septic tank and
nitrification of wastewater is necessary, a filter media must consist of 3 feet
of very fine washed gravel, 100 percent passing a 3/8-inch sieve with an
effective size between 3 and 5 millimeters and a uniformity coefficient of 2 or
less. Washed means that negligible fines (less than 1.0 percent) pass a No. 10
sieve.
(B) Where additional removal
of BOD5 and denitrification is intended or required, a treatment media may
consist of 2 feet of very coarse washed sand, 100 percent passing a 3/8-inch
sieve with an effective size between 1.5 and 2.5 millimeters and a uniformity
coefficient of 2 or less. Washed means that negligible fines (less than 4.0
percent) pass the No. 100 sieve.
(C) Sieves of 3/8 inch, 1/4 inch, and Nos. 4,
6, 8, 10, 50, and 100 must be used in gradation analysis.
(D) The permittee must provide fresh samples
of the intended media for each project before shipment to the project site. A
laboratory gradation analysis must be performed and the gradation data plotted
on semi-log paper as a gradation curve. The permittee must submit lab data,
gradation curve, and a 5-pound sample of the media to the agent for approval.
Only approved media may be used.
(c) Filter media must be overlain by a 3-inch
bed of 1/2-inch to 3/4-inch washed gravel. The media and gravel may only
lightly cover the distribution piping. Unless otherwise authorized, each
orifice must be covered by an orifice shield to prevent aerial spray drift.
(d) Filter dosing must use a low
pressure distribution piping system operating under adequate head to pressurize
the system. The operating head must be a minimum of 5 feet at the remotest
orifice and have no more than 10 percent flow variation between the nearest and
remotest orifice in an individual unit. Each lateral pipe end must terminate
with a screwed plug or cap accessible for removal and flushing. Wherever
practical, a valved backflush system must be installed to flush groups of
laterals back to a septic tank or elsewhere.
(e) Pressure-distribution piping must be
spaced 2 feet center to center in a parallel grid. Orifice spacing must be
every 2 feet on laterals. Piping grid edges should be within 1 foot of the
filter basal edge.
(f) Filter media
must be underlain by a 6-inch bed of a 1/2 to 3/4-inch washed gravel underdrain
media. No filter fabric may cover the underdrain media.
(g) Perforated collection pipes must meet
requirements in OAR 340-073-0060(2)
and be bedded in the underdrain media. Pipes must be at least 4 inches in
diameter with no filter fabric wrap. At least 15 lineal feet of collection pipe
is required for each 225 square feet of filter basal area.
(h) The filter container must be watertight
to suit the design conditions. Underflow must be contained. Groundwater must be
excluded. A concrete container may be used. Other materials may be used if
equivalent function, workmanship, watertightness, and at least a 20-year
service life can be expected.
(3) Recirculation/dilution tank.
(a) A recirculation tank receives septic tank
effluent and underflow from the filter. A pumping system at this tank delivers
flow to the filter dose piping network according to a project design. The
recirculation tank volume measured from tank floor to tank soffit must be at
least equal to the projected daily sewage flow volume.
(b) The recirculation ratio at design flow
must be at least 4. Recirculation ratio is the daily volume of recycle divided
by design daily volume of the wastewater. A fabricated "T" or "Splitter T"
float valve located in the recirculation tank must be used whenever possible.
Minimum recirculation tank liquid volume must be at least 80 percent of the
gross tank volume when a float valve is used. Alternatively, where required and
reasonable, a splitter basin using orifice or weir control may be used to
divide underflow 20 percent to the absorption field and 80 percent to recycle
on a daily basis. This alternative must use orifice control wherever possible.
Minimum recirculation tank liquid volume must be at least 50 percent of the
required tank volume when a splitter basin is used.
(c) Evaluation of and design for overflow and
surge control at the recirculation tank must be included in the design plans.
(d) An audible or visual high
water alarm must be included in the recirculation tank immediately below the
overflow level. A latching electrical relay must retain the audible or visual
alarm until a site attendant acknowledges it.
(e) Parallel pump start/stop electric
controls (usually floats) must be installed to correct any unforeseen high
liquid level event and keep sewage contained. This pump start function
precludes overflow and must operate in parallel with the start/stop function of
a timer and must not interfere with or depend upon a timer position.
(f) All areas of the filter must be wetted 48
times a day or every 30 minutes to achieve the recirculation ratio of at least
4 unless the agent authorizes otherwise.
(g) Testing must demonstrate the
recirculation tank is watertight. The designer must witness the testing. Test
protocol must be included in the design plans.
(h) A fence or other effective means must
restrict access onto the filter. Design and construction must prevent surface
water entry onto the filter.
(i)
Access openings to the recirculation tank must be provided at each end. Larger
tanks must have additional openings. The smallest dimension of any access must
be 18 inches. Larger openings must be provided if partially obstructed with
piping or other objects. Provisions must be made to remove dregs (settleable
solids). Pumps must be readily removable and replaceable without demolition of
piping or other components.
(4) Operation and Maintenance standards. The
owner of an onsite system using an RGF must ensure the RGF and all other
components of the onsite system are properly operated and timely maintained or
decommissioned.
(5) Operation and
maintenance manual. The designer of an RGF system must ensure that
comprehensive and detailed operation and maintenance instructions are provided
to the onsite system owner at the time of installation. The instructions must
emphasize operating and maintaining the entire system within the parameter
ranges for which it is designed. The information must be presented in a manner
that can be easily understood by the owner and include at a minimum:
(a) As-built plans with the name and contact
number of the installer;
(b) A
description of how the process functions, including diagrams illustrating basic
system design and flow path;
(c) A
maintenance schedule for all critical components;
(d) Requirements and recommended procedures
for periodic removal of residuals from the system;
(e) A detailed procedure for visually
evaluating the function of system components;
(f) A description of olfactory and visual
techniques for confirming correct process parameters and system performance;
(g) A recommended method for
collecting and transporting effluent samples;
(h) Safety concerns that may need to be
addressed; and
(i) Emergency
contact numbers for maintenance providers and pumpers.
(6) Service contracts. The owner of an RGF
system must maintain a contract, under OAR
340-071-0130(23),
with a maintenance provider to serve and maintain the onsite system. A service
contract must be entered before the system is installed and must be maintained
until the system is decommissioned.
Notes
Statutory/Other Authority: ORS 454.625 & 468.020
Statutes/Other Implemented: ORS 454.615, 454.780, 468B.050 & 468B.055
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