Current through Register Vol. 41, No. 3, September 23, 2024
A.
The contactor, or media filled reactor, utilized for attached growth biological
processes shall be preceded by primary clarification equipped with scum and
grease collecting devices. Other pretreatment facilities equivalent to primary
clarification may be proposed for evaluation by the department. The media shall
provide sufficient surface area to support the attached biological growth
necessary to achieve the desired performance standard. Recirculation of treated
wastewater back to the contactor influent should be provided to maintain design
loadings.
B. Trickling filters.
Biological contactors called trickling filters shall be designed so as to
provide either the reduction in biochemical oxygen demand required by the
issued certificate or permit, or the treatment necessary to properly condition
the sewage for subsequent treatment. This section provides performance criteria
to achieve final effluent limits to meet federal secondary equivalency
requirements for trickling filters. Such biological contactors may be designed
to achieve higher degrees of treatment or used in conjunction with other unit
operations. Where the design intent is to achieve other than secondary
equivalency levels, the proposed design parameters shall be thoroughly reviewed
during the preliminary engineering conference.
1. The hydraulic loading used for design of
standard rate trickling filters shall be between two and four million gallons
per acre per day with an organic loading between 400 and 800 pounds of
BOD5 per acre foot per day.
2. The hydraulic loading used for design of
high-rate filters shall be between 10 and 30 million gallons per acre per day
with an organic loading between 1,200 and 3,300 pounds
BOD5 per acre foot per day.
3. Other design loadings that are based on
pilot studies and related to design and performance parameters through rational
design equations or models will be evaluated by the department.
4. The performance of biological contactors
can be detrimentally affected by diurnal loading conditions. The volume of
media as determined from either pilot plant studies or from acceptable design
equations shall be based upon the design peak hourly organic loading rate
rather than the average rate. An alternative for reducing the design peak flow
would involve provision of adequate flow equalization prior to the
contactor.
5. Consideration should
be given to the use of two-stage biological contactors in series operation
where single stage reactors may not accomplish the required removals. Expected
treatment efficiencies shall be calculated and documented.
C. Features. All hydraulic factors involving
proper distribution of sewage on the contactor media shall be carefully
calculated. For reaction type distributors, a minimum head of 24 inches between
the low water level in siphon chamber and the horizontal elevation of the
center of distribution arms shall be required. Surge relief to prevent damage
to distributor seals shall be provided where sewage is pumped directly to the
distributors. A minimum clearance of six inches between the media surface and
the bottom of distributor arms shall be provided.
1. The sewage may be distributed over the
contact reactor media surface by rotary distributors or other suitable devices
that will permit reasonably uniform distribution to the surface area. At design
average flow, the deviation from a calculated uniformly distributed volume per
square foot of the filter surface shall not exceed plus or minus 10% at any
point.
2. Sewage may be applied to
the contactor media by siphons, pumps or by gravity discharge from preceding
treatment units when suitable flow characteristics have been developed.
Application of sewage should be continuous. In the case of intermittent dosing,
the dosing cycles shall normally vary between five to 15 minutes with
distribution taking place approximately 50% of the time. The maximum rest
should not exceed five minutes based on the design average flow. Consideration
shall be given to a piping system that will permit recirculation.
3. Underdrains with semi-circular inverts or
equivalent shall be provided and the underdrainage system shall cover the
entire floor of the filter. Inlet openings into the underdrains shall have an
unsubmerged gross combined area equal to at least 15% of surface area of the
filter. The underdrains shall have a minimum slope of 1.0%. Effluent channels
shall be designed to produce a minimum velocity of two feet per second at the
average daily rate of application to the filter. Provision shall be made for
flushing the underdrains. The use of a peripheral head channel with vertical
vents is acceptable for flushing purposes. Inspection facilities shall be
provided.
4. The underdrainage
system, effluent channels and effluent pipe shall be designed to permit free
passage of air. The size of drains, channels, and pipe shall be such that not
more than 50% of their cross-sectional area will be submerged under the design
hydraulic loading. Provision shall be made in the design of the effluent
channels to allow the possibility of increased hydraulic loading. Consideration
should be given to the use of forced ventilation, particularly for covered
trickling filters and deep (10 feet or more) contactors filled with a
manufactured media.
5. The design
should provide for variable rates of recirculation for various purposes; for
example, to prevent drying of a standard rate filter between dosing. Devices
shall be provided to permit measurement of flow to the filter process,
including recirculated flows. The design should include provisions to flood
filter structures where applicable.
6. All distribution devices, underdrains,
channels and pipes shall be installed so that they may be properly maintained,
flushed or drained. Mercury seals shall not be permitted. Ease of seal
replacement shall be considered in the design to ensure continuity of
operation.
7. A freeboard of four
feet or more should be provided for all deep bed contactors with manufactured
media that also utilize fine spray distributors, so as to maximize the
containment of windblown spray.
8.
Protection such as covers or windbreaks shall be provided to maintain operation
and treatment efficiencies when climatic conditions are expected to result in
problems due to cold temperatures.
D. Reactor media. Contact reactor media may
be crushed rock, stone or specially manufactured material. The media shall be
durable, resistant to spalling or flaking and relatively insoluble in sewage.
The top 18 inches of rock or stone media shall have a loss by the 20-cycle,
sodium sulfate soundness test of not more than 10% (as prescribed by ASCE
Manual of Engineering Practice, "Filtering Materials for Sewage Treatment
Plants," Manual of Engineering Practice No. 13, ASCE, New York, New York), the
balance to pass a 10-cycle test using the same criteria. Stone media shall be
free from iron. Manufactured media shall be chemically and biologically inert.
The media shall be structurally stable to allow for distributor maintenance or
a suitable access walkway shall be provided.
1. Rock or stone filter media shall have a
minimum depth of five feet above the underdrains. Manufactured contactor media
should have a minimum depth of 10 feet to provide adequate contact time with
the wastewater. Rock and stone filter media depth should not exceed 10 feet and
manufactured filter media should not exceed 30 feet except where special
construction is justified through performance data or pilot plant
studies.
2. Rock, stone, and
similar media shall not contain more than five percent by weight of pieces
whose longest dimension is three times the least dimension. They shall be free
from thin elongated and flat pieces, dust, clay, sand or fine material and
shall conform to the following size and grading when mechanically graded over
vibrating screens with square openings:
a.
Passing 4-1/2 inch screen-100% by weight
b. Retained on three-inch screen-95-100% by
weight
c. Passing two inch
screen-0-2% by weight
d. Passing
one inch screen-0-1% by weight
e.
Maximum dimensions of stone--five inches
f. Minimum dimensions of stone--three
inches
3. Applications
of manufactured media such as wood, plastic, etc., will be evaluated on a
case-by-case basis. The handling and placement of the media should be
specified.
E. Roughing
reactors. Roughing contact reactors are used to reduce the organic load applied
to subsequent oxidation processes. They are particularly applicable preceding
an activated sludge process or a second stage filter in a treatment works
receiving high strength wastewater (excessive organic loadings). Roughing
filter designs differ from other contactors principally on the basis of the
deeper depths and media design utilized for given loadings in comparison to
high rate trickling filters. Since it is used to reduce the downstream organic
loading rather than to provide a stabilized effluent, it is designed to receive
organic loadings exceeding those applied to conventional biological
contactors.
F. Granular media
filters. Intermittently dosed biological sand filters utilized to process
septic tank effluent to meet secondary treatment standards should be limited to
schools, day camps and other installations that have part-time usage. These
reactors should also be limited to those installations generating a sewage flow
of 20,000 gallons per day or less and provide lengthy rest periods for filter
operation. Biological sand filters may serve year-round residential dwellings
if the design capacity is restricted to 1,000 gallons per day or less.
1. Biological sand filters shall not be used
to treat raw wastewater and shall be preceded by a minimum of pretreatment
designed to produce a settled sewage with adequate grease management. The use
of biological sand filters designed to enhance effluent from other sewage
treatment reactors shall be evaluated on a case by case basis.
2. Sand filter media beds shall consist of
level areas of sand beneath which there are graded layers of gravel surrounding
the underdrains. Each filter bed shall have an impervious bottom. Sewage is
discharged onto the beds through rotary distributors or pipes onto splash
plates or, in the case of subsurface filters, through lines of drain tile laid
with open joints. Open sand beds shall be surrounded by a concrete, brick or
cinder block wall extending above the sand and at least one foot above ground
level. For subsurface sand filters, the surrounding wall is not necessary
except to prevent caving of the earth walls while the sand and gravel are being
placed. The underdrainage system shall consist of open joint or perforated pipe
tied together into a manifold and vented to the atmosphere. The minimum size
for the underdrain shall be four inches in diameter. The underdrain pipes
should be placed on a slope of not less than 1.0%.
3. Rock, gravel and sand media components
shall be clean and free of organic matter, clay or loam soils and fine
limestone material.
a. The media depth shall
not be less than 30 inches. Sand media for intermittently dosed and
recirculated effluent, shall have an effective size of 0.30 mm to 1.0 mm and
0.8 mm to 1.5 mm, respectively. The uniformity coefficient should not exceed
4.0. No more than 2.0% shall be finer than 0.177 mm (80 mesh sieve) and not
more than 1.0% shall be finer than 0.149 mm. No more than 2.0% shall be larger
than 4.76 mm (4 mesh sieve). Larger granular media up to 5 mm in effective size
may be considered on a case by case basis.
b. The gravel base for sand media shall
conform to the Virginia Department of Transportation's Road and Bridge
Specifications (1974). The base gravel shall consist of No. 3 sized gravel with
at least a three-inch depth above the sloped underdrains. The middle layer
shall consist of at least three inches of No. 68 gravel, and the top layer
shall consist of at least three inches of No. 8 gravel.
4. Dosing tanks with either siphons or pumps
for sand filters shall have the capacities to effect the dosage volumes
required. The siphons and the rotary distributor should be supplied by the same
manufacturer. The influent line to the rotary distributor shall be equipped
with a valved drain.
5. Sand
filters designed for intermittent flooding should be divided into at least two
beds for small filters and three beds for the larger filters. Distribution
boxes must be provided for diverting the sewage onto the filter bed or beds
desired, as it is often necessary to take one filter bed out of operation
during scheduled rest periods. Providing such rest periods will prevent surface
clogging that results in sewage ponding above filter media. When three filters
are employed, only two beds are normally used at any one time, the other bed
being held out of operation for rest periods or maintenance, if
required.
6. In the design of
intermittently flooded sand filters the area of the filter beds is normally
based upon a rate of application of 2.3 gallons per square foot per day. Also,
a sufficient amount of settled sewage should be discharged onto the sand bed
surface to cover the sand to a depth of two inches.
7. A rotary distributor will accomplish
uniform application of settled sewage over the sand filter surface. A uniform
application will maintain the design treatment efficiency of the filter so that
a relatively higher dosage rate may be utilized or, for equal sewage flows, the
area of sand bed required may be less than other designs. The design of the
area of the filter beds equipped with rotary distributors should be based on an
application rate of 3.5 gallons per square foot per day. The amount of sewage
applied to the sand filter at each discharge of the dosing siphon should be
equal to a depth exceeding one-half inch over the entire sand bed area being
dosed.
8. The rate of dosage onto a
buried sand filter shall not exceed 1.15 gallons per square foot per day of
settled sewage. Settled sewage shall be applied to the filter through lines of
drain tile laid with open joints, with the tile placed in a 12-inch layer of
No. 3 stone. The top of the filter may be finished with a 12-inch layer of
stone. Where it is not feasible or desirable to finish the top of the
subsurface filter with stone, a 3-inch layer of straw covered with a four to
eight inch layer of top soil may be used. Open joint underdrain tiles shall be
sloped one inch per 10 feet and shall be installed in the base gravel and
connected to the effluent pipe. The ends of the distribution lines should be
tied together into a manifold and should be vented to the atmosphere. All open
joints shall be covered with collars of asphalt paper or other suitable
material.
Distribution boxes must be provided for diverting sewage
onto the filter beds through headers, with each header connecting to not more
than four distribution lines, where multiple units are used. Each application
must completely fill the tile lines in use.
9. Consideration should be given to providing
recirculation for granular media filters to improve treatment performance.
Recirculating sand filters should be designed using a hydraulic loading rate of
3-1/2 gallons per day per square feet, based on average daily flow, with an
organic loading rate not to exceed 0.005 pounds of BOD5
per day per square foot of surface area. A recirculation ratio greater than 3:1
shall be provided. The use of granular media filters for nutrient removal will
be evaluated on a case by case basis based on evaluation of performance data.
Granular media filters shall be timer dosed and adjustable from one to 10
minutes of dosing per 30 minutes on time.
Statutory Authority
§ 62.1-44.19 of the Code of Virginia.
