Wisconsin Administrative Code › Department of Natural Resources › NR 800 - Environmental Protection - Water Supply › Chapter NR 811 - Requirements For The Operation And Design Of Community Water Systems › Subchapter VII - Treatment › Section NR 811.49 - Filtration - gravity

Current through June 24, 2024
The application of any type of gravity filter and media
					 shall be supported by water quality data representing a period of use
					 sufficient to characterize any variations in water quality. Experimental or
					 pilot plant treatment studies may be required to demonstrate the applicability
					 of the method or rate of filtration proposed. Pressure filters will not be
					 approved for surface water applications. The following specific requirements
					 shall be met:
(1) RAPID RATE GRAVITY
				  FILTERS.
(a)
Pretreatment.
Rapid rate gravity filters may only be utilized after coagulation, flocculation
				  and sedimentation.
(b)
Number.
At least 2 filter units or cells shall be provided.
				  Provisions shall be made to meet the plant design capacity at the approved
				  filtration rate with one filter out of service. If only 2 units or cells are
				  provided, each shall be capable of meeting the plant design capacity, normally
				  the projected maximum daily demand.
(c)
Rate of filtration.
The
				  permissible rate of filtration shall be determined after consideration of
				  factors such as raw water quality, degree of pretreatment provided, filter
				  media, water quality control parameters, competency of operating personnel, and
				  other factors required by the department. If effective coagulation,
				  flocculation, sedimentation, and filtration processes are to be utilized with
				  relatively clean water sources, the following filtration rates may be approved:
Filtration Rate
Filter Media Type
2 gpm/ft2
Single Media
3 gpm/ft2
Dual Media
4 gpm/ft2
Tri Media
In all cases, the filtration rate shall be proposed and
					 justified by the design engineer and shall be approved by the department prior
					 to the preparation of final plans and specifications. The department may
					 approve higher rates than indicated in this paragraph with sufficient
					 justification by the design engineer or through pilot testing under s.
					 NR
					 811.44.
(d)
Structural details and
				  hydraulics.
The filter structure shall be designed to provide:
1. Vertical walls within the
				  filter.
2. No protrusion of the
				  filter walls or other structures into the filter media or the area between the
				  top of the media and the high water line during backwashing.
3. Cover by superstructure.
4. Head room to permit normal inspection and
				  operation.
5. Minimum filter box
				  depth of 8.5 feet.
6. Minimum water
				  depth over the surface of the media of 3 feet.
7. Trapped effluent pipe to prevent backflow
				  of air to the bottom of the filters.
8. Prevention of floor drainage to the filter
				  with a minimum 4-inch curb around the filters.
9. Prevention of flooding by providing an
				  overflow if this is not provided in a pretreatment unit.
10. Maximum velocity of treated water in the
				  pipe and conduits to the filter of 2 feet per second.
11. Cleanouts and straight alignment for
				  influent pipes or conduits where solids loading is heavy or following lime-soda
				  softening.
12. Washwater drain
				  capacity to carry maximum backwash flow.
13. Walkways around filters not less than 24
				  inches wide.
14. Safety handrails
				  or walls around the filter areas adjacent to walkways.
15. Construction to prevent cross connections
				  and common walls between potable and nonpotable water.
16. Washwater troughs.
(e)
Washwater troughs.
Washwater troughs shall be designed to provide:
1. A bottom elevation above the maximum level
				  of expanded media during washing.
2. A 2-inch freeboard at the maximum rate of
				  wash.
3. A top or edge which is all
				  at the same elevation.
4. Spacing
				  so that each trough serves the same number of square feet of filter
				  area.
5. A maximum horizontal
				  travel of suspended particles not exceeding 3 feet in reaching the
				  trough.
(f)
Filter material.
The media shall be clean silica sand or other
				  natural or synthetic media approved by the department and shall meet the
				  following general requirements: a depth of not less than 24 inches; an
				  effective size of the smallest material no greater than 0.45 mm to 0.55 mm,
				  depending upon the quality of the raw water; a uniformity coefficient of the
				  smallest material not greater than 1.65; a minimum of 12 inches of media with
				  an effective size range no greater than 0.45 mm to 0.55 mm; and a specific
				  gravity greater than other filtering materials within the filter. The following
				  specific requirements shall be met:
1. 'Sand.'
				  Sand shall have an effective size of 0.45 mm to 0.55 mm, a uniformity
				  coefficient of not greater than 1.65, specific gravity greater than 2.5 and an
				  acid solubility less than 5 percent.
2. 'Anthracite.' Filter anthracite shall
				  consist of clean, hard, and durable anthracite coal particles of various sizes.
				  Non-anthracite material may not be blended. Anthracite used as the only media
				  shall have an effective size from 0.45 mm to 0.55 mm and a uniformity
				  coefficient not greater than 1.65. Anthracite used to cap sand filters shall
				  have an effective size from 0.8 mm to 1.2 mm and a uniformity coefficient not
				  greater than 1.7. Effective size of anthracite for iron and manganese removal
				  from potable groundwater shall be a maximum of 0.8 mm. Effective sizes greater
				  than 0.8 mm may be approved by the department based upon onsite pilot plant
				  studies. Anthracite shall have a specific gravity greater than 1.4 and an acid
				  solubility less than 5%.
3.
				  'Granular activated carbon (GAC).'
a. Granular
				  activated carbon as a single media may be considered only after pilot or full
				  scale testing and with prior approval of the department.
b. The media shall meet the basic
				  specifications for filter media as provided in this paragraph except that
				  larger size media may be allowed by the department where full scale tests have
				  demonstrated that treatment goals can be met under all conditions.
c. There shall be provisions for a free
				  chlorine residual and adequate contact time in the water following the filters
				  and prior to distribution.
d. There
				  shall be means for periodic treatment of filter material for control of
				  bacterial and other growth.
e.
				  Provisions shall be made for frequent replacement or regeneration of granular
				  activated carbon if used for filtration.
4. 'High density sand.' High density sand
				  shall consist of hard durable, and dense grain garnet, ilmenite, hematite,
				  magnetite, or associated minerals of those ores that will resist degradation
				  during handling and use and shall meet all of the following:
a. Contain at least 95% of the associated
				  material with a specific gravity of 3.8 or higher.
b. Have an effective size of 0.2 to 0.3
				  mm.
c. Have a uniformity
				  coefficient of not greater than 1.65.
d. Have an acid solubility less than
				  5%.
5. 'Other media.'
				  Other media may be approved, but only on the basis of pilot tests and
				  experience which demonstrate that the requirements of this chapter will be
				  met.
6. 'Supporting media.' Torpedo
				  sand and gravel shall be provided as supporting media except when proprietary
				  filter bottoms are used. In that case, the department, on the basis of
				  substantiating information provided by the owner, may allow elimination of
				  certain layers of supporting media or a reduction in the depth of the layers.
				  Otherwise, the following apply:
a. A 3-inch
				  layer of torpedo sand shall be used as a supporting media for the filter sand.
				  The torpedo sand shall have an effective size of 0.8 mm to 2.0 mm, and a
				  uniformity coefficient not greater than 1.7.
b. Gravel, when used as the supporting media,
				  shall consist of hard, rounded silica particles and may not include flat or
				  elongated particles. The coarsest gravel shall be 2.5 inches in size when the
				  gravel rests directly on the strainer system, and shall extend above the top of
				  the perforated laterals or strainer nozzles. Not less than 3 layers of gravel,
				  in addition to the layer of torpedo sand, shall be provided in accordance with
				  the following size and depth distribution when used with perforated laterals or
				  strainer nozzles. Reduction of gravel depths may be considered upon
				  justification to the department when proprietary filter bottoms are specified.
Gravel Size
Gravel Depth
2 1/2 to 1 1/2 inches
5 to 8 inches
1 1/2 to 3/4 inches
3 to 5 inches
3/4 to 1/2 inches
3 to 5 inches
1/2 to 3/16 inches
2 to 3 inches
3/16 to 3/32 inches
2 to 3 inches
(g)
Filter bottoms and strainer
				  systems.
Departures from these standards by using proprietary bottoms
				  may be approved by the department on a case-by-case basis if the effectiveness
				  of the method is demonstrated. Porous plate bottoms may not be used where iron
				  or manganese may clog them or with waters softened by lime. The design of
				  manifold type collection systems shall:
1.
				  Minimize loss of head in the manifold and laterals.
2. Assure even distribution of washwater and
				  even rate of filtration over the entire area of the filter.
3. Provide a ratio of the area of the final
				  openings of the strainer systems to the area of the filter of about
				  0.003.
4. Provide a total
				  cross-sectional area of the laterals about twice the total area of the final
				  openings of the strainer system.
5.
				  Provide a cross-sectional area of the manifold
				  at 1.5 to 2 times the total
				  cross-sectional area of the laterals.
6. Lateral perforations without strainers
				  shall be directed upwards.
(h)
Surface wash.
Surface
				  wash facilities consisting of either fixed nozzles or a revolving mechanism are
				  required unless air scour equipment is provided. All surface wash devices shall
				  be designed with:
1. Water pressures of at
				  least 45 psi.
2. Volume of flow of
				  2.0 gallons per minute per square foot of filter area with fixed nozzles and
				  0.5 gallons per minute per square foot with revolving arms.
3. A vacuum breaker installed above the high
				  water elevation in the filter or other approved device to prevent back
				  siphonage.
(i)
Air scouring.
Air scouring may be provided in place of surface
				  wash. The following requirements apply:
1. Air
				  flow for air scouring the filter shall be 2 to 5 standard cubic feet per minute
				  per square foot of filter area when the air is introduced in the underdrain.
				  Air scour distribution systems placed above the underdrains shall use the lower
				  end of the range.
2. A method for
				  avoiding excessive loss of the filter media during backwashing shall be
				  provided.
3. Air scouring shall be
				  followed by a fluidization wash sufficient to restratify the media.
4. Air shall be free from
				  contamination.
5. Air scour
				  distribution systems shall normally be placed below the media and supporting
				  bed interface; if placed at the interface the air scour nozzles shall be
				  designed to prevent media from clogging the nozzles or entering the air
				  distribution system.
6. Piping for
				  the air distribution system may not be flexible hose which will collapse when
				  not under air pressure and may not be a relatively soft material which may
				  erode at the orifice opening with the passage of air at high
				  velocity.
7. Air delivery piping
				  may not pass down through the filter media nor may there be any arrangement in
				  the filter design which would allow short circuiting between the applied
				  unfiltered water and the filtered water except if all of the following criteria
				  are met:
a. The vertical piping is double
				  wall, welded at top and bottom, schedule 40 stainless steel for the internal
				  pipe and schedule 5 stainless steel for the external pipe.
b. The annulus between the double-wall is
				  pressurized on-site to 80 psi.
c.
				  An air connection to the double-wall annulus shall be provided including piping
				  with a pressure gauge, regulator, flow switch and ball valve along with an air
				  reservoir and compressor.
d. The
				  flow switch shall alarm and trigger filter shutdown if a pressure drop of over
				  10 psi is detected.
8.
				  The backwash delivery system shall be capable of 15 gallons per minute per
				  square foot of filter surface area; however, when air scour is provided, the
				  backwash rate shall be variable and may not exceed 8 gallons per minute per
				  square foot unless operating experience shows that a higher rate is necessary
				  to remove scoured particles from filter surfaces.
9. The filter underdrains shall be designed
				  to accommodate air scour piping when the piping is installed in the
				  underdrain.
10. Backwash facilities
				  shall meet the requirements of par. (k).
(j)
Appurtenances.
The
				  following shall be provided for every filter:
1. Sampling faucets on the individual and
				  combined influent and effluent lines. Combined filter effluent sample faucets
				  shall be located upstream of subsequent treatment processes.
2. Indicating loss-of-head gauge with
				  appropriate cross-connection protection.
3. Indicating flow rate controls. A modified
				  rate controller which limits the rate of filtration to a maximum rate may be
				  used. However, equipment that simply maintains a constant water level on the
				  filters will not be approved unless the rate of flow onto the filter is
				  properly controlled. A pump in each filter effluent line may be used as the
				  limiting factor for the rate of filtration only with approval from the
				  department.
4. For surface water
				  and groundwater under the direct influence of surface water, provisions for
				  filtering to waste with appropriate measures for backflow prevention.
5. For surface water and groundwater under
				  the direct influence of surface water, on-line continuous turbidimeters shall
				  be installed on the effluent from each filter. All turbidimeters shall
				  consistently determine and indicate the turbidity of the water in nephelometric
				  turbidity units (NTUs). Each turbidimeter shall report to a recorder that is
				  designed and operated to allow the operator to accurately determine the
				  turbidity at least every 15 minutes. Turbidimeters on individual filters shall
				  be designed to accurately measure low-range turbidities and trigger an alarm
				  when the effluent level exceeds 0.3 NTU. Access to the filter interior through
				  wall sleeves shall be provided in several locations to allow the installation
				  of sampling lines, pressure sensors and other devices, at different depths in
				  the filter media.
6. A 1 to
				  1.5-inch pressure hose and rack at the operating floor for washing the filter
				  walls.
(k)
Backwash.
Backwashing facilities shall be designed to provide:
1. A minimum rate of 15 gallons per minute
				  per square foot, consistent with water temperatures and specific gravity of the
				  filter media. The department may approve a reduced rate of 10 gallons per
				  minute per square foot for full depth anthracite or granular activated carbon
				  filters, if justification is provided. A reduced rate of backwashing is
				  acceptable when air scouring is provided that meets the requirements of par.
				  (i).
Note:
A rate of 20 gallons per minute per
					 square foot or a rate necessary to provide for a 50% expansion of the filter
					 bed is recommended.
2.
				  Backwashing by filtered water at the required rate from washwater tanks, a
				  washwater pump from a reservoir or a high service main, or a combination of
				  these.
3. Washwater pumps in
				  duplicate unless an alternate means of obtaining washwater is
				  available.
4. Backwashing of not
				  less than 15 minutes wash of one filter at the design rate of wash.
5. A washwater regulator or valve on the
				  washwater line to obtain the desired rate of filter wash with the washwater
				  valves on the individual filters open wide.
6. A rate-of-flow indicator and totalizer on
				  the main washwater line, located for convenient reading by the operator during
				  the washing process.
7. Backwashing
				  by a method which prevents rapid changes in the backwash water flow.
8. Backwash shall be operator initiated.
				  Backwash systems with automated sequencing shall be operator
				  adjustable.
9. The backwash
				  discharge shall terminate above a collection basin with a free air break.
				  Backwash discharges may not be directly piped to a wastewater collection unless
				  a breather pipe is installed that provides adequate backflow prevention.
(l)
Miscellaneous.
Roof drains may not discharge into the filters
				  and basins or the conduits preceding the filters.