Section NR 811.49 - Filtration - gravity 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.(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.(2) SLOW RATE GRAVITY FILTERS. The use of slow rate gravity filters is not allowed without prior engineering studies to demonstrate the adequacy and suitability of this method of filtration for the specific raw water supply. The following standards shall be applied: (a)Quality of raw water. Slow rate gravity filtration shall be limited to waters having maximum turbidities of 50 nephelometric turbidity units (NTUs) and maximum color of 30 units; turbidity may not be attributable to colloidal clay. Raw water quality data shall include examinations for algae.(b)Structural details and hydraulics. Slow rate gravity filters shall be designed to provide: 1. Not less than 2 filter units. If only 2 units are provided, each shall be capable of meeting the plant design capacity, normally the projected maximum daily demand, at the approved filtration rate. If more than 2 filter units are provided, the filters shall be capable of meeting the plant design capacity at the approved filtration rate with one filter removed from service.2. A cover or superstructure.3. Headroom to permit normal movement by operating personnel for scraping and sand removal operations.4. Adequate manholes and access ports for handling of sand.5. Filtration to waste and overflow at the maximum filter water level.(c)Rates of filtration. The permissible rates of filtration shall be based on the quality of the raw water as determined from experimental data. Proposed rates shall be submitted to the department for approval. The design rate shall be 45 to 150 gallons per day per square foot of sand area. However, the department may approve design rates of 150 to 230 gallons per day per square foot if effectiveness is demonstrated to the satisfaction of the department.(d)Underdrains. Each filter unit shall be equipped with a main drain and an adequate number of lateral underdrains to collect the filtered water. The underdrains shall be so spaced that the maximum velocity of the water flow in the lateral underdrain will not exceed 0.75 feet per second. The maximum spacing of the laterals may not exceed 3 feet if pipe laterals are used.(e)Filtering material. A minimum depth of 30 inches of filter sand, clean and free of foreign matter, shall be placed on graded gravel layers. The effective size shall be between 0.30 and 0.45 mm, and the uniformity coefficient may not exceed 2.5.(f)Filter gravel. The supporting gravel shall conform to the size and depth distribution requirements in sub. (1) provided for rapid rate gravity filters.(g)Depth of water on filter beds. The design shall provide a depth of at least 3 feet of water over the sand. Influent water shall be distributed in a manner which will not scour the sand surfaces.(h)Control appurtenances. Each filter shall be equipped with: 2. An orifice, Venturi meter or other suitable metering device installed on each filter to enable measurement of the rate of filtration.3. An effluent pipe located at an elevation which will maintain the water level in the filter above the top of the sand.Wis. Admin. Code Department of Natural Resources NR 811.49
CR 09-073: cr. Register November 2010 No. 659, eff. 12-1-10; renumbering of (1) (e) made under s. 13.92(4) (b) 1, Stats., Register November 2010 No. 659.Amended by, CR 22-074: am. (1) (b), (c), (f) 6. b., (j) 1., cr. (1) (k) 9. Register January 2024 No. 817, eff. 2/1/2024