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

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.

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.

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).'

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:

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:

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.

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.

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:

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).

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.

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.

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.

1. A loss-of-head gauge.

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.