NR 811.49(1)(j)5.5. 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.

NR 811.49(1)(j)6.6. A 1 to 1.5-inch pressure hose and rack at the operating floor for washing the filter walls.

NR 811.49(1)(k)1.1. 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).

NR 811.49(1)(k)2.2. 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.

NR 811.49(1)(k)3.3. Washwater pumps in duplicate unless an alternate means of obtaining washwater is available.

NR 811.49(1)(k)4.4. Backwashing of not less than 15 minutes wash of one filter at the design rate of wash.

NR 811.49(1)(k)5.5. 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.

NR 811.49(1)(k)6.6. A rate-of-flow indicator and totalizer on the main washwater line, located for convenient reading by the operator during the washing process.

NR 811.49(1)(k)7.7. Backwashing by a method which prevents rapid changes in the backwash water flow.

NR 811.49(1)(k)8.8. Backwash shall be operator initiated. Backwash systems with automated sequencing shall be operator adjustable.

NR 811.49(1)(k)9.9. 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.

NR 811.49(2)(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:

NR 811.49(2)(a)(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.

NR 811.49(2)(b)1.1. 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.

NR 811.49(2)(b)2.2. A cover or superstructure.

NR 811.49(2)(b)3.3. Headroom to permit normal movement by operating personnel for scraping and sand removal operations.

NR 811.49(2)(b)4.4. Adequate manholes and access ports for handling of sand.

NR 811.49(2)(b)5.5. Filtration to waste and overflow at the maximum filter water level.

NR 811.49(2)(c)(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.

NR 811.49(2)(d)(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.

NR 811.49(2)(e)(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.

NR 811.49(2)(f)(f) Filter gravel. The supporting gravel shall conform to the size and depth distribution requirements in sub. (1) provided for rapid rate gravity filters.

NR 811.49(2)(g)(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.