Idaho Administrative Code
Title IDAPA 58 - Environmental Quality, Department of
Rule 58.01.08 - IDAHO RULES FOR PUBLIC DRINKING WATER SYSTEMS
Section 58.01.08.523 - FACILITY AND DESIGN STANDARDS: SURFACE WATER TREATMENT: SLOW SAND FILTRATION
Current through August 31, 2023
The use of these filters shall require prior engineering studies to demonstrate the adequacy and suitability of this method of filtration for the specific water supply. Slow Sand Filtration and Diatomaceous Earth Filtration for Small Water Systems, Manual on Slow Sand Filtration, and Slow Sand Filtration referenced in Subsection 002.02, may be used as guidance in design of slow sand filtration facilities. (3-24-22)
01. Quality of Raw Water. Slow rate gravity filtration shall be limited to waters having maximum turbidities of ten (10) nephelometric units and maximum color of fifteen (15) units; such turbidity must not be attributable to colloidal clay. Raw water quality data must include examinations for algae. For source water having variable turbidity, the potential use of a roughing filter or other pretreatment technology should be evaluated. The Department may allow the use of a pretreatment technology on raw waters that exceed the normal limits for turbidity and color, if it can demonstrated to the Department's satisfaction that pretreatment will enable slow sand filtration to properly operate and comply with these Rules. (3-24-22)
02. Number of Units. A minimum of two (2) units for redundancy shall be provided for filtration such that plant design capacity can be maintained with any component out of service for maintenance or repairs. The Department may allow a single bed filter if it can be demonstrated to the Department's satisfaction that an alternative water source is available such that the water system can provide plant design capacity with the filter taken out of service for maintenance and repairs. (3-24-22)
03. Structural Details and Hydraulics. Slow rate gravity filters shall be so designed as to provide a cover, unless otherwise approved by the Department based on documentation provided by the design engineer, headroom to permit normal movement by operating personnel for scraping and sand removal operations, adequate access hatches and access ports for handling of sand and for ventilation, filtration to waste, an overflow at the maximum filter water level, and protection from freezing. A permanent means of determining sand depth shall be provided. (3-24-22)
04. 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 underdrain will not exceed three-fourths (0.75) feet per second. The maximum spacing of laterals shall not exceed three (3) feet if pipe laterals are used. (3-24-22)
05. Filter Material. The following requirements apply: (3-24-22)
06. Filter Sand Support. (3-24-22)
Size of 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 |
(3-24-22)
07. Depth of Water Over Filter Beds. The design shall provide a depth of at least three (3) to six (6) feet of water over the sand. Influent water shall not scour the sand surface. (3-24-22)
08. Control Appurtenances. Each filter shall be equipped with a loss of head gauge, an orifice, Venturi meter, or other suitable means of discharge measurement installed on each filter to control the rate of filtration, and an effluent pipe designed to maintain the water level above the top of the filter sand. The effluent piping must not be directly interconnected with the other filter beds. A sample tap shall be provided for each filter bed. (3-24-22)
09. Ripening. Slow sand filters must be filtered-to-waste until they are biologically mature before being put into service following construction, scraping, re-sanding, or reopening after extended shutdown. The period of filter-to-waste shall be as follows: (3-24-22)
10. Supernatant Drain Required. Filter beds shall be equipped with a supernatant drain to allow for quick removal of water standing over sand that has become impermeable because it requires scraping or rebedding. (3-24-22)
11. Filter Bed Control and Minimum Rate of Flow. Each filter bed shall be controlled separately and filters must be operated at a constant filtration rate with any changes made gradually. The minimum rate of filtration shall be at least two hundredths (0.02) gallons per minute per square foot. (3-24-22)