Section 58.01.08.523 - SURFACE WATER TREATMENT: SLOW SAND FILTRATIONThe use of slow sand filters requires 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 of Design for Slow Sand Filtration, Slow Sand Filtration, and Recommended Operations and Optimization Goals, Slow Sand Filtration referenced in Subsection 002.02, may be used as guidance in design and operation of slow sand filtration facilities.
01.Quality of Raw Water. Slow rate gravity filtration must 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 must 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.02.Number of Units. A minimum of two (2) units for redundancy must 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 PWS can provide plant design capacity with the filter taken out of service for maintenance and repairs.03.Structural Details and Hydraulics. Slow rate gravity filters must be designed to provide a cover, unless otherwise approved by the Department, 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 must be provided.04.Underdrains. Each filter unit must be equipped with a main drain and an adequate number of lateral underdrains to collect the filtered water. The underdrains must 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 is three (3) feet if pipe laterals are used.05.Filter Material. The following requirements apply:a. A minimum depth of thirty (30) inches of filter sand must be placed on graded gravel layers.b. The effective size must be between fifteen hundredths (0.15) of a millimeter and thirty-five hundredths (0.35) of a millimeter. Larger sizes may be considered by the Department based on the results of a pilot study in accordance with Subsection 501.19.c. The uniformity coefficient must not exceed three point zero (3.0).d. The sand must be cleaned and washed free from foreign matter.e. The sand must be rebedded to the original minimum depth of thirty (30) inches when scraping has reduced the bed depth to no less than twenty-four (24) inches. Where sand is to be reused in order to provide biological seeding and shortening of the ripening process, rebedding must utilize a "throw over" technique whereby new sand is placed on the support gravel and existing sand is replaced on top of the new sand. The maximum filtration rate must not exceed zero point one (0.1) gallon per minute per square foot for each individual bed.06.Filter Sand Support. a. A three (3)-inch layer of sand must be used as a supporting media for filter sand. The supporting sand must have an effective size of zero point eight (0.8) millimeters to two point zero (2.0) millimeters and a uniformity coefficient not greater than one point seven (1.7).b. Gravel must consist of cleaned and washed, hard, durable, rounded rock particles and may not include flat or elongated particles. The coarsest gravel must be two and one-half (2.5) inches in size when the gravel rests directly on a lateral system and must extend above the top of the perforated laterals. Not less than four (4) layers of gravel may be provided in accordance with the size and depth distribution specified in the table below. Reduction of gravel depths and other size gradations may be considered upon justification to the Department. 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 |
07.Depth of Water Over Filter Beds. The design must provide a depth of at least three (3) to six (6) feet of water over the sand. Influent water must not scour the sand surface.08.Control Appurtenances. Each filter must 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 must be provided for each filter bed.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 must be as follows: a. Filters must be filtered-to-waste after scraping or cleaning until the effluent turbidity falls consistently below the pre-cleaning level, unless otherwise approved by the Department.b. Filters must be filtered-to-waste following construction, re-sanding, or extended shutdown based on project specific protocols approved by the Department and incorporated into a Department approved operation and maintenance manual. These protocols may be based on factors from standard literature such as those listed in Subsection 002.02 but typically include factors such as minimum filter-to-waste time periods, bacteriological testing, and effluent turbidity. Sampling results from the filter-to-waste period must be provided to the Department for review and the Department must provide authorization prior to restarting service to the public.10.Supernatant Drain Required. Filter beds must 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.11.Filter Bed Control and Minimum Rate of Flow. Each filter bed must be controlled separately and filters must be operated at a constant filtration rate with any changes made gradually. The minimum rate of filtration must be at least two hundredths (0.02) gallons per minute per square foot.Idaho Admin. Code r. 58.01.08.523