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

Universal Citation: ID Admin Code 58.01.08.523

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)

a. A minimum depth of thirty (30) inches of filter sand shall be placed on graded gravel layers. (3-24-22)

b. The effective size shall 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. See Subsection 501.19 for general information on conducting pilot studies. (3-24-22)

c. The uniformity coefficient shall not exceed three point zero (3.0). (3-24-22)

d. The sand shall be cleaned and washed free from foreign matter. (3-24-22)

e. The sand shall 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 shall 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 shall not exceed zero point one (0.1) gallon per minute per square foot for each individual bed. (3-24-22)

06. Filter Sand Support. (3-24-22)

a. A three (3)-inch layer of sand shall be used as a supporting media for filter sand. The supporting sand shall 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). (3-24-22)

b. Gravel shall consist of cleaned and washed, hard, durable, rounded rock particles and shall not include flat or elongated particles. The coarsest gravel shall 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 shall 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

(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)

a. Filters shall 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 based on documentation provided by the design engineer. (3-24-22)

b. Filters shall be filtered-to-waste following construction, re-sanding, or extended shutdown based on project specific protocols that have been approved by the Department and then 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 shall be provided to the Department for review and the Department must provide authorization prior to restarting service to the public. (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)

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