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)
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)