Section 58.01.08.521 - SURFACE WATER TREATMENT: RAPID RATE GRAVITY FILTERS01.Pretreatment. The use of rapid rate gravity filters requires pretreatment in the form of coagulation, flocculation, and sedimentation.02.Rate of Filtration. The filter rate must be proposed and justified by the design engineer in the Department approved PER.03.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. Where declining rate filtration is provided, the variable aspect of filtration rates, and the number of filters must be considered when determining the design capacity for the filters.04.Structure and Hydraulics. The filter structure must be designed to provide for: a. There may be no protrusion of the vertical filter walls into the filter media.b. Cover by superstructure with sufficient headroom to permit normal inspection and operation.c. Minimum depth of filter box of eight and one-half (8.5) feet.d. Minimum water depth over the surface of the filter media of three (3) feet.e. Trapped effluent to prevent backflow of air to the bottom of the filters.f. Prevention of floor drainage to the filter with a minimum four (4) inch curb around the filters.g. Prevention of flooding by providing overflow.h. Maximum velocity of treated water entering the filters of two (2) feet per second.i. Cleanouts and straight alignment for influent pipes or conduits where solids loading is heavy, or following lime-soda softening.j. Washwater drain capacity to carry maximum flow.k. Walkways around filters to be not less than twenty-four (24) inches wide and equipped with safety handrails or walls.l. Construction so as to prevent cross connections and common walls between potable water and non-potable fluids.05.Wash Water Troughs. Washwater troughs must be constructed to have: a. The bottom elevation above the maximum level of expanded media during washing.b. A two (2) inch freeboard at the maximum rate of wash.c. The top edge level and all at the same elevation.d. Spacing so that each trough serves the same number of square feet of filter area.e. Maximum horizontal travel of suspended particles to reach the trough not to exceed three (3) feet.06.Filter Material. The media must be clean silica sand or other natural or synthetic media free from detrimental chemical or bacterial contaminants, approved by the Department, and having the following characteristics: a. A total depth of not less than twenty-four (24) inches and generally not more than thirty (30) inches.b. An effective size range of the smallest material no greater than forty-five hundredths (0.45) of a millimeter to fifty-five hundredths (0.55) of a millimeter.c. A uniformity coefficient of the smallest material not greater than one and sixty-five hundredths (1.65).d. A minimum of twelve (12) inches of media with an effective size range no greater than forty-five hundredths (0.45) of a millimeter to fifty-five hundredths (0.55) of a millimeter and a specific gravity greater than other filtering materials within the filter.e. Types of filter media are as follows: i. Clean, crushed anthracite or a combination of anthracite and other media may be considered on the basis of experimental data specific to the project. The anthracite must have the following characteristics:(1) Effective size of forty-five hundredths (0.45) of a millimeter to fifty-five hundredths (0.55) of a millimeter with uniformity coefficient not greater than sixty-five hundredths (1.65) when used alone.(2) Effective size of eight tenths (0.8) of a millimeter to one and two-tenths (1.2) millimeters with a uniformity coefficient not greater than one and eighty-five hundredths (1.85) when used as a cap.(3) Effective size for anthracite used as a single media on potable groundwater for iron and manganese removal only must be a maximum of eight tenths (0.8) of a millimeter (effective sizes greater than this may be approved based upon onsite pilot plant studies or other demonstration acceptable to the Department). See Subsection 501.19 for general information on conducting pilot studies.ii. Sand media must have the following characteristics:(1) Effective size of forty-five hundredths (0.45) of a millimeter to fifty-five hundredths (0.55) of a millimeter.(2) Uniformity coefficient of not greater than one and sixty-five hundredths (1.65).(3) Larger size sand media may be allowed by the Department where full-scale tests have demonstrated that treatment goals can be met under all conditions.iii. Granular activated carbon (GAC) as a single media may be considered for filtration only after pilot or full-scale testing and with prior approval of the Department in accordance with Subsection 501.19. The design must include the following: (1) The media must meet the basic specifications for filter media as given in Subsections 521.06.a. through d., except that larger size media may be allowed where full scale tests have demonstrated that treatment goals can be met under all conditions.(2) There must be a means for periodic treatment of filter material for control of bacterial and other growth.(3) Provisions must be made for frequent replacement or regeneration.iv. Other media will be considered based on experimental data and operating experience.v. A three (3) inch layer of torpedo sand must be used as a supporting media for filter sand where supporting gravel is used, and must have an effective size of eight-tenths (0.8) millimeters to two (2.0) millimeters, and a uniformity coefficient not greater than one and seven-tenths (1.7).vi. Gravel, when used as the supporting media, must consist of cleaned and washed, hard, durable, rounded silica particles and must 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 must 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 when proprietary filter bottoms are specified. 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.Filter Bottoms and Strainer Systems. Departure from the standards set out in Subsection 521.07 may be acceptable for high rate filters and for proprietary bottoms. Porous plate bottoms must not be used where iron or manganese may clog them or with waters softened by lime. The design of manifold-type collection systems must: a. Minimize loss of head in the manifold and laterals.b. Ensure even distribution of wash water and even rate of filtration over the entire area of the filter.c. Provide the ratio of the area of the final openings of the strainer systems to the area of the filter at about three-thousandths (0.003),d. Provide the total cross-sectional area of the laterals at twice the total area of the final openings.e. Provide the cross-sectional area of the manifold at one and one-half (1.5) to two (2) times the total area of the laterals.f. Lateral perforations without strainers must be directed downward.08.Surface or Subsurface Wash. Surface or subsurface wash facilities are required except for filters used exclusively for iron or manganese removal, and may be accomplished by a system of fixed nozzles or a revolving-type apparatus. All devices must be designed with:a. Provision for water pressures of at least forty-five (45) pounds per square inch.b. A properly installed vacuum breaker or other approved device to prevent back siphonage if connected to the treated water system.c. Rate of flow of two (2.0) gallons per minute per square foot of filter area with fixed nozzles or one-half (0.5) gallon per minute per square foot with revolving arms.d. Air wash can be considered based on experimental data and operating experiences.09.Air Scouring. Air scouring can be considered in place of surface wash provided the following conditions are met:a. Air flow for air scouring the filter must be three (3) to five (5) standard cubic feet per minute square foot of filter area when the air is introduced in the underdrain; a lower air rate must be used when the air scour distribution system is placed above the underdrains.b. A method for avoiding excessive loss of the filter media during backwashing must be provided.c. Air scouring must be followed by a fluidization wash sufficient to restratify the media.d. Air must be free from contamination.e. Air scour distribution systems must be placed below the media and supporting bed interface with the following exception: if placed at the interface the air scour nozzles must be designed to prevent media from clogging the nozzles or entering the air distribution system.f. Piping for the air distribution system must not be flexible hose which will collapse when not under air pressure and must not be a relatively soft material which may erode at the orifice opening with the passage of air at high velocity.g. Air delivery piping must not pass down through the filter media nor may there be any arrangement in the filter design which allows short circuiting between the applied unfiltered water and the filtered water.h. The backwash water delivery system must be capable of fifteen (15) gallons per minute per square foot of filter surface area (37 m/hr); however, when air scour is provided the backwash water rate must be variable and not exceed eight (8) gallons per minute per square foot (20 m/hr) unless operating experience shows that a higher rate is necessary to remove scoured particles from filter media surfaces.i. The filter underdrains must be designed to accommodate air scour piping when the piping is installed in the underdrain.10.Filter Appurtenances. The following must be provided for every filter:a. Influent and effluent sampling taps.b. A gauge capable of indicating loss of head.c. A meter indicating rate-of flow. A modified rate controller which limits the rate of filtration to a maximum rate may be used. However, equipment that simply maintains a constant water level on the filters is not acceptable, unless the rate of flow onto the filter is properly controlled. A pump or a flow meter in each filter effluent line may be used as the limiting device for the rate of filtration only if approved by the Department on a site-specific basis.11.Backwash. Provisions must be made for washing filters as follows: a. A minimum backwash rate such that a fifty (50) percent expansion of the filter bed is achieved.b. Filtered water provided at the required rate by wash water tanks, a wash water pump, from the high service main, or a combination of these.c. Wash water pumps in duplicate unless an alternate means of obtaining wash water is available.d. Not less than fifteen (15) minutes wash of one filter at the design rate of wash.e. A wash water regulator or valve on the main wash water line to obtain the desired rate of filter wash with the wash water valves on the individual filters open wide.f. A rate-of-flow indicator, preferably with a totalizer, on the main wash water line, located so that it can be easily read by the operator during the washing process.g. Design to prevent rapid changes in backwash water flow. Backwash must be operator initiated. Automated systems must be operator adjustable.12.Roof Drainage. Roof drains must not discharge into the filters or basins and conduits preceding the filters.Idaho Admin. Code r. 58.01.08.521