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Oklahoma Administrative Code (Last Updated: March 11, 2021) |
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TITLE 252. Department of Environmental Quality |
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Chapter 626. Public Water Supply Construction Standards |
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Subchapter 9. Treatment |
SECTION 252:626-9-9. Filtration
Latest version.
- (1) Gravity sand filtration(A) Slow sand filtration(B) Rapid sand filtration(2) Pressure sand filtration(3) Membrane processes(b) Appurtenances.(1) Design filters with filter rate controllers that ensure the rated capacity of the filter will not be exceeded.(2) Head loss through the filter media is monitored by differential pressure-cell devices that measure the water pressure above and below the filter media. The head loss sensor connection to the filter box should be located approximately four inches (4") above the top of the washwater collection trough to prevent the wash water from entering the sensor. A sediment trap with a drain shall be installed on the sensor line to capture any sediment that may enter the line. The end of the sensor shall be turned up, keeping a full column of water in the line at all times to minimize air entrainment. A fine mesh stainless steel screen shall be installed on the end of the sensor to prevent clogging of the filter media.(3) Include provisions for draining the filters to waste with appropriate measures for backflow prevention.(4) Provide a means to sample from the combined filter effluent line.(5) Provide continuous online turbidimeters with a recording device on the effluent line of each filter.(c) Slow sand filter design.(1) Slow sand filters are allowed for water supplies where raw water turbidity is less than 10 NTU or where this turbidity value can be obtained by pretreatment. Slow sand filters consist of 24 to 48 inches of sand, which has an effective size of 0.15 to 0.30 mm and a uniformity coefficient equal to or less than 2.5 supported by torpedo sand and graded gravel.(2) A minimum of two filters shall be provided. Design capacity shall be achievable with the largest filter out of service.(3) Provide for piping for ripening of the filter media.(4) Water depth above the filter sand surface must be 3 to 5 feet with 6 inches of freeboard.(5) Provide an underdrain system consisting of a manifold and collector laterals. Construct laterals of open joint, porous or perforated pipe or conduits with even spacing between laterals.(6) Support media must conform to OAC 252:626-9-9(d)(6).(7) Provide an orifice on the outlet line from each filter, limiting the flow of water through the system to 50 gal/day/ft of surface area. Size orifices in accordance with Appendix E, Table II. Locate the orifice at least 1 inch above the initial height of the filter sand surface. Alternatively, the outlet line equipped with an orifice may terminate in a weir box with the weir elevation at least 1 inch above the initial height of the filter sand surface.(8) Equip each filter effluent line with a rate of flow indicator.(9) Each filter shall be equipped with an indicating loss of head gauge or other means to measure head loss.(d) Rapid rate gravity filters.(1) Pretreatment. Pretreat water to be processed in rapid rate gravity filters prior to filtration by flocculation, coagulation and sedimentation.(2) Rate of filtration.(A) The maximum filtration rates:(i) single media filter is 2 gal/min/ft surface area,(ii) dual media filter is 3 gal/min/ft surface area, and(iii) multi-media filter is 4 gal/min/ft surface area.(B) Higher filtration rates will be considered only after pilot studies show that a higher rate is suitable for the raw water source. Approval of higher rates will require continuous monitoring of raw, settled and finished water for turbidity.(3) Structural details and hydraulics.(A) Define the hydraulic gradient across the rate-of-flow controller on the plans and specifications. Provide for a positive head at the throat of the controller when operating at the design flow rate. Show the entire hydraulic gradient from top of filter to clear well on the plans.(B) Provide for the following:(i) vertical walls within the filter,(ii) no protrusion of filter walls into filter media,(iii) head room to permit normal inspection and operation,(iv) minimum depth of filter box of 8.5 feet,(v) minimum water depth of 3 feet above the surface of filter media,(vi) trapped effluent to prevent backflow of air to the bottom of the filters,(vii) a minimum curb height of 4 inches must surround the filters to prevent the entrance of floor drainage,(viii) overflow with discharge to backwash wastewater facilities,(ix) maximum water velocity of 2 ft/s in pipes and conduits to filters,(x) cleanouts and straight alignment for influent pipes or conduits where solids loading is heavy, or following lime-soda softening,(xi) washwater drain capacity sufficient to carry maximum flow and equipped with an air gap a minimum of 2 times the diameter of the drain line,(xii) walkways around filters to be a minimum of 24 inches wide,(xiii) safety handrails or walls around filter areas adjacent to walkways,(xiv) for each filter unit, a meter indicating the instantaneous rate of flow.(4) Washwater troughs. Design washwater troughs as follows:(A) bottom elevation above the maximum level of expanded media during washing,(B) a 2-inch freeboard at the maximum rate of wash,(C) the top edges to be level,(D) spacing so that each trough serves an equal number of square feet of filter area, and(E) do not exceed a 3 foot maximum horizontal travel of suspended particles to trough.(5) Filter material. The media must be clean silica sand or other natural or synthetic material meeting AWWA standard specifications.(A) Silica sand (single media) must be a total depth of not less than 24 inches and generally not more than 30 inches, an effective size of 0.45 - 0.55 mm and a uniformity coefficient not greater than 1.65.(B) Anthracite coal (single media) must be a total depth of 30 to 36 inches of clean crushed anthracite coal, an effective size of 0.45 - 1.2 mm and a uniformity coefficient not greater than 1.65.(C) Silica sand and anthracite coal (dual media) must be a total depth of 24 to 30 inches with at least 12 inches of sand.(i) Sand must have an effective size of 0.45 - 0.55 mm and a uniformity coefficient not greater than 1.65.(ii) Anthracite must have an effective size of 0.45 - 1.2 mm and a uniformity coefficient not greater than 1.85.(D) Garnet, silica sand, and anthracite (multi-media) must have a total depth of media of at least 30 inches with a minimum of 4.5 inches of garnet, 9 inches of silica sand, and 16.5 inches of anthracite.(i) Garnet must have an effective size of 0.15 - 0.35 mm.(ii) Silica sand must have an effective size of 0.45 - 0.55 mm and a uniformity coefficient not greater than 1.65.(iii) Anthracite must have an effective size of 0.45 - 1.2 mm and a uniformity coefficient not greater than 1.85.(E) Granular activated carbon as a single media may be considered for filtration only after pilot or full scale testing and with prior approval of the DEQ. Granular activated carbon use is covered in 252:626-9-5.(6) Supporting media.(A) Torpedo sand. Provide a 3-inch layer of torpedo sand, with an effective size of 0.8 - 2.0 mm, and a uniformity coefficient not greater than 1.7, as a supporting media for filter sand.(B) Gravel. Gravel, when used as supporting media, must consist of hard, rounded particles and not include flat or elongated particles. The coarsest gravel must be 2-½ inches in size when the gravel rests directly on the strainer system, and extends above the top of the perforated laterals.(i) Provide at least 4 layers of gravel in accordance with Appendix E.(ii) Reduction of gravel depths may be considered upon justification to the DEQ when proprietary filter bottoms are specified.(7) Underdrainage system.(A) Design all filter piping based on a minimum flow rate of 5 gal/min/ft of surface area.(B) Design underdrainage system to collect water with minimum uniform loss-of-head over the filter bed during filtration and for uniform upward velocities throughout the entire filter bed during the backwash process.(C) Do not use porous types of underdrainage systems where the water has appreciable iron or manganese content, or where softening by lime is considered.(D) For underdrainage systems using strainers, the maximum ratio of area of strainer openings to area of filter is 0.003.(E) Direct laterals perforations without strainers downward.(F) Total cross-sectional area of laterals on underdrain systems must be twice the cross-sectional area of the final openings.(G) Design the cross-sectional area of the manifold to be twice the cross-sectional area of the laterals in order to minimize friction loss.(H) Design the manifold so that air cannot accumulate as the result of slope or connection to effluent piping.(8) Rate of flow controllers. Equip each filter with a rate of flow controller to ensure that the rated capacity of each filter is not exceeded during operation of other filters.(9) Surface wash 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.(A) Install a reduced pressure zone (RPZ) back-flow preventer on surface wash units and the potable water supply.(B) Minimum water pressure is 45 psi on the high side of the pressure-reducing valve.(C) Install a pressure regulator on the surface wash supply line.(D) The minimum design flow rate is 2.0 gal/min/ft of filter area for fixed nozzle designs and 0.5 gal/min/ft for revolving arm designs.(E) Air-operated surface wash systems are allowed.(10) Air scouring.(A) Design underdrain to accommodate air scour piping, when piping is installed in the underdrain.(B) Air flow for scouring the filter must be 3-5 standard ft/min/ft of filter area when introduced in the underdrain.(C) Make provisions to avoid excessive loss of filter media during backwashing.(D) Air must be free from contamination.(E) Place the air scour distribution systems below the filter media and supporting bed interface.(F) Do not use flexible hose piping that is capable of collapsing when not under pressure, or of materials easily eroded at the orifice by high velocity air.(G) To prevent short-circuiting, do not place air delivery piping in the filter media.(H) Design for ease of maintenance and replacement of air delivery piping.(I) Design the backwash water delivery system for 15 gal/min/ft of filter surface area. Where design includes air scour, backwash water rate must be variable with a normal operating range up to 8 gal/min/ft unless experience shows that a higher rate is necessary to remove scoured particles from filter media surfaces.(11) Backwash. Make provisions for backwashing filters as follows:(A) a minimum backwash rate of 15 gal/min/ft, or at a rate necessary for 50 percent expansion of the filter bed. A reduced rate of 10 gal/min/ft may be acceptable for full depth anthracite or granular activated carbon filter,(B) filtered water must come from washwater tanks, washwater pump(s), high service main, or a combination of these,(C) duplicate washwater pumps unless an alternate means of obtaining washwater is available,(D) sufficient water to backwash 1 filter for at least 15 minutes at design backwash rate,(E) regulator or control valve for each filter to obtain desired rate of filter wash,(F) rate-of-flow indicator on main washwater line, located so that it can be easily read by the operator during the back-washing process, and(G) design to prevent rapid changes in backwash water flow.(e) Rapid rate pressure filters.(1) General. Rapid rate pressure filters are only allowed for iron and manganese removal for ground water systems.(2) Details of design. Provide the following:(A) pressure gauges on inlet and outlet pipes of each filter,(B) filtration and backwashing of each filter individually,(C) minimum side wall shell height of 5 feet. A corresponding reduction in side wall height is acceptable where proprietary bottoms permit reduction of the gravel depth,(D) top of washwater collectors to be at least 18 inches above surface of media,(E) underdrain system to efficiently collect the filtered water and to uniformly distribute the backwash water at a rate no less than 15 gal/min/ft of filter area,(F) an air release valve on the highest point of each filter,(G) an accessible manhole to facilitate inspections and repairs,(H) means to observe wastewater during backwashing,(I) construction to prevent cross-connection,(J) rate of filtration must not exceed 3gal/ft of filter area, and(K) sufficient information on the filter media to allow review and approval on a case-by-case basis.(f) Membrane filtration. There are four categories of membrane filtration: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO). For the purposes of this design standard, membranes shall be defined strictly by pore size as follows: MF, 0.1 - 0.2 µm; UF, 0.01 - 0.1 µm; NF, 0.001 - 0.01 µm; and RO 0.0001 - 0.001 µm. Using these membrane sizes, MF and UF are used for particle and microbial removal, while NF and RO reject most dissolved contaminants.(1) Source water testing. Source water shall be tested for all parameters that may affect membrane filtration and finished water quality. Historic information shall be reviewed to determine water quality extremes that may be expected. Tabulated results of tests done, summaries and conclusions shall be submitted as part of the engineering report proposing membrane filtration.(2) Pilot plant verification study. Prior to initiating the design of a membrane treatment facility, the DEQ must be contacted to determine if a pilot plant study is required to determine the best membrane to use, type of pretreatment, type of post treatment, the blending ratio (blending is not allowed if the source water is surface water or GWUDI), the amount of reject water produced, process efficiency, process control monitoring, cold and warm water flux, fouling potential, operating and TMP, differential pressure and other design criteria. The DEQ must be contacted prior to iniating any pilot study to establish a protocol. The use of membrane processes for treatment of surface water shall require a pilot study for a minimum duration of three (3) months during the time period identified as having the historically poorest water quality for contaminants tested.(3) Log inactivation. Challenge testing removal efficiency for Giardia and Cryptosporidium shall be accepted by the DEQ if the system or modules meet the following:(A) Challenge testing shall be conducted according to the criteria established by 40 CFR § 141.179 (b)(2) and show at a minimum 2.5 log removal of Giardia and 2.0 log removal for Cryptosporidium.(B) At least 0.5 log removal credit shall be from approved disinfection process using chlorine, chlorine dioxide, ozone or UV.(4) Membrane materials. Provide for compatibility of membrane material and the use of oxidants in the engineering report.(5) Pretreatment. Membrane processes treating surface water shall require pretreatment in accordance with OAC 252:626-9-2. NF or RO processes treating surface water shall require pre-sedimentation in accordance with OAC 252:626-9-2 (d).(6) Post treatment. Post treatment shall be addressed in the engineering report, which shall demonstrate the degasification of carbon dioxide, hydrogen sulfide removal, organic removal, pH, hardness adjustment for corrosion control, and disinfection as a secondary pathogen control for the distribution system.(7) Cross-connections. Membrane systems piping for feed water, filtrate, backwash water, waste and chemical cleaning shall be designed to prevent any cross connection with any potable water supply, in accordance with OAC 252:626-5-15.(8) Flow meters. Flow meters shall be provided on the source water influent piping, the plant finish water piping and on membrane backwash piping.(9) Pressure gauges. Pressure gauges shall be provided on the influent and effluent piping to each membrane unit.(10) Turbidity monitoring. Turbidity monitoring equipment shall be required on all membrane processes treating surface water and GWUDI. Turbidity monitoring equipment shall be installed on all influent and effluent piping of membrane units. Continuous turbidity recording equipment shall be provided on the effluent piping and connected to an alarm system to warn operators of an excessive turbidity breakthrough.(11) Membrane cleaning. A schedule and procedure for proper membrane cleaning shall be developed based on manufacturer's recommendations to prevent contamination of both raw and finished water. The Clean-In-Place procedures shall be approved by the DEQ. Chemicals shall meet AWWA, ANSI and/or NSF requirements, where applicable. Only treatment devices approved by ANSI or NSF shall be used.(12) Direct testing equipment. Equipment for direct testing shall be provided to monitor membrane integrity and to detect and locate defects or breaches that could allow raw water to be diverted around the membrane process.(13) Indirect testing. The membrane system shall be designed to conduct and record indirect integrity continuously on each membrane unit.(14) Redundancy. Redundancy of control components, including, but not limited to, valves, air supply and computers shall be required. Provide membrane units to meet the design capacity with the largest unit out of service.(15) Flux rates. The design engineer shall address the following factors in the engineering report and/or pilot study:(A) Flux rate shall be based upon the coldest average monthly temperature anticipated and the reference temperature (20° Celsius for MF/UF and 25° Celsius for RO/NF).(B) Chemical cleaning strategy shall be determined to restore membrane permeability and acceptable flux without damaging the membrane integrity.(C) Backwash strategies shall be implemented for MF/UF membranes to enhance membrane flux and to extend intervals between chemical cleanings.(D) Flux rate shall be guaranteed by the manufacturer for a minimum of one (1) year.(16) Backwashing. Provisions for backwashing shall be included in the design according to the manufacturer's recommendations.(17) Disinfection. The system shall be properly disinfected and water shall be run to waste each time the membrane units are opened for maintenance. Certain disinfectants shall not be used through the membranes, if prohibited by the manufacturer.(18) Reject water and solids. Waste from membrane filtration shall be handled in accordance with the requirements in OAC 252:626-13.(19) Operation and maintenance requirements. Operation and maintenance manuals for membrane filtration systems shall be in accordance with OAC 252:626-3-7 (c)(6).