 |
Virginia Administrative Code (Last Updated: January 10, 2017) |
 |
Title 12. Health |
|
Agency 5. Department of Health |
|
Chapter 590. Waterworks Regulations |
Section 900. Softening
-
Softening shall not be used as the sole treatment method for surface waters or bacteriologically contaminated groundwater. The softening process selected shall be based upon the mineral qualities of the raw water and the desired finished water quality in conjunction with requirements for disposal of sludge or brine water, cost of plant, cost of chemicals, and plant location.
A. Lime, excess lime, and excess lime soda processes.
The applicable design standards for mixing, flocculation, and sedimentation are the same for the lime, excess lime, and excess lime soda processes as for conventional clarification except that the minimum flash mix time is five minutes, flocculation time is 40 minutes, and settling time is two hours. Where softening is included as a treatment process in conjunction with clarification, the greater detention time criteria shall govern. For criteria pertaining to softening with solids contact units, see 12VAC5-590-870 F.
1. Mechanical sludge removal equipment shall be provided in the sedimentation basin.
2. Determinations shall be made of the CO2 content of the raw water. When concentrations exceed 10 milligrams per liter, the economics of removal by aeration as opposed to removal with lime should be considered.
3. Equipment for stabilization of water softened by the excess lime and excess lime soda processes is required.
4. Staging shall be considered when the excess lime soda process is employed.
5. Provision shall be included for proper disposal of softening sludges.
6. The use of excess lime shall not be considered an acceptable substitution for chlorination.
B. Cation exchange process.
Iron, manganese, or a combination of the two, in the oxidized state or unoxidized state, should not exceed 0.3 milligrams per liter in the water as applied to the ion exchange material. Pretreatment shall be required when the content of iron, manganese, or a combination of the two, is one milligram per liter or more.
1. The units may be of pressure or gravity type, or either an upflow or downflow design, using automatic or manual regeneration. Automatic regeneration is suggested for small plants.
2. The design capacity for hardness removal should not exceed 20,000 grains per cubic foot when resin is regenerated with 0.3 pounds of salt per kilograin of hardness removed.
3. The depth of the exchange material should not be less than three feet.
4. The rate of softening should not exceed seven gallons per square foot per minute and the backwash rate should be six to eight gallons per square foot per minute.
5. The freeboard shall depend upon the specific gravity of the media and the direction of the water flow.
6. The bottoms, strainer systems and support for the exchange material shall conform to criteria provided for rapid rate gravity filters.
7. Facilities shall be included for even distribution of brine over the entire surface of both upflow and downflow units. Backwash, rinse and air relief discharge pipes shall be installed in such a manner as to prevent any possibility of backsiphonage.
8. A bypass shall be provided around softening units to produce a blended water of desirable hardness. Meters shall be installed to measure total water delivered to the system and on each softener unit. An automatic proportioning or regulating device and shutoff valve should be provided on the bypass line. In some installations it may be necessary to treat the bypassed water to obtain acceptable levels of iron and manganese in the finished water.
9. Waters having five units or more turbidity shall not be applied directly to the cation exchange softener. Silica gel materials should be used for water having a pH above 8.4 and should not be used when iron is present. When the applied water contains a chlorine residual, the cation exchange material shall be a type that is not damaged by residual chlorine. Phenolic resin shall not be used.
10. Smooth nose sampling taps shall be provided for the collection of representative samples for both bacteriological and chemical analyses. The taps shall be located to provide for sampling of the softener influent, softener effluent, and the blended water. The sampling taps for the blended water shall be at least 20 feet downstream from the point of blending.
11. Brine measuring or salt dissolving tanks and wet salt storage facilities shall be covered. The makeup water inlet shall have a free fall discharge of two pipe diameters but not less than two inches above the maximum liquid level of the unit or be protected from backsiphonage. Water for filling the tank should be distributed over the entire surface by pipes above the maximum brine level in the tank. The salt shall be supported on graduated layers of gravel under which is a suitable means of collecting the brine. Wet salt storage basins must be equipped with manhole or hatchway openings having raised curbs and watertight covers having overhanging edges similar to those required for finished water reservoirs. Overflows, where provided, shall be turned down, have a proper free fall discharge and be protected with noncorrodible screens or self-closing flap valves.
12. Wet salt storage basins shall have sufficient capacity to store at least 30 days operating supply.
13. Stabilization for corrosion control shall be provided.
14. Suitable disposal must be provided for brine waste.
15. Pipes and contact materials shall be resistant to the aggressiveness of salt.
16. Salt storage tanks and feed equipment should be enclosed and separated from other operating areas in order to prevent damage to equipment.
Historical Notes
Derived from VR355-18-009.06 § 3.27, eff. August 1, 1991.