Design Guide for Community Water Systems - The Water, Sanitation ...

. Pre-treatment requirements. Iron, manganese, or a combination of thetwo, should not exceed 0.3 milligram per liter in the water as applied tothe ion exchange resin. Pre-treatment is required when the content ofiron, manganese, or a combination of the two, is one milligram per literor more (see Section 4.6). Waters having five units or more turbidityshould not be applied directly to the cation exchange softener.c. Design. The units may be of pressure or gravity type, of either anupflow or downflow design. Automatic regeneration based on volumeof water softened should be used unless manual regeneration is justifiedand is approved by the department. A manual override shall beprovided on all automatic controls.d. Exchange capacity. The design capacity for hardness removal shouldnot exceed 20,000 grains per cubic foot when resin is regenerated with0.3 pound of salt per kilograin of hardness removed.e. Depth of resin. The depth of the exchange resin should not be less thanthree feet.f. Flow rates. The rate of softening should not exceed seven gallons perminute per square foot of bed area and the backwash rate should be sixto eight gallons per minute per square foot of bed area. Rate-of-flowcontrollers or the equivalent must be installed for the above purposes.g. Freeboard. The freeboard will depend upon the specific gravity of theresin and the direction of water flow. Generally, the washwatercollector should be 24-inches above the top of the resin on downflowunits.h. Underdrains and supporting gravel. The bottoms, strainer systems andsupport for the exchange resin shall conform to criteria provided forrapid rate gravity filters.i. Brine distribution. Facilities should be included for even distribution ofthe brine over the entire surface of both upflow and downflow units.j. Cross connection control. Backwash, rinse, and air relief dischargepipes must be installed in such a manner as to prevent any possibility ofback-flow.k. Bypass. A bypass must be provided around softening units to producea blended water of desirable hardness. Totalizing meters must beinstalled on the bypass line and on each softening unit. The bypass linemust have a shut-off valve and should have an automatic proportioningor regulating device. In some installations, it may be necessary to treatthe bypassed water to obtain acceptable levels of iron and/ormanganese in the finished water.l. Additional limitations. Silica gel resins should not be used for watershaving a pH above 8.4 or containing less than 6 milligrams per litersilica and should not be used when iron is present. When the appliedwater contains a chlorine residual, the cation exchange resin shall be ofa type that is not damaged by residual chlorine. Phenolic resin shouldnot be used.Aug. 29, 2003Page 74