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The City of Deerfield Beach, Florida, needed to expand its existing Lime Softening Plantat the West Water Treatment Plant by 10.5 mgd. Membrane Softening was selected asthe water treatment process for the West Water Treatment Plant expansion due to thesuperior water quality which could be achieved and the enhanced ability to meet futureprojected drinking water regulations. The primary water quality parameters whichare of concern for this project include hardness, color, disinfection byproducts (DBPs), and iron. The average rawwater hardness in the City's well water supply is approximately 235 mg/l as calciumcarbonate. Several years ago, an American Water Works Association committee set 80mg/l of hardness as a desirable goal for finished water hardness. The City has been ableto produce water with hardness in the range of 70 to 90 mg/l using the lime softeningprocess. However, with the lime softening process the addition offree chlorine to reduce color levels can create challenges in meeting disinfectionbyproduct regulations. Membrane softening offers the advantage of being able to removecolor while at the same time reducing the levels of disinfection byproduct precursors.Membranes are very effective in reducing hardness; however, for corrosion controlconsiderations, the City would prefer to maintain finished water hardness in the range of70 to 90 mg/l. To achieve this objective, the project specifications require that permeatecalcium levels be in the range of 20 to 33 mg/l as calcium ion. The City's raw water is generally high in color with values ranging from 30 to 70 colorunits. Historically the City has reduced color by the addition of up to 20 mg/l of chlorineprior to softening. The addition of high chlorine dosage rates with the relatively highTOC (7.6 mg/l) results in DBPs including total trihalomethane (TTHM) values thatrange from 25 to 90 ug/l and total haloacetic acid (THAA) values of 60 to 90 ug/l in thefinished water. Since chlorine addition for color removal must be balanced against theneed to maintain acceptable DBP levels in the distribution system, color values in thefinished water from the lime softening plant typically are in the range of 11 CU. Whilethis color value is below the secondary standard of 15 CU, it is perceptible to asignificant number of consumers. For this reason, lowering finished water color levels isan important treatment objective for the City in order to improve customer satisfaction.One of the primary advantages of the membrane treatment process is the ability toremove both color and DBP precursors in a single treatment step. Depending onmembrane selection, color should be reduced to 1 to 2 CU in the permeate. In addition,with membrane treatment, THM formation potential (THMFP) and HAA formationpotential (HAAFP) should be reduced below the specification requirements of 40 and 30ug/l respectively.Another treatment concern for the City is the high levels of iron in the raw water, whichhistorically have averaged 1.6 mg/l with values as high as 2.2 mg/l. The City isrehabilitating four of its wells, which will be dedicated to the membrane softening plant.Raw water iron levels for the membrane plant are anticipated toaverage 1.5 mg/l. Since the rejection of iron should be similar to the rejection of calcium,if higher iron removal is required, the overall permeate hardness will decrease. Thismeans that with high raw water iron levels in the membrane softening process, finishedwater hardness objectives must be balanced against iron removal objectives. Becausethere is an MCL for iron but not for hardness, it was considered more important to selectthe membrane performance to meet the MCL for iron than maintaining a minimumpermeate hardness greater than 50 mg/l as CaCO3. In addition, the permeate will beblended with lime softened water in approximately a 60:40 ratio, which will help to keepthe finished water hardness in the desirable range. A permeate iro

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Edition: Vol. - No. Published: 03/05/2003 Number of Pages: 12File Size: 1 file , 820 KB