AWWA MTC57638

The City of Deerfield Beach, Florida, needed to expand its existing Lime Softening Plant at the West Water Treatment Plant by 10.5 mgd. Membrane Softening was selected as the water treatment process for the West Water Treatment Plant expansion due to the superior water quality which could be achieved and the enhanced ability to meet future projected drinking water regulations. The primary water quality parameters which are of concern for this project include hardness, color, disinfection byproducts (DBPs), and iron. The average raw water hardness in the City's well water supply is approximately 235 mg/l as calcium carbonate. Several years ago, an American Water Works Association committee set 80 mg/l of hardness as a desirable goal for finished water hardness. The City has been able to produce water with hardness in the range of 70 to 90 mg/l using the lime softening process. However, with the lime softening process the addition of free chlorine to reduce color levels can create challenges in meeting disinfection byproduct regulations. Membrane softening offers the advantage of being able to remove color while at the same time reducing the levels of disinfection byproduct precursors. Membranes are very effective in reducing hardness; however, for corrosion control considerations, the City would prefer to maintain finished water hardness in the range of 70 to 90 mg/l. To achieve this objective, the project specifications require that permeate calcium 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 color units. Historically the City has reduced color by the addition of up to 20 mg/l of chlorine prior to softening. The addition of high chlorine dosage rates with the relatively high TOC (7.6 mg/l) results in DBPs including total trihalomethane (TTHM) values that range from 25 to 90 ug/l and total haloacetic acid (THAA) values of 60 to 90 ug/l in the finished water. Since chlorine addition for color removal must be balanced against the need to maintain acceptable DBP levels in the distribution system, color values in the finished water from the lime softening plant typically are in the range of 11 CU. While this color value is below the secondary standard of 15 CU, it is perceptible to a significant number of consumers. For this reason, lowering finished water color levels is an 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 to remove both color and DBP precursors in a single treatment step. Depending on membrane selection, color should be reduced to 1 to 2 CU in the permeate. In addition, with membrane treatment, THM formation potential (THMFP) and HAA formation potential (HAAFP) should be reduced below the specification requirements of 40 and 30 ug/l respectively. Another treatment concern for the City is the high levels of iron in the raw water, which historically have averaged 1.6 mg/l with values as high as 2.2 mg/l. The City is rehabilitating four of its wells, which will be dedicated to the membrane softening plant. Raw water iron levels for the membrane plant are anticipated to average 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. This means that with high raw water iron levels in the membrane softening process, finished water hardness objectives must be balanced against iron removal objectives. Because there is an MCL for iron but not for hardness, it was considered more important to select the membrane performance to meet the MCL for iron than maintaining a minimum permeate hardness greater than 50 mg/l as CaCO3. In addition, the permeate will be blended with lime softened water in approximately a 60:40 ratio, which will help to keep the finished water hardness in the desirable range. A permeate iro