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Drinking water plants in Florida that use brackish groundwater and surface waters rely onreverse osmosis for treating these water sources. The typical process train utilized in the stateincludes: chemical pretreatment; cartridge filtration; reverse osmosis (RO); degasification; corrosioncontrol; and, disinfection. Pretreatment chemicals are used to manage scaling in the ROmembranes and reduce the pH to optimize hydrogen sulfide stripping in the degasifiers. Posttreatment chemicals are used to adjust the pH and alkalinity for corrosion control and fordistribution system disinfection. The objective of this paper was to present the methods used insouthwest Florida for pre- and post-treatment chemical addition and the associated costswith each option, and to present a method for optimizing plant chemical applications andminimizing operational costs. The cost model prepared was compared to a full-scale operatingplant.The traditional method of pretreatment uses acid and anti-scalant to suppress the pH of thefeedwater prior to the RO elements to avoid scaling. The pH remains suppressed entering thedegasifier at the optimum pH for hydrogen sulfide stripping. Following recent developments inreverse osmosis elements and manufacturer changes in scale inhibitors, a new approach is todelay pH correction until just prior to degasification, thus maintaining an ambient pH through theRO trains and relying on the scale inhibitor to control scaling in the membranes. This paper presents the results of the chemical optimization study which included a literature review, datafrom regional plants, the methods used to evaluate the whole plant chemistry, and the resultingannual operating costs for the year 2008 and year 2033 water quality conditions. The pH andalkalinity profiles through the plant were examined using water chemistry and unit processanalyses. The study found that acid consumption could be reduced but required a simultaneousincrease in chemicals for post treatment. Results are highly dependent on the alkalinity of thesource water. The results of the analysis will be compared to a full-scale operating plant. Thecost savings for a new 12 mgd plant at low feedwater alkalinity could be as high as $150,000/yearin operating costs for the optimum solution. The paper illustrates a method for wholeplant evaluations during engineering to optimize the placement of chemical injection points,which is critical for utilities considering new plants or alternate water supplies. Includes 3 references, tables, figure.

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Edition: Vol. - No. Published: 03/01/2007 Number of Pages: 9 File Size: 1 file , 300 KB