AWWA MTC64629

Drinking water plants in Florida that use brackish groundwater and surface waters rely on reverse osmosis for treating these water sources. The typical process train utilized in the state includes: chemical pretreatment; cartridge filtration; reverse osmosis (RO); degasification; corrosion control; and, disinfection. Pretreatment chemicals are used to manage scaling in the RO membranes and reduce the pH to optimize hydrogen sulfide stripping in the degasifiers. Post treatment chemicals are used to adjust the pH and alkalinity for corrosion control and for distribution system disinfection. The objective of this paper was to present the methods used in southwest Florida for pre- and post-treatment chemical addition and the associated costs with each option, and to present a method for optimizing plant chemical applications and minimizing operational costs. The cost model prepared was compared to a full-scale operating plant. The traditional method of pretreatment uses acid and anti-scalant to suppress the pH of the feedwater prior to the RO elements to avoid scaling. The pH remains suppressed entering the degasifier at the optimum pH for hydrogen sulfide stripping. Following recent developments in reverse osmosis elements and manufacturer changes in scale inhibitors, a new approach is to delay pH correction until just prior to degasification, thus maintaining an ambient pH through the RO 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, data from regional plants, the methods used to evaluate the whole plant chemistry, and the resulting annual operating costs for the year 2008 and year 2033 water quality conditions. The pH and alkalinity profiles through the plant were examined using water chemistry and unit process analyses. The study found that acid consumption could be reduced but required a simultaneous increase in chemicals for post treatment. Results are highly dependent on the alkalinity of the source water. The results of the analysis will be compared to a full-scale operating plant. The cost savings for a new 12 mgd plant at low feedwater alkalinity could be as high as $150,000/year in operating costs for the optimum solution. The paper illustrates a method for whole plant 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.