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Kimley>>> Horn Page 9 <br />For years, raw water blending and zinc orthophosphate (ZO) were the methods used to stabilize the <br />membrane treated product water. Although this method was used to both inhibit corrosion and increase <br />alkalinity and hardness in the finished water, the alkalinity was still low (less than 20 mg/I as CaCO3). <br />Finished water quality fluctuations would sometimes create turbid conditions in the finished water when the <br />zinc orthophosphate would come out of solution, a phenomenon which only occurred when pH exceeded <br />8.3. Raw water blending also resulted in an increase in other constituents which were already removed <br />through the membrane softening process, such as sodium, chlorides, and specifically organics which <br />resulted in increased disinfection byproducts (DBPs) into the distribution system. <br />In order to address these issues, a finish water stabilization study was prepared for Indian River County <br />which outlined alternatives for post-treatment stabilization. The results of the study recommended pilot <br />testing and subsequent construction of a carbonic acid solution and lime slurry addition to the degasified <br />permeate at both water treatment facilities (RO Plants Lime Slurry Addition, completed in spring 2014). <br />The intent of the project was to primarily increase finish water alkalinity and improve overall stability through <br />remineralization without the use of zinc orthophosphate corrosion inhibitor. This project was successfully <br />implemented and both WTPs have been operating with a combination of lime slurry and carbonic acid <br />solution for approximately seven (7) years. Design water quality goals for alkalinity and hardness have <br />been achieved and maintained since this project was completed, with values ranging from 60-70 mg/L as <br />CaCO3 and 80-100 mg/L as CaCO3, respectively. The following table presents the current finished water <br />quality values for Hobart and Oslo, as well as the goals that were established during the previous <br />investigation and PDR for the finished water stabilization system: <br />Tahiti 1 - Finished Water Oiiality (mals <br />Water Quality Parameter <br />Goal <br />Current Water Quality <br />State Designated <br />Optimal Water Quality <br />Parameters @ POE*** <br />Hobart <br />Oslo <br />pH <br />8.3 <br />8.15 <br />8.2 <br />7.9-8.5 <br />Calcium Hardness (mg/L as CaCO3) <br />70 <br />90 <br />85 <br />60-100 <br />Total Hardness (mg/L as CaCO3) <br />120 <br />115 <br />110 <br />N/A <br />Alkalinity (mg/L as CaCO3) <br />70 <br />66 <br />62 <br />40-80 <br />LSI* <br />0.1-0.2 <br />0.07 <br />0.07 <br />N/A <br />CCPD* <br />0.5— <br />10.0 <br />0.43 <br />0.38 <br />N/A <br />TDS (mg/L) <br />300-350 <br />275-315 <br />275-315 <br />N/A <br />Dissolved Inorganic Carbonate (mg C/L) <br />17 <br />16 <br />16 <br />N/A <br />* Calculated using RTW Model <br />** Predicted utilizing EPA Guidance Manual for Selecting Lead and Copper Control Strategies <br />*** From IRC reporting Format 62-550.730(4)@ <br />POST-TREATMENT STABILIZATION DESCRIPTION <br />Carbonic acid solution (H2CO3) is used for re -carbonation and to assist the calcium addition increase <br />alkalinity of the product water. This system reduces pH of the product water stream and converts hydroxide <br />to bicarbonate and carbonate species to enhance lime and/or caustic addition. The H2CO3 feed system <br />provides buffering capacity for the product stream but does not add alkalinity by itself. The combination of <br />this system with lime helps increase alkalinity and hardness of the finished water. H2CO3 lowers pH of the <br />degasified permeate to make it possible for the water to dissolve or uptake more lime without the dramatic <br />pH increase that would otherwise occur through lime addition by itself. <br />69 <br />