FINAL REPORT Evaluation of Seawater Desalination Projects ...

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EVALUATION OF SEAWATER DESALINATION PROJECTS PROPOSED FOR THE MONTEREY PENINSULA 4 Projected Performance This section discusses the following topics for each proposed project: • TDS objective(s) • Title 22 drinking water standards (i.e., primary standards, pathogen control, DBP minimization, etc.) • Corrosion control in the distribution system • Blending with existing distribution system water • Disinfection practices sufficient 4.1 Coastal Water Project (CWP) In general, the Coastal Water Project (CWP) Conceptual Design Report (CDR) 25 specifies appropriate, conceptual-state treatment process information for assessing desalination plant performance relative to drinking water quality with no significant gaps or deficiencies. However, there are some potential issues that warrant more detailed planning as the project enters the pilot stage. (See Table 1 for project intake and outfall locations.) For example, the CDR indicates that 3.0 mg/L of free chlorine will be added just prior to the coagulation and flocculation pretreatment processes. Although not explicitly specified in the CDR, this disinfection step is likely intended to satisfy the various state and federal requirements for primary disinfection for surface water treatment plants. No information is provided in the CDR to justify the sufficiency of this dose for achieving the 0.5-log Giardia inactivation credit that will almost certainly be required by the California Department of Health Services (CDHS). In addition, data provided by Duke Energy Power Services 26 from its National Pollution Discharge Elimination System (NPDES) permit renewal sampling in 1999 indicate that total organic carbon (TOC) levels in the power plant Units 6 and 7 intake and discharge are approximately 10 mg/L, an amount that is unusually high for a surface water source as well as for seawater. This level of TOC, coupled with a 3.0 mg/L chlorine dose and a combined 21 minutes of contact time in the coagulation and flocculation processes as well as additional contact time in the submerged membrane filtration basins, could result in the formation of significant chlorinated disinfection by-products (DBPs), which are strictly regulated in drinking water systems. The reaction of this TOC with the 25 RBF Consulting, California American Water, Coastal Water Project, Conceptual Design Report (Draft), September 16, 2005. 26 California American Water, CWP Source Water Monitoring Documents, transmitted from Lela Adams at California American Water to Larry Gallery, RBF Consulting, December 14, 2004. Monterey Peninsula Water Management District 4-1
EVALUATION OF SEAWATER DESALINATION PROJECTS PROPOSED FOR THE MONTEREY PENINSULA applied chlorine would diminish the disinfection potential for inactivating pathogens. Both the efficacy of primary disinfection and the potential for DBP formation, as well as the possible removal of these DBPs via the reverse osmosis (RO) processes need to be explicitly evaluated during the pilot phase, as noted in the CDR. Note that while the feed for the seawater desalination plant is planned to be withdrawn from the discharge for Units 1 and 2 prior to the point at which the cooling water flow is combined with that from Units 6 and 7 prior to discharge, Units 1 and 2 and Units 6 and 7 utilize intakes in Moss Landing Harbor and may have similar water quality. The CDR also does not specify how the physical pathogen removal credits for Giardia, Cryptosporidium, and viruses would be allocated to the various treatment processes by the CDHS; however, it is likely that the combination of membrane filtration, cartridge filtration, and RO would achieve the required pathogen removal objectives. Another potential water quality issue is the possible presence of synthetic organic chemicals (SOCs) in the watershed. A report developed by The Watershed Institute at California State University Monterey Bay 27 indicated the detection of the pesticides chloropyrifos (up to 0.145 μg/L) and diazinon (up to 0.682 μg/L) in Moss Landing Harbor. While there are no maximum contaminant levels (MCLs) for these two compounds, the levels detected are in the same range as the MCLs for some other regulated SOCs, which also could be present in the watershed that drains into Moss Landing Harbor. Because the ability of the RO process to remove various SOCs can vary depending on the compound and may not be well documented in the literature, the pilot phase should include a full screen for SOCs (as well as for all regulated drinking water parameters) in both the feed and RO permeate water. Note that the 1999 NPDES permit renewal sampling did not detect the presence of any regulated SOCs in the intake water for power plant Units 6 and 7. The CDR specifies that the hardness, alkalinity, and pH of the RO permeate would be adjusted via chemical applications both for aesthetic considerations and to protect the distribution system piping. The CDR also indicates that a corrosion inhibitor may be needed. In addition, the PEA 28 indicates that RO post-treatment would be applied with consideration for blending with other water supplies. No total dissolved solids (TDS) target is specified, however, nor is the potential impact of these chemical additions on the ability of the treatment process to meet that target. The CDR states an assumption of five percent downtime for maintenance, but indicates an annual average daily capacity that is 97 percent of the design daily capacity. Nonetheless, 27 California State University, Monterey Bay, Watershed Institute, Monitoring Chloropyrifos and Diazinon in Impaired Surface Waters of the Lower Salinas Region, March 31, 2004. 28 RBF Consulting, California American Water, Coastal Water Project – Proponent’s Environmental Assessment for the Coastal Water Project, CPUC Proceeding A.04-09-019, July 14, 2005. Monterey Peninsula Water Management District 4-2
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