FINAL REPORT Evaluation of Seawater Desalination Projects ...

EVALUATION OF SEAWATER DESALINATION PROJECTS
PROPOSED FOR THE MONTEREY PENINSULA
filtration provided by beach wells, RO, and disinfection using free chlorine (via sodium
hypochlorite) should be sufficient to achieve the 4-log virus and 3-log Giardia reduction
required by the CDHS using a combination of physical removal and chemical inactivation.
Although Cryptosporidium reduction would also need to be achieved, it is expected that the
CDHS would award the process the 3-log reduction in conjunction with the virus and
Giardia reduction (notwithstanding any additional Cryptosporidium reduction required under
the newly promulgated federal Long-Term 2 Enhanced Surface Water Treatment Rule
(LT2ESWTR), if applicable). The Board Review Draft EIR also acknowledges that this
process includes the capacity to comply with the likely CDHS requirement for a minimum of
2-log virus inactivation using 10 minutes of free chlorine contact time after the RO
membranes. However, the CDHS typically requires the more conservative disinfection
requirement of either 2-log virus or 0.5-log Giardia inactivation, and with the use of free
chlorine the Giardia benchmark is the more stringent requirement. In any case, with a
treated water storage tank of approximately 2.5 million gallons and a treatment plant flow of
7.5 mgd, the contact time in this tank should be sufficient to achieve either of these
inactivation requirements for typical chlorine doses applied for primary disinfection.
Although no source water quality information is provided, the TOC levels are generally low
in seawater and may be somewhat lower using a beach well intake; thus, the precursor
material for disinfection by-product (DBP) formation is expected to be minimal. The Final
Phase 1 Technical Memorandum notes that occasional non-point source pollution could
potentially cause the beach wells to become infiltrated with enteric viruses, synthetic organic
chemicals (SOCs), pharmaceutical residuals, and/or endocrine disruptors. Because there are
no test wells constructed at this stage of project development, the potential for such
contamination cannot be accurately assessed. While no available documentation regarding
the Sand City Desalination Project specifically called for increased monitoring these
contaminants and the ability of the proposed treatment process to remove them during either
a piloting stage or at full scale, acknowledgement and awareness of this possible
contamination is important at this early stage of project development.
Both the Board Review Draft EIR and Final Phase 1 Technical Memorandum indicate that
lime and carbon dioxide would be used for post-treatment conditioning to produce “noncorrosive
water.” The Final Phase 1 Technical Memorandum also notes that the TDS of the
RO permeate (product water) are expected to be in the range of 200 to 300 mg/L. However,
neither document accounts for matching the finished water to the receiving distribution
system in terms of pH, alkalinity, and TDS (including the addition of post-treatment
chemicals for conditioning).
37 Jones & Stokes Associates, Monterey Peninsula Water Management District Water Supply Project, Board
Review Draft Environmental Impact Report, December 2003.
Monterey Peninsula Water Management District 4-6