FINAL REPORT Evaluation of Seawater Desalination Projects ...

EVALUATION OF SEAWATER DESALINATION PROJECTS
PROPOSED FOR THE MONTEREY PENINSULA
water. A critical environmental advantage of this process is the discharging of the exit
water at the surface, rather than through diffusers at the bottom.”
Offshore discharge of brine from a movable vessel offers different opportunities and
challenges than a traditional fixed bottom discharge. As with the proponent’s information
regarding the intake system, limited engineering specifics are presented to support claims by
the proponent of the benefits of the brine discharge system. Nonetheless, sufficient
information is provided to make it apparent that, as presented, the Salinity Plume Deterrent
Systems and the Multi-Port Dispersion Systems may have critical flaws that could prevent
issuance of a discharge permit under the National Discharge Pollutant Elimination System
(NDPES) established by EPA and implemented by the Regional Water Quality Control
Board. The areas of concern discussed below need not be fatal flaws but are issues that need
to be addressed. The corresponding studies required to site such a system are much more
complex than implied in the documentation provided by the proponent.
From a regulatory perspective, the Salinity Plume Deterrent System as proposed would
minimize the exposure of marine organisms to high brine concentrations; however, the
proposal appears to be entirely dependent on dilution, and EPA and the Regional Water
Quality Control Board policies and regulations do not consider dilution to be an acceptable
form of treatment. The policy implications of issuing a new NPDES permit on this basis are
very significant. State and federal regulations would apply for operation within State and
U.S. waters. Applicable U.S. Coast Guard requirements would also apply. Proponents do
not describe disposal of pretreatment sludges, which are treated by land-based desalination
plants. Regulatory agencies would likely have a difficult time changing their policies to
allow for a treatment system that is, in reality, a dilution system. In the unlikely event
dilution was allowed in an NPDES permit, the volume required would be significantly
greater than the amount of product water produced due to the natural levels of salinity in the
intake dilution water 23 .
The proponent claims that there are significant benefits of a surface water discharge
compared to a fixed bottom water discharge. Although this could conceptually be correct,
the proposed system does not demonstrate an understanding of NPDES permitting
regulations and agency policies. Most existing outfalls are located on the bottom to avoid
conflicts with navigation and because most NPDES discharges into the marine environment
are either freshwater or heated cooling water. In the majority of both cases, the effluent
discharge could be expected to be less dense than seawater, and a rising plume adds to farfield
dilution. In contrast, discharge of brine from desalination is generally denser than
seawater, and, all else being equal, more far-field dilution of brine could be expected from a
23 Proponent’s comments on the draft GEI/B-E report suggest a dilution ratio of a half part of raw seawater to
every one part of brine. Technical support for this opinion was requested from proponent, but was not
provided.
Monterey Peninsula Water Management District 3-21