FINAL REPORT Evaluation of Seawater Desalination Projects ...

EVALUATION OF SEAWATER DESALINATION PROJECTS
PROPOSED FOR THE MONTEREY PENINSULA
surface discharge from a sinking plume than from a bottom plume. However, all else is
rarely equal and much more information about discharge and receiving water characteristics
is required to ensure protection of aquatic organisms. Near- and far-field dilution of
discharged effluents are impacted by many additional factors, such as initial discharge
velocity, discharge and receiving water density, near- and far-field water currents and
flushing, angle of discharge relative to currents and other physical features. The proponent
states that the discharge would be a low-velocity discharge of diluted brine to near-ambient
density and would remove many of the features that enhance far-field dilution. Instead, the
proponent’s approach would leave a “plume of effluent” in surface waters (typically
considered some of the most sensitive areas of a water body) that would have reduced
tendencies to disperse by forces other than far-field advection. This goes against most
accepted regulatory policies and criteria for designing an outfall.
The proposed brine discharge system could be modified for a high-velocity discharge without
dilution in the Salinity Plume Deterrent Systems. Initial velocity would create dilution and
the density could cause a sinking plume (assuming temperatures were controlled so as to not
neutralize the effect of density on far-field dilution). However, the potential for recirculation
between the discharge and the intake system under a variety of sea conditions is a
concern. A considerable amount of design, impact evaluations, and operational controls are
likely to be required to create an acceptable discharge system.
The desalination treatment process proposed to be implemented in the Water Standard
Company Seawater Desalination Vessel consists of the use of commercially available
treatment components. The overall treatment process is based on the use of reverse osmosis
to accomplish the desalination treatment objectives of the project. A low-pressure membrane
pretreatment system (microfiltration) has been included to provide suitable feed water to the
RO process and post-treatment is provided to condition the product water to meet aesthetic,
compatibility and regulatory objectives. Documentation of the Water Standard Company
project has been provided to the reviewer under the terms of a Non-Disclosure Agreement.
Certain observations presented here cannot be explained in complete detail without violating
this Agreement.
Pretreatment System
Water Standard Company has provided a general description and process flow diagram of the
proposed pretreatment process, which indicates the use of Pall microfiltration. As discussed
regarding the Coastal Water Project, the use of membrane filtration is considered an
appropriate selection for an open intake seawater supply. While existing full-scale
implementation of this technology on seawater is not extensive, the track record as RO
pretreatment on other challenging source waters (e.g., municipal wastewater) is substantial.
Additionally, several long-term seawater pilot studies have provided strong indication of
successful application of membrane filtration on seawater. However, it is still standard
practice that membrane pretreatment (microfiltration or ultrafiltration) be pilot tested on local
Monterey Peninsula Water Management District 3-22