The Monterey Peninsula Regional Water Board (MPRWA) in California still has to decide between three competing seawater desalination projects for the area after its Technical Advisory Committee accepted on 17 September 2012 a report by consultants Separation Processes Inc of Carlsbad.
The consultant’s initial scoping and constraints analysis looked at reverse-osmosis (RO) projects from California American Water (Cal-Am), the DeepWater Desal group (DWD), and the People’s Moss Landing project (PML) to replace water currently abstracted from the Carmel river which must be reduced by 60% by the end of 2016.
Lack of comparable information was the main reason given by the consultant for making no decision at this stage. “Each candidate project has attendant risk, with none considered to be disqualifying at this juncture,” says the report.
Only a balanced evaluation on a uniform set of criteria could provide the necessary information, the consultant said, and recommended the inclusion of each proponent project for full evaluation. However, the report did collect more technical information on each project than had been previously available.
The most developed project would appear to be that proposed by Cal-Am, which is proposing two sizes of plant – 5.4 MGD (20,400 m³/d) and 9 MGD (34,000 m³/d).
Water would be treated with single-stage dual-media filtration in either gravity beds or pressure filters. The pretreated water would be stored and fed to first and second pass RO trains.
The second-pass trains would treat 40-50% of the product water produced by the first-pass trains, resulting in a final treated product with a 40-50% blend of second-pass product to first pass product. The blended RO product supply would be stabilized through calcite contact beds and supplemental carbon dioxide.
Disinfection would be through sodium hypochlorite addition, although potentially it could use UV disinfection in series on a temporary basis. Solids from the pretreatment system would be sent to a settling basin with decant sent along with RO brine to the Monterey Regional Water Pollution Control Authority’s outfall.
The consultant particularly questioned the lack of long-running experience with slant wells.
DWD proposes to draw seawater through a new screened ocean intake installed at a depth of approximately 65 ft (20m) depth in Monterey Bay, beneath the photic zone. The raw water would be pumped through a proprietary warming system located in an existing structure to increase its temperature from 8°C up to 29-35°C.
The consultant expressed reservations about this “black-box” system – “with no track record of successful operation it could prove unreliable and affect performance of the entire treatment system.”
The heated supply would be treated with single-stage dual-media filters, possibly with pre-oxidation and coagulant addition. The product from a single-pass RO would be stabilized through calcite contact beds and supplemental carbon dioxide. Disinfection would be through sodium hypochlorite addition.
Two facility designs are proposed, broken into two phases. The first 9.1 MGD (34,400 m³/d) phase would supply the Monterey Peninsula. The second phase would expand the plant to 22.3 MGD (87,000 m³/d) to supply water to northern customers.
PML is considering two alternative intake approaches – one to use an existing open-intake pumping system in the Moss Landing Harbor and existing supply pipelines; and a second to modify an existing outfall and use it as a supply line drawing water from the bay, similar to the DWD intake.
With either intake alternative, they propose to treat the incoming supply with single-stage granular-media filters and store the filtrate in a clear well ahead of the RO units. No details are given about the number of RO passes.
The SPI report says, “The lack of specificity in the PML treatment approach is a concern, but not considered disqualifying.”
PML has proposed two facility design capacities – 4.8 MGD (18,200 m³/d) and 9.4 MGD (35,600 m³/d).
Indpendent consultant Kris Helm, who presented the SPI report to the committee, is quoted in the Monterey Herald as criticising the California Public Utilities Commission’s preferance towards the Cal-Am proposal. He said all three proposals included difficulties which could present obstacles during environmental review and result in project delays.
He argued that CPUC should make every effort to ensure that these fundamentally different approaches was given equal footing in the environmental review.