A Match Made at Sea
Desalination process partners with power generation to lower costs for both
California's population is expected to increase more than 30 percent over the next 23 years. Even with aggressive reuse and conservation, that many more people will require more than 1 billion gallons of drinking water. Recognizing this need, the California Department of Water Resources has charted a new course for desalination. Currently, more than 24 new, large seawater desalination projects are in various stages of development.
Poseidon Resources of Stamford, Conn., is developing three of the largest desalination projects in California:
- the 50-million-gallon-per-day (mgd) Carlsbad Project
- the 50-mgd Huntington Beach Project and
- the 20-mgd Monterey Bay Regional Project in Moss Landing.
All the projects are public-private partnerships between the company and local utilities and municipalities.
The Carlsbad and Huntington Beach projects are at the most advanced stage of development. They are located adjacent to coastal power generation plants that use seawater for once-through cooling. The environmental impact assessments, local land use permits, and concentrate discharge permits for these projects have been approved by the host municipalities and pertinent regulatory agencies. The California Coastal Commission is reviewing these projects, and contingent upon favorable outcome, plant construction should begin this year. When completed, the projects will replace 6 percent to 8 percent of the imported water supplied to San Diego and Orange counties from the Colorado River and the Sacramento Bay-San Joaquin River Delta.
Breaking Cost and Environmental Barriers
Challenges associated with wider use of seawater desalination for drinking water supply include the high cost of water production and the safe disposal of plant concentrate. Safe disposal is defined as disposal that does not result in death or inhibit reproductive capacity of the marine organisms living in the discharge area. The salinity of seawater concentrate usually is two times higher than the ambient ocean water salinity (67,000 milligrams per liter mg/L versus 33,500 mg/L). Not all marine organisms can adapt to live in this high salinity.
Poseidon Resources, supported by engineering and marine science experts, built the Seawater Desalination Demonstration Facility at the Encina Power Plant in Carlsbad, Calif., to determine the best ways to overcome these challenges. This facility, operating since 2003, consists of a seawater intake feed pump station, two pretreatment filtration systems configured to operate in parallel, filtered water transfer pumps, a membrane system feed seawater storage tank, two single-stage reverse osmosis membrane systems, and a permeate conditioning system. The research completed at this facility yielded several creative solutions.
Power Plant Co-location
One of those solutions is power plant co-location. The company's demonstration facility in Carlsbad is directly connected to the discharge outfall of the Encina Power Plant. This approach allows the desalination plant to use warm power plant discharge as its source water and as a blending water to reduce salinity of the concentrate prior to ocean disposal.
Long-term testing at the Carlsbad plant indicates that the co-location approach will
- save 15 percent to 20 percent in capital expenditures by avoiding construction of separate ocean intake and discharge pipes;
- reduce desalination plant energy demand by 5 percent to 10 percent through the use of warmer seawater;
- achieve 10 percent to 15 percent additional savings as a result of the synergies of desalination and power plant operations.
Desalination and Power Plant Synergies
Under the co-location configuration, the power plant discharge serves both as an intake and discharge to the desalination plant. Three key environmental benefits stem from this approach:
- The configuration eliminates the need for the construction of a separate desalination plant intake pipe and discharge outfall, and thereby avoids disturbing benthic marine organisms.
- Plant co-location reduces the salinity of the desalination plant discharge as a result of the mixing and dilution of the membrane concentrate (67,000 mg/L) with the power plant cooling water (ambient seawater salinity of 33,500 mg/L). The resulting blend is about 40,000 mg/L, which is not a threat to aquatic life surrounding the discharge.
- Blended plant discharges dissipate more quickly, and this results in a 40 percent smaller footprint for the thermal plume from the power plant into the ocean.
New Test for Assessing Discharge Impacts
The methodology commonly used to evaluate the impact of desalination concentrate discharge on marine organisms is the U.S. Environmental Protection Agency's whole effluent toxicity test. This test, however, only predicts the salinity levels above which marine organisms die. Research completed at the Carlsbad demonstration plant indicates that some organisms lose reproductive capacity at salinity levels lower than those determined by the whole effluent toxicity test.
The project team has developed a new methodology that establishes the site-specific salinity level (40,000 mg/L) at which marine organisms not only survive but also maintain healthy growth and normal reproductive ability. The new methodology was used for the environmental review of the Carlsbad and Huntington Beach desalination projects.
The validity of the new methodology was proven at the Carlsbad demonstration facility's Marine Aquarium. The aquarium test was conducted by a marine biologist with expertise and extensive knowledge of the aquatic life at the power plant discharge. The tested marine species have adapted seamlessly and have shown healthy growth after more than 30 months of continuous exposure to the elevated salinity discharge. The average elevated salinity of the discharge is 36,500 mg/L (elevated from ambient salinity of 33,500 mg/L) and the maximum salinity of the discharge is 40,000 mg/L. The salinity concentration varies with the amount of cooling water discharged by the power plant, which varies with the amount of electricity the power plant produces.
Chemically Enhanced Boron Removal
Seawater contains an order-of-magnitude higher level of boron than fresh water sources. Although boron is mostly rejected by seawater reverse osmosis membranes, its level in the desalinated water may exceed regulatory requirements, especially when the source seawater is warm. To address this issue, the project team investigated the effectiveness of chemically enhanced boron removal from seawater. Research completed at the Carlsbad facility has proven that adjustment of the seawater pH from 7.8 to 8.5 by adding a relatively small dosage (10 to 20 mg/L) of sodium hydroxide enhances the boron rejection ability of the reverse osmosis system by 10 percent to 20 percent and produces high-quality potable water under all operating conditions.
In recognition of the originality and the value of the work competed at the Carlsbad Seawater Desalination Demonstration Facility, the American Academy of Environmental Engineers awarded the project the 2006 Grand Prize for Excellence in Environmental Engineering Research. This prize is the nation's premier award for achievement in applied research in the field of water, wastewater, and environmental engineering.
In addition, the International Water Association (IWA) awarded the company the 2006 Global Grand Prize in the Applied Research category for advancing seawater desalination science.
Poseidon Resources received a patent for this technology in 2005 (US Patent Number 6,946,081).
This article originally appeared in the January/February 2007 issue of Water & Wastewater Products.
This article originally appeared in the 01/01/2007 issue of Environmental Protection.