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| Figure 1 - General View of the Carlsbad Pilot Seawater Desalination Plant |
Carlsbad, CA -- Over the last two decades production of potable water from seawater has evolved into a viable water supply alternative due to a number of breakthroughs in desalination technology, dramatic reduction of water production costs and decreasing availability of traditional surface and groundwater resources. Today, the affordable cost of water produced at large-scale desalination plants allows tapping the largest water reservoir in the world – the ocean. Seawater desalination technology, widely used for decades in many other arid parts of the World, is now making great strides in the coastal United States, including Florida, Texas and California.
To date, only a limited number of desalination plants have been constructed along the California coast, primarily because the cost of desalinated water has been higher than the cost of traditional sources of water supply – groundwater, interstate and out-of-state water transfers. Prolonged drought, dwindling alternative water sources such as Colorado River and Bay Delta water are fueling the drive for accelerated development of new seawater desalination plants.
In response to the drive for local drought-proof water resources to supplement Southern California’s traditional water supply alternatives, Poseidon Resources is advancing the development of several large seawater desalination plants, including these in the City of Huntington Beach and Carlsbad. The two desalination plants would be located at existing coastal electrical power generation stations. The Huntington Beach and Carlsbad desalination plants are projected to have product water capacity of 50 MGD each. The two projects are in a process of environmental feasibility review and permitting, and are planned to begin construction in 2005.
To demonstrate the feasibility of coastal desalination and the benefits of co-location of large desalination plants with existing power plants in Southern California, Poseidon Resources assisted by a local specialty contractor (Enaqua) and Hydranautics, have constructed a 40,000 gallons per day desalination demonstration plant located at the Encina Power Plant in Carlsbad.
This seawater desalination demonstration facility consists of raw water intake feed pump station; two pretreatment filtration systems configured to operate in parallel; filtered water transfer pumps; membrane system feed seawater storage tank; 5-micron cotton cartridge filter; 45 gpm high-pressure reverse osmosis (RO) feed pump; a single-stage RO system; permeate lime conditioning system; and UV disinfection system (See Figure 1).
The pilot plant also has a potable water sampling station that allows visitors to taste the desalinated water. In addition, this facility is equipped with a number of ports for water quality sample collection. The pilot plant is fully automated and is designed for remote monitoring and control via the Internet.
The source of feed seawater for the planned full-scale plant and for the pilot facility is the warm cooling water of the Encina power plant. This once-through power generation station withdraws cooling water from the Pacific Ocean via the Agua Hedionda Lagoon. After passing through the power plant intake structure, trash racks and traveling screens, the cooling water is pumped through the condensers of the power plant generation units. The power plant has a total of five power generators and depending on the number of units in operation pumps between 200 MGD and 820 MGD of cooling water through the condensers. The warm cooling water from all condensers is directed to a common discharge tunnel and lagoon leading to the ocean. The full-scale desalination facility, is planned to tap to this discharge tunnel for both desalination plant feed water and for discharging high-salinity concentrate downstream of the intake area.
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| Figure 2 – Sand Media Pretreatment System |
The pilot system intake withdraws warm water from a small lagoon which is located at the end of the power plant discharge tunnel. The power plant discharge cooling water is typically 5 to 10 0F warmer than the ocean seawater. Intake seawater’s total dissolved solids (TDS) concentration varies between 33,000 milligrams per liter (mg/L) and 34,500 mg/L, and averages 33,500 mg/L. The pilot plant’s dry-weather intake water turbidity is usually between 1 and 4 nephelometric turbidity units (NTU). During wet-weather conditions, which are usually brief and occur mostly in the winter, raw seawater turbidity varies from 6 to 12 NTU, with occasional hourly spikes of up to 24 NTU.
The intake seawater is conveyed to a feed storage tank from where it is pumped to the pilot plant pretreatment systems. Currently, the two pretreatment systems undergoing testing are a Parkson’s two-stage, continuous backwash sand media filtration system (Figure 2) and the Hydranautics’ HydraSub© immersed microfiltration (MF) system (Figure 3).
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| Figure 3 – Microfiltration Pretreatment System |
The sand-media pretreatment system includes two Parkson Dynasand© continuous backwash filters in series. The first filter has a coarse (9-mm) sand media bed. The second filter contains finer (5-mm) sand media. Both filters have instrumentation for continuous turbidity monitoring and data logging. The second filter is also equipped with a particle counter.
The microfiltration system consists of a test vessel which contains several immersed membrane modules and typically operates under less than 0.1 bars of vacuum. This system is also equipped with instrumentation for feed water and filtrate turbidity monitoring, and for automated data reporting and acquisition.
The two pretreatment systems are operated independently and typically produce between 40 and 45 gpm of filtered water each. The pilot Dualsand pretreatment system is designed for loading rates, sand media size and filter media depth comparable to these of the full-scale pretreatment filters of the Tampa Bay Seawater Desalination Plant. The feed water to the Dualsand filtration system is conditioned using ferric sulfate. At this time, the MF system does not use chemicals for raw seawater conditioning and is tested at filter effluent production rates similar to these of the Dualsand system. No other chemicals are currently used to condition the treated seawater.
The pilot reverse osmosis system consists of two 4-element pressure vessels in series (Figure 4). This RO system configuration allows permeate to be collected from one or both ends of each vessel and to test different number of membrane elements. The seawater reverse osmosis membrane elements tested are 8-inch diameter, high salt-rejection units provided by Hydranautics. The RO system is designed to run in a range of 45 to 55 percent recovery and typically operates at 50 % recovery. The effects of operation at higher and lower than 50 percent recovery on key system parameters such as permeate TDS concentration, feed pressure and other parameters are planned to be tested over the course of the demonstration study.
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| Figure 4 – Reverse Osmosis System |
The pilot desalination plant has been in continuous 24-hours/day operation since August 2003. The quality of the produced permeate is consistently high - TDS concentration of 200 and 300 mg/L at RO system feed pressure between 780 and 900 psi. The two pretreatment systems are performing well and the filtered seawater of these systems typically has silt density index (SDI) of less than 4 and turbidity below 0.1 NTU.
In both the pilot scale and the planned full-scale desalination plants, the high-salinity concentrate, which is a side product of the desalination process, is blended with the power plant cooling water discharge prior to discharge to the ocean. A recently completed chronic and acute whole effluent toxicity testing of a blend of pilot plant concentrate and power plant cooling water in a ratio corresponding to a full-scale worst-case discharge conditions, indicates that the blended discharge is environmentally safe.
An additional confirmation of this conclusion is an ongoing marine aquarium test where aquatic organisms endogenous for the area of the power plant discharge are exposed to a blend of desalination plant concentrate and power plant cooling water discharge blend reflective of the full-scale facility operations. Seawater leaving the Encina power station’s cooling system (33,500 mg/L of TDS) is combined in the aquarium with pilot plant concentrate (67,000 mg/L of TDS) to maintain salinity concentration within the expected range of the combined power plant/desalination plant discharge (35,700 to 36,200 mg/L of TDS). The marine species in the aquarium such as the barred sand bass, the California halibut, the red sea urchin, the green abalone, and others are representative of a diverse and healthy endogenous marine environment and most of them are of economic or recreational fishing significance.
The aquarium test is designed to assess the effect of prolonged exposure of the aquatic biota on the elevated salinity conditions and is conducted by a marine biologist with expertise and extensive knowledge of the aquatic life in the vicinity of the power plant intake and discharge. The marine species have adopted seamlessly and after more than six months of continuous exposure to the elevated salinity concentration are healthy and tolerate the desalination plant discharge very well.
For more information contact:
Mr. Nikolay Voutchkov
Senior Vice President
Poseidon Resources Corporation
1055 Washington Boulevard
Stamford, CT 06901
Telephone: 203-327-7740, ext. 126
Email: nvoutchkov@poseidon1.com
Web site: http://www.poseidon1.com/
© Copyright 1998 - 2008 Water and Wastewater.com
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