Figure 1. General schematic of collocated seawater desalination plant.
Figure 1. General schematic of collocated seawater desalination plant.
Figure 2. Large-size wedge-wire screen.
Figure 2. Large-size wedge-wire screen.
Figure 3. Wedge-wire screen of West Basin Desalination Demonstration Facility. (Courtesy: West Basin Municipal Water District).
Figure 3. Wedge-wire screen of West Basin Desalination Demonstration Facility. (Courtesy: West Basin Municipal Water District).

Water production costs

Currently, the cost of desalinating seawater in the USA is relatively high compared to that of traditional low-cost water sources (groundwater and river water) and to production costs for water reclamation and reuse for irrigation and industrial use. Indeed, the cost of traditional local groundwater water supplies in some parts of the USA is as low as US$0.13–0.24/m3 (US$0.50–0.90/kgal). However, the quantity of such low-cost sources in coastal urban centres of California, Texas, Florida, South Carolina and other parts of the USA exposed to recent long-term drought pressures is very limited. For example, notwithstanding that over 40% of the current Orange County, California water supplies are in this category, the county water agencies have embarked on exploring seawater desalination because practically all available fresh aquifers delivering this low-cost water in the county are tapped-in and over-drafted. Most of the utilities in Southern California currently purchase imported water from the Bay Delta and Colorado River at a rate of which is within 30% of the cost of desalinated seawater. The costs for seawater desalination are comparable to the future total costs for delivery of new incremental water supplies to many parts of the coastal United States, such as municipalities and utilities in Southern California and Texas utilities along the Gulf of Mexico.

The generally lower costs for production of reclaimed water and for implementation of water conservation measures have often been used as an argument against the wider use of seawater desalination. However, this argument is flawed by the fact that water conservation and reuse do not create new sources of drinking water – they are merely a rational tool to maximise the beneficial use of the available water supply resources. Under conditions of prolonged drought when the available water resources cannot be replenished at the rate of their use, aggressive reuse and conservation can help but may not completely alleviate the need for new water resources and water rationing.

While in the early 1990s extensive conservation and reuse were uncommon for the majority of the municipalities in California, Texas, and other coastal states, the prolonged drought during this period forced many utilities to implement low-cost water reuse and conservation measures that now constitute 10% and 20% of their water portfolios. Utilities which already have comprehensive water reuse and conservation programmes will not be able to ‘squeeze’ another 10–15% of water savings via the same low-cost reuse and conservation measures. Implementing the next tier of more sophisticated equipment and technology-intensive reuse and conservation measures to reach water-saving goals of 25–35% comes at a price which in some cases may near that of desalination.

Table 1: Energy use of various water supply alternatives
Water supply alternativeEnergy use - kWh/m³
Conventional treatment of surface water0.2-0.4 (0.8-1.5)

Raw water imported by state water project in California (w/o Treatment)

2.4-2.8 (9.0-10.6)
Raw water imported from Colorado River in California (w/o Treatment)1.6-2.1 (6.0-8.0)
Water reclamation0.5-1.1 (2.0-4.0)
Indirect potable reuse1.3-2.0 (5.0-7.5)
Brackish water desalination0.8-1.3 (3.0-5.0)
Desalination of Pacific Ocean Water2.6-3.7 (10.0-14.0)

 

Typically, seawater desalination cost benefits extend beyond the production of new water supplies. If seawater desalination is replacing the use of over-pumped coastal or inland groundwater aquifers, or is eliminating further stress on environmentally sensitive estuary and river habitats, then the higher costs of this water supply alternative would also be offset by its environmental benefits. Similarly, seawater desalination provides additional benefits in the time of drought where traditional water supplies may not be reliable and their scarcity may increase their otherwise relatively low costs.

Energy use

Salt separation from seawater requires a significant amount of energy to overcome the naturally occurring osmotic pressure exerted on the reverse osmosis membranes. This in turns makes seawater desalination several times more energy intensive than conventional treatment of fresh water resources. Table 1 presents the energy use associated with various water supply alternatives. The table does not incorporate the costs associated with raw water treatment and product water delivery. For example, a number of water agencies and municipalities in San Diego County, Los Angeles County and Orange County in Southern California, have to import and convey a portion of their untreated source water at an additional energy expenditure of 1.6–2.8 kWh/m3 (6.0–10.6 kWh/kgal). When this energy use for conveyance of source water is added to the energy needed for water treatment, the total power demand for production of fresh water from imported sources in some cases (i.e. water supply to a number of utilities and municipalities in Southern California) could be comparable to that of desalinating seawater locally (see Table 1).

While energy use for seawater desalination is projected to decrease by 10–20% in the next five years as a result of technological advances, the total energy demand for conventional water treatment would likely increase by 15–20% in the same time frame because of the energy demand associated with the additional treatment (such as micro- or ultra-filtration, ozonation, or UV disinfection, for example) which would be needed in order to meet the most recent regulatory requirements for production of safe drinking water in the USA.

Often the opponents of seawater desalination in California argue that the energy used for desalination would create a significant negative impact on the energy use in the state. According to a report prepared recently by the California Energy Commission, the current power demand of the water sector in California (including both water and wastewater conveyance and treatment) totals 13,341,000 MWh. Assuming a conservative unit energy use for seawater desalination of 2.9 kWh/m3 (11 kWh/kgal), the total energy needed to produce 450 MGD of drinking water planned in the next 10 years, is 4,950 MWh, which is only a 0.037% increase of the current California water sector energy demand. These facts clearly indicate that the California desalination initiative would not ‘break the back’ of the state's electrical energy supply system any time soon.

Seawater intakes

A number of the seawater desalination projects under consideration in California and Florida are proposed to be collocated with power generation plants which currently use seawater for production of electricity. Under the collocation configuration the desalination plant does not have a separate intake and discharge to the ocean and both the desalination plant intake and desalination plant discharge are connected to the exiting power plant discharge outfall or canal. Figure 1 depicts a general schematic of the configuration of collocated desalination plant intake and discharge.

Collocation yields a number of benefits mainly because it avoids construction and permits for new intake and concentrate discharge facilities, and because of the energy cost savings associated with the desalination of warmer source water. However, collocation has been considered undesirable by some environmental groups due to the potential loss of marine organisms caused by the impingement of marine organisms against the screens of the power plant intake and their entrainment inside the power plant conveyance and cooling system and subsequently inside the desalination plant.

The actual significance of the loss of marine organisms due to once-through cooling as compared to other beneficial uses of the ocean, such as commercial and recreational fishing, has been a subject of debate and a flurry of recent federal and state regulations.

Based on recently introduced regulatory requirements, the 21 once-through cooling plants along the California coast are required to prepare comprehensive plans for discontinuation of their use of open intakes and switching to air-cooling towers or to water close-circulation cooling towers in order to reduce impingement and entrainment of marine organisms.

Opponents of collocated seawater desalination plants have often presented the argument that if the power plant changes its cooling system in the future, seawater desalination under collocated configuration at the particular location would no longer be available. However, this argument is unfounded in reality, because most collocated desalination facilities have already executed long-term agreements with their power plant hosts to reserve the right to use their outfall and intake systems and equipment even if the power plant no longer needs them in the future. The main benefit the collocated desalination plants would lose in this case is the availability of warmer source water and associated energy savings of 5–15%.

Even if the host power plants abandon once-through cooling in the future, the desalination projects will still retain the main cost-benefits of collocation – avoidance of the need to construct a new intake and outfall. The cost savings from the use of the existing power plant intake and outfall facilities would be over 25%, resulting in a significant net benefit with or without the power plant in operation.

Recent studies of wedge-wire screens in Santa Cruz, California indicate that this type of open intake may prove to be a viable alternative for dramatic reduction in impingement and entrainment of marine organisms. Wedge-wire screens are passive intake systems, which operate on the principle of achieving very low approach velocities at the screening media (see Figure 2). Wedge-wire screens installed with small slot (1 to 5 mm) openings reduce impingement and entrainment and are an EPA approved technology for compliance with the US EPA 316(b) Phase II rule provided the following conditions exist:

• The cooling water intake structure is located in a freshwater river or stream;

• The cooling water intake structure is situated such that sufficient ambient counter currents exist to promote cleaning of the screen face;

• The through screen design intake velocity is 15cm/s (0.5 ft/s) or less;

• The slot size is appropriate for the size of eggs, larvae, and juveniles of any fish and shellfish to be protected at the site; and

• The entire water flow is directed through the technology.

Typically, wedge-wire screens are designed to be placed in a water body where significant prevailing ambient cross flow current velocities [≥ 30 cm/s (≥ 1 ft/s)] exist. This cross high flow velocity allows organisms that would otherwise be impinged on the wedge-wire intake to be carried away with the flow.

Some wedge-wire screen systems are air burst back-flush systems, which direct a charge of compressed air to each screen unit to blow off debris and impinged organisms back into the water body where they would be carried away from the screen unit by the ambient cross flow currents.

A 2-mm cylindrical wedge wire screen intake is also planned to be tested for one year at the West Basin Municipal Water District's Temporary Ocean Water Desalination Demonstration Facility in Redondo Beach. This demonstration facility is currently under construction and is projected to be in operation in late 2010. The wedge-wire intake configuration planned for the West Basin Demonstration Facility is shown in Figure 3. This screen is planned to be installed on the top of the velocity cap of one of the existing intake structures of the Redondo Beach power generation station.

Various forms of sub-surface intakes (i.e. beach wells, horizontally directionally drilled and slant wells and innovative infiltration gallery configurations) have been heavily promoted by the California Coastal Commission and local environmental groups as a viable alternative to power plant collocation and construction of new open intakes along the California coast. Ongoing long-term studies of sub-surface intakes in Long Beach and Dana Point, California are expected to provide comprehensive data that would allow completing a scientifically-based analysis of the viability and performance benefits of sub-surface intakes.

Concentrate management

Seawater desalination plants along the US coastline would produce concentrate of salinity that is approximately 1.5 to 2 times higher than the salinity of the ambient seawater (i.e. in a range of 52 ppm to 67 ppm). While most marine organisms can adapt to this increase in salinity, some aquatic species such as abalone, sea urchins, sand dollars, sea bass and top smelt, are less tolerant to high salinity concentrations. Therefore, thorough assessment of the environmental impact of the discharge of concentrate and of any other byproducts of the seawater treatment process is a critical part of the evaluation of project viability.

At seawater desalination projects that are proposed to be collocated with power plants, the desalination plant discharge is planned to be diluted with the cooling water of the power plant to salinity levels that typically do not have a significant impact on aquatic life. The magnitude and significance of impact, however, mainly depend on the type of marine organisms inhabiting the area of the discharge and on the hydrodynamic conditions of the ocean in this area, such as currents, tide, wind and wave action, which determine the time of exposure of the marine organisms to various salinity conditions.

Extensive salinity tolerance studies completed over the past several years at the Carlsbad seawater desalination demonstration facility in California indicate that after concentrate dilution with power plant cooling seawater down to 40 ppm or less, the combined discharge does not exhibit chronic toxicity on sensitive test marine species. Recent acute toxicity studies at this facility further show that sensitive marine species can even tolerate salinity of 50 ppm or more over a short period of time (2 days or less).

Some seawater desalination projects are planning to use deep injection wells to discharge the high-salinity seawater concentrate generated during the reverse osmosis separation process. However, the full-scale experience with this concentrate disposal method to date is very limited.

A third disposal alternative, besides injection wells and co-disposal with power plant cooling water, currently under consideration for implementation at a number of seawater desalination projects in the US, is the discharge of the concentrate through existing wastewater treatment plant ocean outfall. International experience with such co-located discharges is fairly limited. However, this technology may have a number of merits similar to these derived from the collocation of desalination and power generation plants.

Proving that concentrate discharge from a seawater desalination plant is environmentally safe requires thorough engineering analysis including: hydrodynamic modeling of the discharge; whole effluent toxicity testing; salinity tolerance analysis of the marine species endogenous to the area of discharge; and reliable intake water quality characterisation that provides basis for assessment of concentrate's make up and compliance with the numeric effluent quality standards applicable to the point of discharge. Comprehensive pilot testing of the proposed seawater desalination system is very beneficial for the project environmental impact analysis.

Conclusions

Over the past decade seawater desalination has experienced an accelerated growth driven by advances in membrane technology and environmental science. While conventional technologies, such as sedimentation and filtration have seen modest advancement since their initial use for potable water treatment several centuries ago, new more efficient seawater desalination membranes and membrane technologies, and equipment improvements are released every several years. Similar to computers, the RO membranes of today are many times smaller, more productive and cheaper than the first working prototypes. Although, no major technology breakthroughs are expected to bring the cost of seawater desalination further down dramatically in the next few years, the steady trend of reduction of desalinated water production costs coupled with increasing costs of water treatment driven by more stringent regulatory requirements, are expected to accelerate the current trend of increased reliance on the ocean as an attractive and competitive water source. This trend is forecast to continue in the future and to further establish ocean water desalination as a reliable drought-proof alternative for many coastal communities in the United States and worldwide.

Although seawater desalination projects in the US face a number of environmental challenges, these challenges can be successfully addressed by carefully selecting the project site, by implementing state-of-the art intake and concentrate discharge technologies and by incorporating energy efficient and environmentally sound equipment and systems.