By Nikolay Voutchkov
One of the key limiting factors for the construction of new seawater desalination plants is the availability of suitable conditions and locations for disposal of the high-salinity sidestream generated during the desalination process, commonly referred to as concentrate or brine. The three most widely used options for concentrate disposal today are:
- direct discharge through new outfall;
- discharge through existing wastewater treatment plant outfall;
- co-disposal with the cooling water of an existing coastal power plant.
Herewith, key advantages, disadvantages, environmental impact issues and possible solutions are presented for each option.
Discharge of seawater desalination plant concentrate through a new ocean outfall is widely used for projects of all sizes. Over 90 percent of the large seawater desalination plants in operation today dispose of concentrate through a new ocean outfall specifically designed and built for that purpose. Examples of large membrane reverse osmosis (RO) seawater desalination plants with ocean outfalls for concentrate discharge are the 86 mgd plant in Ashkelon, Israel (Figure 1); the 36 mgd Tuas Seawater Desalination Plant in Singapore; the 14 mgd Larnaka desalination facility in Cyprus and most of the large plants in Spain, Australia and the Middle East.
The main purpose of every ocean outfall is to dispose of desalination plant concentrate in an environmentally safe manner, which mainly means to minimize the size of the zone of discharge in which the salinity is elevated outside of the typical range of tolerance of the marine organisms inhabiting the discharge area. The two key options available to accelerate concentrate mixing from an ocean outfall discharge are to either rely on the naturally occurring mixing capacity of the tidal (surf) zone or to discharge the concentrate beyond the tidal zone and install diffusers (at the end of the discharge outfall) in order to improve mixing. Although the tidal zone carries a significant amount of turbulent energy and usually provides much better mixing than end-of-pipe type of diffuser outfall systems, this zone has a limited capacity to transport and dissipate the saline discharge load into the open ocean. If the mass of the saline discharge exceeds the threshold of the tidal zone’s salinity load transport capacity, the excess salinity would begin to accumulate in the tidal zone and could ultimately result in a long-term salinity increment in this zone beyond the level of tolerance of the aquatic life. Therefore, the tidal zone is usually a suitable location for discharge only when it has adequate capacity to receive, mix and transport the salinity discharge from the plant into the open ocean.
This salinity threshold mixing/transport capacity of the tidal zone can be determined using hydrodynamic modeling. If the desalination plant total dissolved solids (TDS) discharge load is lower than the tidal zone threshold mixing/transport capacity, then concentrate disposal to this zone is preferable and much more cost effective than the use of a long, open outfall with a diffuser system. The Ashkelon seawater desalination plant is an example of discharge in the tidal zone.
For small plants (i.e., plants with production capacity of 0.1 mgd or less), the ocean outfall is usually constructed as an open-ended pipe that extends up to 300 feet into the tidal zone. This type of discharge usually relies on the mixing turbulence of the tidal zone to dissipate the concentrate and to quickly bring the discharge salinity to ambient conditions.
The ocean outfalls for large seawater desalination plants usually extend beyond the tidal zone. The length, size and configuration of the outfall and diffuser structures for large desalination plants are typically determined based on hydrodynamic or physical modeling for the site-specific conditions of the discharge location.
The key challenges associated with selecting the most appropriate location for ocean outfall discharge are: finding an area void of endangered species and stressed marine habitats; identifying a location with strong ocean currents that allow quick and effective dissipation of the concentrate discharge; avoiding areas with busy naval vessel traffic, which could damage the outfall facility and/or change mixing patterns; and selecting a discharge location in relatively shallow waters close to the shoreline in order to minimize construction expenditures.
Advantages and disadvantages
Key advantages related to using a new ocean outfall are that this option can accommodate practically any size seawater desalination plant and that it provides more freedom in plant location selection, as compared to the other two disposal approaches (where existing wastewater plant or power plant outfalls are used and therefore, the desalination plant location and capacity are driven by the location and size of those existing facilities).
Key disadvantages of this approach are that it is usually very costly and that implementation requires extensive environmental and engineering studies. Depending on the site-specific conditions, the costs for a new ocean outfall typically range from five to 30 percent of the total desalination plant construction expenditures. The higher end of this range applies to large desalination plants (i.e., facilities with fresh water production capacity of 10 mgd or more) where the construction and operation of a new concentrate outfall may contribute up to 20 percent of the production costs for desalinated seawater.
There are two alternatives for discharge through existing wastewater treatment plant outfall: 1) direct disposal through the plant outfall and 2) indirect disposal through a discharge to the nearby wastewater collection system which conveys the concentrate to a treatment plant with an existing ocean outfall. The key feature of this combined discharge method is the benefit of accelerated mixing that stems from blending the heavier-than-ocean water concentrate with the lighter wastewater discharge. Depending on the volume of the concentrate and on how well the two waste streams are mixed prior to the point of discharge, the blending may allow reduction of the size of the wastewater discharge plume and dilute some of its constituents. Co-discharge with the lighter-than-seawater wastewater effluent would also accelerate the dissipation of the saline plume by floating this plume upwards and expanding the volume of the ocean water with which it mixes.
Direct discharge through an existing wastewater treatment plant outfall has found a limited application to date, especially for medium and large seawater desalination plants. This disposal method had been practiced during the short-lived operations of the Santa Barbara seawater desalination plant in California (Figure 2). There, the desalination plant concentrate discharge volume was comparable to that of the wastewater treatment plant effluent discharge (i.e., 5.5 mgd).
Key considerations related to the use of existing wastewater treatment plant outfall for direct seawater desalination plant concentrate discharge are the availability and cost of wastewater outfall capacity and the potential for whole effluent toxicity (WET) of the blended discharge that may result from ion imbalance of the blend of the two waste streams. First, for this concentrate disposal option to be feasible there has to be an existing wastewater treatment plant in the vicinity of the desalination plant and second, this plant has to have available extra outfall discharge capacity. Third, the fees associated with the use of the wastewater treatment plant outfall have to be reasonable and fourth, the wastewater treatment plant utility that would allow the use of their outfall for concentrate discharge has to be comfortable with the arrangement of handling and separation of liability for environmental impacts of the blended discharge between the owner of the desalination plant and the owner of the wastewater treatment plant. Usually, this beneficial combination of conditions is not easy to find, especially for discharging large seawater concentrate volumes.
Bioassay tests completed on blends of desalination plant concentrate and wastewater effluent from the El Estero wastewater treatment in Santa Barbara, Calif. indicate that this blend can exhibit toxicity on fertilized sea urchin (Stron-gylocentrotus purpuratus) eggs (See Figure 3). Parallel tests on desalination plant concentrate diluted to similar TDS concentration with seawater rather than wastewater effluent did not show such toxicity effects on sea urchins. Long-term exposure of red sea urchins to the blend of concentrate from the Carlsbad seawater desalination demonstration plant and ambient seawater discharged by the adjacent Encina power plant confirm that sea urchins can survive elevated salinity conditions when the discharge is void of wastewater.
The most likely factor causing the toxicity effect on the sensitive marine species is the difference in ratios between the major ions (Ca, Mg, Na, Cl and SO4) and total dissolved solids (TDS) that occur in the wastewater effluent-concentrate blend as compared to the concentrate-seawater blend and the ambient ocean water. Since the seawater reverse osmosis (SWRO) membranes reject all key seawater ions at approximately the same level, the ratios between the concentrations of the individual key ions that contribute to the seawater salinity (Ca, Mg, Na, Cl and SO4,) and the TDS in the concentrate are approximately the same as these ratios in the ambient seawater.
Therefore, marine organisms are not exposed to conditions of ion-ratio imbalance if this concentrate is directly disposed to the ocean. Since wastewater effluent has fresh water origin and fresh water often has very different ratios of key ions (Ca, Mg, Na, Cl and SO4) to TDS, blending this effluent with seawater concentrate may yield a discharge which has ratios of the key ions to TDS significantly different from the ambient seawater. This significant ion make-up shift (ion ratio imbalance) caused by blending of the two waste streams is considered the most likely cause for the toxicity effect of the concentrate-wastewater blend on sensitive marine species. This ion-imbalance effect has to be investigated in order to ascertain that marine organisms in the vicinity of the discharge are not negatively affected by the combined wastewater-concentrate discharge.
Discharge to the nearby wastewater collection system is one of the most widely used methods for disposal of concentrate from small brackish water desalination plants worldwide. This indirect wastewater plant outfall discharge method, however, is only suitable for disposal of very small volumes of concentrate into large-capacity wastewater treatment facilities, mainly because of the potential negative effects of the concentrate’s high TDS content on the operations of the receiving wastewater treatment plant.
Discharging concentrate to the sanitary sewer is regulated by the requirements applicable to industrial discharges and the applicable discharge regulations of the utility/municipality which is responsible for wastewater collection system management. Feasibility of this disposal method is limited by the hydraulic capacity of the wastewater collection system and by the treatment capacity of the wastewater treatment plant receiving the discharge. Typically, wastewater treatment plants’ biological treatment process is inhibited by high salinity when the plant influent TDS concentration exceeds 3,000 mg/L. Therefore, before directing desalination plant concentrate to the sanitary sewer with the increase in the wastewater treatment plant influent, salinity must be assessed and its effect on the plant’s biological treatment system has to be investigated. Taking into consideration that wastewater treatment plant influent TDS may be up to 1,000 mg/L in many facilities located along the ocean coast and that the desalination plant concentrate TDS level would be above 65,000 mg/L, the capacity of the wastewater treatment plant has to be at least 30 to 35 times higher than the daily volume of concentrate discharge in order to maintain the wastewater plant influent TDS concentration below 3,000 mg/L. This means, for example, that a 10 mgd wastewater treatment plant would likely not be able to accept more than 0.25 mgd of concentrate (i.e., serve a seawater desalination plant of capacity higher than 0.25 mgd), which is twice the salinity of the ambient seawater.
If the effluent from the wastewater treatment plant is used for water reuse, the amount of concentrate that can be accepted by the wastewater treatment plant is limited not only by the concentrate salinity, but also by the content of sodium, chlorides, boron and bromides in the blend. All of these compounds could have a profound negative effect on the reclaimed water quality, especially if the effluent is used for irrigation. Treatment processes of a typical municipal wastewater treatment plant (such as sedimentation, activated sludge treatment and sand filtration) do not remove a measurable amount of these concentrate constituents.
A number of crops and plants cannot tolerate irrigation water that contains over 1,000 mg/L of TDS. However, TDS is not the only parameter of concern in terms of irrigation water quality. High levels of chloride and sodium may also have measurable negative impacts on the irrigated plants. Most plants cannot tolerate chloride levels above 250 mg/L. The typical wastewater plant effluent has chloride levels of less than 150 mg/L, while the seawater treatment plant concentrate would have chloride concentration of 50,000 mg/L or more. For example, using the chloride levels indicated above, a 10 mgd wastewater treatment plant cannot accept more than 0.02 mgd of seawater desalination concentrate if the wastewater plant’s effluent would be used for irrigation. This limitation could be even more stringent if the wastewater effluent is used for irrigation of salinity-sensitive ornamental plants.
Although the use of existing wastewater treatment plant outfalls or concentrate discharge to the sanitary sewer system may seem attractive for its simplicity and low construction costs, this disposal method has a number of limitations. Due to the potential toxicity effects of the concentrate-wastewater effluent blend, the direct discharge of the seawater concentrate through existing wastewater discharge outfalls may be limited to relatively small concentrate discharge flows. Similarly, indirect discharge of the concentrate through the wastewater collection system may be severely constrained or practically impossible, especially if the wastewater plant effluent is reused for irrigation of salinity-sensitive crops and ornamental plants.
Desalination plant co-location with coastal power plant
The key concept of the co-location approach is the direct connection of the membrane desalination plant intake and discharge facilities to the discharge outfall of an adjacently located once-thru coastal power generation plant. This approach allows using the power plant cooling water both as source water for the seawater desalination plant and/or as a blending water to reduce the salinity of the desalination plant concentrate prior to discharge to the ocean. The co-location approach has found full-scale implementation at the Tampa Bay seawater desalination plant in Florida and is proposed for a number of large seawater desalination plants along the Pacific coast of California. In the case of the Tampa project, the 25 mgd desalination plant is co-located with the Tampa Electric Company (TECO) power generation station which uses approximately 1,400 mgd of seawater for cooling (Figure 4).
Under typical operational conditions, seawater enters the power plant intake facilities and after screening is pumped through the plant’s condensers to cool them and thereby to remove the waste heat created during the electricity generation process. The cooling water discharged from the condensers is typically five to 15°C warmer than the source ocean water and is conveyed to the ocean via a separate discharge canal. This seawater concentrate is returned to the power plant discharge canal for blending with the cooling water prior to conveyance to the ocean.
In order for the co-location concept to be cost-effective and possible to implement, the power plant cooling water discharge flow has to be larger than the desalination plant capacity and the power plant outfall configuration has to be adequate to avoid entrainment and recirculation of concentrate into the desalination plant intake. It is preferable that the length of the power plant outfall downstream of the point of connection of the desalination plant discharge is adequate to achieve complete mixing prior to the point of entrance into the ocean.
A similar co-location configuration is planned for a number of other large seawater desalination plants in California, such as the 50 mgd Carlsbad seawater desalination project, which is currently is under development (see Figure 5).
As shown in Figure 5, under typical operational conditions approximately 600 mgd of seawater enters the power plant intake facilities and after screening is pumped through the plant’s condensers to cool them and thereby to remove the waste heat created during the electricity generation process. The Carlsbad intake structure is connected to the end of this discharge canal and under normal operational conditions would divert 106 to 130 mgd of the 600 mgd of cooling water for production of fresh water.
Approximately 50 mgd of the diverted cooling seawater would be desalinated via RO and conveyed for potable use. The remaining 50 mgd would have salinity approximately two times higher than that of the ocean water (67 ppt versus 33.5 ppt). This seawater concentrate would be returned to the power plant discharge canal downstream from the point of intake for blending with the cooling water prior to conveyance to the Pacific Ocean. Under average conditions, the blend of 500 mgd of cooling water and 50 mgd of concentrate would have discharge salinity of 36.2 ppt, which is within the 10 percent natural fluctuation of the ocean water salinity (36.9 ppt) in the vicinity of the existing power plant discharge. (Parts per thousand is the preferred unit for desalination plant concentrate.—Editor’s note.)
Advantages and disadvantages
The co-location with existing once-thru cooling coastal power plant yields four key benefits: 1) the construction of a separate desalination plant outfall structure is avoided thereby reducing the overall cost of desalinated water; 2) the salinity of the desalination plant discharge is reduced as a result of the mixing and dilution of the membrane concentrate with the power plant discharge, which has ambient seawater salinity; 3) because a portion of the discharge water is converted into potable water, the power plant thermal discharge load is decreased, which in turn lessens the negative effect of the power plant thermal plume on the aquatic environment; 4) the blending of the desalination plant and the power plant discharges results in accelerated dissipation of both the salinity and the thermal discharges.
Under a typical co-location configuration, the desalination plant uses the power plant discharge water both as feed water for the desalination facilities and as dilution water for the desalination concentrate. An example of co-location configuration where the power plant discharge is used only for dilution of the concentrate is the 32 mgd Carboneras desalination plant in Spain (Figure 6), which is currently the largest SWRO plant in Europe. The plant’s concentrate is discharged to the cooling water canal of a nearby coastal power generation plant and thereby diluted to an environmentally safe level before its return to the sea. The Carboneras seawater desalination plant has a separate open intake independent from the intake and discharge of the power plant.
Summary and conclusions
Ocean water discharge is the most widely practiced method for disposal of concentrate from seawater desalination plants today. New ocean outfalls are typically used for this purpose. However, co-discharge of concentrate with power plant cooling water has gained significant attention over the last five years due to the cost advantages and environmental benefits of this disposal method. Co-disposal with wastewater effluent is relatively less attractive than the other two concentrate management methods and it is usually viable for small-size seawater desalination 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 characterization that provides a basis for assessment of the 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 concentrate environmental impact analysis.
About the author
Nikolay Voutchkov has over 20 years’ experience in seawater desalination and water and wastewater treatment. He is a Senior Vice President and Corporate Technical Director for Poseidon Resources, a US company specializing in development of large water infrastructure projects. His areas of expertise include pilot testing and full-scale implementation of membrane treatment technologies for production of potable water from seawater and industrial water reuse; assessment of the effects of seawater desalination plant discharges on the marine environment; and product water quality integration of desalinated water with other sources of potable water. Voutchkov has authored over 40 technical publications and co-authored several books in the field of desalination, water and wastewater treatment and water reuse. He is one of the principal authors of the American Water Works Association’s newly updated “Manual of Water Supply Practices (AWWA M46) on Reverse Osmosis and Desalination”. Voutchkov is a registered Professional Engineer, Diplomate of the American Academy of Environmental Engineering and has received a number of prestigious awards and a patent for his work in the field of desalination. He is a member of the American Membrane Technology Association, the International Desalination Association, the American Water Works Association and the International Association on Water Quality. Contact him at Poseidon Resources Corporation, 1055 Washington Boulevard, Stamford, CT 06901, USA; telephone (203) 327-7740; fax (203) 327-5563; email: firstname.lastname@example.org or visit the company’s website at www.poseidonresources.com.