Environmental Concerns Temper Enthusiasm for Desalination
Environmental interests and desalination intersect in sometimes surprising ways. Take for example the Ciénega de Santa Clara, a wetland in Mexico that is an ecologically important wetland, habitat for endangered species, home to migratory birds and shorebirds, and a source of ecotourism. The Ciénega is also the natural resource that will be most affected by operation of the Yuma Desalting Plant (YDP). Karl Flessa, professor of Geosciences at UA, focused on this intersection in his presentation “Environmental Monitoring of the Ciénega de Santa Clara.” During its recently completed pilot run, YDP diverted up to 30 percent of the water that would normally flow to the Ciénega and instead flushed the processed effluent, much saltier water, into the Ciénega. Flessa’s team has been monitoring the inflows to the wetland, water quality, vegetation, and marshbirds to see if the YDP run will significantly affect the Ciénega habitat. Jennifer Pitt of the Environmental Defense Fund reported that the YDP pilot run at one-third capacity resulted in a 20 percent increase in salinity and a 25 percent decrease in flow to the Ciénega. So far, research has shown that the area is accustomed to some variation in water supply and is fairly resilient, but the full effect of the YDP run is unknown. More long-term monitoring is necessary.
The more common environmental concerns are about ocean desalination, both from sucking the seawater into the facility and returning the concentrated brine waste to the ocean. Care must be taken to avoid harming both large and small organisms by trapping them in the intake screens. New designs for ocean water intake specifically address this issue. Passive intake screens can withdraw a large amount of water at a low, uniform velocity with no moving parts, which leaves aquatic life and debris in the water source.
Concentrate, the highly saline effluent produced, must be dispersed in a way that minimizes ecosystem harm. Remedies for ocean outfalls to date depend on site specific designs for rapid diffusion of the brine, but more research is needed into impacts on the marine environment and methods for preventing damage. Issues concerning the ocean environment were mentioned in the Yuma conference presentations, including “San Diego Water Authority’s Seawater Desalination Program Update” and “The Master Plan for Desalination in Israel.” In both of these talks, speakers identified major environmental issues, which include ocean intake and outfall concerns and the need for monitoring impacts on coastal ecosystems. One speaker also mentioned the importance of using local recycled materials and environmentally-friendly design guidelines in building desalination plants.
Inland desalination is not without its environmental concerns. Of first importance is what to do with the concentrated brine. Currently, many methods exist for concentrate management: disposal to surface water, disposal to sewers, land application, deepwell injection, and evaporation ponds. Disposal to surface water is most common because it is relatively inexpensive and convenient. However, the increase in salinity may be detrimental to freshwater habitats. Disposal to sewers requires wastewater treatment plants to be able to handle the increase in salinity, possibly requiring more advanced treatment such as reverse osmosis. Land application can be beneficial when used to water landscaping; however, vegetation must be salt-tolerant. Deep-well injection puts the concentrate deep underground, so it is necessary to monitor the effects on groundwater, and it cannot be used in areas prone to earthquake and other ground instability. Evaporation ponds work well in arid climates where solar energy is abundant; however, ponds typically require large land area and impervious lining to protect the groundwater, which increase costs significantly. Blending with wastewater treatment plant effluent or power plant cooling water may facilitate concentrate disposal and be used in conjunction with above methods. Concentrate has also been used for dust suppression, roadbed stabilization, and soil remediation in small quantities.
In addition to these common concentrate management strategies, there are several newer technologies that fall under the category of “zero liquid discharge.” Thermal brine concentrators, crystallizers, and spray dryers reduce concentrate to a solid product for landfill disposal. However, the costs for these technologies are currently too high for most plants. Because every ecosystem is different, planning for proposed desalination plants must include adequate research about the area in order to find the most environmentally benign solution for that particular facility.
On the other hand, desalination can also have environmental benefits. Desalination plants can provide a secure source of water, albeit salty water, for environmental purposes. Salt tolerant wetlands plants can thrive on concentrate from inland desalination and provide an ecosystem supporting marsh birds and other wildlife. James Lozier, of CH2MHill, reported that the Oxnard, California Concentrate Treatment Wetlands demonstrates the ability of an engineered natural treatment system to utilize concentrate for environmental benefit. This is accomplished by employing salt-tolerant, brackish marsh species to remove nutrients and heavy metals and to provide volume reduction. With appropriate methods, it may be possible to restore or create new wetlands habitat using concentrate as a sustainable water source.