Desalination can boost U.S. water supplies

to supplement water — such as diverting freshwater from sensitive ecosystems — but definitive conclusions cannot be made without further research. The report says that researchers should investigate the extent to which fish and other creatures get trapped in saltwater intake systems in various settings, and seek ways to mitigate this and other impacts. Studies also should examine the long-term ecological effects of disposing of the salt concentrate that remains after desalination in rivers or the sea, a common practice. In addition, environmental evaluations of new desalination plants should be conducted, including ecological monitoring before and after the plant starts operating. The results should be synthesized with existing data in a national assessment that can guide future decision making, the report says.

Desalination also has raised concerns about greenhouse gases because it uses large amounts of energy. Seawater reverse osmosis uses about ten times more energy than traditional treatment of surface water, for example, and in most cases uses more energy than other ways of augmenting water supplies. Researchers should investigate ways to integrate alternative energy sources — such as the sun, wind, or tides — in order to lower emissions from desalination, the report says (we note that in the Middle East and North Africa, nuclear energy is the preferred way to power large desalination plants, as the cases of Lybia and India shows; see HSDW story). Recent improvements in technology have lowered desalination’s costs and energy requirements, which used to be prohibitively high. Meanwhile, other ways to augment water supplies have grown more expensive, making desalination more competitive. Finding ways to further lower costs should be another goal of the research effort, the report says.

Developing cost-effective, environmentally sustainable ways to dispose of salt concentrate should be a priority. The cost of disposing of this waste varies widely by site and has generally risen. Inland plants, in particular, have few or no cost-effective and environmentally sustainable disposal methods. Making the membranes used in reverse osmosis more permeable could lower desalination’s energy use and costs further, as can improving the pre-treatment of water to remove sediments that can hinder membranes’ efficiency, the report says. Even with improved technologies, however, the energy used by reverse osmosis probably cannot be reduced more than 15 percent below current levels. Larger reductions in energy costs may be possible using other desalination methods that could be powered with low-grade heat left over from other industrial processes, which would otherwise go to waste. Thermal desalination is one such method, and it may be possible to develop other novel approaches. Even if costs are lowered, the report notes, conserving water or transferring it from one use to another will in most cases remain a less expensive option than adding water through desalination or other methods.