Desalination Part 4 of 4: Measuring the Impact

Continued from Desalination Part 3: Getting Better All the Time.........

A bigger problem may be the leftover brine, which typically contains twice as much salt as seawater and is discharged back into the ocean. So far little scientific information exists about its long-term effects. In the past, most big seawater-desalination plants were built in places that did not conduct adequate environmental assessments, says Peter Gleick, president of the Pacific Institute, a think-tank based in California that published a report on desalination in 2006. But as plants are built in areas with tighter environmental restrictions, more information is becoming available.

Some recent measurements from Perth are encouraging. Initially scientists from the Centre for Water Research feared that the brine discharge from the plant would increase the saltiness of the coastal environment. But a monitoring study found that salinity returns to normal levels within about 500 metres of the plants’ discharge units. “The brine discharge is a problem that can be overcome with good design,” says Dr Antenucci.

A separate problem may be that some metals or chemicals leach into the brine. Thermal-desalination plants are prone to corrosion, and may shed traces of heavy metals, such as copper, into the waste stream. Reverse-osmosis plants, for their part, use chemicals during the pre-treatment and cleaning of the membranes, some of which may end up in the brine. Modern plants, however, remove most of the chemicals from the water before it is discharged. And new approaches to pre-treatment may reduce or eliminate the need for some chemicals.

Based on the limited evidence available to date, it appears that desalination may actually be less environmentally harmful than some other water-supply options, such as diverting large amounts of fresh water from rivers, for example, which can lead to severe reductions in local fish populations. But uncertainties over the environmental impacts of desalination make it hard to draw definite conclusions, the National Research Council concluded. Its report suggested that further research on the environmental impacts of desalination, and how to mitigate them, should be a high priority.

The reverse-osmosis process is increasingly being used not just for desalination, but to recycle wastewater, too. In Orange County, California, reclaimed water is being used to replenish groundwater, and in Singapore, it is pumped into local reservoirs, which are used as a source for drinking water. In both cases, the treated water is also available for tasting at local water-recycling facilities. This “toilet-to-tap” approach may leave some people feeling queasy, but wastewater is a valuable resource, says Sabine Lattemann, a researcher at the University of Oldenburg, Germany, who studies the environmental impacts of desalination. “Energy demand is lower compared to desalination,” she explains, “and you can produce high-quality drinking water.”

As water becomes more scarce, people will want to find several ways to secure their supplies. Many parts of the world also have enormous scope to use water more efficiently, argues Dr Gleick—and that would be cheaper than desalination. But sometimes, making desalination part of the approach to water management may be the only way to ensure a steady supply of drinking water.

In drought-ridden Western Australia, which ordered conservation years ago, the Water Corporation has adopted what it calls “security through diversity”, otherwise known in the industry as the “portfolio” approach. At the moment, Perth’s residents receive about 17% of their drinking water from seawater desalination. Desalination makes sense as one of several water sources along with conservation, agrees Dr Antenucci. But, he adds, “to say it is the silver bullet is wrong.”