In a water-stressed world, governments are on the lookout for a solution to declining fresh water stocks. This situation is already critical in many arid regions and will be increasingly important as we continue to witness unpredictable climate change. It is very likely that the need for fresh water will soon be considered, in the same category as oil and energy resources, and to be one of the determining factors of world stability and the prosperity of nations.
A regular dependable supply of water is needed in order for communities to grow crops and live healthy lives. As our global population skyrockets, we must be able to guarantee this basic need and avoid shortages of fresh water. One fix growing in popularity is called desalination – extracting from the abundant supply of sea water and turning the salty water into freshwater for drinking.
But is desalination the answer …?
A recent report from the Pacific Institute examined the environmental effects of desalination plants. Modern reverse-osmosis desalination plants take in large volumes of seawater – generally two gallons are withdrawn for every gallon of freshwater produced – and pass it through fine-pored membranes to separate freshwater from salt. The highly concentrated brine is then typically disposed of back into the ocean.
With the majority of desalination plants extracting water directly through open water intakes in the ocean, there is a direct impact on marine life. The three key environmental impacts with desalination plants are impingement, entrainment and brine release.
Impingement is when fish and other marine organisms are killed on the intake screens. Entrainment is when organisms small enough to pass through, such as plankton, fish eggs, and larvae, are killed during processing of the salt water.
Brine release is when highly concentrated salt brine is flushed back into the ocean. The brine contains several other chemicals used throughout the process. Twice as saline as the ocean, the brine is denser than the waters into which it is discharged and tends to sink and slowly spread along the ocean floor, where there is typically little wave energy to mix it. Changing the salinity of an ecosystem has the potential to modify its functions. There are several proven methods to disperse concentrated brine, such as multi-port diffusers placed on the discharge pipe to promote mixing. Brine can also be diluted with effluent from a wastewater treatment plant or with cooling water from a power plant or other industrial user, although these approaches have their own drawbacks that must be addressed.
The impacts on the marine environment, even for a single desalination plant, may be subject to daily, seasonal, annual, and even decadal variation, and are likely to be species- and site-specific. More research is needed, especially to understand the long-term impacts. We do, however, know that there are several operational, design, and technological measures available to reduce the marine impacts of open water intakes. In particular, subsurface intakes can virtually eliminate impingement and entrainment, as they extract seawater from beneath the seafloor or a beach. The sand acts as a natural filter, providing a level of pre-filtration that can reduce plant chemical and energy use and long-term operating costs.
The Big Question
Do we really want to be living in a world where we’ve depleted and polluted our fresh water sources so much that we must desalinate seawater to survive? Wouldn’t it be more sustainable and environmentally conscious to protect the precious resources we have now?
One sustainable solution would be to harvest and store rainwater. This is a great fix especially for climate regions which see great variability in precipitation throughout the year. Even some very arid regions have enough rainfall in the rainy season to provide fresh water to their population year round.
What’s very clear is that we must manage our freshwater resources more carefully and immediately begin planning national water policies to avoid a water crisis in the near future!