Ask Pablo: What's the Problem With Desalination?

Exterior of a desalination plant with water in the foreground

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Dear Pablo: My city wants to put in a desalination plant for drinking water. Environmentalists are opposed, which makes me wonder: What's so bad about desalination?
Desalination is any process whereby salt and/or minerals are removed from water to make it potable. In most cases desalination is used in arid coastal regions to turn sea water into drinking water but it is also used inland, where local ground or surface water is brackish. The main area in the US for desalination include Southern California, the Gulf Coast and Florida but about 75% of the world's desalination capacity is located in the Middle East.

How Does Desalination Work?

Two main processes are used to desalinate water; membrane filtration and distillation. Membrane filtration is gaining popularity and includes reverse osmosis (RO) filtration. Because RO requires forcing the sea water through progressively smaller membranes it also requires a lot of energy for pumping. The other method, distillation, which currently accounts for 85% of global distillation capacity, uses heat to evaporate and condense water, leaving salt and minerals behind. This process obviously requires a lot of heat energy, but generating a vacuum in the distillation chamber can lower the boiling point of water and increase efficiency.

So What's The Problem With Desalination

It's obvious that the massive amounts of energy used in desalination contribute to climate change-causing greenhouse gas emissions, possibly exacerbating the local drought conditions that require use of desalination in the first place. There are additional issues with the incoming and outgoing (waste) water. Inlet water from the ocean often contains fish and other sea life and passing through the desalination plant kills these organisms. Slowing the speed of the inlet water by using larger pipes can allow fish to escape by simply swimming back out.

On the outlet side the effluent of desalination plants is a brine that is far too salty for the marine life that it comes into contact with. Some desalination plants create sea salt for additional revenue, eliminating the need for any effluent. Another solution is to dilute the brine with the cooling water of a nearby power plant, or just with ocean water.

Are There Renewable Distillation Technologies?

Aside from the obvious use of solar photovoltaic or wind-generated electricity desalination can use waste heat from a nearby power plant or solar energy can be used directly in solar distillation. Similar to a solar still that you might use in an emergency survival situation in the desert or on a life raft, solar distillation uses the suns energy to evaporate water and then condense it. The drawback of this technology is that it yields relatively little fresh water and requires a large area. On a small scale a solar still is very effective but it isn't feasible for supplying water to a city or for irrigation of fields.

As with energy, the cheapest form of drinking water is water that is conserved. For a fraction of the cost of building and operating a desalination plant a community can support and fund water conservation efforts. These can include assistance with drought resistant landscaping, incentives for the removal of lawns, free replacement shower heads and subsidized replacement of water-using appliances, allowing and encouraging the use of graywater, and progressive water rates that punish heavy users.