Turning Seawater into Drinking Water
AAs rainfall becomes less predictable and coastal cities grow, a number of dry regions have turned to the sea for their drinking water. Desalination, the removal of salt and other minerals from seawater, was once regarded as a costly technology of last resort. Today large plants supply a substantial share of the water used in several countries in the Middle East and North Africa, in parts of Australia, and on many small islands.
BThe oldest large-scale method is thermal distillation. Seawater is heated until it evaporates, and the vapour, which leaves the salt behind, is condensed as fresh water. Modern plants reduce their energy use by boiling water at low pressure and by passing it through a series of chambers, each using heat recovered from the one before. Even so, distillation remains energy-hungry, and it is found mainly where cheap fuel or waste heat from power stations is available.
CMost new plants instead use reverse osmosis. Here, seawater is pushed at high pressure through membranes with pores so small that water molecules pass through but salt does not. The process needs no heating, and devices that recover energy from the high-pressure waste stream have greatly reduced its electricity needs over recent decades. The membranes, however, are delicate: seawater must first be filtered to remove sand, algae and other material that would otherwise block them.
DWhichever method is used, desalination produces a by-product known as brine: water with roughly twice the salt concentration of the sea. Because brine is denser than seawater, it can sink and spread along the seabed if it is released in one place, harming organisms that live on the bottom near outfall pipes. To limit this, plants commonly discharge brine through diffusers that mix it rapidly with the surrounding water, and some researchers are investigating ways of extracting saleable minerals from it.
EWater intakes cause a separate problem. Open pipes drawing in seawater can trap fish against their screens and suck in eggs and larvae. Engineers have responded by slowing the speed at which water enters, or by drawing it from wells beneath the beach, where the sand acts as a natural filter.
FCost remains the main argument against desalination. Although prices have fallen, desalinated water is generally more expensive than water taken from rivers or underground sources, largely because of the energy involved. Critics also point out that a plant built for drought years may stand partly idle once the rains return, while the loans that paid for its construction must still be repaid.
GFor these reasons, many water planners see desalination as one part of a wider strategy rather than a complete solution. Repairing leaks in ageing pipes, recycling treated wastewater and encouraging households to use less can often provide additional water more cheaply. Desalination's chief advantage is simply that, unlike rainfall, the sea does not run dry.