Reading 7 — turning seawater into drinking water

Sample question

Reading Passage — Turning seawater into drinking water A Of all the water on Earth, about ninety-seven per cent is in the sea, and every litre of it carries roughly thirty-five grams of dissolved salt — far too much to drink or to irrigate crops with. Removing that salt is not difficult in principle; boiling seawater and condensing the steam has been done on ships for centuries. The difficulty is doing it cheaply enough, and on a large enough scale, to supply a city. Two families of technology compete for the task. Thermal plants evaporate seawater using heat, and were long the standard choice in the Gulf states, where energy was cheap. Most plants built today, however, use reverse osmosis, which forces seawater through a membrane that lets water molecules pass and holds the salt back. A reverse osmosis plant is less a single machine than a chain of stages, each of which exists largely to protect the one after it. B The chain begins at the intake. The simplest design is an open pipe some distance offshore, fitted with screens that keep out fish, seaweed and debris; the water is often drawn in slowly, so that swimming animals can escape the current. An alternative is to draw water from wells sunk into the beach or the seabed, so that it has already passed through a layer of sand before it reaches the plant. Such subsurface intakes filter the water naturally and draw in far fewer living things, but they depend on suitable geology and cannot easily supply the very largest plants. C Whatever the intake, the water that arrives still contains fine particles, algae and bacteria, and these are the membrane's greatest enemies. Pretreatment removes them. A coagulant is usually added first, causing the smallest particles to cling together into clumps large enough to be caught, and the water then passes through filters — beds of sand and crushed coal in older plants, and in many newer ones membranes with far larger pores than those that follow. Chemicals may also be added to stop minerals from forming a crust on the membrane surface. If this stage fails, the membranes clog, the plant must push harder to force water through them, and they have to be cleaned or replaced far sooner than planned. D The reason for all this care becomes clear at the next stage. If fresh water and salt water are separated by a suitable membrane, water naturally moves towards the salty side, a process called osmosis. The pressure needed to stop that movement for seawater is about twenty-seven times the pressure of the atmosphere. To reverse it — to drive water out of the seawater — high-pressure pumps must push harder still, typically at between fifty-five and seventy times atmospheric pressure. At that pressure the water is fed into long tubes packed with membrane sheets wound in a spiral. Somewhere between two-fifths and a half of it passes through as fresh water; the remainder, now up to about twice as salty as the sea, leaves the tubes as brine. E That brine is still at almost the full pressure the pumps gave it, and for many years the pressure was simply thrown away through a valve. Much of the improvement in the economics of desalination has come from energy recovery devices, which pass the pressure of the outgoing brine directly to the incoming seawater, so that the main pumps have far less work to do. Together with better membranes, they explain why a modern plant typically needs three to four kilowatt-hours of electricity to produce a cubic metre of fresh water — a fraction of what the first membrane plants consumed. F The water that leaves the membranes is, if anything, too pure. Stripped of almost all its minerals, it tastes flat and, more importantly, it is chemically aggressive: sent straight into a distribution network, it would dissolve metal and concrete from the pipes. Post-treatment therefore puts some minerals back, usually by passing the water over limestone or dosing it with lime and carbon dioxide, which adds calcium and makes the water more stable. It is then disinfected, in the same way as water from any other source, before it enters the public supply. G What remains is the brine. Because it is denser than seawater, it tends to sink and spread along the seabed, where high salinity can harm organisms that cannot move away. Plants therefore release it through diffusers that mix it quickly with the surrounding water, and outfalls are sited away from sensitive habitats such as seagrass meadows. None of this makes desalination harmless, and none of it makes it cheap: even a modern plant uses several times more energy per cubic metre than one treating river or ground water. For a coastal city whose other sources are exhausted, that is a price worth paying. For one that still has rivers and reservoirs it could manage better, it seldom is. Flow chart of the fresh-water route through the plant: seawater intake → pretreatment → high-pressure pumps → membranes → [ ? ] → public supply — What happens at the stage marked [ ? ]?

• Fine particles are clumped together with a coagulant
• The brine's pressure is passed to incoming seawater
• Sand filters organisms out of the water
• Minerals are added back and the water is disinfected