Buổi 132: Reading – T/F/NG + diagram: khử mặn nước biển

Turning seawater into drinking water

(Bài đọc tự viết; thành phố và nhà máy là giả định; mô tả đơn giản hóa.)

A As populations grow and droughts become more frequent in some regions, a number of coastal cities have turned to the sea for their water supply. Desalination – the removal of salt from seawater – is not a new idea, but improvements in technology have made it increasingly common. Today, the most widely used method is known as reverse osmosis.

B The process begins at an intake, where seawater is drawn from the ocean through large pipes. Because seawater contains sand, seaweed and small organisms, it must first go through pre-treatment. In this stage, the water passes through a series of filters that remove larger particles. This step is essential, since any solid material that remained would quickly damage the delicate membranes used later in the process.

C The filtered water is then pumped at very high pressure towards the membranes themselves. These are thin sheets with extremely small pores, through which water molecules can pass but salt and most other impurities cannot. As a result, the water is divided into two streams: fresh water on one side of the membrane, and a much saltier liquid, known as brine, on the other. Finally, minerals are added back to the fresh water to make it suitable for drinking, and it is sent to storage tanks before entering the city’s supply. The brine, meanwhile, is discharged back into the sea.

D The main advantage of desalination is reliability. Unlike rivers and reservoirs, the ocean does not run dry during a drought, so a desalination plant can provide a steady supply regardless of rainfall. For cities with limited freshwater sources, this security can be extremely valuable.

E The drawbacks, however, are significant. Pushing water through membranes at high pressure is energy-intensive, which makes desalinated water considerably more expensive to produce than water from conventional sources. If the energy comes from fossil fuels, the process also adds to greenhouse gas emissions. A second concern is the brine. Because it is saltier and sometimes warmer than the surrounding seawater, releasing it in large quantities may harm marine life near the outlet, particularly organisms on the sea floor.

F Several approaches aim to reduce these problems. Some plants are powered partly by solar or wind energy, which lowers their emissions, although not necessarily their costs. Others release brine through long pipes with many small openings, so that it mixes quickly with seawater rather than collecting in one place. In the hypothetical city of Port Azura, engineers are also testing whether minerals can be recovered from the brine and sold, which could turn a waste product into a source of income.

G For most cities, desalination is unlikely to replace conventional water sources entirely. Reducing leaks in existing pipes, reusing treated wastewater and encouraging people to use less water are often cheaper ways of increasing supply. But where other options have been exhausted, desalination offers a dependable, if costly, alternative.