Reading: True / False / Not Given: Question 8

As freshwater supplies come under increasing strain from population growth, irrigated agriculture and prolonged drought, many coastal regions have turned to desalination, the removal of salt from seawater, as a way of securing a reliable water supply that does not depend on rainfall. Although several techniques exist, the method used to produce roughly two-thirds of the world's desalinated water today is reverse osmosis, a process that forces seawater at high pressure through a semi-permeable membrane whose microscopic pores allow water molecules to pass through while blocking the larger dissolved salt ions.

Before seawater reaches the membranes, it must first pass through a pre-treatment stage that removes larger particles, algae and suspended sediment, since these would otherwise clog the membrane and shorten its working life. The pre-treated water is then pressurised, typically to between fifty and eighty times normal atmospheric pressure, and pumped through banks of membrane modules. Because pressurising such large volumes of water demands a considerable amount of electricity, energy consumption has historically been the single largest ongoing cost associated with reverse-osmosis plants. More recently, the introduction of energy-recovery devices, which capture pressure from the concentrated leftover brine and use it to help pressurise incoming seawater, has cut the electricity required per litre of fresh water produced by roughly half since the 1990s.

The process leaves behind a concentrated by-product known as brine, a highly saline liquid that is typically discharged back into the sea near the plant's outflow. Marine biologists have raised concerns that, if not sufficiently diluted before release, brine can settle near the seabed and reduce oxygen levels in the surrounding water, potentially harming bottom-dwelling organisms close to the outflow. Some newer plants address this by mixing the brine with treated wastewater before release, or by dispersing it through diffusers designed to spread the discharge across a wider area of seabed.

Despite these environmental trade-offs, reverse-osmosis desalination has expanded rapidly since the early 2000s, and several arid coastal cities now rely on it for a substantial share of their municipal water supply, a dependence that was rare before large-scale membrane plants became commercially viable.

Do the following statements agree with the information given in the passage? Write TRUE, FALSE or NOT GIVEN.

1. Reverse osmosis is used to produce roughly two-thirds of the world's desalinated water today.
2. In reverse osmosis, dissolved salt ions pass through the membrane while water molecules are blocked.
3. Seawater passes through a pre-treatment stage that removes particles and algae before reaching the membranes.
4. Reverse-osmosis plants are cheaper to build than plants using other desalination methods.
5. Energy-recovery devices work by capturing pressure from the leftover brine to help pressurise incoming seawater.
6. Energy-recovery devices have increased the electricity required per litre of fresh water produced since the 1990s.
7. Dispersing brine through diffusers is a cheaper method than mixing it with treated wastewater.