It is said that each person in UK uses 150 litres of water per day, while each person in the US uses 570 litres. This huge consumption of water means that the demand for water globally is rising by 640bn litres each year. Agriculture accounts for 70% of the use of water around the world, but sanitation and drinking water needs also put a burden on the water system. In addition, between 1000 and 4000 litres of water are needed to make one litre of biofuel to replace fossil fuels.
Droughts are becoming more common with climate change changing patterns of rainfall and countries are now turning to desalination to fulfil their water needs. Desalination refers to the removal of salt from seawater and subterranean water along with the treatment of waste water that makes it drinkable. Desalination isn’t commonplace as it requires large amounts of energy and the effect of removing large amounts of water from the sea aren’t fully known. But advances in techniques and technology recently have meant that reverse osmosis is the most common way to desalinate water. This requires water to be forced through cartridges that contain thin polyamide membranes, trapping salt and tiny particles but allowing the water to flow through.
Cartridges originally contained 300 sq ft of membrane to be housed inside each, but they are now able to hold 450 sq ft. Despite this, seawater needs more pressure than subterranean or wastewater to make it drinkable and that’s why it requires much more energy too. Sea water in different parts of the world can also make a difference as some areas are more salty than others.
Saudi Arabia relies the most on desalination, while the US comes second on the list. Later this year one of the largest seawater desalination plants will open in Carlsbad, San Diego. Areas such as Singapore, the Canary Islands and the Caribbean have no choice but to rely on desalination, while Europe, China and Japan will use desalination as a way prevent droughts.
Desalination costs around $3 per cubic meter, about twice as much as if treating rainwater or wastewater. However, desalinating one cubic meter of water requires less energy than to pump a cubic meter of fresh water for more than 200km. As well as this, desalination is often performed more intensively at night so the plants use electricity when there is less demand and it would go to waste.
Water should be affordable to everyone, but in developing countries the use of desalination can cause problems. Maintenance of the membranes in the cartridges is required often and some countries have subterranean water with lots of impurities, meaning the membranes can become blocked more quickly. In addition, desalination plants are expensive to run and developing countries often can’t afford the maintenance and running costs. As well as this, waste saltwater can be injected back into the ground in developing countries, meaning there are environmental impacts of desalination plants as nearby soil and rivers can have increased salinity.
The answer to these problems could come in the form of solar panels that allow plants to make their own electricity affordably, or large saltwater greenhouses which pump water over cardboard pads that allow for evaporative cooling. A recently invented form of desalination called manipulated osmosis could also be used which removes impurities in water before it enters the osmosis process. This can reduce the amount of energy required by up to 30% and reduce maintenance costs as the membranes will not become clogged so quickly.
Manipulated osmosis is already being used in several countries and China hopes to quadruple its seawater desalination by 2020 as it has 7% of the world’s fresh water but one fifth of the world’s population. There is even a desalination plant in London but currently no wastewater recycling centre which may prove more cost effective and offer a way to improve levels of drinking water all year round. Some have argued that recycling wastewater for irrigation and sanitation in many countries and cities may help to keep fresh water demands at a level that can be coped with, and it remains to be seen whether desalination technology will be the answer to long term water shortages.
