Difference between revisions of "Fundamental resources/Water"

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Revision as of 20:07, 11 May 2011

With world population growing, demand for food (and hence water for farming ) expected to grow by 70% ^[1], rivers becoming polluted and one in eight people already without clean drinking water ^[2], some have warned that we are heading for a 'peak water' crisis with people lacking the necessary water to survive, and wars breaking out over the access to water supplies ^[3].

A grim picture, indeed. But fortunately, an entirely avoidable one. Water is one of the most abundant resources available to us on this blue planet. The only problem we may face is synthetic scarcity; this article aims to show that there is no real shortage of water, nor of ways to purify and manage it.

Abundance of water

Over 70% of the surface of the Earth is covered by water, with the average depth of the oceans being 3.8 kilometres (12,430 ft) – we live on what is primarily a water world. Wasting the fresh water that falls on the surface of the land should of course be avoided wherever possible, but having enough fresh water in any place ultimately comes down to energy, rather than water itself — energy to transport water and energy to purify it.

As long as we have large amounts of renewable energy at our disposal (which we do - see the energy section), we can always create fresh water as required from seawater —

Desalination

97% of Spaceship Earth's water is in its oceans. Warnings of water shortages invariably dismiss desalination a priori as too expensive or energy-intensive ^[4][5][6]. Of course, ignoring 97% of a resource will make that resource seem really scarce. If the problem is that current methods of desalination are too inefficient, the obvious solution is to find better methods.

Currently desalination is done on a large scale with reverse osmosis  and membrane processes and these are indeed very energy intensive. However, as the energy article shows, we are not short on energy. But if we want a more energy-efficient means of desalination, solar desalination, nanofiltration and microbial desalination are promising technologies.

• Solar desalination  is a simple and ancient method of desalination that uses no electricity at all, just the heat of the sun to evaporate seawater, which seperates the salt from the water. It is only feasible in very hot countries, but these tend to be the ones most in need of water.
• Nanofiltration uses filters made of carbon nanotubes, small enough to let water molecules pass though, while blocking salt particles, impurities and pathogens. Nanofiltration uses only a quarter the energy of conventional methods of desalination^[7]. IBM are conducting research into nanofiltration-based desalination ^[8].
• Microbial desalination is particularly promising because, far from being energy-intensive, it actually generates electricity. It uses electrically active microbes to suck sodium and chlorine ions out of the water, simultaneously desalinating water and generating a flow of electrons.

How to get pure water

The intelligent use of mankind's water resources is not just an environmental issue; it is a public health issue. Drinking contaminated water is one of the most common causes of cholera, typhoid, diarrhoea, hepatitis A, dysentery and river blindness. Fortunately, using modern technologies, it is now possible to cheaply purify even the filthiest water — and even urine — into safe drinking water.

Suspended particles can be removed from water by passing them through a slow sand filter  with layers of gravel and sand. This is within the reach of all the World's people, as it requires no input of energy and no technological sophistication. A slow sand filter removes many, but not all, microscopic pathogens. The remaining ones can be removed by ultraviolet LEDs

It would be a relatively simple matter to fit homes with the means to catch rainwater, pass it through a slow sand filter and sterilize it with UV light. In all but the driest climates, this would provide the inhabitants of the home with an abundance of water. Decentralizing water-production would eliminate the need for a complicated water-grid and would make people more independent and resilient in the case of disasters.

Atmospheric water generators  condense water from the air. This water is clean and ready to drink. It is interesting to note that even very dry air contains about 5ml water per cubic meter of air, so even in a desert, you could condense enough water from the air to sustain a person or to grow food.

An interesting article by Patrick G. Salsbury analyses the enormous potential of atmospheric water generators to help meet humanity's water needs. He calculates that a device smaller than a microwave oven (using a crude prototype design and a very conservative estimate) provides 110l of clean water a day. Aqua Sciences have a condensor the size of a truck trailer that makes up to 4500l a day - enough for 90 people.

Nanofilters are an effective way of filtering out both suspended particles (which cause unpleasant tastes, smells and discoloration) and pathogens. The Tata Swach is a device that uses nanofilters to give very pure water. It retails for only $21.

Microbial fuel cells can be fitted to sewage water treatment facilities to generate electricity. One investigation found that the organic impurities in sewage contain 9.3 times as much energy as is needed to treat the water^[9]. It should be possible to integrate microbial energy-harvesting systems with water treatment facilities to create a self-sustaining facility that creates both clean water and clean electricity.

Using less water

In the USA in 2006, the average water use per person per day was 575 litres. Compare this with 149 litres in the UK and 4 litres in Mozambique. 50 litres of water is sufficient for a person^[10][11], without requiring sacrifices in lifestyle, just by improving system design.

Greywater  is the run-off from showers, sinks etc. It is water that has been used, but is not so dirty it cannot be used for purposes like gardening or washing clothes. Houses can be fitted with greywater recycling systems, where, for example, the output of the shower becomes the input of the dishwasher. This significantly reduces the amount of water needed to run a home.

Using composting toilets rather than flush toilets would save 26.7%^[12] of water use in the home. Composting toilets are also a source of fertilizer for growing food - and electrodes could even harvest electricity from the bacteria they contain (see Energy).

With the concern about conserving water in recent years, designers have developed toilets, sinks, dishwashers and washing machines that use a fraction of the water used by conventional designs.

However, reducing the domestic use of water is only a small part of the puzzle. Agriculture is the main cause of water use —