Difference between revisions of "Fundamental resources/Water"
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Revision as of 20:08, 11 May 2011
) 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.
Contents
Abundance of water
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 —
"We need a 'Blue Revolution' in agriculture that focuses on increasing productivity per unit of water — more 'crop per drop'.
70% of world water use goes to irrigation for agriculture. The single most important thing we can do if we want to reduce water demand is to increase the water-efficiency of agriculture. Along with protecting the environment and securing food supplies, this is a third reason to move to sustainable agriculture, which consistently effects large reductions in water used [13]. The various types of food-production suggested in the Food article are all very economical with water. There are several very simple ways to reduce agricultural water use.
A combination of these techniques — applied intelligently and with a sensitivity to local needs and resources — can enable even rather dry climates to secure their own food supply without the need to pump in water from elsewhere. Rainfed agriculture currently provides about 60% of the world's food [14]. This could be expanded to nearly 100% using the methods mentioned here, while we can also make irrigation at least twice as efficient. This would massively unburden the world's water needs and can be done very cheaply, as most of the methods are very simple, low-tech interventions.
The recent tragic famines in Niger, Sudan, Chad and the Horn of Africa have been blamed on droughts, but they have more complex causes than a simple lack of rain. While drought was the immediate cause of the crop failure, the drought would not have done the same damage to an agricultural system that used water more effectively. By building soil and using groundcover, earthworks and trees, we can build agricultural systems that can maintain their productivity through times of drought and put an end to catastrophic crop failures.
Building soil
Sustainable farming practices emphasize building the structure of the soil, not disturbing it with tilling or digging. This builds soil infiltration (the ability of water to enter soil) and water retention (the amount of water a soil can hold, which is mostly determined by the amount of humus in the soil).
Several practises typical of sustainable agriculture have the effect of dramatically increasing soil infiltration and retention capacity, thus dramatically decreasing the need for water to be applied —
- No-till farming , which is gaining popularity around the world, increases infiltration and retention capacity while reducing labor and increasing yields.
- Permaculture designs generally avoid leaving earth bare. The ground is covered either with cover crops (such as clover and grass), or with mulch such as straw or woodchips. When ground is covered, water does not easily evaporate from it, and the layer of groundcover acts as a sponge, holding extra water after rainfall. This method alone reduces water requirements 25-50%.
- Compost, the basic fertilizer of sustainable farming, which is essentially manmade humus, can increase soil infiltration 125%[15]
- Mob-grazing (a method of raising livestock that emulates natural grazing patterns), has been shown to increase infiltration as much as 775%[16]
- Burying charcoal in the soil improves its structure and thus increase water retention capacity. This method is gaining attention recently.
Using trees
Sustainable agriculture emphasizes polyculture; rather than growing a single crop, many different species grow together and interact with one another. Trees have a role to play in nearly all agroecosystems. Planting trees between crops reduces the need for water in at least six ways —
- Tree roots secrete humic acid, which creates humus in the soil. This humus has a spongy texture and helps the soil hold more water.
- Trees have much deeper roots than food crops, so they can draw up deep groundwater and bring it into the field. This moisture then enters the water cycle on the farm.
- The shade provided by the trees reduces evaporation by two-thirds [17]
- Trees may increase orographic rainfall by deflecting cool winds upwards. This would mean that planting forests can actually increase the amount of rainfall an area receives.
- Air rising off trees contains pseudomonas syringae bacteria, which create ice crystals in clouds, leading to rain[18].
- Trees condense moisture from the air and drip it down to the soil below. This is known as occult precipitation. Trees are natural atmospheric water generators.
Earthworks
Very simple earthworks can turn arid areas into productive farmland. This is exemplified in Syria and Jordan, where there are productive rainfed farms in areas with as little as 120mm of rainfall per year. The simple structures include stone walls built on contour to stop water draining downhill, hollows dug into the ground to accumulate rainwater, swales, which are a combination of these two methods, and mini-dams to redirect water towards productive areas. A combination of agroforestry and earthworks can make any area productive, as has recently been demonstrated in Burkina Faso, Niger and Kenya.
Alternative food production methods
- Aeroponics uses a tiny fraction of the water of drip irrigation. It is by far the most water-efficient means of growing food yet devised.
- Seawater agriculture can grow food in coastal regions, requiring no fresh water at all.
- Aquaponics recycles the same water over and over, so that a given amount of plants can be grown in only 10% of the water a garden would need.
