In addition to water scarcity, chemical and biological contamination of drinking water supplies is a major global problem. A large proportion of the world's population do not have access to good quality drinking water, and around 80 per cent of the world's diseases are attributable to inadequate water supplies, sanitation, and water treatment. As the human population of the world grows, particularly in developing countries, it will become increasingly difficult to keep up with the demand for proper sanitation and water treatment. Provision of reliable supplies of water for drinking, washing and other needs is a cornerstone of global sustainable development. Small-scale desalination in rural communities in under-developed countries could play a major part in improving quality of life for a significant proportion of the world's population. Even in the developed countries, however, drinking water contains a range of contaminants which can cause chronic health problems, and many people want to take independent action to reduce the health risk to themselves and their families.
For vast numbers of people, particularly in rural communities, the best solution to the problem is not to wait for the installation of modern municipal water distribution and treatment facilities, but to look after their own needs. Public water supplies are always desirable and deserve the highest priority, but individuals can take action themselves to provide a local supply of good quality drinking water. One of the simplest things they can do is to construct their own solar powered water distiller, or solar still.
This is a very simple device for evaporating and condensing water, using only the power of the sun. Clearly it will work best in areas blessed with lots of good sunshine, but the device can operate effectively in most parts of the world. It can be constructed out of inexpensive and readily available materials, and has no moving parts, so that life expectancy is very good. One of the most significant aspects, however, is that the source water can be of any quality, including badly polluted, highly saline or even pure sea water, and yet the resulting product is still of the highest quality. Thus finding the source water is seldom going to present any problem. Even kitchen waste water can be used.
Solar stills use exactly the same processes which in nature generate rainfall, namely evaporation and condensation. Water is placed into a flat shallow water-tight bath, in an outdoor situation fully exposed to the sun. It can be advantageous to elevate the bath on a frame, but it will also operate perfectly satisfactorily on the roof of a house. The bottom of the bath is painted black to increase heat absorption. Above this flat tank, there is a sheet of glass, inclined at a suitable gentle angle, and oriented towards the south in the northern hemisphere, and towards the north in the southern hemisphere. Solar radiation penetrates the glass, heating the water and gradually vaporizing it to maintain super-saturated air in the space between the water and the glass. The water then condenses on the glass and rolls down to collect in a trough, along which it trickles into a suitable pipe and thence into a collecting vessel. The feed into the bath can be adjusted so that it enters at about or just less than the rate of condensation, to allow for the occasional manual top-up. The feed should be slow and controlled to prevent any possibility of direct contamination of the condensate.
A bath with standard dimensions of 87×193 cm will on average produce 3 gallons of water per day. With 10 hours of hot sunshine per day, this can increase to 5 gallons. Two or three such units should provide all the water which a family will require for all their drinking, cooking and washing requirements, particularly if recycled water is used for flushing toilets and garden irrigation.
The quality of the distillate is very high because all the salts, inorganic and organic components and microbes are left behind in the bath. Under reasonable conditions of sunlight the temperature of the water will rise sufficiently to kill all pathogenic bacteria anyway. A film or layer of sludge is likely to develop in the bottom of the tank and this should be flushed out as often as necessary. It is useful to have the glass hinged from the top in order to provide access for the occasional more thorough cleaning of the bath. Apart from this the only maintenance required is cleaning of the glass, which is a very quick operation. It is a simple task to provide an adequate reservoir and plumb the system into the house so that distilled water can come out of the kitchen tap. Experience gained with simple solar stills in Australia, Greece and India is discussed in section 10 (See: Configuration, Theoretical Analysis and Performance of Simple Solar Stills).
Solar stills use no energy other than that from the sun, and the whole operation is environmentally healthy. The resulting water is of the highest quality, and the operators of the still have the confidence that they do not need to rely on anybody else for their supply. In much of the world the device could literally be life-saving. As all the components of a solar still are commonplace and readily available in virtually all parts of the world, people can make their own at very low cost. Several suppliers are now offering ready-made units, and the typical cost of a system capable of supplying 3 gallons per day is $750. There is no reason why solar stills should not be multiplied up to provide a large unit for a whole community. The system is particularly well suited to small tropical islands where fresh water is in short supply, as nothing but seawater need be used.
In view of the dubious quality of domestic water supplies in much of the world, many people are taking steps to treat their water, particularly before drinking. One of the simplest things to do is to fill plastic bottles with water and leave them outside in the sun for a day. Most bacteria and viruses are killed by ultraviolet light, but protozoan cysts generally survive this treatment. It is still common practice over much of the world for people to boil water before use. This kills most pathogens, and will drive off most volatile organic chemicals but it does little to remove most heavy metals and nitrates. In fact, boiling serves to increase their concentration. There are powerful arguments, therefore, for consumers to install a proprietary treatment system. This can be either at the point of use (usually the kitchen) or at point of entry, e.g. to a building.
The use of simple counter-top gravity filters is becoming very widespread. These simple and inexpensive devices certainly improve the quality of drinking water, but they cannot eliminate all chemical or biological contaminants. A more effective solution is to install a small reverse osmosis unit. These operate under pressure to force water through a synthetic semi-permeable membrane. The pores in such membranes can be small enough to prevent the passage of nearly all inorganic and organic molecules, in addition to bacteria and other pathogens. Reverse osmosis (RO) is best suited to treatment of water which is already of good quality, as, in most units the life of the membrane can be dramatically reduced by using poor quality water. Recently, however, domestic products have appeared on the market, capable of desalinating seawater. Incorrect use of small RO units can damage the membranes so that they operate at reduced efficiency and become unreliable.
For most people, therefore, a better solution is to install a small electrical distillation unit. This operates on the same principle as the solar still except that electrical power is used to boil the water, and condensation can take place much more rapidly. Various designs are available but they all use some form of cooler to accelerate the condensation. One of the most common systems allows the steam to rise into an inverted cone, which is water-cooled from above. In other appliances the steam enters a coil which is cooled by a fan.
The benefit of distillation is that it provides a form of treatment superior to any kind of filtration, RO or ultraviolet system. Boiling of the water kills all pathogens, and virtually all biological and chemical components remain behind in the boiling chamber. Distillation units are particularly effective at eliminating hardness, nitrates and heavy metals. The only contaminants which might pass through the steam into the distillate are chlorine (at levels unlikely to cause any problem) and organics with a low boiling point and high solubility in water.
After a period of use limescale builds up in the boiling chamber, and this may contain high concentrations of contaminants such as heavy metals. This, therefore, needs to be removed from time to time, perhaps with a mild acid or proprietary descaler, and discharged to the sewage system. This should be the only form of maintenance required. After purchase of the unit, the only cost involved is that of the power, which for most consumers would be a very modest addition to the electricity bills. For long-term use, the cost of distillation is less than that of any point-of-use filtration or reverse osmosis system and much less than bottled water. The typical purchase price of a point-of-use distillation unit capable of producing 9 gallons of water per day is about US$1500. After this capital outlay the only cost in producing distilled water is for the power consumed. A gallon of fresh distilled water can thus be produced at home for about 25 cents (US). The equivalent cost of bottled mineral water varies from 69 cents to $1.19, on top of which must be added the cost of transportation and handling.
Exactly the same principle of evaporation and condensation can be used on a commercial scale for industrial applications or hospitals where very high quality water is required. Medium to large scale distillation plants usually have more than one boiling chamber and the steam is super-heated under pressure in the first chamber. Each chamber operates at a successively lower temperature and pressure. For ultra-pure water a carbon filter can also be added which will remove all the volatile organic compounds.
Figure 1 a-h shows the patterns of cost of small and medium sized distillation units for home and community applications. Some units are for tap water, some for brackish water, and some for seawater. The smaller units are for home applications and are generally for tap water. This information, that has been compiled from the catalogues of different manufacturers, may generally be helpful in the choice of appropriate units for specific applications. The different appliances vary in price in accordance with the salinity, the degree of automation, protection and safety of each unit.