Water Purification Using Solar Energy

A very large proportion of people living in rural areas, and poorly run municipalities do not have access to safe drinking water. In addition, South Africa is currently in the grip of a periodic drought, which is especially severe in the Western Cape, where even some relatively well run cities and municipalities, such as Cape Town, are experiencing a severe water crisis. Unfortunately these periodic droughts can historically last for many years, so there is no certainty when the rain patterns will return to “normal”. While safe drinking water is available in the form of bottled water at most shops, it’s very expensive in comparison, perhaps too much for many poor families. In addition, bottled water in shops also tends to run out quickly during any form of real water shortage or crisis, so it may not be available when you really need it.

Many home owners in affected areas are thus faced with a growing problem of finding cost effective ways to purify contaminated water to make it safe for their family to drink. Their source of contaminated water may differ depending on their location and situation. It could be untrustworthy council water, a polluted river or stream, polluted or brackish ground water, or even sea water in coastal regions.

Common Domestic Water Purification Methods

Two commonly used methods by home owners for effectively purifying contaminated water, are reverse osmosis filtration, or distillation.

Please note that simply boiling water (which is common advice when dealing with unsafe water) might kill biological contaminates such as  bacteria, parasites and viruses, but it will not remove salt, nor harmful metals such as lead or arsenic, or other harmful chemicals. You cannot simply boil sea water, for instance, to make it drinkable. The same goes for adding chemicals such as chlorine which kill harmful viruses, parasites or bacteria, but cannot remove salt and other non-biological contaminates.

The process of reverse osmosis requires that contaminated water is forced through a special membrane (or series of filters and membranes) that have molecular sized openings, which removes almost all harmful impurities. The openings in the membrane are so small that dissolved salts, viruses, bacteria, harmful metals are all removed to a very large extent. This process requires high pressure to force the water through the membrane and thus consumes energy (usually in the form of electricity to drive pressure pumps). The membranes also need to be replaced or flushed periodically as they become clogged.

Distillation is the process in which contaminated water is made to evaporate or boil, and then the rising steam, or water vapour, is directed away from the contaminated water and then allowed to condense back into liquid water, where it is collected in a different clean container. As the contaminated water is heated, water molecules escape into the air, as they change phase to steam or water vapour, leaving behind the harmful contaminates. When the steam or water vapour condenses back to liquid water on a separate clean surface, it will be quite pure, and thus safe to drink. Like reverse osmosis, distillation also requires energy to heat the water enough to release sufficient water vapour, or convert it to steam.

One point to keep in mind is both these processes remove virtually all minerals from the drinking water, which may not be beneficial for long term health. A certain small amount of calcium, and magnesium is beneficial in drinking water, for example. You may need to supplement your diet accordingly, if your drinking water has been de-mineralised for long periods of time.

South Africa is Blessed with Abundant Solar Energy

Luckily in many places here in SA, we have plenty of energy reaching the ground from the sun on most days. Solar insolation thus represents a useful source of free energy for purifying water for many people in SA. Solar energy can be used in two different ways to purify water. It can be converted into electricity using conventional PV panels, which is then used to drive a high pressure pump for reverse osmosis. In the case of distillation, electricity from solar panels can also be used to heat the water. Alternatively, for distillation, solar energy can be used directly to heat the water, which will probably be simpler and cheaper, in many domestic applications.

Which Approach is Right For You?

For many middle class families, who can tolerate moderate installation and operating costs, reverse osmosis is probably the most practical solution. Many commercial kits are available that can be installed under the kitchen counter which use normal council water pressure. This can have the added advantage of not needing additional energy for the process. If the council’s water pressure is greater than 3 bar, that is usually sufficient for most commercially available reverse osmosis systems to work, but if not, a booster pump will be necessary. Obviously, if you don’t use council supplied water as your drinking water source, a booster pump will be necessary to create the required pressure to force the water through the membranes. For example, if you intend to use ground water, river water or sea water. In off-grid environments, the pressure pump can be driven by a suitably sized conventional domestic solar PV system. The downsides of reverse  osmosis can be that water is wasted when periodically flushing the system, which can be an issue when the source water is very scarce. In addition, if you are using sea water as your water source, you may need a specialised (and thus potentially more expensive) system due to the much higher salt concentration of sea water.

In poorer communities, or if you do not want to pay higher installation and operation costs, simple distillation systems that use the sun’s heat directly, are a practical option. Many of these can be built on a DIY basis from very rudimentary materials and thus need not cost very much at all. They are also very simple devices that don’t need much maintenance, apart from ensuring the condensing surface and water collection containers etc. remain clean.

This interesting presentation describes the benefits of such a system when used by a rural family in Mexico.

Distillation can be quite efficient in terms of the proportion of contaminated water converted to drinkable water (depending entirely of the design of your system), but you will need to periodically get rid of the contaminates which will accumulate.  However, the simplicity of most of these systems may mean getting rid of accumulated contaminates is simply a matter of draining and washing the evaporation basin from time to time.