Purifying Drinking Water

Are you travelling to a country where the only water that you’ll have access to is potentially contaminated? Do you know how safe the water at your destination is to drink. Waterborne disease is a serious problem in many parts of the world and can potentially make your holiday much less enjoyable.

How to make water safe to drink - a travellers guide

Practically everyone acknowledges the importance of an adequate supply of drinking water of a quality sufficiently high to avoid a risk to health. While we now take this luxury for granted, in many parts of the world neither the quality nor quantity of drinking water can be guaranteed. Using water unfit for drinking can at worst be fatal, at best lead to discomfort and mild illness. Insecurity over a drinking supply and illness are major causes of culture shock. Mild illness leaves an individual vulnerable to more serious illness. Any decline in the mental or physical health will lead to discomfort and a loss of performance.



Types of Contamination
Suspended Solids
Selection and Treatment
Order of Treatment
Storage of Water



Practically everyone acknowledges the importance of an adequate supply of drinking water of a quality sufficiently high to avoid a risk to health. While we now take this luxury for granted, in many parts of the world neither the quality nor quantity of drinking water can be guaranteed.

Using water unfit for drinking can at worst be fatal, at best lead to discomfort and mild illness. Insecurity over a drinking supply and illness are major causes of culture shock. Mild illness leaves an individual vulnerable to more serious illness. Any decline in the mental or physical health will lead to discomfort and a loss of performance.

Mystery, misunderstanding and disagreement surround the treatment of water for drinking. The opinion of experts seems to vary: Mann and Williamson in their book "Water Treatment & Sanitation" state that boiling should be carried out for not less than 20 minutes while the National Water Council says there is no need to boil water for longer than one minute. Readers will find boiling times from 30 minutes to merely boiling mentioned regularly.

Sometimes manufacturers and distributors of water filters make extravagant claims for their products. Often the less suitable a filter the more elaborate the prose that describes how ideal it is for solving all your water treatment needs.In response to these problems we at SafariQuip have studied the arguments and produced this guide.The aim of the guide is to provide the layman with basic information on the treatment of water for drinking. The guide contains sufficient detail to understand the main reasons for treatment, the effects of different types of treatment and will provide the reader with enough knowledge to make an objective appraisal of the methods and products available. In emergency the reader will have sufficient knowledge to improvise a treatment system that will reduce if not eliminate the risk of illness.

The guide covers all the methods of treating drinking water that are commonly available to travellers and small groups such as family units. It does not attempt to cover large scale commercial treatment systems nor does it cover desalination systems or the treatment of chemically poisoned water such as might result from industrial pollution.It is useful to understand from the outset three points about advice on water treatment that cause misunderstanding:

  1. There is no need to completely sterilise or 'purify' water (kill or remove all the micro-organisms) in order to make it safe to drink. Germs in drinking water do not necessarily cause disease. Many diseases are not transmitted by drinking water and although their germs may be present they remain harmless when drunk. Only the germs that are responsible for diseases transmitted by drinking water need be treated. Some germs are very difficult to kill and remain largely unaffected by normal chemical treatment and boiling. Fortunately these are not responsible for diseases transmitted by drinking water. Even some water borne diseases are harmless when drunk. For example, Legionnaires' disease is caught by breathing in droplets of water containing the bacteria and not by drinking them. Some descriptions of water treatment are based on standards higher than those necessary for safe drinking water.

  2. In theory there is no normal treatment method that produces infinitely safe drinking water. There is always the chance, however small, that a germ might, by virtue of small size or resistance to chemicals or heat, survive and cause disease. The more exacting your water treatment process, the less the risk becomes. Although always present in theory, there comes a time when the risk is so tiny that in practice it can be discounted. It is one of the skills of the experts to assess when water is in practice safe to drink. Unfortunately different experts set their standards at different levels depending on their varying perception of the risks.

  3. Beware the use of words like 'pure', 'disinfect' and 'protection'. These words and many similar are used by manufacturers in carefully written prose. Read the descriptions carefully and critically and you will find that many are not offering absolutely safe water as it first appeared but rather a relative improvement and no more.The standards SafariQuip has adopted are those of Britain's leading experts concerned with the establishment and maintenance of safe water supplies and public health. Where there has been a distinction between what is necessary in Britain and what might be required overseas the higher standard has been applied.


Types of Contamination

Suspended Solids

If you put dirty water in a glass the suspended solids are the tiny particles that drift about and do not readily sink to the bottom. In addition to those particles that are visible there can be tremendous concentrations of particles that are simply too small to be seen. The resolution of the human eye is about one tenth of a millimetre, a particle one twentieth that size (5uM) is of course totally invisible to the naked eye. There can be over ten million such particles in a single litre of water without any visible trace! Suspended solids usually consist of fine particles of organic material such as decaying vegetable matter or fine mineral matter such as mud and clay. Normally mud and clay contamination is harmless. Sometimes however there will be extremely fine rock particles in the water that do not settle out, this is most likely to be true for glacier water and water running through some types of clay. Generally these particles are harmless but they can in some areas include mica or asbestos. If in doubt or you start to suffer from discomfort, filter the water.

Chemical contamination including taste and odour

It is often the case that extremely small concentrations of certain chemicals can make water unpleasant or completely undrinkable. Most people will have experienced the taste of chlorine sometimes noticeable in our tap water, others the metallic taste from water out of jerricans or the taste from water out of plastic containers. These tastes, and many others including those from stagnant water, are caused by minute quantities of chemicals that can easily be removed from water by charcoal or carbon filtration.

Microbiological contamination

  1. Eggs, worms, flukes, etc.
    Into this category come the organisms that lead to infections of, amongst others, roundworm (Ascaris), canine roundworm (Toxocara canis), guinea worm (Dracunculus) and bilharzia (Schistosomiasis). In the scale of things they are quite large although still microscopic. Their large size makes the task of removal relatively simple with even crude forms of filtration. The very tiny black things that you sometimes see wriggling in still water are insect larvae, not harmful and not germs as sometimes thought. Practically any form of pre-treatment will remove them.

  2. Protozoa
    These are small single-celled animals. Into this group are the organisms that cause Giardiasis (Giardia lamblia), an unpleasant form of chronic diarrhoea, and amoebic dysentery (Entamoeba histolytica). Both of these protozoa have a cyst stage in their life cycle. The cysts are inert and resistant to some forms of chemical treatment, but can quickly become active and develop when they encounter suitable conditions such as the human digestive tract. They are sufficiently large to be separable from the water by the careful use of some types of pre-filter.

  3. Bacteria
    Very small single-celled organisms responsible for many of the illnesses that beset travellers. At the more serious end of the scale are cholera, salmonella, typhoid and bacillary dysentery caused by Shigella, less serious are the many forms of diarrhoea known to travellers under a variety of names such as Montezuma's revenge and Delhi belly. There is a relationship between the dose of bacteria and the risk of infection. The greater the dose the greater the risk. For a healthy person, typically thousands of a particular bacterium need to be drunk to cause disease. (See the reference to bacteria under 'Filtration' and beware the atypical!) The harmful bacteria transmitted by drinking contaminated water are fortunately fairly "soft" and succumb to chemical treatment. Because of the extremely small size only very few filters can be relied upon to remove all harmful bacteria from water.

  4. Viruses
    These exceptionally small organisms live and multiply within host cells. Some viruses are transmitted through drinking water. Hepatitis A and a variety of intestinal infections are amongst this group. Even the finest filters are too coarse to retain viruses. The polio and hepatitis viruses are about 50 times smaller than the pore size in even the finest ceramic filter.


Selection and treatment of a water supply

Whatever method of water treatment you plan to use it is essential to start with the best possible supply available to you. The following list may be used as a guide to assess the potential suitability of a water supply.

  • Good Points - Ground water, eg wells, boreholes, springs.- Water away from or upstream of human habitations.- Quickly running water.- Water above a sand or rock bed.- Clear, colourless and odourless water.

  • Bad Points- Water close to sources of industrial, human or animal contamination.- Stagnant water.- Water containing decaying vegetation.- Water with odour or a scum on its surface.- Discoloured or muddy water.

  • Wells and boreholes can be contaminated by debris and excreta falling or being washed in from the surface. With wells it is perhaps worth considering how protected the top is from such contamination. A narrow wall around a well will stop debris from falling or being washed in. A broad wall is not so good as it will be stood on, consequently dirt from feet gets carried up on to the wall and subsequently falls into the well. Any wall is better than no wall at all.

  • Quickly running water is a hostile environment for the snails that support Bilharzia.



If your water source is very poor there are a few ad hoc steps that can be taken to improve it. If you are using water from a river, pool or lake try to draw the water carefully without gathering extra dirt from the bottom or any floating debris from the surface.

If the source is surface water such as a lake or river and very poor, some benefit may even be gained by digging a hole adjacent to the source and letting the water seep into it for collection. In seeping through, a form of pre-filtration will take place and at least the coarsest contamination should be left behind.

Filtration can to some extent be improvised. Pouring the water through finely woven fabrics will remove some of the larger contamination. If you have fine clean sand available, perhaps taken from a stream or lake bed, an improvised sand filter can be made using a tin can or similar container with a hole in the bottom, even a sock will do. Pour the water into the top, over the sand. Take care to disturb the surface of the sand as little as possible. Collect the water that has drained through the sand. The longer the filter is used the better the quality of the water will become so re-filter or discard the first water through. Discard the contaminated sand after use.

If you are able to store the water without disturbing it another option is open to you, that of sedimentation. Much of the dirt in water will eventually settle out on the bottom if left over a long enough period. The cleaner water can then be drawn off at the top. Very great care will be needed not to disturb the dirt at the bottom. Siphoning is the best method of drawing off the cleaner water. The flukes of Bilharzia die after about 48 hours so water kept longer than this will be safe from this parasite.

If the water you are using has an unpleasant taste or smell, perhaps because the water was stagnant, an improvement can be achieved by using lumps of wood charcoal, perhaps taken from the previous night's fire, wrapped in a piece of cloth and coarsely crushed. When the 'bag' of charcoal is placed in the water or the water run through the charcoal (like a sand filter) the organic chemicals responsible for practically all of the unpleasant taste and smells will be removed. Some colour improvement may also be noticed. The water these techniques will provide is not safe to drink without further treatment but since some of the harmful contaminants are quite large it is fairly certain that you will have gained some benefit from their use.


Water Supply Treatment


Boiling at 100°C kills all of the harmful organisms found in water. There are a few organisms such as slow viruses and spores that can survive even extended periods of boiling but there is no danger from drinking these.

There is a catch however - As your altitude above sea level increases, the weight of the atmosphere above you decreases and air pressure drops. The drop in air pressure has the effect of reducing the temperature at which water boils. A rule of thumb method for calculating this is that water boils at 1°C less for every 300 metres of altitude; thus if you are on the summit of Kilimanjaro at 5895m the water you boil for a drink will only be 80°C.

The consequence of this for the traveller is that although the water is boiling it is not so hot and therefore germs take longer to die and food takes longer to cook. The reverse of this phenomenon is demonstrated by the pressure cooker - the high pressure causes a boiling point much higher than 100°C and so food cooks more quickly.

At temperatures below 100°C most organisms can still be killed but this takes longer. At temperatures below 70°C an important change starts to take place, some of the harmful organisms start to be able to survive indefinitely and as the temperature continues to drop they not only survive but flourish.

There is one more important consideration. When water is boiling vigorously there is a lot of turbulence and all the water is at the same temperature. While water is coming to a boil even if bubbles are rising there is not only a marked and important difference between the temperature of the water and the temperature at a full boil but there can also be a substantial difference in temperature between water in different parts of the pan with the result that harmful organisms may still be surviving.

To make water safe for drinking you should bring water to a full boil. We strongly recommend that you continue to boil the water for two minutes as a safety margin. Boil water for one minute extra for every 300 metres above sea level. Do not cool water down with untreated water.



To start to cover the various types of filtration it is necessary to understand that filters differ an enormous amount. The key to understanding the usefulness of a filter is:

How small are the particles that the filter will reliably separate and what dirt load can the filter take before it clogs up?

If the pores in the filter are too large then harmful particles can pass through. If the pores are small enough to stop harmful particles in the water they are so small they can get quickly blocked up preventing any more water from being filtered.

To reduce this problem filter manufacturers employ ingenious means to increase the filter area and filter in progressively smaller stages through the thickness of a filter. The problem remains however; for example even in 1 litre of water that appears quite clean there can be a hundred thousand million (1011) particles the same size or larger than bacteria. To stop a bacterium the filter has to take out all the other particles as well.

If the filter is small such as the drinking straw types, or the water is at all visibly dirty then the filter will block in next to no time. The situation is not hopeless.

There are three solutions:

  • Water can be filtered first through a coarse filter to remove most of the dirt and then again through a fine filter to remove the harmful bacteria, a re-cleanable filter can be used or, finally, only apparently clean water could be used with the filter. The use of a coarser initial filter is called pre-filtration. Viruses are so small they cannot be filtered out of drinking water by normal means (see 'Viruses') and the risk from viruses will persist in any water that has been treated by filtration only. However because viruses that are with their host infected cells can be filtered out along with the cells it is possible that the finest filters are able to reduce the risk of some virus infections in drinking water. A filter collects quite a lot of miscellaneous debris on its surface and in order to prevent this providing a breeding ground for bacteria the filter needs to be sterilised from time to time.

    Some filters are self-sterilising and need no action but other filters should be sterilised by boiling for 20-30 minutes at least once every 2 weeks. Where filters are described as combining a chemical treatment this is for self-sterilisation. The chemical is in such small concentrations and in contact with water passing through the filter for such a short period that its use to improve the quality of the filtered water is negligible.

    Pre-filtration Pre-filters should remove particles larger than 5uM-10uM in size and be very simple to maintain. They will be more resistant to clogging since they take out only the larger particles. These filters will remove the larger microbiological contamination including protozoal cysts, flukes and larger debris that might form a refuge for bacteria and viruses. Pre-filters are an ideal preparation for further filtration and chemical treatment thus making water considerably safer for drinking and normally adequate for washing. Further treatment is essential for completely safe drinking supplies.

  • Fine Filtration To remove all harmful bacteria from water a filter must remove all particles larger than 0.5uM (some harmless bacteria are as small as 0.2uM). Filters may be disposable or cleanable. Filters using a disposable cartridge are generally more compact and have high initial flow rates but are more expensive in operation. Alternatively there are ceramic filters that use porous ceramic 'candles'. These filters have low flow rates and are fairly heavy. Some need special care in transport to ensure the candles do not get chipped or cracked enabling untreated water to get through. Ceramic filters can be easily cleaned and are very economic in use.

  • Activated Carbon/Charcoal Filters Carbon filters have the ability to remove a very wide range of chemicals from water including chlorine and iodine. Since all of the nasty tastes and smells in water are caused by traces of chemical the carbon filter can improve the quality and palatability of water a great deal. A word of warning: carbon filters do not kill or remove germs and may even provide an ideal breeding ground unless self-sterilising. Some filters combine carbon with other filter elements to make a filter that improves the taste as well as removing harmful organisms.


Chemical Treatment

There are broadly three germicidal chemicals used for drinking water treatment. For ease of use, efficiency and storage life the active chemical is usually in the form of a compound and made up as a tablet in a dose suitable for a fixed volume of water. The chemicals are used up as organisms are killed and absorbed by other organic material in the water and consequently with heavy contamination a larger dose is required. Germs can be embedded in other matter and protected from the effects of a chemical so in the worst cases of contamination where water is visibly dirty there is no alternative but to pre-filter first. Chlorine and iodine have no lasting germicidal effect so on no account should untreated water be added to water already treated.

  • Silver. Completely harmless, taste free and very long lasting effect. Protects stored water for up to six months. Particularly useful if the water is to be stored or the taste of chlorine and iodine are found unpleasant. The sterilisation process is quite slow and it is necessary to leave water for at least two hours before use. Silver compounds are not effective against cysts of Amoeba and Giardia and therefore we strongly recommend pre-filtration first if poor quality water supplies are likely to be used.

  • Chlorine. Completely harmless and very widely used in water treatment plants. Fast acting and 100% effective if used correctly. A minimum of ten minutes is required before water can be used. The cysts of Amoeba and Giardia are about 10 times more resistant to chlorine than the bacteria but both are reliably killed if treatment time and dose are adequate. The normal recommended dosage given for most water treatment tablets is 100% effective against Amoeba and Giardia cysts if used in clear water.

    If in doubt we recommend that the period before use be extended to at least twenty and preferably thirty minutes. If heavy contamination is suspected or visible we recommend you also double the dosage.

    An alternative to double dosage and/or longer treatment times is pre-filtration. Some people find the taste of chlorine unpleasant particularly if larger doses are being used. The concentration of chlorine quickly drops over several hours and more so in warm temperatures so there is very little lasting effect. Excess chlorine may be removed by using Sodium Thiosulphate or a carbon filter.

  • Iodine Fast acting and very effective. Normally 10 minutes is stated as the treatment period before water is safe to use. It has a quicker action against cysts than chlorine. Double dosage and extended treatment times are still very strongly recommended if the water is dirty or heavy contamination is suspected, alternatively pre-filtration is most effective. Iodine is more volatile than chlorine and the lasting effect is negligible. Excess iodine may be removed by Sodium Thiosulphate or a carbon filter.

NOTE: There is medical evidence that iodine can have serious lasting physiological side effects if used regularly. Groups that are particularly at risk are those already with thyroid problems and the unborn foetus of pregnant women using iodine. Thyroid problems may only become apparent when the gland is faced with excess iodine. We recommend that iodine should not be used over an extended period. In the unlikely event of the use of iodine compounds being unavoidable ask your doctor to arrange for a thyroid test beforehand or use a good carbon filter to remove excess iodine from the treated water.


Order of Treatment

If chemical treatment and filtration are being combined it is normal and best practice to filter first. The reason is that filtration removes organic matter which would absorb the chemical and make it less effective, if of a carbon type, the filter will also absorb the chemical leaving none for residual treatment. In some cases the filter may also be a source of contamination. If water is being stored prior to treatment then it is worthwhile chemically treating as soon as the water is collected and again after filtration. The first chemical dose prevents algae growing in the stored water.


Storage of Water

It is advisable to try and use separate containers for treated and untreated water. If you are able to do this mark the containers accordingly and do not mix them. If you are unable to use separate containers take particular care to sterilise the area round the filler and cap before treated water is stored or at the time treatment takes place. Containers for untreated water should in any case be sterilised every 2-3 weeks. Treated water should never be contaminated with any untreated water. Treated water should never be stored in an open container. Treated water left uncovered and not used straight away should be regarded as suspect and re-treated.

© Copyright 1986 J McIntosh

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