To effectively purify water it is first necessary to have some understanding and appreciation for the ways in which water can be contaminated and what methods can be used to remove these contaminants to make the water drinkable
again .
Types of Contaminants
The various types of contaminants can be classed as either chemical or biological with different subdivisions existing within these two broad classes of impurities . Although these contaminants are discussed singly it is quite possible or even probable that water supplies will be polluted with more than one type of impurity .
Chemical Contaminants
These can be broken down into either suspended or dissolved impurities ,the easiest way to differentiate between the two is to remember that suspended impurities can be filtered out dissolved ones can't .
A. Suspended impurities - These are materials that are not soluble in water , they may be solids of varying particle sizes such as sand , clay or earth particles ; mineral fibres such asbestos , rockwool or fibreglass or liquids such as oil or oil emulsion . Suspended impurities usually advertise their presence by discolouring the water as in muddy water or altering its appearance such as the rainbow surface on oil contaminated water . As a class suspended impurities are common contaminants of water .
B. Dissolved Impurities - These can be broken down into three sub-classes -
1.Heavy Metals - These are the nightmare contaminants of industrial pollution such as zinc , lead , copper , cadmium or mercury . If present in large enough amounts they can give the water a metallic taste or discolour it but may be there in sufficient quantities to be poisonous without giving any warning . Fortunately contamination of the water with heavy metals is unlikely unless you are down stream from chemical or heavy industrial processing or a waste tip .
2.Organic Solvents - Which have some solubility in water and can be cumulatively poisonous ( build up in the body ) or cause cancer or both . Some examples are benzene , toluene , chloroform or polychlorinated biphenyls and as with the heavy metals unless you are downstream from industry or a waste dump they are unlikely to be a problem .
3.Inorganic Salts - These include nitrates from fertilizers , cyanides from industrial processes and the intentional addition of fluoride . These contaminants are more widespread than heavy metals or the organics and are difficult to detect .
Biological contaminants
These are living organisms which cause disease after they have been taken into the body with the water that is drunk . They may be broken down into three large groups -
a. Bacteria - cause such diseases as cholera, typhoid or gastroenteritis, they are the most commonly found biological contaminant.
b. Viruses - can cause infectious hepatitis or polio .
c. Protozoans - which can cause dysentery eg Giardia Lamblia or Entamoeba Histolytica .
None of these contaminants may advertise their presence and water that looks sparkling clear and inviting may be heavily contaminated with any number of disease causing organisms .
Methods of Purification
There are many methods of purifying contaminated water , most will only remove one or two types of contamination but reverse osmosis and fractional distillation will remove all impurities in one operation . Simple distillation is not far behind removing all contaminants but the volatile organic solvents , which are rarely encountered . These methods will be discussed first followed by the methods for removing chemical contaminants and then the ways of removing biological contaminants (ie sterilising the water) .
Reverse Osmosis
In this process the water is forced under pressure through a semipermeable membrane , only the water molecules are small enough to get through so all the impurities are left on the other side . There are large units around that are expensive and require 240v power for operation but there are smaller portable ones that are worked by muscle power . The smaller units are a bit difficult to find , the throughput of water is small and quite a bit of muscle power is required to work them . For a personal ,portable purifier reverse osmosis could work well but a number of units would be needed
to supply more than a survival ration of water for several people . The biggest drawback is that you either have one or you don't , they are impossible to build from scratch using low technology .
Fractional Distillation
This is the process whereby contaminants of differing boiling points are taken off separately . Heat is applied to the water to boil it and the resulting steam is taken through a vertical column of glass or metal filled with small shards ,rings or balls of glass , metal or ceramic . This is called the fractionating column and it brings the rising steam into contact with falling condensed vapour which scrubs out any high boiling point liquids from the steam . The scrubbed steam is then lead into a water or air cooled tube where it condenses into liquid again . Any low boiling point contaminants come
off first , followed by pure water at exactly 100oC then the high boilers come over above that . All insolubles , bugs etc. are left as a sludge in the boiling vessel.
This method , as mentioned earlier , removes all impurities but the drawbacks are that it is more difficult to construct than simple distillation apparatus and needs a thermometer accurate in the area of 100oC at the top of
the fractionating column to ensure that only the pure water is collected .
Simple Distillation
This is the same as fractional distillation without the fractionating column . It is very simple to build requiring only a heatproof container over a heat source to boil the water and a coil of metal or glass tubing to condense the steam back to liquid. All the impurities except some volatile organics are left behind in the boiling vessel and in practice it would be rare to come across water contaminated in this way.
A simple ,if low output, way to distil water is to make a shallow pan of black plastic , fill it with contaminated water then lean two sheets of glass over the top of it to for an A shape . The sun will evaporate the water which condenses on the glass and runs down into a trough on each side of the contaminated pan put there to collect the distilled water . This method has been used to desalinate sea water .
Removing Chemical Contamination
Filtration
This is the process of passing the water through a material with very fine holes in it , the finer the holes the more of the contaminants are left on the filter . Filters have been constructed of various materials such as cloth , paper , metal , plastic and ceramics but the most common water filtration materials are activated carbon and the sand filter .
Activated Carbon
This is the bit that does the work in 90% of home water filters , it is cheap and very effective in removing the things that you usually buy a filter for - unpleasant tastes and odours . Activated carbon will also remove chlorine taste and smell and reduce organic and heavy metal contaminants . What it won't remove is dissolved salts like fluoride or nitrates and won't remove biological contaminants , in fact it acts as an ideal habitat for the growth of pathogenic ( disease causing ) bugs . Any suspended solids will also tend to clog an activated carbon filter so a pre-filter of paper or cloth is a good idea if the water to be filtered is very murky .
Due to the cheapness and ability of activated carbon to remove off tastes and smells it is worth having a water purifier of this type plus spare carbon cartridges to clean up water filtered by other means and disinfected .
Sand Filter
This is a filter system that can be home constructed . The larger the area of the filter bed the faster the throughput but a 20 litre bucket is a good start . The bottom of the bucket should be drilled with 6mm holes to allow drainage then a layer of 25mm to 50mm of coarse washed gravel put in the bottom,this layer should then be covered with a circle of porous cloth . On the cloth is placed a 50mm layer of charcoal then another circle of cloth , if no charcoal is available use more sand or sandy clay. Next goes another circle of cloth then a 250mm layer of sand or sandy clay , topped finally by a layer of cloth . The water poured in the top is filtered as it passes slowly through the different layers , the top layer of cloth can be removed and any coarse sediment washed off as it clogs up .
This filter will remove most suspended contaminants but won't touch biological contamination or dissolved contaminants . It is ideal where the water is muddy or turbid but not suspected to be contaminated by chemical nasties . After filtration the water should be sterilized to remove biological contaminants .
Removing Biological Contamination
This process is also referred to as disinfection or sterilization of the water supply , these methods remove biological contamination only and will have no effect on chemical impurities . There are two main methods used to
sterilize water ;heat and chemicals . Ultraviolet light and ozone have also been used to achieve this but require 240v electricity supply and are impossible to home produce .
Ultraviolet Light
Heat
Boiling water for at least 15 minutes can sterilize even heavily infected water , but the energy must be available to keep the water at a rolling boil for the full time . Boiled water tends to taste " flat " but this may be remedied by pouring the water from one container to another to re-aerate it .
Chemical Additives
A.Chlorine and its compounds
1.Chlorine gas - is a very efficient disinfection agent and is used by water authorities to purify our water but in its free state it is unavailable to the public because of its highly poisonous nature . It can be manufactured but is very difficult to handle and would prove more of a health hazard than the contaminated water .
2.Hypochlorites - These are cheap and efficient germicides , easily available in the form of household bleach or pool chemicals . Calcium hypochlorite (also called chloride of lime) is available as a powder or granules , it can be a bit aggressive on its container over time but will keep its activity for a long time . It can be a bit of a problem to store though because if it gets damp it can cause a fire ,it needs to be kept absolutely dry and away from flammable solvents like petrol and metho.
3.Sodium hypochlorite is more reactive than the calcium salt and so is only supplied as a solution in water , usually 5% to 6% sodium hypochlorite . It is safer to store than calcium hypochlorite but the amount of chlorine per kilo is less and the solutions tend to lose activity over long storage , particularly if the lid is left off the container .
4.Isocyanurates - eg Sodium dichloroisocyanurate (PuritabsR) These are more expensive than the hypochlorites but also much more stable . They are amenable to transport for use as a portable water sterilizer while camping or travelling . Where they are packaged correctly a shelf life of 5 years is possible .
5.Halazone - Again this is a more stable , more expensive product designed for use as a personal water sterilizer for the traveller . One 4 milligram tablet of halazone will disinfect one litre of lightly contaminated water or 500 mls of heavily contaminated water .
In general chlorine compounds are comparativley safe , cheap and effective water disinfectants . After sufficient chlorine has been added to the water supply to satisfy the chlorine demand (kill all existing bugs) a residual chlorine content of 0.2 to 0.4 parts per million (ppm) should be maintained .The chlorine demand can be worked out adding the chlorine , letting the water stand then measuring the free chlorine with a pool test kit . Once the initial load of bugs have been killed sufficient chlorine compound needs to be added to keep the free chlorine within the above levels . The initial amount of chlorine to be added will need to be increased if organic matter ( leaves , insects , grass etc) or sediment is present so it is better to filter the water supply before disinfecting it .
The amount of chlorine available from the above chemicals varies depending on which one is used , this is illustrated in the table below -
Chemical % available Cl
Sodium Hypochlorite 96.5
Calcium Hypochlorite 65.0
Sodium Dichloroisocyanurate 64.5
Halazone 50.0
So that if 1000 litres of water requires 0.3 ppm of residual free chlorine this means adding 4.6 grams of calcium hypochlorite , 6 grams of halazone etc. to the water once all the bugs already in it have been disposed of .
The unpleasant chlorine taste introduced to the water by this form of disinfection can be removed by adding small amounts of sodium thiosulphate (photographic "hypo") or citric acid to the water before drinking .
B.Iodine
Iodine is available from chemists as a solution in alcohol (iodine tincture) which contains about 2% iodine . It can sometimes be bought in its solid crystalline form but this is more difficult to store as it tends to evaporate over time . A level of 1 ppm iodine is sufficient to disinfect water, this translates as about 1 to 2 drops of iodine tincture per litre .
Thus the first step in water purification is to discover what type of contamination may be present in potential sources of drinking water , and then considering the resources available , decide on the method of purification which will do the job efficiently
