It is important to have good quality of water for drinking and washing purposes.
This is achieved near the source by keeping reservoirs clean and in treatment plants throughout the distribution network.
Normal hard water is not a risk to health but can cause scale build-up in boilers and hot water systems, also hard water does not lather soap well.
Simple Chemistry of Water
Water is a chemical compound. It is a liquid which boils at 100oC and freezes at 0oC, and consists of water molecules.
Each molecule contains two hydrogen atoms joined to one oxygen atom.
It has the chemical formula H20.
It is an excellent solvent and as a result normal drinking water contains dissolved substances such as iron (Fe), manganese (Mn), calcium (Ca), magnesium (Mg), sulphate (S04) and fluoride (F).
It will also contain dissolved gases including oxygen (02), nitrogen (N) and carbon dioxide (C02).
Many of the above minerals are essential for the health and proper growth of our bodies, but their content must be carefully controlled as must the possible content of harmful bacteria.
The presence of these minerals usually imparts a much more pleasant taste to water than would be the case with the "pure" compound.
The ability of rain to dissolve gases present in the atmosphere produces the formation of weak acids, e.g. -
H20 + C02 H2C03, carbonic acid
H20 + S02 H2S03, sulphurous acid
H20 + C03 H2C04, sulphuric acid
On penetrating layers of soil the water dissolves carbonates, chlorides, and sulphates of calcium and magnesium. It may also dissolve ammonia, silica and iron oxides. The amounts of each of these materials dissolved depends on the thickness and type of layers through which the water passes and on the solubility of the material. The presence of acids in the rain water increases its ability to dissolve many of the materials. In addition to chemicals, water may also contain micro-organisms e.g., bacteria, parasites, viruses and algae.
The pH of natural water varies from approximately 6.0 to 8.0 and depends on the type of rock through which the water passes. In some areas it may be as low as 4.0, e.g., due to acid rain. The pH scale which ranges from 0 to 14 measures acidity and alkalinity. The lower the value the more acidic the water.
pH less than 7 - solution is acidic
pH greater than 7 - solution is alkaline
Hard And Soft Water
Soft water contains little or no dissolved solids.
It is often brownish or yellowish in colour.
Hard water, on the other hand, will have a high calcium or magnesium salt content.
Soft rainwater that percolates through certain types of rock strata e.g., chalk or limestone, will become "hardened" in this way. Hard waters do not form a lather readily which shaken with soap solution, whereas soft water lathers easily.
The table below sub-divides hard and soft waters into several classes depending on the hardness.
0 - 50
50 - 100
100 - 150
150 - 200
Hard water can be further sub-divided into two types - TEMPORARY and PERMANENT.
Temporary hardness can be removed by heating to temperatures above 60oC, whereas permanent hardness requires removal by chemical methods.
Temporary hardness can lead to the formation of scale deposits on heating pipes, boilers, kettles, etc.
Physical properties of water are relatively easy to measure and some are readily observable.
1. Temperature - this is important as it can have a marked effect on other properties - rate of chemical reactions, solubility, tast, etc.
2. Colour - pure water has a pale green-blue tint in large volumes. However, matter suspended in water can alter the apparent colour. The use of chlorination in water treatment will normally remove all traces of the original source colour.
3. Solids content - solids may be present in suspension or solution or both. The larger suspended solids will normally settle out fairly quickly, whereas finer solid material may require the addition of chemicals to assist in their removal. The turbidity of water is the cloudy appearance due to the presence of very fine solids e.g., when clay and/or algae are present.
4. Taste - pure water does not have a particularly pleasant taste, nor, frequently, does tap water as a result of the use of chlorine disinfectant. Water which contains quantities of naturally dissolved salts, on the other hand, can be quite pleasant and sweet tasting. On the other hand the presence of organic derived compounds (algae, peat) can lead to an unpleasant taste.
This process kills off harmful organisms in water so that infection by disease will not occur when the water is used for domestic purposes.
Generally this is achieved by chlorination that is the addition of chlorine into the water.
The organisms in water which it may be necessary to kill by disinfection include bacteria, viruses and protozoa.
The resistance of these organisms to the effects of disinfection varies according to the type of organism present.
Chlorine itself which is a very strong oxidising agent and its level of dosing must be strictly controlled otherwise the consumer will experience taste and odour problems in the supply.
An alternative method of disinfection is by using ozone (O3).
This is, however, a very expensive method if applied on a large scale.
The ozone must be produced at the point of treatment and its production involves the use of large quantities of electricity and its sterilizing effect is not as persistent as that of chlorine.
Disinfection is generally the last step in the treatment process.
Other Forms Of Water Treatment
The above methods of water treatment are a general guide to the steps which can be applied to obtain a potable water supply.
The quality of water and hence its treatment requirements can vary dramatically depending on for example the nature of the source. Therefore, there is not one standard system of water treatment.
Each water will have its own requirements.
A few of the other possible types of treatment are mentioned briefly below.
(a) Softening Of Water
Hard waters are produced from a source where chalk or limestone is present.
Hard water causes a difficulty in obtaining a lather with soap and will lead to a deposit forming in kettles or hot water pipes.
The hard water may be softened by adding lime or sodium carbonate which will lead to a precipitate being formed which can be removed by filtration.
Alternatively the hard water may be passed through ion-exchange resins which will effectively remove the compounds which initially caused the hardness.
(b) Iron And Manganese Removal
Traces or iron and manganese can be found in many waters.
Iron, while not being harmful in small amounts can impart a bitter taste to water and can cause brown stains to form on laundry. The presence of manganese can also cause taste problems and it can react with chlorine to produce an objectionable black slime.
A combination of chlorine and lime followed by filtration is now the common treatment for manganese removal.
(c) Addition Of Fluoride
The addition of fluoride to the water supply has been the subject of much public debate.
It is now generally accepted that the addition of fluoride in small amounts to the water supply is effective in reducing dental cavities. Fluoride in the form of hydrofluorosilic acid can be added to the water after all other forms of treatment have been completed.
(d) Taste, Odour And Organic Removal
Activated (i.e., heat treated) carbon, either in powdered (PAC) or granular (GAC) form, is increasingly used in water treatment for taste, odour and organic removal.
It can be added (as PAC) at various treatment stages but the most common application is as filters containing GAC prior to the disinfection stage.
(e) ph Correction
If necessary, e.g., in soft water areas, the pH of the water is adjusted by the addition of an alkali.
Lime and calistic soda can be used.
The purpose of pH adjustment is to make the water non-aggressive to metal pipework - both in the distribution network and in the home.
(f) New Developments
New processes are being developed by the Water Industry e.g., sirofloc, membrane filtration.
The Sirofloc process uses finely divided magnetite to attract particulate (including collodial colour) material.
The magnetite itself is removed by a magnetic process.
Water can be softened by several alternative methods; base exchange method, addition of lime-soda and use of inhibitors.
In domestic installations the base exchange method is used and water is passed through a medium called 'zeolite', which converts the calcium salts in the water to sodium salts.
For industrial plant, water may be softened by the addition of lime or lime-soda. In this process large volumes of sludge needs to be removed.
Additives such as 'Calgon' may be used to stop 'fur' in boilers and these may be regarded as inhibitors rather than true softeners.
Water contains dissolved and suspended solids which are small particles of animal and vegetable debris. Water from chalk or from salt-bearing strata may contain over 1000 p.p.m. (parts per million) solids, whereas water from upland sources may contain less than 50 p.p.m. (parts per million).
The water in treatment plants is filtered and sterilised with 'chlorine' to reduce harmful effects of bacteria in water.
The pH value of water should be considered i.e. its acidity or alkalinity. Soft acid waters are derived from hard insoluble rocks or from peaty uplands, they have a pH less than 7.0 and may corrode pipes and tanks unless passed through a cylinder packed with limestone to neutralise the acidity.
Waters with a pH of more than 7.0 are alkaline and are not likely to attack metals.