Water Quality - Physical Parameters

The quality of water is determined by the impurities present in it. The impurities may be physical, chemical or bacteriological in nature. In order to ascertain the quality of water, it is subjected to various tests viz., physical, chemical and bacteriological tests. It is essential for devising water quality management programme to properly use water in any project. It gives information for following decisions to be taken.

• Helps in identifying the present and future problems of water pollution.
• Identifying the present resources of water as per various usages.
• It helps in developing plans and setting priorities for water quality management programme so as to meet future water requirements.
• It helps in evaluating the effectiveness of present management actions being taken and devising future course of actions.

Impurities in Water

It is not possible to find pure water in nature. The rain water as it drops down to the surface of earth absorbs dust and gases from the atmosphere. It is further exposed to organic matter on the surface of earth and by the time, it reaches the source of water supply, it is found to contain various other impurities also. For the purpose of classification, the impurities present in water may be divided into the following three categories.

1. Physical impurities
2. Chemical impurities
3. Bacteriological impurities

Analysis of water

In order to ascertain the quality of water, it is subjected to various tests. These tests can be divided into the following three categories.

1.  Physical tests
2.  Chemical tests
3.  Bacteriological tests

Collection of Water Samples

The sampling is the most important part of any analysis because the final results obtained, even from the most accurate analysis, will be misleading, if the samples on which such analysis is carried out, are not representative ones of the liquids to be tested. As a matter of fact, it will be ideal to carry out all the analysis immediately after the collection of samples and quicker the analysis, the more representative will be the results of analysis of the liquid at the time the samples are taken. These precautions while sampling of water are as follows.

• The water should be collected in bottles, especially of white glass, having well fitted stoppers. Bottles having holding capacity of about 2 liters of water are necessary for chemical analysis. For bacteriological examination, bottles with smaller capacities will be sufficient.
• Bottles should be thoroughly cleansed, filled thrice with water and thrice emptied before collecting the sample. However, it will not be necessary to carry out such process, if the sealed bottles are directly obtained from the laboratories.
• When the sample of water is to be collected from a pipe, the water tap should be turned on and the water should be allowed to go waste for at least two minutes so as to prevent the entry of impurities of the pipe in the sample of water. If the sample is to be collected for conducting a bacteriological analysis, the nozzle of the tap should be flamed and made unbearably hot and then cooled by the running water before the bottle is filled.
• For the collection of water for bacteriological tests, the person who collects the water must be free from any disease. The containers and bottles must be cleaned with sulphuric acid, potassium dichromate or alkaline permanganate and then, they should be thoroughly rinsed with distilled water and finally sterilization should be done. Immediately after collection of the samples, bottles should be closed and covered with clot to prevent accumulation of dirt, etc.
• For collecting the sample of water from lake, streams, spring or well the whole bottled with stopper closed should be immersed deep into the surface of water and then only the stopper of the bottle should be removed by means of a clean piece of string and the bottle is filled. Thus the entry of floating materials will be prevented in the bottle.
• The bottle should be held as far away from its neck as possible. In no case, the water entering the bottle should come into contact with the hand.
• In case the water is being collected from the ground sources i.e. through well or tube well, sufficient quantity of water should be pumped out before collecting the samples.
• After collecting the sample, the stopper of bottle should be well secured and the bottles containing samples of water should be labelled stating the source, date and time of collection.

Physical Tests

Under this category, tests are carried out to examine water for the following properties.

1) Colour

The colour of water is usually due to presence of organic matter in colloid condition and due to the presence of mineral and dissolved organic and inorganic impurities. An undesirable appearance is produced by colour in water. It spoils the clothes and affects various industrial processes. The measurement of colour in water is carried out by means of a tintometer. The instrument has an eye piece with two holes. A slide of standard coloured water is seen through one hole and in the other hole, the slide of water to be tested is inserted. The intensity of colour in water is measured on an arbitrary scale.

The unit of colour on cobalt scale is the colour produced by one milligram of platinum cobalt in one litre of distilled water. The slide of standard numbers is kept ready in the laboratory. For public water supply, the number on cobalt scale should not exceed 20 and should preferably less than 10.

Transparent water with a low accumulation of dissolved materials appears blue. Dissolved organic matter such as humus, peat or decaying plant matter, etc. produce a yellow or brown colour. Some algae or dinoflagellates produce reddish or deep yellow waters. Water rich in phytoplankton and other algae usually appears green. Soil runoff water has a variety of yellow, red, brown and grey colours.

The colour in water is not harmful but it is objectionable. The colour of a water sample can be reported as Apparent or True colour. Apparent colour is the colour of the whole water sample and consists of colour from both dissolved and suspended components. True colour is measured after filtering the water sample to remove all suspended material. Before testing the colour of the water, first of all total suspended matter should be removed from the water by centrifugal force in a special apparatus. When multicolour industrial wastes are involved, such colour measurement is meaningless. It should be remembered that the examinations of colour by matching with slides of standard colours will be sufficient for most of the purposes and it is obvious that the results will be influenced by the personal factor, the conditions of lighting under which the tests are carried out etc.

2) Temperature

The test for temperature of water has no meaning in the sense that it is not possible to give any treatment to control the temperature in any water supply project. The temperature of water to be supplied from storage reservoir depends on the depth from which it is drawn. The most desirable temperature for public supply between 4.4°C to 10°C. The temperature above 35°C is unfit for public supply, because it is not palatable.

The multiplication of bacteria in the waters is more rapid at higher temperatures than in the waters at lower temperature. Hence, when waters with a temperature of about 15°C are collected for bacteriological analysis, they should be cooled down as quickly as possible. It should further be remembered that the air temperature at the time of taking the water sample should always be recorded.

The measurement of temperature of water is done with the help of ordinary thermometers. From the study of temperature, the characteristics of water such as density, viscosity, vapour pressure and surface tension can be determined. It also helps in determining the saturation values of solids and gases which can be dissolved in water and also the rates of chemical, biochemical and biological activity.

Density, viscosity, vapour pressure and surface tension of water are all dependent upon the temperature. The saturation values of solids and gases that can be dissolved in water and the rates of chemical, biochemical and biological activity are also determined on the basis of temperature. The temperature of surface water is generally same as the atmospheric temperature while that of ground water may be more or less than atmospheric temperature.

3) Taste and Odour

The water possesses taste and odour due to various causes and they make the water unpleasant for drinking. Tastes and odours in water are due to the presence of any of the following.

1.  Dead or living microorganisms
2.  Dissolved gases such as hydrogen sulphide, methane, carbon dioxide or oxygen combined with organic matter
3.  Mineral substances such as sodium chloride, iron compounds
4.  Carbonates and sulphates

The test is conducted for odour by inhaling through two tubes of osmoscope. One tube is kept in a flask containing diluted water and other one in a flask containing water to be tested. The odour of water also changes with temperature. The odour may be classified as sweetish, vegetable, greasy etc. The odour of both cold and hot water should be determined.

The intensities of the odours are measured in terms of threshold odour number (TON). In this method, water to be tested is diluted with odour free water and mixture at which odour becomes detectable is determined. It indicates threshold number and other intensities of odour are then worked out. The results of test are greatly affected by the sensitiveness of the observer. For public water supply, the threshold number should not be more than 3.

4) Turbidity

Mainly, the colloidal matter present in water imparts turbidity to water. It is also caused due to presence of suspended matter in the water. Turbidity is a measure of the resistance of water to the passage of light through it. Turbidity is expressed as NTU (Nephelometric Turbidity Units) or ppm (parts per million) or Milligrams per litre (mg/l). The turbidity in water may also be due to clay and silt particles, discharges of sewage or industrial wastes, presence of large numbers of micro-organisms etc. and the cloudy appearance developed in water due to turbidity is aesthetically unattractive and it may also be harmful to the consumers.

The standard unit of turbidity is the form of finely divided silica in a million parts of distilled water. The permissible turbidity for drinking water is 5 to 10 ppm. The measurement of turbidity in the field is done by means of a turbidity rod. For laboratory, various turbidimeters are found out to measure the turbidity of water, the most common being Jackson turbidimeter and Baylis turbidimeter. Jacksons Turbidimeter is used to measure turbidity when it is more than 100 ppm. Bali's Turbidimeter is used to measure the turbidity of the sample when it is less than 5 ppm.

Fig. 1 Jacksons Turbidimeter

Fig. 2 Bali's Turbidimeter

Ground waters are generally less turbid than the surface water. The character and amount of turbidity depends on the type of soil over which the water has moved. Earlier, the turbidity produced by one milligram of silica in one litre of distilled water was considered as the unit of turbidity. Turbidity was previously determined by Jackson candle Turbidity units (JTU). This unit is now replaced by more appropriate unit called Nephelometric Turbidity unit (NTU) which is the turbidity produced by one milligram of formazin polymer in one litre of distilled water. Nephelometry method has better sensitivity, precision and applicability over a wide range of particle size and concentrations as compared to older methods.

5) Specific Conductivity of Water

The total amount of dissolved salts present in water can be estimated by measuring the specific conductivity of water. The specific conductivity of water is determined by means of a portable ionic water tester and is expressed as micro-mho per cm at 25°C. ‘mho’ is the unit of conductivity and it equals to 1 Ampere per volt. The specific conductivity of water in micro mho per cm at 25°C is multiplied by a coefficient generally 0.65 so as to directly obtain the dissolved salt content in mg/L or ppm. The actual value of this coefficient depends upon the type of salt present in water.