Environmental Engg.

Lesson-7 Drinking water quality - physical properties

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

Before we take up the discussion of various tests, it will be necessary to note the precautions which are to be taken while collecting the sample of water to be analysed.  In fact, 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 are as follows:

1. The water should be collected in bottles, especially of white glass, having well-fitted stoppers.  Bottles having holding capacity of about  2 litres of water are necessary for chemical analysis.  For bacteriological examination, bottles with smaller capacities will be sufficient

2. 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.

3. 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.

4. 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.

5. 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.

6. After collecting the sample, the stopper of bottle should be well secured and the bottles containing samples of water should be labeled stating the source, date and time of collection.

Physical test

Under this category, tests are carried out to examine water fro the following:

1. Colour

2. Taste and Odour

3. Temperature

4. Turbidity

Other physical characteristics for which tests are sometimes carried out are density, electrical conductivity, radioactivity and viscosity.

Colour

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 a 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 are kept ready in the laboratory.  For public water supply, the number on cobalt scale should not exceed 20 and should preferably less than 10.

It should, however, 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.

Taste and odour

The water possesses taste and odour due to various causes and they make the water unpleasant for drinking.  The test is conducted 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 taste and odour of water may also be tested by threshold number.  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.

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 desirable temperature of potable water is 10°C while temperature of 25°C is considered to be objectionable.

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.

Turbidity

The colloidal matter present in water imparts turbidity to water.  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 turbidity is expressed in terms of parts of suspended matter per million parts of water or shortly written as ppm.  It is to be noted that the expression ppm is equivalent to mg per litre.  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.