Ssc Je Paper 2

Physical Characteristics of sewage and Their Testing 

Physical examination of sewage is carried out in order to determine its physical characteristics.

This includes :

1.      Turbidity

2.      Colour

3.      Odour

4.      Temperature

        1.       Turbidity :  Sewage is normally turbid , resembling dirty dish water or wastewater from baths having other floating matter like fecel matter , pieces of paper , cigarette-ends , match-sticks , greases , vegetable debris , fruit skins , soaps etc . The turbidity increases as sewage becomes stronger .

       The degree of turbidity can be measured and tested by turbidity rods or by turbiditymeters , as is done for raw water supplies .

      2.     Colour : The colour of sewage can normally be detected by the naked eye , and it indicates the freshness of sewage . If its colour is yellowish , grey or light brown , it indicates fresh sewage . However , if the colour is black or dark brown , it indicates stale and septic sewage . Other colour , may also be formed due to presence of some specific industrial waste .

     3.     Odour : Frersh sewage is practically odourless . But , however , in 3 to 4 hours , it becomes stale with all oxygen present in sewage being practically exhausted . It then starts omitting offensive odours , especially that of hydrogen sulphide gas , which is formed due to decomposition of sewage .

    The odour of water or wastewater can be meqasured by a term called the Threshold odour number (TON) which represents the extent of dilution required to just make the sample free of odour .

     4.     Temperature : The temperature haws an effect on the biological activity of bacteria present in sewage , and it also affects the solubility of gases in sewage . In addition , temperature also affects the viscosity of sewage , which , in turn , affects the sedimentation process in its treatmnent.

       The normal temperature of sewage is generally slighty higher than the temperature of water , because of additional heat added during the utilization of water . The average termperature of sewage in India is 20° C , which is near about the ideal temperature for the biological activities . However , when the temperature is more , the dissolved oxygen content (D.O) of sewage gets reduced .