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 .
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