Sewage contain contains 99.9% of water only.the small portion of solids present in sewage pose threat as they are offensive in nature,undergo changes by biodegradation,cause nuisance and pollution.
What are characteristic of sewage
a. Physical characteristics:
1.Colour: Yellow has fresh sewage,grey or light brown colour.The state or septic sewage has black or dark brown colour other colour in sewage are due to presence of industrial waste.
2.Odour: Fresh domestic sewage has slightly soapy or oily smell but the sewage is offensive odour due to the liberation of hydrogen, sulphide and the other sulphur compounds.
3.Temperature: The temperature is slightly higher than the water supplied increase the temperature in biological activity.
4.Turbidity: The sewage is normally a dry water or dish water from bath having other floating material faecal matter etc.
b.Chemical characteristics
1.Solids: The sewage contain both organic and inorganic solids.The solids are present in the form of settable suspended solids and other collidial partials and dissolved solids.By the sedimentation we can remove the solids particals but the removal of dissolved and collidial solids required some biological treatment.Estimated that 1000kg of sewage contains approximately 0.45kg of solids of which 0.225kg is in dissolved form 0.112kg in suspension and 0.112kg in settable form mainly solids are contains approximately 45 parts of organic and 55 parts of inorganic solids.
2.Other chemical substances: Sewage may be also contains proteins, carbohydrates,fat,oils,greases,phenols etc.The sewage may be contains chlorides, compounds of nitrogen, phosphorus, sulphur,alkaline substances,toxic compounds and the heavy metals.
3.Gaese: Sewage contains dissolved oxygen, hydrogen, sulphide, carbon dioxide,methane etc.
c:Biological characteristics: Sewage may be contains a range of micro organisms.These micro organisms includes the viruses bacteria,algae,fungi etc.These organisms be aerobic, anaerobic or facultative in nature."Facultative bacteria" are those which can survive and grow both in the presence and absence of oxygen.