99.9 % of Sanitary sewage is water. The remaining 0-1% is solids responsible for causing foul nature to sanitary sewage. They include carbohydrates, proteins, vitamins and trace elements of human origin. Also inorganic salts as sodium chloride find their way into sewage.
Colour : Fresh domestic sewage is of light grey or yellow in colour. Its colour becomes darker with time. Fresh sewage becomes stale in 2 to 6 hours depending on temperature.
Odour :
Fresh sewage is odourless or of earthy odour. It becomes odourous with time. This is because of the decomposition of sewage and evolution of gases of decomposition as Hydrogen sulphide, Ammonia, Methane, etc.
Temperature : Fresh sewage has a temperature of 37°c i.e human body temperature. As temperature increases biological activity accelerates. A particular biochemical reaction takes place in the least time at a particular temperature range only.
In general biochemical activity gets doubled for every 10°c rise in temperature. If domestic sewage gets digested in 24 hours in a tropical (hot) country it may take more than 72 hours in a temperate (cold) country.
pH :
It is potential hydrogen i,e concentration of free (undissociated) Hydrogen ions of a sample. A pH value less than 7 is said to be Acidic and a value between 7 to 14 is Alkaline. Domestic sewage is alkaline when fresh and becomes acidic with time.
Dissolved Oxygen : Aquatic life breathe the dissolved oxygen present in water. The solubility of oxygen in water depend on (i) temperature (ii) atmospheric pressure and (iii) impurities. Higher concentration of dissolved oxygen exist when temperature is low.
It decreases with increase in temperature. Dissolved oxygen is 14 mg/1 at 0°c. With increase in temperature, dissolved oxygen concentration decreases as solubility of oxygen decreases. Similarly impurities consume oxygen and D.O reduces. Thus fresh pure water has more D.O than water with more dissolved solids, liquids or gases as sea water.
Biochemical Oxygen Demand (B.O.D): BOD is the amount of oxygen required by the sample to yield stable end products as C02 and H20 at a particular time and temperature. It takes infinite time for the completion of the reaction. 68% of the sanitary sewage is supposed to be stabilized at 20°c and in 5 days is taken as the standard temperature and time for a BOD test.
BOD refers to the 'nuisance potential' of the sample. Treatment is aimed at the reduction of
BOD.
Chemical Oxygen Demand (COD) : Because the BOD test needs 5 days at 20°c a simpler test taking only 3 hours for the assessment of the 'nuisance potential' is deviced.
It is performed by oxidizing the waste water sample by a strong oxidizing agent under acidic conditions. COD is higher than BOD as it oxidizes (inorganic) reducing agents, and the organic matter otherwise slowly oxidizable, besides the readily biodegradable organic matter.
BOD/COD ratio is called "Biodegradability Index". Biodegradability Index determines the nature of treatment suited for the wastewater.
If the Biodegradability Index is less than 0.3, the waste cannot be treated biologically. If the Biodegradability Index > 0.3 but < 0.6 then the waste can be treated biologically if it is seeded (i.e readily biodegradable organics are added to the wastewater). If the Biodegradability index is > 0.6, it can be effectively tteated biologically.
