Characteristics of Sewage
Aerobic Decomposition
(i) Nitrogenous organic matter
Nitrate
(ii) Carbonaceous organic matter
Carbon dioxide
(iii) Sulphurous organic matter
Sulphate
Nitrogen Cycle under Aerobic Decomposition
Anaerobic Decomposition
(i) Nitrogenous organic matter
(ii) Carbonaceous organic matter
(iii) Sulphurous organic matter
(iv) Organic acids
Threshold Odour Number (TON)
The volume of the sewage
The volume of distilled water or odourless water.
Total Solids, Suspended Solids and Settleable Solids
(i) 
Where,
Dissolved solids plus colloidal or filterable solids in mg/lit
Non-filterable solids in mg/lit
The total amount of solids in mg/lit
Volatile suspended solids, in (mg/lit.)
Fixed solids
(ii) 
Total Solids
50% → Dissolved
25% → Suspended
25% → Settlable
Chemical Oxygen Demand
(i) Biodegradable + non Biodegrable O.M.
(ii) 
added and used is measured.
Theoretical Oxygen Demand
Benzene
Glucose
Biochemical Oxygen Demand
·
Where,
Biochemical oxygen demand in ppm or mg/lit.
Initial dissolved oxygen in mg/lit.
Final dissolved oxygen in mg/lit.
· 
(i) 
Where,
k = Rate constant signifying the rate of oxidation of organic matter and it depends upon the nature of organic matter and temperature. Its unit is per day.
the equivalent of organic matter present after t days.
(ii) 
Where,
Deoxygenation constant.
Organic matter present at 
(iii) 
(iv) 
Where,
The total amount of organic matter oxidized int days i.e. BOD.
(v) 
(vi) 
Where,
Ultimate B.O.D of 
days.
(vii) 
(viii) Laboratory Estimations of 
and L values
(Thomas Method)
Where,
m = Slope of the line
C = Intercept of the line on the y-axis.
Relative Stability (s)
Where time in days at 20oC.
time in days at 37oC.
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