Biochemical Oxygen Demand
When biodegradable organic matter is released into a body of water, microorganisms especially bacteria feed on the waste, breaking it down into simpler organic and inorganic substances .when this decomposition takes place in an aerobic environment, i.e. in the presence of oxygen, the process produces non–objectionable, stable end product such as carbon dioxide, sulphates, ortho-phosphate and nitrate. A simplified representation of aerobic decomposition is given by following.
Organic Matter + O2 MICROORGANISMS CO2 + H2O + New cells + Stable Products
The amount of oxygen required by microorganism to oxidize organic waste aerobically is called as biochemical oxygen demand (BOD). It is generally expressed in mg of oxygen required per litre of waste.
MODELLING BOD AS FIRST ORDER REACTION:-
Rate of decomposition of organic wastes is proportional to the amount of waste available. If Lt represent the amount of oxygen demand left after time t, then, first order reaction equation will be,
dLt/dt = -k Lt (1)
Where k = BOD reaction rate constant
Lt = Lo e –kt (2)
Where Lo = Ultimate BOD.
The ultimate BOD is the sum of the amount of oxygen consumed by the waste in the first t days (BODt), plus the amount of oxygen remaining to be consumed after time t. i.e.
Lo = BODt + Lt (3)
Combining eq. (2) and (3)
BODt = Lo (1- e–kt) (4)