Chemical Oxygen Demand (C.O.D.) means the amount of oxygen from chemical oxidants required for the complete oxidation of substances dissolved or suspended in water. By C.O.D. determination it is possible to quantitatively evaluate substances which can be oxidised by strong oxidants like potassium dichromate in highly acidic solutions by concentrated sulphuric acid. Both organic and inorganic substances can be oxidised.
The COD analysis environmental equipment allows the determination of substances which are not oxidised by the corresponding biological method (B.O.D. = Biological Oxygen Demand), for example cellulose.Chemical oxidation is more complete than biological oxidation and therefore C.O.D. values for a given sample are generally higher than B.O.D. values.
Biochemical Oxygen Demand. (B.O.D) measures the amount of oxygen consumed by micro-organisms in decomposing organic matter in wastewater or stream water. The micro-organisms in, or inoculated on purpose into, a sample of water containing a biodegradeable organic substance, consume oxygen owing to their metabolic activity and generate a corresponding volume of carbon dioxide.
If we work in an enclosed system and make the carbon dioxide produced by a highly alkaline substance become absorbed, we will get a progressive reduction of the gas pressure that can be measured by appropriate manometers.Today, conventional incubation time at 20oC is considered to be 5 days, and the B.O.D. value obtained is called BOD 5 . This corresponds with good reproducibility considering all of the technique's aspects at 70% of what is called BOD last , which is the complete oxidation value that results in 21 - 28 days.
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