Chemical Oxygen Demand (C.O.D)
The amount of oxygen required to carryout the chemical decomposition of organic matters in sewage is known as chemical oxygen demand (C.O.D).
It indicates the amount of biologically active as well as biologically inactive organic matter present in the sewage. In general, chemical oxygen demand(C.O.D) of sewage is higher than Biochemical Oxygen Demand(B.O.D)
This test is carried out to measure the organic matter present in the sewage. To perform this test, a known quantity of sewage is mixed with a known quantity of standard solution of potassium dichromate (K2Cr2O7) and the mixture is heated.
The organic matter is oxidized by K2Cr2O7. The resulting solution of K2Cr2O7 is titrated and the oxygen used for oxidizing the sewage is determined. This is called C.O.D.