Determination of Chemical Oxygen Demand
INTRODUCTION
The Chemical Oxygen Demand is a measurement of the amount of material that can be oxidized in the presence of a strong chemical oxidizing agent. COD often is used as a measurement of pollutants in wastewater and natural waters. Other related analytical values are Total Oxygen Demand (TOD), Biochemical Oxygen Demand (BOD), total organic carbon (TOC). BOD is a measure of oxygen consumed by microorganisms under specific conditions, TOC is a measure of organic carbon in a sample, and TOD is a measure of the amount of oxygen consumed by all elements in a sample after complete oxidation. Some organic materials such as cellulose, phenols, etc resist biodegradation along with pesticides, pyridine, some nitrogenous heterocyclic and polychlorinated biphenyl which are toxic to microorganisms these materials are oxidized in COD test. A strong oxidizing agent is used to oxidize the material instead of microorganism as in case of BOD. The COD value will always be higher than the value of BOD for waste water.
COD is one of the most used parameter because of the quick result and it take into account those organic materials which are not easily biodegradable. In COD, a strong oxidizing agent such as potassium dichromate in acid medium is used for chemical oxidation of organic matter. It is measured in mg/l or ppm. Chemical Oxygen Demand is an important water quality parameter because, similar to BOD, it provides an index to assess the effect discharged wastewater will have on the receiving environment. The COD test is often used as an alternate to BOD due to shorter length of testing time.
Empirically for municipal wastewater, BOD5 ≈ 0.4 to 0.6 COD. Even though COD does not represent exactly the biodegradable organics, it is sometimes taken as measuring parameter of organic waste, as a COD value can be determined in 3-4 hours as against 3-5 days for a BOD value. COD can be determined titrimetrically or photometrically.
A blank sample is usually created by adding all reagents to a volume of distilled water. To ensure a true measurement of organic matter, COD is measured in both polluted water and blank samples, and the oxygen demand in the blank sample is subtracted from the COD in the original sample.
Relevant Indian Standard for Chemical Oxygen Demand Test :
IS 3025 (Part 44)-1993: Method of Sampling and Test (Physical and Chemical) for Water and Wastewater, Biochemical Oxygen demand, First Revision.