In this article, I discuss why COD is a critical factor for the treatment of wastewater.
Wastewater COD (Chemical Oxygen Demand) is the amount of oxygen necessary to oxidize the organic carbon completely to CO2 and H2O.
Some organic chemicals are not completely oxidized.
However, COD is measured in approximately 3 hours via oxidation with potassium dichromate (K2Cr2O7) in the presence of Sulfuric Acid and Silver Sulphate.
During the Wastewater COD test, other reduced substances (e.
g.
, Sulfides, Sulfites, Fe++) are also oxidized and included in the waste water COD.
Furthermore, reduced forms of organic Nitrogen are converted to Ammonia in the COD test and more oxidized forms of Nitrogen (e.
g.
, Nitrite) are converted to Nitrate.
If the Wastewater COD value is much higher than the BOD value, it means that the sample contains large amounts of organic compounds that are not easily biodegraded.
For some wastewaters, COD in wastewater can be correlated with BOD.
For example, COD of a 500 mg/l solution of Phenol (C6H6O) is 1191.
5 mg/l.
C6H6O + 7O2 -> 6CO2 + 3H2O Wastewater COD = {[7(32)x500]/94} mg/l = 1191.
5 mg/l Relationship with ultimate BOD: For wastewater with readily degradable organics (e.
g: Dairy wastes) the COD is given by Wastewater COD = BODult / 0.
92 where, BOD is the ultimate BOD (mg/l).
I.
For untreated domestic wastewaters: COD range = 250 - 1000 mg/l (Metcalf and Eddy, 1991).
II.
Industrial wastewaters: COD range = 200 - 350,000 mg/l (Eckenfelder, W.
W.
Jr.
, 1989).
Ref: I.
Metcalf and Eddy, inc.
,1991.
Wastewater Engineering: treatment, disposal and reuse, 3rd Ed.
, Mcgraw-hill, New York).
Ii.
(Eckenfelder, W.
W.
Jr.
, 1989.
Industrial water pollution control, 2 Ed.
, Mcgraw-hill, New York).