Ask the Expert Question-and-Answer Archive (Wastewater Treatment)

by Mike McGinness, EcoShield Environmental Systems, Inc.
April, 2012

COD Value

Q. The value is 7 thousand, i.e. 7,000 mg/L (sorry, Im in South América). The COD value is round 15,000 mg/L. The sample correspond to a mixture of all waste streams (the final effluent). Does it make sense for you? I have doubts of the reliability or validity of the samples.

A. The data may be valid, It sounds extremely high on both numbers for an overflowing rinse, however, if it were a combination of dead rinse bath dumps, it would make sense. If it were a cleaning bath dump I would expect numbers like 30,000-150,000 mg/l of COD for a used detergent-alkaline cleaner bath loaded with oil and grease and detergents. The COD to BOD ratio of 2:1 is typical at that concentration.

BOD is a 5 day oxygen demand by bacteria used to eat as much of the organics as they can in five days. The COD is the amount of oxygen used by an aggressive chemical oxidation process, somewhat equal chemically to thermal combustion (oxidation), that gives a max reading of total possible oxygen demand in a biological or chemical oxidation process. A BOD of 10 and COD of 1000 or 10,000 would indicate a very toxic, and not biodegradable mix.

Hope that helps!

Best Regards,

Mike McGinness