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

by Mike McGinness, EcoShield Environmental Systems, Inc.
June, 2003

COD Defined

Q. I understand that COD is a measure of the food available to microorganisms, but I dont make the connection as to how this is useful for process control in a wastewater treatment plant. Is it useful for adjusting the amount of dissolved oxygen in the aeration tank? Anyway, if you have the time to give me a quick explanation I would appreciate it.

A. COD stands for Chemical Oxygen Demand. It is basically a wet incineration chemical test method that measures the total amount of oxygen that can be combined (under extreme reaction conditions) with the chemical compounds (organic and inorganic) in the water sample. It is en extremely repeatable and reliable test of the total amount of pollution in the sample. It is typically used to measure the amount of waste in waste water and is used for setting treatment requirements (limits) in discharge permits. It is also useful in process monitoring (incoming loads, concentration) and process control in waste water plants. It does not measure oxygen in the water. Dissolved oxygen (D.O.) is the amount of dissolved oxygen in the water and is measured and used for controlling air or oxygen feeds to waste water treatment systems.