Waste Water Treatment – COD vs BOD as a Measure of Waste Water Strength

BOD, Biochemical Oxygen Demand, is the standard measurement for determining the strength of waste water.    It is also a common regulatory tool used around the world to measure the strength of industrial waste water.   Most municipalities will charge a fee per 1,000 gallons of waste water when the BOD is considered to be high.    High BOD is generally anything over 1,000 mg/l where household BOD is around 350 mg/l.   Industrial waste water is often well above this threshold resulting in high fees from the sewer authority.   The sewer authority charges theses fees to cover increased cost associated with the high strength waste water.  The additional costs can come from increased electrical costs to run the aerators at higher speed,  longer retention times and increased sludge handling. Other problems include odor and killing off the microbial population needed for treatment of the waste water.  High BOD can also deplete the Dissolved Oxygen in the receiving body of water, causing fish kills and odor problems.

Measuring BOD is a bioassay procedure that measures the oxygen consumed by bacteria to decompose the organic matter in the waste water stream.    The procedure takes five days to complete.   This is called a Five-day BOD and is by generally written as BOD5.

Controlling your process on a daily basis is not practical using BOD.  Most industrial users need same day results, so COD (Chemical Oxygen Demand) is used instead.  The COD test can be run in a couple of hours using a portable analyzer.  Since BOD is a subset of COD, COD is always higher than BOD.  For example, BOD from brewery waste water is usually around 50% to 60% of the COD.   Once the percentage is established, the BOD can be estimated from the COD.

On site measuring of COD is a quick, easy and inexpensive way to monitor and control your process waste water stream on a daily basis.  Running the test for BOD will also be required as most municipalities will not accept COD as a surrogate for BOD.   Check your pre-treatment permit for requirements.