BOD vs. COD- The best of buddies

Nikkishia Singh | 16 May 2016

Biological Oxygen Demand vs. Chemical Oxygen Demand.

To the average man, any referral to the term “BOD” might summon up images of intimidating bodybuilders or recent (often failed!) resolutions to visit the gym for that perfect swimsuit bod. To scientists, however, the term “BOD” holds a far greater importance. BOD, otherwise referred to as Biological Oxygen Demand is defined as the amount of oxygen consumed by microorganisms whilst decomposing organic matter. But why exactly would this information be so vital to us? Well, let’s just say the entire fate of the aquatic universe depends on it.

Ok, so perhaps that was a little exaggerated- but when you consider that BOD is an indication of the relative amount of organic pollution in the water, it may not be too far off the mark!

Domestic sewage contains an assortment of organic compounds such as carbohydrates, lipids and proteins which all exert an “oxygen demand” on the system. Some of these compounds can be complex, and it is only the readily metabolized organics that are quickly degraded by microorganisms as BOD, while other more complex compounds may be retained in the system. These other less-readily biodegradable compounds need to be calculated for as well, as they can negatively impact the overall treatment efficiency. An alternate measurement, the Chemical Oxygen Demand (COD) can be used to determine the total oxidizable organics and inorganics in the system. Thus, where BOD is a measure of all biodegradable organic pollutants in a wastewater stream, Chemical Oxygen Demand (COD) is a measurement of all chemicals in a wastewater stream. Both BOD and COD represent important regulatory parameters necessary to estimate the:

BOD and COD are two of the most widely used methods for measuring oxygen demand. They are both relatively simple and reliable methods and both hold their own advantages and disadvantages. COD allows for short analysis times and are not affected by the presence of toxic materials, however the method calls for the use of expensive apparatus and the use of hazardous chemicals. Conversely, BOD more closely resembles the natural environment and can be performed with minimal equipment and expense. However there is a relatively long incubation time. Furthermore since BOD is a microbially driven process, toxic materials may hinder the process, and it can only provide information on the biodegradable organic fraction in the system (1).

Both BOD and COD represent complementary methods whose results each paint a side of the whole picture. Doing each individually is valuable, but together they can provide a far better idea of how your system is actually functioning, allow you to make predictions and assess whether your system is “in-shape” for summer.  

Previous blog post Next blog post