Soluble BOD5 vs ultimate BOD20

BOD bottle. Source: Wikimedia commons.

We don't often get back to an old test such as BOD very often. However, I have seen posts where people are seeing BOD5 removal drop and they are wondering why their treatment systems is running well, yet effluent BOD is high.

BOD5 is also called soluble BOD. This means that BOD5 represents oxygen demand by most common wastewater components such as sugars, fatty acids, starches, etc. What remains in the BOD5 bottle after 5 days tends to be insoluble or recalcitrant organic compounds. This could be particulates, FOG, long chain hydrocarbons, xenobiotics, pharmaceuticals, lignin, and other slow to degrade or insoluble organics. In theory, if we continued to run the BOD test for 20 days, the microbes would continue to consume oxygen (although at a slower rate) as they degraded these compounds. 

The BOD20 vs BOD5 can be related to BOD5 removal efficiency by considering how non-soluble or recalcitrant compounds are processed by  microbes. For example, consider grease (a very common insoluble in wastewater). If you do not remove grease in pretreatment, the following happens in the biological unit:

• Option 1 - the grease can float across treatment system straight to the effluent
• Option 2 - insoluble grease is adsorbed (fixes) into the MLSS - this can also lead too floating scum on the clarifier.

Now we will assume tht the grease is adsorbed into floc. As the soluble BOD decreases, the bacteria begin to use a combination of biosurfactants & exocellular enzymes to carve up the long chain hydrocarbon into forms capable of being transported across the cell wall. In the case of long chain fatty acids (FOG), biodegradation takes place via Beta-Oxidation where fatty acid length is decreased two carbons each cycle through the beta-oxidation pathway. Once the fatty acid drops to less than 10 carbons in length, solubility increases and degradation rates increase as you have converted the organic into soluble BOD. This is how microbes can increase the soluble BOD5 while degrading organics that were previously not detected by the 5 day test.

​Now back to case of BOD5 being possibly generated across a aeration basin. If you expect potential conversion of insoluble organics into soluble (BOD5) forms across your biological unit, you an check by running COD or filtered COD. Besides being a quick test, COD is effective in rapid detection of things contributing to ultimate BOD. 

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