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 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.