Ask the Expert Question-and-Answer Archive (Wastewater Treatment)
by Mike McGinness, EcoShield Environmental Systems, Inc.
pH and Temperature Effect on COD
This liquor manufacturing industry uses an activated sludge treatment process. Can pH neutralization process reduce COD levels? What's the correlation between pH level and COD, temperature and COD? Can high or low pH and temperature affect or contribute to high levels of COD from waste water?
The answer to your question is not simple, but in general, pH and temperature have no effect or simple direct relationship to COD at all. That said, in many waste water streams, pH and temperature of the treatment process can have huge effects OVER TIME on COD, or BOD REDUCTION! Note that BOD and COD lab tests are run at a specific fixed temperature and a fixed, narrow pH range, so the COD TEST itself does require a specific test temperature, pH, and time.
A biological waste water treatment process, such as activated sludge, which is a common way to reduce COD and BOD, is very sensitive to pH and temperature. Oxygen concentration and oxygen transfer rates in aerobic systems are very sensitive to temperature. Look for a dissolved oxygen concentration as a function of water temperature graph online or in a text book, such as.
Also some compounds that add to the COD and BOD, that are water soluble, for example MEK, are also volatile, and if the waste water is heated (versus being at an ambient temperature) and aerated, the MEK (which adds COD to the waste water), can be evaporated, thus reducing the waste water COD and BOD.
Bacteria that treat waster water to reduce the COD and BOD, are extremely sensitive to pH! So pH can also have a huge effect on activated sludge COD and BOD reduction rates.
Also, some compounds can be precipitated, or dissolved, like heavy metals or suspended solids, at various pH ranges. In some cases the increased dissolved heavy metals concentrations will be toxic to bacteria and inhibit biological BOD and COD reduction in the waste water, usually at low pH, and in some cases the heavy metals can be precipitated at high pH ranges. Therefore, pH and temperature are very important in assisting the treatment process to reduce the COD (and BOD), over time, but rapid temperature changes do not have an immediate effect on COD of water, while a pH change can in some cases almost immediately precipitate sources of high COD, and BOD, that can be quickly settled or filtered out of the waste water. While there are bacteria that can survive at extreme pH and temperature (see Thermophiles) ranges, in general, most typical biological waste water treatment operations operate best (reduce COD and BOD the fastest) at mild temperatures (70 F) and a (7-8) pH range.