When ozone is used to treat pollutants in pharmaceutical wastewater, the COD initially decreases; why does it increase later on?
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I would like to ask the experts: I use ozone to treat pollutants in pharmaceutical wastewater. At first, the COD level decreases, but why does it increase later on?
When general wastewater contains some organic substances that are difficult to degrade, ozone first oxidizes some of the more easily oxidizable organic compounds, and at this point the COD level decreases； As further oxidation converts the hard-to-degrade organic substances into smaller organic molecules, the COD level will increase.
Won’t organic substances that have degraded into smaller molecules be easier to degrade then? But when I extended the reaction time, there was no tendency for COD to decrease. What could be the reason?
It is possible that new substances are formed under ozone conditions, which are not further oxidized by ozone, but can be oxidized by potassium dichromate.
I see, thank you. May I ask again, what might this type of small organic molecule be? For example, could it be phenol?
The issue of potassium dichromate not being able to fully measure COD – as mentioned above, it’s possible to conduct chromatographic analysis on water samples taken before and after the experiment. Also, have you left the water samples treated with ozone for some time before taking measurements? The residual ozone in the wastewater might also have an impact
Ozone oxidation of organic pollutants is a process in which they are oxidized into carbon dioxide and water, and this process is irreversible. There may be other reasons as well, such as the manufacturing process, the addition of other chemicals, or errors in the testing data.
Isn’t ozone technology still not very mature? I’ve seen many examples; it’s better to consider electrochemistry instead.
Many people are now using ozone technology to treat wastewater, with significant results and without causing secondary pollution. The only drawback is the cost issue; finding a suitable catalyst has become an urgent task.
In ozone technology, it’s not just a cost issue; the technology itself is also inadequate. I’ve seen many cases where it’s used later on but not actually utilized, mainly because the effects are not noticeable.