High COD is not a single treatment problem. Chemical Oxygen Demand is a measurement of the oxidisable load in wastewater, and the right treatment depends on what is actually creating that load.
The first question should therefore not be “How much ozone do I need?” but “What fraction of this COD is suspended, biodegradable or refractory, and which treatment step is best suited to remove it?”
COD is normally reduced using a combination of physical separation, biological treatment and, where necessary, chemical or advanced oxidation. The correct sequence depends on whether the COD is particulate or dissolved, biodegradable or refractory. Ozone is usually most valuable when it targets difficult residual compounds, improves biodegradability or provides final polishing.
COD is a measurement, not a contaminant
Chemical Oxygen Demand measures the oxygen equivalent required to chemically oxidise organic matter and certain oxidisable inorganic compounds in a water sample. It provides a useful indication of wastewater strength, but it does not identify which substances are creating that COD.
This distinction matters because two wastewater streams can both measure 1,000 mg/L COD while requiring completely different treatment processes. One may contain suspended food solids, fats and easily biodegradable organics. Another may contain dyes, phenols, surfactants or other difficult dissolved compounds.
Before selecting a treatment technology, it is useful to think about the COD as several different fractions.
Remove the easiest COD first
Advanced oxidation is rarely the cheapest place to start if a large proportion of the COD can be removed by screening, separation or biological treatment. The most economical treatment trains normally remove the easiest load first and reserve higher-value polishing technologies for the fraction that remains.
Screening, settlement, filtration or other physical separation can remove particulate organic load before it reaches downstream treatment.
Oil separation, dissolved air flotation or coagulation may reduce a significant part of the COD before biological or oxidation stages.
Aerobic, anaerobic, MBBR or other biological processes can often remove large biodegradable loads more economically than advanced oxidation.
This is where ozone, advanced oxidation, activated carbon, membranes or other polishing technologies become more relevant.
Do not spend ozone on COD that another treatment step can remove more efficiently.
The best role for ozone is often to target the residual fraction that conventional treatment cannot remove easily, rather than trying to oxidise the entire wastewater load.
COD treatment methods compared
There is no single best technology for COD removal. The appropriate process depends on wastewater composition, required discharge quality, available footprint, operating cost and what treatment infrastructure already exists on site.
| Treatment method | Best suited to | Main strength | Main limitation |
|---|---|---|---|
| Physical separation | Suspended and particulate COD | Low-cost bulk removal | Dissolved COD remains |
| Coagulation / DAF | Colloids, fats, oils and suspended organics | Effective pre-treatment | Chemical use and sludge generation |
| Biological treatment | Biodegradable organic load | Efficient bulk COD and BOD removal | Refractory compounds may remain |
| Activated carbon | Selected dissolved organics | Effective polishing | Media exhaustion and replacement |
| Membrane processes | Dissolved contaminants and reuse duties | High separation performance | Creates a concentrate stream |
| Fenton / advanced oxidation | Refractory dissolved organics | Strong oxidation potential | Chemical or energy demand can be significant |
| Ozone | Selected refractory compounds, colour, odour and polishing | On-site oxidation and potential improvement in biodegradability | Economics depend strongly on ozone demand and process conditions |
Where ozone fits in COD reduction
Ozone becomes particularly useful when the easy COD has already been removed and the remaining load contains compounds that respond to oxidation. It can be used either before biological treatment to alter difficult compounds, or after conventional treatment as a polishing stage.
Ozone may deserve consideration where biological treatment has reached its practical limit, residual colour or odour is still present, refractory organic compounds remain, biodegradability needs to be improved, or the final effluent requires additional oxidation before discharge or reuse.
Our industrial wastewater application page explains where ozone can sit within the wider treatment train, including direct ozonation, AOP, mass transfer, contact design and full system integration.
For streams containing phenols, sulphides or difficult petrochemical organics, see our guide to ozone treatment for petrochemical wastewater . Where colour is the main treatment objective, our wastewater colour removal page covers that application in more detail.
BOD/COD ratio and biodegradability
Biological Oxygen Demand represents the oxygen consumed by microorganisms while degrading biodegradable material. COD is broader and includes both biodegradable and non-biodegradable oxidisable substances.
The relationship between the two measurements can therefore provide a useful indication of how amenable a wastewater may be to biological treatment. A relatively low BOD/COD ratio can suggest that a larger proportion of the organic load is difficult to biodegrade, although the ratio should always be interpreted alongside the wastewater source and actual process data.
Ozone does not necessarily need to mineralise every organic molecule to be useful. Partial oxidation can sometimes convert larger or refractory compounds into smaller intermediates that are more accessible to downstream biological treatment. In these cases, the process objective may be to improve treatability rather than achieve maximum immediate COD destruction.
A modest immediate reduction in COD can still be valuable if ozonation makes the remaining load easier to biodegrade.
This is why ozone performance should be judged against the complete treatment objective, not only against a single COD result taken immediately after oxidation.
Why is COD still high after treatment?
If COD remains high after an existing treatment process, simply adding another oxidation stage may not solve the underlying problem. The first step is to identify where the residual load is coming from.
When ozone is not the first treatment step
Ozone is a high-value oxidation technology, but that does not mean it should be used against every organic load. It normally requires careful review where the wastewater still contains a large amount of easily biodegradable COD, high suspended solids, heavy fats or oils, or where the treatment objective has not yet been defined properly.
If inexpensive physical or biological treatment can remove most of the load, it is usually more sensible to use those processes first and reserve ozone for the residual fraction where oxidation provides a specific benefit.
The same principle applies to system sizing: high COD does not automatically mean a larger ozone generator is the correct answer.
How ozone treatment is assessed for COD reduction
A wastewater ozone system should not normally be sized from COD and flow rate alone. The required ozone dose depends on which compounds are present, how quickly they react, competing ozone demand, contact conditions and the required treatment result.
For complex industrial wastewater, laboratory or pilot testing can be particularly useful before final equipment selection. Testing can establish dose-response behaviour, changes in COD and biodegradability, colour or odour response, ozone demand and the likely role of pre-treatment or downstream polishing.
For more detail on ozone demand, see Ozone Demand in Water . For equipment sizing principles, see How to Size an Ozone Generator , and for hydraulic reaction time see Ozone CT & Contact Time .
Your COD is high. What should you check next?
What information is needed to review a COD problem?
If an industrial effluent is not reaching its COD target, the following information is normally enough to begin a useful technical review.
Send us your flow rate, water analysis, existing treatment process and target effluent quality. We can review whether the remaining COD is more likely to be addressed through upstream process optimisation, ozone oxidation, advanced oxidation or another polishing stage.