Chemical Oxygen Demand (COD) Removal with Ozone

Chemical Oxygen Demand (COD) is a water quality parameter that measures the amount of oxygen required to oxidize organic and inorganic matter in water. It represents the total pollution load, including both biodegradable and non-biodegradable substances.

COD is expressed in milligrams per liter (mg/L) and is used to assess wastewater strength, treatment efficiency, and environmental impact.  COD is a generic term that measures total oxygen demand, including both biodegradable and non-biodegradable pollutants.

Ozone for COD Elimination:

Ozone eliminates COD in water by oxidizing organic and some inorganic compounds into simpler, less oxygen-demanding forms—often carbon dioxide (CO₂), water (H₂O), and inorganic ions.  Ozone actively reacts with pollutants in real time, reducing the total amount of oxidizable material left in the water. This lowers the COD value by degrading the substances that would otherwise consume oxygen during chemical or biological processes.

Due to the fact that COD is not measuring any one specific compound in water it is impossible to attribute a specific ratio of ozone to COD reduction.  However, some guidelines are offered below along with specific case studies.

COD In Water Sources:

The COD level in water varies significantly based on its origin and composition. Different water sources contain varied concentrations of organic and inorganic pollutants, which impact the amount of oxygen needed for oxidation.

How Ozone Oxidizes COD in Water:

Direct Oxidation (Molecular Ozone Reaction)

- Ozone molecules attack electron-rich sites on organic compounds—like double bonds in alkenes, aromatic rings (e.g., benzene in phenols), or functional groups (e.g., alcohols, amines). For example:

- A compound like phenol (C₆H₅OH) gets oxidized to intermediates like catechol or hydroquinone, then further to organic acids, and eventually CO₂ and H₂O.

- Reaction: C₆H₅OH + O₃ → intermediates → CO₂ + H₂O.

- This reduces COD by converting complex, oxygen-hungry molecules into smaller, fully oxidized products that no longer register as COD contributors.

Indirect Oxidation (Hydroxyl Radical Formation)

- In water, ozone decomposes, especially at higher pH, forming hydroxyl radicals (•OH), which are even more reactive (oxidation potential 2.8 V). These radicals are non-selective and attack a broader range of organic and inorganic pollutants:

- Example: A pesticide like atrazine breaks down into fragments, then mineralizes to CO₂, H₂O, and chloride/nitrate ions.

- Reaction: O₃ + H₂O → •OH + O₂ → aggressive breakdown of organics.

- This amplifies COD reduction, hitting compounds ozone alone might miss.

Targeted Pollutant Breakdown

- Organics: Ozone cleaves long carbon chains (e.g., in fatty acids), aromatic structures (e.g., dyes), and biodegradable matter (e.g., sugars), reducing the total oxidizable carbon.

- Inorganics: It oxidizes reduced species like sulfides (H₂S → SO₄²⁻) or ammonia (NH₃ → NO₃⁻), though these are less common COD contributors unless significant.

- The result: fewer molecules remain to demand oxygen in a COD test.

Summary

- Post-ozonation, the water’s COD drops because the organic load is either fully mineralized (to CO₂ and H₂O) or converted to simpler compounds (e.g., acetic acid) that require less oxygen to oxidize completely. For instance, a wastewater with 500 mg/L COD might fall to 50–100 mg/L, depending on the dose and contact time.

- Ozone does not remove everything—some oxidation byproducts (e.g., aldehydes, ketones) may persist, contributing residual COD.

Relationship between BOD and COD in Water and Ozone Reactions:

Water with high COD values typically also have high BOD (Biological Oxygen Demand) levels in water.  When ozone is used it is common that BOD is oxidized first, and causes in increase in COD values.  Then as ozone dosage rates rise, or treatment time is extended COD values begin to decrease.   Specifically here is what is happening:

- Initially, COD values increase While BOD values decrease

- Ozone initially oxidizes the organic matter in the water. The BOD drops because ozone quickly breaks down the biodegradable compounds (like sugars or proteins) that microbes would normally eat. It turns them into simpler compounds like carbon dioxide and water, or into smaller organic pieces.

- But here’s the twist: those smaller compounds (like organic acids, aldehydes, or ketones) are still oxidizable by chemicals—they count toward COD. In fact, ozone might break big, complex molecules into more of these smaller, oxidizable fragments. Therefore, while the microbe-food (BOD) is disappearing, the amount of chemical oxidant (COD) actually increases for a while.

- COD drops after BOD has decreased dramatically

- When the biodegradable compounds are mostly oxidized, ozone oxidizes smaller fragments left behind eventually turning them into carbon dioxide and water, which don’t count toward COD anymore.

- At this point, with the easy microbial food gone, ozone’s focus shifts to finishing off the remaining oxidizable bits. The COD starts falling because there’s less and less left that can react with a chemical oxidant.

Ozone-to-COD Ratios for Wastewater Treatment:

The ozone dosage required for COD removal depends on wastewater characteristics, pollutant types, and treatment goals. Below are typical O₃-to-COD ratios for various applications:

- Low-COD Wastewater (150-400 mg/L): Requires 0.5-1.5 g O₃ per g COD for moderate COD reduction.

- Medium-COD Wastewater (500-2,000 mg/L): Needs 1.5-3.0 g O₃ per g COD, common in textile, food, and chemical industries.

- High-COD Wastewater (2,000-10,000 mg/L): Requires 3.0-10.0 g O₃ per g COD, found in landfill leachate, pharmaceuticals, and heavy industry.

Factors Influencing Ozone Dosage:

Wastewater Composition: Presence of refractory organics (e.g., phenols, pesticides, dyes) requires higher ozone doses.

pH and Alkalinity: Higher alkalinity can lead to ozone scavenging, reducing efficiency.

Temperature: Higher temperatures accelerate ozone decomposition, affecting treatment performance.

Contact Time: The reaction rate of ozone to COD will be dependent upon water temperature, PH, and other oxidizable compounds in water.

Pre-Treatment: Coagulation, filtration, or other water treatment before ozonation can reduce ozone demand.

Synergistic Technologies: Ozone may oxidize COD into a filterable state either with standard filtration, water settling, or biofiltration.  Ozone oxidation may also be benefited with an AOP reaction implementing H2O2 or UV Lights.

Advantages of Ozone for COD Removal:

Powerful Oxidation – Breaks down complex organics that biological treatments cannot.
Reduces Secondary Pollution – Unlike chlorine, no toxic byproducts are formed.
Enhances Biodegradability – Converts refractory COD into biodegradable forms, improving biological treatment performance.
Removes Color & Odor – Used in textile and dye wastewater to eliminate color compounds.

Summary

- Ozone is highly effective for COD reduction, particularly in industrial wastewater with refractory organics.

- Typical O₃-to-COD ratios range from 0.5 to 10 g O₃ per g COD, depending on wastewater type.

- Ozonation is best used as a pre-treatment for biological processes, enhancing overall efficiency.

Ozone Treatment for Biorefractory COD Removal

Authors: S. Baig; P.A. Liechti

Abstract

Studied the effectiveness of ozone in treating industrial wastewater with biorefractory COD. Ozone treatment achieved COD reduction of up to 60%, particularly in highly polluted effluents. Found that ozone alone was effective, but combining it with biological treatment improved overall removal efficiency.

Benefits of Ozone:

- Reduces Biorefractory COD – Helps degrade persistent organic pollutants.

- Improves Biological Treatment Efficiency – Converts COD into biodegradable fractions.

- Effective for High-Pollution Wastewater – Works well in heavily contaminated industrial effluents.

Read the full study here.

Case Analysis on Textile Wastewater Treatment with Ozone

Authors: Yin, H., uo, H., Qiu, P.

Abstract

The large amount of wastewater produced by the textile industry necessitates a cost-effective technology for enhanced wastewater treatment. In this study, a combined processing method was established to enhance discharge water quality. This process incorporated a pretreatment system, a biological contact oxidation unit, an ozone oxidation unit, and an intensive treatment system. Through this treatment approach, the ozonation of textile wastewater was examined to determine the effects of ozone dosage, ozonation time, and color/chemical oxygen demand (COD) of feed wastewater. Results revealed that the color and COD removal rates increased with increased ozone dosage. Color and COD decreased whereas NH₃–N slightly increased with the progress of ozonation. Color removal rate decreased whereas ozone dosage increased with increased feed color and COD. Feed color greatly influenced ozone dosage but not COD. Color removal rate during ozonation can be controlled to 50%–55% at a response time of approximately 2 h. The average ozone dosage was 51 g m^–3. After treatment by the combined process, the final discharge water was able to meet the national first-grade emission standard (GB4287-2012). The total removal rates of COD and color reached 95.2% and 95.4%, respectively. The cost of wastewater treatment amounted to only approximately 1.70 Yuan RMB m^–3 wastewater.

Benefits of Ozone:

- Removes Persistent Organic Pollutants – Targets dyes and textile chemicals.
- Enhances Water Reuse Potential – Reduces pollutants to meet discharge regulations.
- Reduces Chemical Treatment Dependency – Minimizes use of additional coagulants.

Read the full study here.

Ozone Treatment in Pulp and Paper Wastewater

Authors: Mainardis, M., Buttazzoni, M., De Bortoli, N. 

Abstract

Pulp and paper wastewater ozonation was tested at pilot-scale in a medium-potentiality wastewater treatment plant (143,000 population equivalent). Single wastewater lines (bleaching and process water), as well as wastewater mixture (before and after secondary biological treatment) were investigated to evaluate the best ozonation conditions. The aim of the work was to evaluate the feasibility of substituting current tertiary physicochemical treatment with an ozonation unit, reducing operating costs and improving management operations. The tests were useful to identify the ozone dosage required to achieve the desired COD abatement. Ozone effectiveness was more pronounced in process water (60 % COD removal), rather than bleaching water (28 % COD removal); ozone showed a significantly higher efficiency on pulp and paper wastewater mixture after biological treatment (up to 81 % COD removal) rather than before biological process (46 % mean COD abatement). COD removal efficiency at a dosage of 100 mg O3/L was comparable to existing physicochemical treatment (mean 50 %); a good TSS abatement (up to 20-30 mg/L) was observed as well. Toxicity analysis revealed that ozonation treatment did not modify treated effluent toxicity characteristics (mean Daphnia Magna mortality of 23.3 %). Economic analysis revealed that ozonation installation cost would be 1.5-2.0 M€, for total flowrate to be treated of 1,200 m³/h. Ozonation would lead to an economic saving of about 300,000 €/y compared to actual physicochemical treatment and the investment cost could be recovered in about 7 years. The proposed innovative approach, including plant analysis, pilot-tests, data regression, respirometric fractionation, toxicity and economic analysis, could be exported to other industrial wastewater treatment plants to allow advanced oxidation processes diffusion.

Benefits of Ozone:

- Highly Effective in COD Reduction – Removes organic matter in lignin-heavy wastewater.
- Reduces Color and Toxicity – Improves effluent quality for water reuse.
- Enhances Biological Treatment Efficiency – Supports secondary treatment processes.

Read the full study here.

Ozone Treatment in Pulp and Paper Wastewater

Authors: Mainardis, M., Buttazzoni, M., De Bortoli, N. 

Abstract

The paper industry is adopting zero liquid effluent technologies to reduce freshwater use and meetenvironmental regulations, which implies closure of water circuits and the progressive accumulation of pollutants that must be removed before water reuse and final wastewater discharge. The traditional water treatment technologies that are used in paper mills (such as dissolved air flotation or biological treatment) are not able to removerecalcitrant contaminants. Therefore, advanced water treatment technologies, such as advanced oxidation processes (AOPs), are being included in industrial wastewater treatment chains aiming to either improve water biodegradability or its final quality. A comprehensive review of the current state of the art regarding the use of AOPs for the treatment of the organic load of effluents from the paper industry is herein addressed considering mature and emerging treatments for a sustainable water use in this sector. Wastewater composition, which is highly dependent on the raw materials being used in the mills, the selected AOP itself, and its combination with other technologies, will determine the viability of the treatment. In general, all AOPs have been reported to achieve good organic removal efficiencies (COD removal >40 %, and about an extra 20 % if AOPs are combined with biological stages). Particularly, ozonation has been the most extensively reported and successfully implemented AOP at an industrial scale for effluent treatment or reuse within pulp and paper mills, although Fenton processes (photo-Fenton particularly) have actually addressed better oxidative results (COD removal≈65–75 %) at a lab scale, but still need further development at a large scale.

Benefits of Ozone:

- Works Well in Complex Wastewater – Effective for pharmaceutical and chemical waste.
- Advanced Oxidation Potential – Enhances pollutant breakdown with UV and peroxides.
- Can Be Used in Pre-Treatment or Final Polishing – Adaptable to different wastewater stages.

Read the full study here.

Case Studies of Microbubbles in Wastewater Treatment

Authors: Lau Hao Wen, Azhar Bin Ismail, P.M. Menon, Jayaprakash Saththasivam, Kyaw Thu, Ng Kim Choon

Abstract

In this study, a physical separation method using a flotation system employing microbubbles was designed and tested as an alternative to conventional dissolved air flotation (DAF) systems. The proposed system is an environmentally friendly microbubble treatment using air, ozone, and CO₂ gases and requiring virtually no chemicals. Three case studies have been investigated in this paper which verify the efficacy of microbubbles in treating wastewater, namely, (i) treatment of oily wastewater derived from cleaning the hull of a tanker ship, (ii) treatment of hotel laundry water, and (iii) treatment of fish pond water. After microbubble treatment, the treated water was determined to be suitable for recycling or discharge to the environment. The test results show that both air and ozone microbubble in wastewater treatments achieved large reductions in TSS, BOD and COD in the tested samples.

Benefits of Ozone:

- Improved Ozone Utilization – Microbubbles extend ozone lifespan in water.
- Higher COD Reduction Efficiency – Works better than standard ozone aeration.
- Eco-Friendly & Energy Efficient – Reduces ozone consumption.

Read the full study here.

Biological and Physical-Chemical Treatment of Textile Dyeing Wastewater for Color and COD Removal

Authors: Instanbul Technical University, Dept of Environmental Engineering

Abstract

Biological treatment, ozonation and chemical precipitation were studied at bench scale to investigate color and COD removal efficiencies in three dye waste effluents of cotton, synthetic and woven fabric processing. The aerobic biodegradability of ozonated and unozonated samples was investigated in fill-and­ draw activated sludge systems. An ozone dosage of 0.8 g/L was applied for 30 minutes in a semi-batch  reactor with volume of 10 L. Alum, FeC13 and FeSO4 were studied as coagulants during chemical clarification and anionic polyelectrolyte was used to improve COD removal efficiency. The results indicated that color removal efficiencies of 78 - l 00 percent were obtained with ozonation at a dosage of  0.8 g/L. COD removal efficiencies of 59 - 71 percent were obtained with combined treatment of ozonation  and chemical clarification, while 62 - 82 percent were achieved with a combination of ozonation and biological treatment. A combination of ozonation and biological treatment is proposed for medium strength wastewater to obtain sufficient color and COD removal. For high strength wastewaters, chemical clarification should be used as a pretreatment before the ozonation and biological treatment to be ab

Read the full study here.

Impact of Chemical Oxidatoin on Biological Treatment of a Primary Municipal Wastewater

Authors: F.J. Beltran, J.F. Garcia-Araya and P. Alvarez

Abstract

Municipal wastewaters taken from a primary sedimentation tank were subjected to different chemical oxidation processes (ozonation or UV radiation alone or combined with hydrogen peroxide) to observe the evolution of COD and BOD/COD ratios. Ozonation of wastewater led to different increases of COD level reduction depending on pH and carbonate-bicarbonate ion concentrations. Direct photolysis or hydrogen peroxide alone were found to be inappropriate technologies. On the other hand, advanced chemical oxidation, that is, oxidation with ozone or UV radiation combined with hydrogen peroxide, increased COD level reduction only when wastewater was previously decarbonated. Thus, elimination of carbonate-bicarbonate ions, increase of pH and addition of hydrogen peroxide (10'3 M) yield increases COD level reduction rates. Finally, preozonation also allows improvement of wastewater biodegradability. 

Read the full study here.

Investrigation on the Removal of COD from Colored Aqueous Solutions with O3, H2O2, HCO3 and PAC

Authors: Ensar Oguz, Bulent Keskinler and Cafer Celik

Abstract

The paper investigates the effectiveness of using ozone to remove Chemical Oxygen Demand (COD) from colored aqueous solutions. The study explores various parameters affecting the ozonation process and evaluates the efficiency of COD removal under different conditions. The results indicate that ozone can significantly reduce COD levels, making it a promising method for treating industrial wastewater.

Read the full study here.