From Ozone Molecule to Hydroxyl Radical: How Catalytic Ozonation Strips COD Out of Refractory Wastewater

In modern industrial wastewater treatment, deep removal of chemical oxygen demand (COD) has long been a technical difficulty. Catalytic ozonation, an efficient advanced oxidation technology, markedly improves the oxidising efficiency of ozone through the use of catalysts and has become a key solution for COD removal. Understanding why requires looking at what changes when a catalyst enters the ozone system, because the improvement is not incremental, it is a change of chemistry.

Mechanism: from selective to non-selective

Conventional ozonation relies mainly on direct oxidation by the ozone molecule, whose redox potential is 2.07 eV, which limits oxidation efficiency and leaves pollutants incompletely treated. Catalytic ozonation adds a catalyst to the ozone system, greatly enhancing ozone decomposition and generating hydroxyl radicals of far stronger oxidising power, with a redox potential of 2.8 V. Those radicals oxidise non-selectively and efficiently degrade a wide range of organic pollutants. By lowering the activation energy, the catalyst accelerates the self-decomposition of ozone in water and markedly raises the hydroxyl radical concentration. Under catalysis, ozone and wastewater enter the catalytic oxidation tower together, and as they pass through the catalyst bed a large number of hydroxyl radicals are produced, which break chains and open rings, converting large recalcitrant molecules into smaller ones and finally mineralising them to carbon dioxide and water. The word to keep from that sequence is non-selective: direct ozone picks its targets, radicals attack nearly everything, and refractory COD is precisely the fraction that direct ozone declines to touch.

Catalyst technology: three innovations that matter

SINOKLE has made an important breakthrough in heterogeneous catalysts, using a range of transition metal oxides and precious metals as catalytic components and, through precise formulation optimisation and controlled sintering, developing catalyst systems of high adaptability and catalytic activity. Three innovations carry the performance. The first is the support structure: high-porosity microporous forming raises the specific surface area of the support to 250-300 m2/g, far above the 50-100 m2/g of conventional supports, markedly increasing active site density and raising the catalytic reaction rate by more than 3 times. The second is surface modification: atomic layer deposition is used to make the catalyst hydrophilic, creating a superhydrophilic surface whose resistance to organic fouling is 90% higher, effectively preventing contamination, scaling and plugging and ensuring long-term stable operation. The third is mechanical properties: staged precision temperature-controlled sintering maintains high mechanical strength of 150 N per granule while keeping annual attrition below 0.3%, giving a catalyst service life of more than 5 years. Each innovation answers a real failure mode: low activity, fouled surfaces, or a bed disintegrating into fines that blind the tower.

Process integration: the CDOF unit

Catalyst quality sets the ceiling; reactor design decides how much of it you actually use. SINOKLE has developed the CDOF (cyclonic dissolved-air ozone flotation) unit, which organically combines multiple catalytic ozonation, cyclonic technology and dissolved-air flotation into one highly efficient integrated treatment system. The system offers clear technical advantages. Reaction is much faster: the patented reactor design cuts effective reaction time from the conventional 90 minutes to within 15 minutes, with ozone utilisation above 99.98%. Operating cost is much lower: the ozone dosing ratio is only 1/5-1/2 of the conventional process, operating costs fall by 30%-50%, and the footprint shrinks to 1/3 of the conventional process. Control is intelligent: a DCS/PLC automatic control system monitors tail-gas concentration, effluent ORP and pH in real time and automatically optimises the operating strategy as water quality changes. That last capability matters more than it appears, because refractory wastewater streams rarely hold a steady composition, and an oxidation system that cannot see its own tail-gas and ORP trends will either overdose or underperform.

Verified in hard waters

The technology performs outstandingly in difficult wastewaters such as petrochemical, pharmaceutical and landfill leachate treatment. Operating data show an absolute COD removal of 1.1-1.5 g O3 per g COD, with oxidation efficiency 2-5 times that of conventional processes. In landfill leachate treatment, COD removal exceeds 90% and effluent quality is stably compliant. Leachate is a fair benchmark, since it combines high COD, variable composition and a biological-inhibition history that defeats conventional trains, so a 90%-plus removal there says something about headroom elsewhere.

What this means for an upgrade project

Through technical innovation and system optimisation, catalytic ozonation has solved the problem of treating high-COD wastewater, providing a reliable technical route for advanced industrial wastewater treatment. For plants planning an upgrade, the evaluation checklist follows the mechanism: verify the support surface area and attrition data of the catalyst on offer, confirm the reactor delivers dissolved ozone to the catalyst rather than to the off-gas, and insist on a control system that watches ORP and tail gas on its own. Where those three boxes are ticked, the radical route turns a chronic COD compliance problem into a monitored, budgetable unit operation.