Inside the CDOF Unit: Ejector, Reactor, Catalyst and Off-Gas, Stage by Stage

Catalytic ozonation is often sold on the strength of its catalyst and judged on its electricity bill. The two are connected, but not in the way most proposals suggest. Ozone that never reaches a pollutant is wasted twice — once in the oxygen plant that fed it, and again in the generator that converted it. The CDOF unit addresses that waste at four separate points, and looking at each of them in turn is the quickest way to understand why the integrated design performs the way it does.

Stage one: getting ozone into the water as bubbles

The entry point is the SINOKLE high-efficiency ejector, a device with four functional sections: a nozzle, a suction chamber, a mixing chamber and a diffuser. Pressurised wastewater, boosted by the ozone dosing pump, enters the ejection chamber from the ejector inlet and forms a high-speed jet. That jet creates a pressure drop, and the pressure drop draws high-concentration ozone from the ozone generator in through the suction port and into the wastewater.

The mixture then passes through the diffusion section to the ejector outlet, where the flow slows and the pressure recovers. Pressure recovery is where the work happens: the deceleration generates powerful shear that shatters the ozone into micro-nano bubbles, and those bubbles are injected into the CDOF reactor. Because the ejector and the reactor are joined by piping, the whole path is closed, so ozone gas is drawn directly into the wastewater for reaction with no risk of gas-phase leakage and no need for additional electrical equipment inside the reaction zone — which removes the electrical safety exposure that open systems carry.

The payback of this stage is measured in one number: ozone utilisation can reach as high as 95%. The benefit is not limited to advanced catalytic ozonation performance either. Higher utilisation means the upstream oxygen generation and ozone generation systems can be sized down or run less hard, so the saving appears in two places on the same balance sheet.

Stage two: the main reactor

The main reactor is a cylindrical pressure vessel constructed entirely of 316L material with a design pressure of 0.6 MPa. Ozone-bearing wastewater entering through the high-efficiency ejector passes specially designed swirl guide plates and a catalyst bed. The guide plates do the physical work: they strengthen gas-liquid-solid three-phase contact reaction efficiency, raising contact efficiency between ozone micro-nano bubbles and the catalyst by more than 30% and promoting suspended solids separation with separation efficiency improved by about 50%.

The catalyst does the chemical work, promoting ozone conversion into hydroxyl radicals (·OH) and degrading COD in the water at a catalytic efficiency 2 to 5 times higher than conventional ozone catalysts. Note that the two improvements are of different kinds. Contact efficiency is a hydraulic property of the vessel; catalytic efficiency is a property of the material. A design that improves one without the other leaves half the available gain on the table.

Stage three: the heterogeneous catalyst itself

The catalyst is a high-efficiency heterogeneous ozone catalyst, produced from multiple transition metal oxides and precious metals as active components, carried on a special support structure through a range of complex processes. In service, the modified ozone catalyst is super-hydrophilic, and in the presence of ozone micro-bubbles and water it shows high flowability, an expansion rate of 30% to 50%, resistance to agglomeration and stable operation. After the flow passes through the catalyst bed, ozone utilisation can reach as high as 99%.

The expansion behaviour is not a footnote. A bed that expands within a known range is a bed that can be backwashed and that will not compact into a solid mass; agglomeration is the usual failure mode of catalytic beds treating oily or solids-bearing water, and it shows up as rising pressure drop long before it shows up as poor removal.

Stage four: secondary reactor and the tail gas

The main and secondary advanced catalytic ozonation reactors are joined by piping fitted with a specially designed Venturi structure. The Venturi generates cavitation bubbles from ozone, which continue oxidising pollutants while allowing dissolved ozone gas to be released. A catalyst bed inside promotes continued generation of hydroxyl radicals (·OH) from residual ozone, so the COD that survived the first pass gets a second chance.

At the top, a gas separation zone collects the very small amount of residual ozone and routes it through an exhaust port to a dedicated ozone off-gas destruction system. After that treatment the ozone off-gas concentration is as low as 0.1 ppm and the stream can be vented directly to atmosphere — the point at which an operator stops worrying about the gas and starts thinking only about the water.

What the four stages add up to

The integrated unit, designated CDOF, combines four core advantages: high-efficiency ozone dosing, a closed pressurised system, cyclone separation technology and innovative ozone catalysis. The practical features that follow are worth listing because they are what a plant manager will actually live with.

• High ozone utilisation, with ozone consumption reduced by 70% compared with conventional catalytic ozonation.
• An innovative catalyst bed with catalyst life extended to 3 to 5 years, no agglomeration, long backwash intervals and lower O&M cost.
• Cyclone technology for high mass transfer efficiency, reaction efficiency improved 3 to 5 times, short residence time, and an integrated unit with a small footprint.
• Closed-system operation with no safety hazards.

For evaluation purposes, the numbers to insist on are the design pressure and material of the reactor, the measured ozone utilisation at the design dose rather than at the optimum, the expected backwash interval under the real solids load, and the residual ozone concentration at the exhaust. Those four answers describe the unit far better than a picture of the skid.