CDOF in Refractory Wastewater: Ozone Catalysis Coupled With Flotation

Two unit operations that were always built separately

Ozone oxidation and flotation solve different halves of the same problem. Ozone, activated into hydroxyl radicals, breaks refractory molecules into smaller and more biodegradable fragments. Flotation lifts the resulting flocs, colloidal matter and residual oil out of the water. In conventional design they sit in series, with a reaction tank, a transfer pump, and often a sedimentation or filtration step in between, and each transfer costs head, floor area and residence time.

The CDOF (Cyclonic Dissolved Ozone Flotation Unit) from SINOKLE integrates the two into one vessel. Oxidation happens in the same volume where the bubbles are released, so partially oxidised matter is contacted by microbubbles while it is still dispersed rather than after it has had time to re-aggregate. That coupling is the engineering idea, and it is what the rest of the design follows from. The vessel is built as a single skid, so the ozone generator, the catalyst bed and the flotation cell are commissioned together rather than as three separate packages.

What the catalyst actually changes

Ozone alone is selective. It attacks unsaturated bonds and certain aromatic structures quickly and leaves others almost untouched, which is why a plain ozone stage often raises the BOD5/COD ratio only slightly for the ozone dose consumed. The heterogeneous catalyst in the CDOF activates ozone molecules and converts them into hydroxyl radicals, whose redox potential is far higher than that of ordinary ozone.

That changes the chemistry from selective to largely non-selective. Mineralisation or chain cleavage then proceeds across the majority of refractory organics, including the fraction that gives colour, the toxic fraction, and the trace organics such as antibiotics, pesticides and endocrine disruptors that conventional biology cannot touch. In practice the catalyst raises ozone utilisation and reaction rate at the same time, which shortens the contact time required and reduces the ozone dose, and the operating cost falls with the dose.

Why the flotation side is not an afterthought

Partial oxidation leaves a residue: fine flocs, colloidal matter, inert gas bubbles and partly oxidised organics that are more polar than what went in and therefore less willing to coalesce. A separate clarifier would struggle with that material because the flocs are light and slow to settle. Flotation handles it for the opposite reason — the bubbles attach and the load rises instead of falling.

Because the oxidation reactor and the flotation unit share a vessel, the separation happens while the reaction products are still fresh. The result is removal of dissolved pollutants and of suspended and colloidal matter in one pass, which brings down effluent turbidity, colour and SS together and gives a more stable effluent appearance than a train of separate units. The microbubbles also create a gas-liquid interface environment that contributes a modest additional oxidising effect.

Where it earns its place

The technology is aimed at advanced treatment and reuse of refractory industrial wastewater, and the relevant industries share a property: their effluent contains organics that biology cannot finish. Petrochemical, coal chemical, fine chemical, pharmaceutical including antibiotics and synthetic drugs, pesticide, dye and printing-dyeing, coking, landfill leachate including membrane concentrate, electroplating and electronics.

Four duties recur. As an independent advanced treatment step, it brings effluent to strict discharge or reuse standards, including circulating cooling water make-up. As pretreatment for a biological train, it degrades toxic and inhibitory substances and raises the BOD5/COD ratio so the downstream anaerobic and aerobic units can do their job. As a targeted step it removes residual antibiotics, endocrine disruptors and persistent organic pollutants. And as a decolourisation step it is unusually effective on dye, printing-dyeing and coking wastewater, with removal rates usually above 95%.

The retrofit case deserves separate mention. Adding a CDOF unit after the biological stage improves COD, colour and other compliance indicators without large-scale expansion, and it absorbs the water quality swings that make an existing plant look unstable. For an industrial park plant with no room to build, that is often the only route to compliance.

Why no secondary pollution is a design claim, not a slogan

The main reaction products are CO2, H2O and inorganic salts. No iron-bearing sludge is generated, which is the structural difference from a Fenton stage: there is no ferric hydroxide stream to dewater, and no acid and alkali consumption to manage. Ozone is generated on site from oxygen or air, so there is no hazardous chemical transport or storage risk of the kind that comes with liquid chlorine. The volume of scum produced is relatively small and easy to handle.

Those properties matter more than they sound in a brownfield plant, because the constraint that stops a upgrade is usually not the reactor but the sludge or chemical storage it would require.

Footprint, automation and the honest trade-offs

The integrated design is compact. It occupies less space than the conventional combination of ozone oxidation plus sedimentation or filtration, because the reaction volume and the separation volume are the same volume. Automated control is straightforward once the ozone generation, dissolved gas release and sludge removal loops are instrumented, and the operating skill required is modest compared with a multi-stage chemical train.

The trade-offs are real and worth stating. Ozone generation is electricity-intensive, so the operating cost tracks the local power price and the ozone dose that the water actually demands. Catalyst life and replacement have to be priced into the comparison. And the process is not a substitute for biological treatment where biology can do the work — it is for the fraction that biology cannot finish. Comprehensive evaluation against the specific water quality and economics remains necessary; the integration is an advantage, not an argument for skipping the design calculation.