CDFU Cyclonic Dissolved Air Flotation: Performance Figures and the Key Variables That Move Them

Oily wastewater carries four distinct things that all get called oil: floating oil, emulsified oil, dissolved oil and suspended particulate matter. Discharge them without separation and the consequences are predictable — eutrophication downstream, ecological chain damage, blocked pipework. The usual remedies each cover part of the list. Gravity separation takes the free fraction, standalone dissolved air flotation takes more, biological treatment eventually degrades what remains. None of them is fast, and none of them handles emulsified and dissolved oil with much conviction.

How the cyclonic route differs

The CDFU ( Cyclonic Dissolved Gas Flotation Unit ) process is a dissolved air flotation technology built around swirl intensification. Instead of relying on gentle bubble rise in a quiet tank, it uses two mechanisms together: cyclonic shear and fine-bubble adsorption.

The flow path is straightforward to describe. Pre-treated wastewater enters the cyclonic dissolved air flotation tank, where pressurised dissolved air generates fine bubbles 5-30 um in diameter. The centrifugal force produced by the cyclonic device intensifies the collision and adsorption between oil droplets and bubbles, forming gas-solid-liquid complexes with a density lower than water. Under the combined action of cyclonic shear and buoyancy, those complexes rise quickly to the surface and are scraped off. Oil leaves at the top as a concentrated phase rather than as a thin film spread across a large surface.

What the process delivers

Oil removal

The CDFU process achieves an overall oil removal rate of 90%-95%. That figure covers emulsified oil as well as floating oil, which is the part that conventional flotation struggles with.

COD removal

The process achieves a total COD removal rate of 30%-50%. It is worth being clear that this is a side benefit rather than the design target: the unit is built to separate oil, and by optimising the swirl intensity and dissolved-air parameters the COD removal rate can be pushed even higher where a plant needs it.

Suspended solids and other pollutants

SS removal exceeds 90%, and the same pass also takes out characteristic pollutants such as petroleum hydrocarbons and volatile phenols. That breadth matters for complex oily wastewater, where a process tuned to a single indicator often passes the target pollutant while leaving the rest behind.

Three variables that decide whether the numbers hold

The removal figures above are attainable ranges, not fixed specifications. What a plant actually sees depends on how the following are managed.

• Dissolved-air conditions. Adjusting the dissolved-air pressure and dissolved-air time changes the bubble diameter distribution, and bubble size determines what the bubbles can capture. Optimising the cyclonic rotation speed strengthens droplet collision and coalescence, which gives higher bubble adsorption efficiency.
• Chemical dosing. Combining the process with chemical dosing, a PAC flocculant for instance, strengthens the binding efficiency between flocs and bubbles and further improves cyclonic flotation separation. In many cases a small dose does most of the work, so this is a knob to be tuned rather than a fixed cost.
• Control. Integrating an intelligent control system with online monitoring, such as oil content and COD sensors, allows real-time adjustment of process parameters. The value here is in shock load conditions, when a fixed setting is either too conservative or too late.

Against conventional routes

The comparison that matters is with conventional dissolved air flotation at 70%-85% oil removal and with biological treatment at a cycle longer than 24 h. Against those, three differences stand out.

• Removal efficiency. Cyclonic shear and bubble adsorption act together, so emulsified and floating oil are both separated, with oil removal of 90%-95%.
• Shock-load tolerance. The process tolerates influent oil content fluctuations of about 30% while holding stable effluent quality — a condition that commonly occurs at refineries during unit upsets.
• Cycle time. Treatment takes only 1/5-1/10 of conventional biological treatment, which converts directly into tank volume and therefore into footprint.

Where it fits in a train

A CDFU unit is most useful at the front of a treatment train, ahead of biological or polishing stages, or as the replacement for an undersized flotation cell that has become the bottleneck. Its short cycle and shock tolerance make it a good buffer for a plant whose influent quality changes faster than the downstream biology can adapt. It is not a substitute for biological treatment when the goal is ammonia or dissolved organic removal — the COD reduction it offers is a bonus, not a compliance route on its own.

One more consideration applies to specification. Because the unit carries oxidation and flotation in the same shell, the ozone dose and the hydraulic residence time are no longer independent variables — raising one changes the other. A supplier that quotes both without explaining the coupling has not finished the design. Ask for the dissolution curve and the oxygen demand of the target stream together.

The summary is unglamorous but accurate: the CDFU process gives efficient, stable and low-cost separation of oil from water, and that is enough to support discharge compliance for a wide range of industrial streams.