CDFU Deep Dive: The Engineering Inside Cyclonic Dissolved Gas Flotation

Three Mechanisms in One Vessel

In the field of industrial wastewater treatment, the efficient separation of oily wastewater has always been a technical challenge. CDFU (cyclonic dissolved gas flotation unit), the compact cyclonic dissolved gas flotation technology independently developed by SINOKLE, is a hybrid separation device that has changed how oily wastewater is handled. The word hybrid is deliberate: the unit integrates cyclonic centrifugal separation, dissolved gas flotation separation and ultra-fine bubble generation into a single device, and that integration is what allows it to hold performance in a footprint a conventional flotation tank cannot match.

The CDFU unit is a highly efficient oil-water separation device mainly used to treat industrial wastewater containing dispersed oil, emulsified oil and suspended solids (SS), and is especially suited to petroleum refining, oilfield produced water and oily wastewater from power plants.

Why Integration Beats Series Operation

Running the same three mechanisms in separate vessels is possible and has been done for decades. The problem is that each transfer between vessels costs pressure head, residence time and floor area, and the fluid mechanics that made the first stage work are destroyed before the second stage sees the water. Placing them in one vessel keeps the cyclonic field alive while the bubbles are being generated, so the contact between bubble and droplet happens in the same zone where the droplets are being concentrated. The result is that the flotation stage sees a feed far richer in oil than the raw inlet, which is precisely the condition under which flotation is efficient.

The Core Breakthrough: Bubble Quality

The key to the technology lies in the quality of the bubbles it produces.

Generation method

The unit uses a specially designed multi-stage impeller cyclonic dissolved gas pump, producing micro- and nano-scale bubbles through high-pressure, high-speed shearing. The shear field does two things at once — it dissolves gas into the water under pressure, and it breaks the resulting gas phase into a very fine dispersion rather than a coarse froth.

Bubble size and what it changes

Bubble diameter governs everything. Halving it roughly doubles the interfacial area available for a given volume of gas, and it raises the collision efficiency with fine droplets disproportionately, because the size ratio between bubble and droplet is the dominant variable. Ultra-fine bubbles also rise slowly, which extends their residence time in the contact zone and gives the attachment process more time to complete.

Dual-Zone Flotation Structure

What the Two Zones Do to the Bubble-Droplet Pair

The contact and separation regions are not simply two tanks placed in series. In the first, the dominant process is attachment: bubbles and droplets collide, and the probability of a successful attachment depends on the size ratio between them, which is why ultra-fine bubbles are the enabling condition rather than a refinement. In the second, the dominant process is separation: the agglomerate has to rise faster than the surrounding water is moving downward, and it has to do so without being broken apart by shear. A single-zone vessel cannot satisfy both requirements at once, because the mixing intensity that maximises attachment is also the mixing intensity that destroys the product of attachment. Splitting them is what allows each to be optimised independently.

The vessel is arranged so that separation happens in two stages rather than one. The first zone operates as a high-intensity contact region where bubbles and droplets meet under vigorous mixing. The second operates as a quiescent separation region where the bubble-droplet agglomerates rise without being sheared apart by turbulence. Splitting the two functions is what prevents the classic flotation failure mode, in which the mixing needed to achieve attachment also destroys the agglomerates that were just formed.

Where It Applies

Three duty categories dominate the application list. Dispersed oil removal, where droplets are large enough for conventional flotation but where footprint or retention time rules a settling route out. Emulsified oil removal, where the cyclonic shear field provides the physical disruption that the interfacial film would otherwise resist. And suspended solids removal, where the flotation mechanism lifts fine particles that would settle only over a very long period.

The Operating Case for the Unit

The commercial argument for CDFU rests on two figures that a plant can verify directly: the absence of a reagent bill, and low O&M demand. Because separation is achieved physically, no flocculant or demulsifier appears in the operating budget, and no oily sludge leaves site as regulated hazardous waste. Maintenance falls to routine inspection of the dissolved-gas circuit and the cyclonic inlet, which is a materially lighter burden than the periodic reagent handling and sludge dewatering that a chemical route demands.

Performance in capture terms is set by the bubble field. The unit's microbubble generation produces a distribution centred well below the conventional flotation range, which is what allows it to capture droplets that would otherwise pass through a conventional unit. For a stream whose target is a bubble size of roughly 25 um in the conventional baseline, the unit operates with a far finer dispersion and correspondingly higher collision efficiency — the practical result being that a single compact vessel does the work that would otherwise need a larger tank and a longer retention time.

What to Expect in Service

Performance in the field follows the bubble physics closely, which makes behaviour predictable once the unit is set up correctly. Start-up shows a short commissioning period while the recycle quality stabilises; the performance curve is flat afterwards until the dissolved-gas stream degrades. Because the separation depends on the cyclonic field rather than on residence time, turndown is generous — the unit can operate well below design flow without the falling efficiency a settling tank displays, which suits industrial plants where the feed varies through the day. The characteristic failure mode is not mechanical but hydraulic: insufficient recycle pressure produces coarse bubbles and a visible loss of capture efficiency long before any equipment fault appears.

Design Notes and Limits

Two practical points follow from the design. First, the unit is sensitive to the availability of a clean pressurised dissolved-gas stream; sand or scale in the recycle line will degrade bubble quality quickly, so upstream filtration is worth the cost. Second, the cyclonic inlet imposes a pressure drop that must be accounted for in the pumping design. Within those limits, the unit delivers the combination that makes it useful: three separation mechanisms, one vessel, and no chemical addition anywhere in the chain.