CDFU vs Conventional Dissolved Gas Flotation: Where the Real Gap Opens Up

Two Devices, One Duty, Very Different Physics

Both a conventional dissolved gas flotation cell and a CDFU unit are asked to do the same job: lift oil and solids out of water with bubbles. The difference is where the bubble meets the droplet. In a conventional cell that meeting is left to chance inside a large open basin. In the CDFU cyclonic dissolved gas flotation unit it is engineered, by placing the bubble population inside a rotating flow field.

The Triple Synergy, Described Plainly

Three mechanisms are stacked in one vessel. Tangential inlet geometry sets up a stable swirl field at a specific injection angle. Centrifugal acceleration separates by density while the swirl is running. And ultrafine bubbles provide the surface area that makes attachment fast. The swirl also drives droplets toward the centre of the field, concentrating them where bubble concentration is highest. Coalescence and flotation therefore happen in the same volume instead of in sequence.

Bubble Size: The Number That Sets Everything Else

Conventional dissolved gas flotation produces bubbles in the 30-100 μm band, with only moderate uniformity. CDFU generates bubbles as fine as 5-30 μm, with a median at or below 20 μm. Smaller bubbles mean more specific surface area per unit of gas volume, and stronger adsorption once contact occurs. The practical effect is that oil droplets a conventional cell would leave behind become separable.

Structure and the Sand Problem

Two design choices carry most of the mechanical advantage. The first is a dual flotation zone: wastewater enters tangentially, centrifugal force throws the dense phase outward and the lighter phase inward, and both zones work at higher space utilisation than a single open basin. The second is the conical bottom, where particles are pushed against the inner wall of the central cylinder and slide down into the cone while still rotating, which keeps sand in motion instead of letting it settle into a bed.

Conventional cells are typically open reinforced concrete structures. CDFU ships as a skid-mounted pressure vessel. On an offshore deck or inside an existing battery, that difference often decides whether the project is feasible at all.

Performance, Side by Side

• Single-stage oil removal above 90%, against 60%-85% for conventional dissolved gas flotation.
• Suspended solids removal above 80%, which is 10-30 percentage points higher than the conventional range.
• Stable removal of oil droplets smaller than 3 μm with efficiency above 80%, while conventional units effectively stop at 5 μm.
• Retention time under 5 minutes, against 15-40 minutes, a factor of one-eighth to one-third.
• Stable operation across a flow fluctuation range of 10%-120%.

The retention time figure is the one that changes project economics. At one-third of the residence time, the same vessel volume handles three times the flow, or the same flow fits in a third of the footprint.

O&M and Energy

The unit runs fully enclosed with automatic control. Oil and sludge are pushed out by pressure rather than scraped out by a mechanism, which eliminates the failure mode that dominates conventional cell maintenance: the scraper drive. The energy figures: gas consumption is 2%-3% of influent volume, roughly one-tenth of the conventional requirement; system pressure drop is ≤0.05 MPa; energy consumption drops by 40%; and the oil discharge ratio sits at 0.5%-2%, about one-fifth of the conventional figure.

No chemicals are used, so the PAC/PAM dosing skid disappears, and with it the hazardous sludge that PAC and PAM generate in a conventional train. Sludge volume falls by about 90%, and the waste oil can be recovered rather than disposed of.

Field Numbers and Where They Apply

The unit is aimed at emulsified oil that resists gravity separation, including influent oil at or below 5,000 mg/L, and at locations where space or access is hostile. In a field test with influent oil content as high as 3,464.35 mg/L, removal reached 94.98% and effluent oil was 173.83 mg/L.

Conventional dissolved gas flotation still has a place: large onshore sites where land is cheap and efficiency variation is tolerable. Its footprint and its sensitivity to flow swings are what limit it elsewhere.

How It Fits a Larger Train

Flotation is a front-end tool. Pairing CDFU with catalytic ozonation (CDOF) downstream, and with active filter media filtration (KFM) for polishing, is how the same platform reaches zero-liquid-discharge targets. Each stage removes what the previous one cannot: bulk oil and solids first, dissolved refractory organics next, residual fine solids last.

Commissioning Notes That Get Skipped

Three items decide whether the unit delivers its rated numbers in the first month. Flushing the swirl chamber before first feed, because construction debris finds the tangential inlet and distorts the field. Setting the dissolved gas pressure against the actual feed temperature, since solubility drops as the water warms and the bubble population thins. And trending the differential pressure across the oil discharge line, which is the earliest visible sign of a partially blocked discharge rather than a separation problem.

A fourth item is contractual rather than mechanical: agree on the effluent sampling point and the oil analysis method before startup. Most disputes about these units are disputes about where the sample came from.