Four Mechanisms, One Shell: What Actually Happens Inside a CDFU Vessel

Everyone working in industrial wastewater treatment learns early that a single technology never handles complex water quality. Equipment that truly holds up on site is always the result of several technologies working together. That is exactly the case with CDFU, the cyclonic dissolved air flotation unit developed by SINOKLE. Four core technologies, each with a distinct job, running together in one shell.

1. Cyclonic centrifugal separation: the workhorse of coarse separation

Oily wastewater enters the CDFU vessel at high speed through a tangential inlet, forming a powerful cyclonic field inside the cylindrical chamber. Using the density difference between oil and water, oil about 0.8-0.95 g/cm3 against water at about 1.0 g/cm3, centrifugal force drives the less dense droplets toward the centre of the cyclone to form an oil core, while the heavier clean water moves outward along the vessel wall.

This separates most free oil droplets within a few seconds to a few tens of seconds, serving as the first highly efficient coarse filtration stage of the entire unit. At such a loading, speed is not a luxury.

There is a second role that matters just as much. Cyclonic separation creates a turbulent, thoroughly mixed environment for the bubble contact that follows: not stagnant water but dynamic contact within high-speed rotation, where collision probability is far higher than in a static DAF tank.

2. Ultra-fine bubble generation: the trump card against emulsified oil

This is where CDFU differs most from conventional flotation. The bubbles released by dissolved air in a conventional DAF are mostly 50-100 μm in diameter, whereas CDFU uses SINOKLE's in-house new-generation micro-bubble generation technology to control bubble size down to the nanometre-to-micrometre scale.

Why is smaller better? Because the smaller the bubble diameter, the larger the total surface area for the same gas volume, specific surface area being inversely proportional to diameter, and therefore the larger the contact area with emulsified oil droplets in the water. At the same time, micro-bubbles rise more slowly and stay in the water longer, greatly increasing the probability of a droplet-bubble collision ending in adhesion.

Here is a rough analogy. Catch fish with a large net and the small ones slip through; switch to a fine screen and nothing escapes. Ultra-fine bubbles are that fine screen, and emulsified oil is the small fish.

3. Dissolved air flotation: the classic separation driver

Dissolved air flotation is the basic framework of the whole system. Inside the CDFU unit, part of the treated effluent is pressurized to 0.3-0.6 MPa and injected with compressed air or nitrogen to form supersaturated dissolved-air water, which is then released at reduced pressure inside the vessel through a dedicated release device.

The essence of this process lies in controlling two steps, dissolving and releasing. Dissolution efficiency determines the total number of bubbles available; the structure of the release device determines how uniform the bubble size distribution is. SINOKLE has made patented optimizations at both steps, so the CDFU achieves significantly higher dissolved-gas utilization than a conventional DAF, with lower energy consumption at the same treatment capacity.

4. Coalescence and demulsification: making the stubborn ones stick together

A considerable proportion of the oil in industrial wastewater exists in emulsified form. The surface of the droplets is wrapped in surface-active substances, forming a stable oil-in-water emulsion in which droplets repel one another because of electrical charge and are hard to aggregate.

Coalescence and demulsification technology uses special internals or media so that emulsified droplets collide and merge as they flow past, turning small droplets into large ones that are more easily captured by cyclonic centrifugal force or adhered to by bubbles.

This step may look like a supporting act, but it is in fact the critical leap that pushes oil removal efficiency from 90%+ to 99%+. Without demulsification, even the finest bubbles are helpless against charge-protected droplets, because there is nothing for them to attach to.

The logic of four-in-one synergy

These four technologies are not simply connected in series. They are spatially nested and functionally complementary. The cyclone provides a centrifugal force field and mixing energy, coalescence enlarges emulsified oil droplets, ultra-fine bubbles provide an enormous capture surface, and dissolved air flotation supplies both the bubbles and the flotation driving force. All four steps take place simultaneously in the same sealed vessel, reinforcing one another rather than queuing.

What the field data says

Engineering data confirm the effectiveness of that synergy. At a petrochemical plant in Shandong, inlet oil content was 250,000 mg/L with effluent below 150 mg/L, a removal of 99.9%. In an ultra-heavy oil project, inlet oil content was 150,000 mg/L with effluent below 40 mg/L, or 99.97%. At heavy-oil power plants in Sierra Leone and Bangladesh, effluent oil content was below 5 mg/L and 10 mg/L respectively.

Treatment capacities across those installations span the full range from 5 to 250 m3/h, which is the more telling result: the architecture maintains good stability as it scales up by more than an order of magnitude.

Technical summary

CDFU at its core works within a compact sealed space. Cyclonic centrifugation handles the big lumps, coalescence and demulsification break down the stubborn ones, ultra-fine bubbles round up the ones that would slip through the net, and dissolved air flotation completes the final transport. Four approaches at once, purely physical, with no chemicals at any stage. Judging from actual operating data in the field, the competitiveness of this technology route in industrial oily wastewater treatment is not really in doubt.