Smaller Bubbles, Faster Separation: The Physics Behind CDFU Flotation Efficiency

SINOKLE works on this problem in oily wastewater treatment through a physical purification technique that does the opposite of settling. Instead of letting solids sink, flotation makes them rise. Dissolved air flotation (DAF) introduces highly dispersed micro-bubbles into the water to be treated, so that the bubbles act as carriers and adhere to suspended particles such as oil droplets or flocs. These loaded particles form aggregates with an overall density lower than water, and buoyancy carries them to the surface as scum. Solid-liquid and liquid-liquid separation follow.

The Three Stages of Flotation

• Bubble generation. A specific device creates large numbers of micron-scale bubbles in the water.
• Adhesion between bubbles and suspended matter. This is the most critical step, in which bubbles effectively collide with and stick to pollutants in the water.
• Separation and rising. The flocs carry pollutants up to the liquid surface, where they form scum that is skimmed off.

Of the three, adhesion is where most designs succeed or fail. Bubble generation can be made reliable with good hardware. Skimming is mechanical. Adhesion depends on interfacial physics that cannot be compensated for after the fact.

Why Particles Stick to Bubbles

Bubbles capture pollutants mainly thanks to wettability and interfacial energy. Hydrophobic substances, meaning those that dislike water, are more likely to adhere to bubbles. Oil droplets are the classic example, with a contact angle greater than 90 degrees. After adhesion, the total interfacial energy of the system decreases and the structure becomes more stable, which is what makes the attachment durable rather than momentary.

Research indicates that micro-nano bubble surfaces are rich in negative charge, carrying a high zeta potential. When they burst, they generate instantaneous local high temperature and pressure and release strongly oxidising hydroxyl radicals. That contributes to physical separation and also helps degrade organic pollutants in the water, which is a useful secondary effect in streams that carry dissolved organics alongside free oil.

Size Determines Efficiency

In flotation, the higher the bubble concentration and the smaller the bubble size, the higher the removal efficiency. Two mechanisms explain why.

• Collision probability. Reducing bubble size significantly increases the bubble concentration per unit volume, which greatly increases the probability that a bubble and an oil droplet will meet.
• Demulsification capability. Smaller bubbles have stronger surface tension and can more easily tear open the oil film, achieving physical demulsification. That is crucial for treating highly emulsified wastewater, where the oil is held in a stabilised film that larger bubbles cannot break.

The bubbles produced by SINOKLE CDFU technology are only 5-30 microns in diameter with excellent uniformity, far smaller than the 100-500 micron bubbles produced by conventional flotation.

Where Conventional Flotation Runs Out of Room

Conventional dissolved gas flotation has a well-known set of limits. Residence times run 15-40 minutes, equipment is large, and significant amounts of chemical agents are needed to make the process work at all. Those three limitations are connected: because the bubbles are large and the collision probability is low, longer contact time is required, and because physical demulsification is weak, chemistry has to compensate.

The CDFU Configuration

CDFU (Cyclonic Dissolved gas Flotation Unit) from SINOKLE combines cyclonic centrifugal separation and dissolved gas flotation into a single unit. Its working sequence runs as follows.

• Efficient gas dissolution. A specially structured multi-stage impeller dissolved gas pump produces micro- and nano-scale bubbles through high-pressure, high-speed shearing.
• Strong cyclonic collision. After the wastewater is mixed with the dissolved gas water, it enters the tangential cyclone separation zone along the tank wall. Under powerful centrifugal force the water flow accelerates in its rotation, greatly increasing the probability of collision between bubbles and oil droplets.
• Ultra-fast separation. Under the cyclonic action, flocs carrying oil droplets rapidly gather toward the centre and rise, while heavy suspended solids such as sand are flung toward the wall and discharged downward.

Combining the two mechanisms means the same vessel both generates the collisions and performs the separation, so neither duty has to be oversized to compensate for the other.

Measured Advantages Over Conventional Process

• Purely physical demulsification. No chemicals are needed, or only a minimal amount, eliminating the generation of oily sludge classified as hazardous waste.
• Extremely compact. Treatment residence time is only 1-5 minutes, between one eighth and one third of conventional processes, and the footprint is only 10%-20%.
• High efficiency. Single-stage oil removal efficiency exceeds 90%, and the oil content of the effluent can be reduced to below 10 mg/L or even lower.

Choosing a Flotation Technology

The position taken by SINOKLE is that the useful comparison is not between brand names but between bubble size distributions and the residence times they force. A flotation system quoted with a long contact time is telling you something about its bubble quality. When bubble diameter sits in the single-digit to low tens of microns and the distribution is tight, the process shortens, the vessel shrinks, and the dependence on chemical aids falls away. That progression, from passive flotation toward efficient cyclonic active separation, is where the technology is heading.