From CFU to CDFU: Six Changes That Decide Whether Emulsified Oil Leaves

Compact flotation is one of the more reliable unit operations in industrial water treatment, which makes its limits easy to forget. A conventional CFU will remove free oil and suspended solids steadily for years with little attention. It will also pass emulsified oil almost untouched, and no amount of additional chemical dosing changes that in a predictable way.

How the conventional unit works, step by step

The sequence is straightforward. A dedicated gas injection device releases a uniform swarm of microbubbles into the water, forming a gas-water mixture and setting up the conditions for capture. Relying on surface tension, the bubbles adsorb suspended particles and grease, forming aggregates less dense than water. Buoyancy then lifts those aggregates rapidly to the surface, where they gather as a skimmable scum layer. Inside the vessel, a pressurised closed structure with level monitoring and flow-state control allows precise collection and automatic discharge of scum and waste oil, which is what keeps treatment continuous rather than batch. After flotation, suspended solids and oil content fall and clarity improves, providing the foundation for downstream treatment or compliant discharge.

That is a good process. It is also a process whose performance is bounded by two physical facts.

The two limits that chemistry cannot fix

First, bubble size. Conventional CFU bubbles are relatively large, typically 100-1000 μm, and the removal of emulsified oil droplets smaller than 20 μm is poor — often close to zero. The reason is geometric rather than chemical: a bubble moving through water sweeps a path proportional to its cross-section, and a large, fast-rising bubble gives a small droplet very little time to attach. The collision efficiency falls faster than the surface area rises.

Second, the chemistry that sites use to compensate. Conventional flotation usually needs dedicated chemical dosing to assist separation. That raises operating cost and makes performance unstable, and it also generates a large volume of high-water-content waste oil classified as hazardous waste — so a removal problem becomes a disposal problem.

Two further weaknesses appear in specific services. In high-salinity environments the gas injection point is prone to scaling and blockage, which turns routine maintenance into a shutdown. And automatic control precision is relatively poor: when flow fluctuates, level and pressure control become unstable, and effluent quality moves with them.

The six changes in CDFU

Building on conventional CFU and integrating ultrafine bubble generation, dissolved-air flotation, coalescing separation and an integrated flotation-cyclone arrangement, SINOKLE developed the next generation compact flotation unit, CDFU, designated the Cyclonic Dissolved Gas Flotation Unit. The design changes that matter are six, and each addresses one of the limits above.

• Smaller, more evenly distributed bubbles. CDFU generates microbubbles only 5-30 μm in diameter, with a D50 of approximately 25 μm and a uniform distribution, against the 100-1000 μm of conventional CFU. The reduction in size raises specific surface area and collision-adhesion efficiency with oil droplets at the same time.
• Emulsified oil removal in a range that used to be inaccessible. Ultrafine bubble technology gives 80%-90% removal of emulsified oil droplets in the 0.1-20 μm band, where conventional CFU is essentially ineffective. This is the single change that reclassifies the technology from a free-oil separator into an emulsified-oil separator.
• Physical demulsification with no chemicals. The whole process is physical, with no chemical addition, which removes the generation of oily sludge classified as hazardous waste and the secondary pollution that comes with it. Separated oil can be recovered for re-refining, converting a disposal cost into a product stream.
• High treatment efficiency in a compact body. Hydraulic retention time is only 1-5 minutes and single-stage oil removal efficiency is above 90%. The swirl design and compact structure give a footprint about one-third that of conventional equipment, which is what makes the unit deployable on a platform deck or a congested pad.
• Automation and stable operation. The unit runs fully automatically in a closed pressurised mode with high level-control precision, on the order of plus or minus 5 mm. Strong shock resistance lets it absorb large fluctuations in water quality and quantity while still delivering stable effluent.
• Lower maintenance burden. The internal structure is simple and resists scaling and blockage. Energy consumption is low and there are no chemical costs, which is where most of the operating and maintenance saving comes from.

Why the bubble number deserves the most attention

Of the six, bubble diameter is the one that repays scrutiny in a tender evaluation, because it is the one most easily misrepresented. A claim of "microbubbles" without a distribution is not a specification. The relevant questions are what the D50 is, how wide the distribution runs, and what fraction of the gas volume ends up in bubbles above 50 μm — because that tail does very little work while consuming most of the gas.

A related check is dissolution performance. If the gas is not fully dissolved before release, the release device produces large bubbles regardless of its geometry, and the measured D50 will drift upward as flow increases. That is why the pump and the release device have to be evaluated as a pair rather than as separate components.

Where the upgrade does not pay

Three situations should keep a conventional unit in place. If the duty is free oil and grit ahead of a gravity separator, the emulsified fraction is not the constraint and the extra capital buys nothing. If the site has no way to handle recovered oil, the chemical-free advantage inverts: the unit produces a concentrated oil stream that still needs a destination. And if flow is highly intermittent, the closed pressurised mode still needs a minimum recirculation to hold the bubble field, so a batch duty is better served by a simple induced-air unit.

Where the duty is stable and the oil is genuinely emulsified, the comparison resolves in one direction. Droplet diameter, not chemical dose, is the variable that decides whether the oil leaves with the water or with the scum.