DAF vs CFU vs CDFU: A Side-by-Side Look at the Metrics That Actually Differ

Oily wastewater turns up across oil extraction, refining, thermal power and marine operation, and in every one of those sectors the treatment requirement has tightened at the same time as the cost pressure has grown. Choosing between the available separation technologies is therefore not a purely technical decision. The parameters that differ most between them, residence time, bubble diameter, chemical demand and sludge generation, are the same parameters that drive operating cost for the next twenty years.

The Three Contenders

Conventional dissolved air flotation (DAF)

DAF relies on releasing dissolved air water at atmospheric pressure to generate bubbles that carry oil droplets to the water surface. It works, but the treatment residence time is as long as 15-40 minutes, which makes equipment bulky and footprints large. Its other weakness is chemical dependence: demulsifiers and flocculants are used in quantity, producing large amounts of oily sludge that must be handled as hazardous waste, creating both secondary pollution and expensive disposal.

Conventional cyclonic flotation (CFU)

CFU uses cyclonic centrifugal force to assist separation and is relatively compact in structure. The limitations show up in bubble quality. Bubbles produced are usually 100-1000 um in diameter, which gives almost no removal effect on emulsified oil, and stability is poor. Operation often requires frequent maintenance and offers a low degree of automation.

CDFU (Cyclonic Dissolved gas Flotation Unit)

SINOKLE CDFU organically combines cyclonic separation, dissolved gas flotation and ultra-fine bubble technology. Single-stage oil removal efficiency exceeds 90%, with a residence time of only 1-5 minutes, between one eighth and one third of conventional processes. It achieves purely physical demulsification with no chemical agents added, eliminating oily sludge generation at the source, and the recovered waste oil is of high quality and can be directly reused.

Core Metrics Compared

• Oil removal efficiency. Conventional dissolved air flotation 60%-85%; conventional cyclonic flotation 60%-80%; CDFU above 90%.
• Bubble diameter. Conventional DAF 5-100 um with poor uniformity; conventional CFU 300-1000μm; CDFU 5-30 um with good uniformity.
• Residence time. Conventional DAF 15-40 min; conventional CFU under 3 min; CDFU 1-5 min.
• Chemical agents. Required for conventional DAF and CFU; not required for CDFU, which runs zero chemicals.
• Footprint. Conventional DAF serves as the 100% baseline; conventional CFU takes about 20%; CDFU occupies only 10%-20%.
• Sludge generation. Conventional DAF produces large amounts of oily sludge; conventional CFU generates scum; CDFU produces zero sludge with waste oil fully recovered.

Why Demulsification Is the Deciding Battlefield

The hardest part of treating oily wastewater is emulsified oil, because it is held in a stabilised interfacial film rather than floating freely. Conventional technologies produce large bubbles that cannot effectively capture tiny oil droplets, so the emulsified fraction passes through regardless of how long the contact time is extended. Extending residence time only buys a marginal gain at a disproportionate cost in tank volume.

Ultra-fine micro-nano bubbles of 5-30 micron diameter change the arithmetic. Their extremely high collision probability and surface tension let them rupture the protective film at the oil droplet interface quickly, which is physical demulsification in the literal sense. The strong centrifugal force generated by the cyclone then accelerates separation of the oil, gas and water phases, so the process is both fast and precise.

Engineering and Safety Differences

Beyond treatment efficiency, the three technologies differ in ways that matter during construction and operation.

• Compact skid-mounted design. Equipment footprint is only one third to one fifth of conventional systems, which suits space-constrained offshore platforms or plant retrofits.
• Fully enclosed operation. The system runs under pressure with no leakage of hazardous gases such as hydrogen sulfide, meeting the highest HSE environmental standards.
• Unattended operation. Integrated DCS/PLC fully automatic control supports remote monitoring, greatly reducing labour and O&M costs, and the SINOKLE control package is designed so that a single operator can supervise several trains at once.

What the Metrics Mean in Practice

Residence time is the parameter most often dismissed as a technical detail, and it is the one with the widest consequences. A process that needs 15-40 minutes of contact time must hold that volume of water at all times, which sets the tank size, the structural steel, the civil works and the land requirement. Reducing contact time to 1-5 minutes shrinks everything upstream and downstream of the separation vessel, including the pumps, the piping and the maintenance access around them.

Chemical demand matters for a different reason. Every kilogram of demulsifier or flocculant that enters the process leaves it as sludge, and oily sludge is typically classified as hazardous waste with a disposal cost attached. A process that runs zero chemicals does not simply save the reagent purchase price; it removes the disposal line entirely, along with the storage, dosing and handling equipment and the associated operator exposure.

Bubble diameter is the physical reason behind both effects. Uniform 5-30 um bubbles generate a far higher concentration of contact points per unit volume than 300-1000μm bubbles, so collision with fine oil droplets becomes likely rather than occasional. Tight distribution matters as much as small median size, because a few large bubbles do very little while consuming the same gas and energy.

Footprint follows from the same combination. When separation is fast and chemical conditioning is unnecessary, the vessel no longer needs to be oversized to absorb uncertainty, and a skid-mounted arrangement becomes practical. That is what allows the technology to be deployed on offshore platforms and inside existing plant boundaries where a conventional system would not fit.

Reading the Comparison Correctly

The table above points in one direction, but the practical decision needs one more step. Ask which fraction of the oil in a given stream is actually emulsified. Where the emulsified share is small and the water is not space-constrained, a simpler technology may be adequate. Where emulsification is severe, the plant is tight on land, or hazardous sludge disposal is a significant cost line, the parameters that separate these technologies become decisive.

The direction of travel, as SINOKLE sees it, is away from chemical stacking and inefficient settling toward purely physical, highly efficient and intelligent approaches. The metrics worth tracking are the ones that predict cost over a plant's lifetime rather than the ones that look best in a commissioning report.