CDFU or Conventional DAF: Picking a Flotation Route for Emulsified Oil
Two technologies, one confusing name
CDFU and DAF both remove oil by making bubbles lift droplets to a surface where they can be skimmed. Everything else — bubble generation, bubble size, tank geometry, chemical demand, retention time — differs, and those differences decide which projects each technology suits. CDFU stands for compact cyclonic dissolved gas flotation. DAF is the older dissolved air flotation process that most refineries and power plants already operate.
Where the bubbles come from
The CDFU unit makes its bubbles in a multi-stage cyclonic dissolved gas pump built specifically for the duty. Bubbles typically measure 5-30 um in diameter, with a median diameter D50 of about 25 um, and the size distribution stays tight. A narrow band matters more than a small average: predictable rise velocity means the tank can be short.
Conventional DAF relies on a dissolved gas pump or pressurised dissolution to release air from a saturated stream. Bubble diameters span 5-100 um and the distribution is broad, so the tank has to be sized for the slowest fraction in the population rather than the average one.
How contact happens
In CDFU the wastewater mixes with dissolved gas water and enters the flotation tank tangentially. Centrifugal force drives droplets outward while micro-bubbles rise, and the two motions multiply the collision probability between oil droplets and bubbles. Attachment follows, and the resulting aggregate is light enough to float in a hurry. Separation is fast because the geometry forces contact instead of waiting for it.
In DAF, micro-bubbles are introduced into the bulk liquid and act as carriers. They adsorb suspended particles or flocs, the aggregate becomes less dense than water, and buoyancy lifts it to the surface as scum. The mechanism works, but it depends on bubbles finding the particles, which is why DAF trains are long and why they usually need coagulation upstream.
Numbers side by side
A thermal power plant project shows what the compact layout buys. Two CDFU stages followed by filters, rated at 5 m3/h, sit on one skid with a total footprint of about 16 m2 — a plot a conventional DAF train of the same duty would not fit into.
- Droplet band addressed: CDFU removes emulsified oil across 0.1-20 um with single-stage oil removal above 90%, while DAF achieves roughly 60%-85% single-stage removal on conventional oily wastewater.
- Hydraulic retention time: under 5 minutes for CDFU, against 15-40 minutes for DAF.
- Footprint: CDFU occupies about 1/3 of what conventional DAF needs.
- Chemicals: none for CDFU; coagulant or flotation agent for DAF.
Retention time is the real cost
A 15-40 minute retention time is not a design preference, it is a tank. Every minute of holdup means steel, concrete, plot space and the pumping energy needed to push water through it. Cutting holdup to under 5 minutes shrinks all four at once, and it changes how the unit behaves when the feed moves: a small inventory responds within minutes, a large one takes most of an hour to reflect a change at the inlet.
The sludge nobody budgets for
Chemical dosing is the second cost that hides. Coagulants and flotation agents do improve separation in a DAF train, and they also convert part of the oil into a sludge that has to be dewatered, hauled and disposed of as hazardous waste. That is a recurring liability with a gate fee attached. CDFU separates physically, so the oil leaves as oil that can be recovered and returned to the front end of the plant rather than leaving as waste that has to be paid for twice.
Stability under shock load
Oily wastewater rarely arrives at a steady concentration. Tank cleaning, slug discharges and upset conditions push the inlet up by factors rather than percentages. A flotation train with a large inventory and a chemical dose set for the average load has little room to absorb that, because the dose is wrong the moment the load changes. Fully automatic closed operation and short holdup let CDFU ride the excursion, and the closed design also removes the open basin, the odour and the manual scum scraping that come with it.
Where conventional DAF still makes sense
Onshore plants and stations where plot space, weight and structural load are not constrained, treating conventional oily wastewater at modest emulsification, remain a legitimate DAF market. If the equipment is already installed and performing to permit, replacing it is hard to defend on capital alone. The case for CDFU is strongest at the margins: new builds, capacity expansions, tightening effluent limits, and retrofits where the existing tank cannot grow.
Matching the technology to the site
- Offshore platforms and topsides. Weight and deck area are the binding constraints, and the small footprint of a skid-mounted CDFU decides the argument.
- Oilfields with heavily emulsified produced water. The 0.1-20 um droplet band is exactly where chemical-free flotation earns its keep.
- Land-constrained plants. A retrofit inside an existing battery limit rarely has room for a half-hour retention tank.
- Onshore stations with space. Conventional DAF can be the lower-capital choice when the water is easy and the permit is loose.
Three questions before signing
What is the droplet size distribution of the real feed, measured rather than assumed? What single-stage removal is guaranteed at design flow, and at what inlet concentration? What does the train consume each year in chemicals, sludge disposal and power? Those answers usually settle the matter before anyone compares capital numbers, and they are the only comparison that survives contact with an operating plant.