CDFU Cyclonic Flotation: Where Ultrafine Bubbles Beat a Bigger Tank
The problem with flotation tanks is the tank
Dissolved air flotation works on a simple principle: dissolve gas into water under pressure, release it as microbubbles, let the bubbles attach to oil droplets and solids, and skim the load off the surface. It is effective, and it is slow. A conventional flotation cell needs a large surface area so that the rise velocity stays low enough for the bubbles to attach and for the froth to build, which means a large vessel, a long residence time and a large building to put it in.
The CDFU (Cyclonic Dissolved Gas Flotation Unit) attacks that constraint from two directions at once. Cyclonic centrifugal separation shortens the contact time required, and ultrafine bubbles improve the attachment efficiency, so the same duty is done in a fraction of the volume. The unit is a patented design developed by SINOKLE and integrates cyclone, coalescence, demulsification and microbubble generation in one vessel.
How the separation actually happens
Water enters the cyclone section, where rotation generates a centrifugal field. Oil droplets and solids, being less dense than water, migrate toward the core while the water turns. Into that same field, ultrafine bubbles are released, with an average diameter about 25 um. Small bubbles matter here for a physical reason: the smaller the bubble, the larger the surface area per unit volume and the lower the rise velocity, so the bubble has more time to meet a droplet and is less likely to break away from it on the way up.
Bubble and droplet collide, attach, and form a buoyant body lighter than water. That body rises and is removed as scum by the skimming arrangement at the surface. Suspended solids are captured the same way, either directly or by being enmeshed in the oil-bubble aggregate. The combination of centrifugal acceleration and fine bubbles raises the collision probability between bubbles and oil droplets, and it is that probability, not the residence time, that limits a conventional cell.
What the performance numbers are
The oil removal rate exceeds 90% and the suspended solids (SS) removal rate exceeds 85%. A single unit can treat up to 500 m3/h. System pressure drop is low, 0.05 MPa or less, which matters because it sets the duty on the upstream transfer pump and the wear on the whole train.
Read those figures in context. They are achieved without chemical addition, because demulsification is purely physical, and that is the difference that shows up downstream: no oily sludge is generated from coagulant or flocculant, so the secondary pollution problem and the sludge disposal cost both disappear. The process suits difficult industrial wastewater such as petrochemical and coking effluent, where the oil is partly emulsified and a chemical route would create more waste than it removes.
Skid-mounted, enclosed and why that is an operational argument
The skid-mounted arrangement occupies about one third of the footprint of conventional flotation equipment, is easy to install, and has a short construction period, which means it can be brought into service quickly — a real advantage in a retrofit where production cannot stop for long. Equipment operates enclosed and pressurised, which reduces volatile organic compound (VOC) emissions and improves site safety compared with an open cell.
Automation is part of the same package. The unit supports automatic operation and remote expert service, so labour cost falls and operational stability improves, which matters where the operating team is small or the site is remote. The technology also combines cyclonic separation, coalescence, demulsification and microbubble generation in one design, so it handles complex oily wastewater and emulsified oil with an adaptability that a single-mechanism device does not have.
Where it is used
Petrochemical, coking and electric desalting duties are the core applications. In electric desalting wastewater treatment the CDFU is combined with a high-efficiency coalescing oil remover to improve oil-water separation further: the coalescer grows small droplets into larger ones that the flotation stage can lift more easily, and the pair together cope with the surge loads that desalter effluent is known for.
One documented case is the Urumqi Petrochemical project, where the unit was applied to coking black water treatment. The treated water met the standards, which is worth more than a laboratory result because coking black water combines high oil, high phenol and high solids in a single stream.
Comparing it with what it replaces
Against conventional flotation equipment such as parallel-flow flotation or dissolved air flotation, three differences are consistent. Footprint: conventional cells need large open areas, while the compact skid suits space-constrained industrial sites. Treatment speed: the cyclonic field raises collision efficiency and shortens the time from inlet to separated phases. Environmental performance: no chemicals means no secondary pollution, and no chemical sludge to dewater and dispose of.
The honest limitations are worth stating as well. The first cost is higher than a simple flotation cell, and the technology is best applied where the oil is present as free and dispersed droplets; where the emulsion is tight and stabilised by surfactants, a coalescing stage ahead of it is not optional. It is also not a dissolved-pollutant solution — COD and colour remain the job of a downstream oxidation or biological step. The unit is a separation device, and it is a very good one when it is asked to do separation.