Four Flotation Types, One Selection Framework: DAF, CAF, ECF, NAF and Where CDFU Fits

Flotation is one of the core separation technologies in industrial water treatment. Its mechanism is simple to state: fine bubbles act as carriers and adhere to suspended particles, oil droplets and colloids in the water, forming bubble-particle complexes with a density lower than water. Those complexes float quickly to the surface under buoyancy, form scum, and are removed mechanically. High treatment efficiency, a small footprint and flexible operation have made flotation a mainstay for oily wastewater, high-turbidity wastewater and refractory wastewater.

The Four Mainstream Types

Classification follows the way bubbles are generated, and each method suits a different water quality scenario.

• Pressurized dissolved air flotation (DAF). High-pressure dissolved gas is released as micro-bubbles under pressure reduction, producing bubbles of 10-100μm. Applied to oilfield produced water, refinery wastewater and food processing wastewater. Mature technology with high de-oiling efficiency and strong stability.
• Cavitation air flotation (CAF). Micro-bubbles generated by mechanical aeration, 50-200μm in size. Suited to low-oil wastewater and the pre-treatment stage. Low energy consumption, simple equipment and easy maintenance.
• Electrolytic flotation (ECF). Gas produced directly by electrolytic reaction, with bubbles of 50-150μm. Used for slightly polluted water and heavy-metal wastewater. No aeration system needed, with uniform bubble size.
• Nano flotation (NAF). Electrolysis with super-saturated dissolved gas producing nano-bubbles below 1μm. Applied to highly difficult emulsified wastewater and fine water treatment. High pollutant capture efficiency with low energy consumption.

Reading the four together, the pattern is clear: bubble size falls as the difficulty of the wastewater rises, and the equipment required to produce smaller bubbles becomes more specialised.

Where Flotation Is Applied

Industrial oily wastewater treatment is the core application scenario.

• Oilfield water treatment. Produced water, fracturing flowback fluid and offshore platform wastewater, where flotation is the core treatment process.
• Refining industry. Electric desalting wastewater, tank farm black water and sour water, all characterised by high oil content and high emulsification.
• Other fields. Food processing, papermaking, electroplating heavy-metal wastewater and landfill leachate.

Development Trends Worth Tracking

Flotation is evolving toward high efficiency, intelligence, modularisation and customisation. Four directions stand out.

• Ultra-fine bubble technology. Bubble size reduced to 5-30 um, improving collision and adhesion efficiency with emulsified oil droplets.
• Material upgrades. Key components use corrosion-resistant materials such as duplex stainless steel and titanium alloy, suitable for high-salinity and highly corrosive conditions including offshore platforms and high-salinity oilfield water.
• Process integration. Combining swirl centrifugation with dissolved air flotation achieves the dual effect of physical demulsification plus high-efficiency separation.
• Intelligent control. Algorithms adjust dissolved gas pressure and reflux ratio in real time, cutting energy consumption by more than 22%, while remote O&M enables unattended operation.

CDFU Swirl Dissolved Air Flotation

CDFU (Swirl Dissolved Air Flotation unit) is a technology iteration built on the traditional CFU. Its core innovation is the deep integration of swirl centrifugal separation and dissolved air flotation, designed specifically for difficult oily wastewater in oilfields and refineries.

Bubble quality and separation performance

Bubble size is precisely controlled at 5-30 um with a median around 25 um, which matches 1-10 um emulsified oil droplets and increases collision and adhesion efficiency threefold. Single-stage de-oiling efficiency exceeds 90% and suspended solids removal exceeds 85%. The swirl centrifugal unit optimises the flow field through CFD to achieve preliminary separation of oil droplets from water, while the dissolved air flotation unit deeply removes residual emulsified oil, forming a double physical barrier.

Environmental and cost performance

No chemical agents are added throughout the process, eliminating oily sludge generation at the source. Separated waste oil can be directly recovered and reused, reducing hazardous waste disposal costs by more than 30%. The design is dominated by static equipment with no mechanical slag scraping device, which reduces failure points and extends the maintenance cycle to 6 months while cutting manual maintenance costs by 50%.

Skid mounting and materials

The compact skid-mounted design reduces equipment volume by more than 50% compared with traditional flotation, which suits space-constrained offshore platforms and oilfield blocks and shortens the deployment period by 40%. Key components use corrosion-resistant materials such as duplex stainless steel and titanium alloy, withstanding high salinity, high corrosion and high temperature below 80 C.

Control and effluent quality

An IoT intelligent control system supports remote monitoring, parameter adjustment and fault warning, enabling unattended operation. Shock load resistance reaches 110%, so the unit stays stably compliant when influent water quality fluctuates. Effluent oil content can be controlled at 10 mg/L or less and suspended solids at 20 mg/L or less, meeting stringent domestic and international standards such as IMO discharge requirements.

Core Performance Indicators

The parameters below come from the CDFU (Swirl Dissolved Air Flotation unit) product platform from SINOKLE and reflect the range over which the unit can be specified. Note that nano flotation reaches bubble sizes of <1μm, which is where the classification stops and the most difficult emulsified streams begin.

• Treatment capacity. 500-5,000 m3/d single stage, up to a maximum of 4 00m³/h, suitable for small, medium and large oilfield and refinery projects.
• Influent quality. Oil content 1,000 mg/L or less, suspended solids 200 mg/L or less, compatible with high-oil, highly emulsified wastewater.
• Effluent quality. Oil content 10 mg/L or less, suspended solids 20 mg/L or less, meeting stringent reinjection water and sea discharge standards.
• Operating energy consumption. Reduced by 40% compared with traditional flotation.
• Treatment time. 1-3 minutes per single stage.

Three Operating Cases

• Produced water treatment at an oilfield in Africa. Treatment capacity 5,000 m3/d with influent oil content fluctuating greatly and severe emulsification. Effluent oil content stays stably below 10 mg/L, far better than industry standards, with stable operation and maintenance costs reduced by 60%.
• Tank farm black water at a petrochemical enterprise in Xinjiang. Treatment capacity 3,600 m3/d using single-stage CDFU treatment. Shock load resistance reaches 110%, operating energy consumption is reduced by 40%, and effluent oil content stays below 15 mg/L, meeting refinery wastewater discharge standards.
• PetroChina petrochemical electric desalting wastewater. Treatment capacity 2,400 m3/d using a two-stage CDFU combined process. Effluent oil content is 10 mg/L or less and suspended solids 5 mg/L or less, achieving wastewater reuse and compliant discharge.

Matching the Technology to the Scenario

Four scenarios call for SINOKLE CDFU in particular. Offshore oilfield platforms with space constraints, high corrosion and unattended operation suit the skid-mounted design and corrosion-resistant materials. Highly emulsified oily wastewater such as refinery sour water and fracturing flowback fluid benefits from pure-physical demulsification. High-salinity water treatment for oilfield reinjection relies on duplex steel or titanium alloy materials. Retrofits of ageing projects avoid large-scale civil works because the modular quick-install design shortens the retrofit cycle and rapidly boosts treatment capacity.

In process design and project bidding, the technology also functions as a technical highlight rather than a plain treatment device. Its innovative swirl plus flotation process, pure-physical environmental performance and intelligent O&M capability strengthen the technical depth and feasibility of a proposal, which helps a project win on merit and then operate stably over the long term.