From Principle to Engineering Triumph: How SINOKLE’s Two-Stage CDFU + KFM Process Conquered Bangladesh’s HFO Power Plant Wastewater

In the field of industrial oily wastewater treatment, there is a universally recognized "hard nut to crack"—oily wastewater from heavy fuel oil (HFO) power plants. This type of wastewater features high oil content, severe emulsification, and a complex composition, making it extremely difficult for conventional processes to achieve stable discharge compliance within reasonable cost parameters. The project delivered by Shenzhen Clear Science & Technology Co., Ltd (SINOKLE) at an HFO power plant under Bangladesh's BR Powergen Limited serves as a textbook case study on how a high-concentration oily wastewater treatment process transitions from principles-level design to engineering implementation.

This article deconstructs the technical core of the project from two dimensions: equipment principles and process chain integration.

1. Why Must a Two-Stage Series Layout Be Adopted? — Process Logic from the Removal Curve

The influent oil content of the Bangladesh project is 2,000 mg/L, and the effluent target is ≤ 10 mg/L. A removal demand spanning this magnitude dictates that it is virtually impossible for a single-stage treatment device to accomplish the task independently.

The reason lies in the particle size distribution of the oil droplets. In high-concentration oily wastewater at 2,000 mg/L, floating oil (particle size > 100μm) often accounts for less than 30%, while a massive portion of the oil exists in the forms of dispersed oil (10-100μm) and emulsified oil (< 10μm). The design parameters of a single-stage flotation device—whether bubble size, hydraulic retention time, or surface loading—can only be optimized for a relatively narrow particle size range. If bubbles are made extremely fine to target emulsified oil, the removal efficiency for floating oil will drop. Conversely, if priority is given to capturing floating oil, a large amount of emulsified oil will escape.

A two-stage series layout essentially performs segment-by-segment treatment based on the particle size distribution: the primary stage targets large particle sizes and high throughput, while the secondary stage targets small particle sizes and high precision.

2. Primary CDFU Stage: "Coarse Separation" via Cyclonic Centrifugation + Large Bubbles

The core mission of the primary CDFU (Cyclonic Dissolved-gas Flotation Unit) is to rapidly bring the oil content down from the 2000 mg/L level to approximately 300-500 mg/L.

The dominant mechanism in this stage is cyclonic centrifugal separation. Wastewater enters the CDFU tangentially at a specific velocity, forming a high-speed cyclonic flow field inside the equipment. The differences in forces acting upon oil and water within the centrifugal force field are magnified—since the density of oil is slightly lower than that of water, its tendency to converge toward the center under centrifugal force is several orders of magnitude stronger than under static conditions. This process requires zero bubble participation and relies purely on hydrodynamics, yielding highly significant effects on floating oil and larger-particle dispersed oil.

Concurrently, the primary CDFU also releases bubbles, but the bubble size tends toward medium-to-large ranges. Coupled with the turbulent mixing of the cyclonic field, this drastically increases the bubble-to-oil droplet collision probability. Note that this stage pursues "throughput" rather than "precision"—in high-oil concentration zones, the main bulk of the oil is removed first to clear up concentration headroom for subsequent advanced treatment.

3. Secondary CDFU Stage: "Polishing Treatment" via Ultra-Micro Bubbles + Coalescence Demulsification

The operating zone of the secondary CDFU is entirely different, as it faces the residual finely dispersed oil and emulsified oil at concentrations of several hundred milligrams per liter.

The technological focus of this stage shifts to two mechanisms: ultra-micro bubble generation and coalescence demulsification.

The particle size of ultra-micro bubbles is in the range of tens of microns or even smaller. According to Stokes' law and bubble-oil droplet collision models, smaller bubbles possess larger specific surface areas and longer retention times in water, leading to a higher contact probability with microscopic oil droplets. While traditional DAF bubble sizes are typically in the 50-100μm range, the ultra-micro bubbles of the CDFU can achieve a significantly smaller magnitude, which is decisive for capturing emulsified oil droplets with particle sizes < 10μm.

The coalescence demulsification mechanism targets scenarios where "oil droplets are too stable for bubbles to adhere to." Emulsified oil remains stable because surfactant substances or charge layers on the droplet surfaces form a barrier. By altering the surface properties of the oil droplets, coalescence demulsification enables these fine oil droplets to merge and enlarge upon collision—once they grow to a certain size, bubbles can capture them effectively.

It is worth noting that coalescence demulsification is a physical process that does not rely on chemical agents. The significance of this feature regarding operational economics and hazardous waste reduction has been repeatedly mentioned previously and will not be reiterated here.

4. KFM Activated Media Filter: The Final Line of Defense from Tens to Single Digits

The oil content in the secondary CDFU effluent can typically be controlled down to the tens of milligrams per liter level. However, to drop from tens to below 10 mg/L, advanced filtration is required.

The core of the KFM Activated Media Filter does not rely on simple mechanical screening. If it relied solely on screening to achieve micron-level filtration precision, the pressure drop would be massive, and the backwashing frequency would be extremely high. Instead, it relies on the surface adsorption and deep-bed interception of the Activated Filter Media.

The Activated Filter Media features a high specific surface area and specific surface oleophilicity. When the water stream carrying trace oil droplets flows through the media bed, the oil droplets are adsorbed onto the media surface under Van der Waals forces and hydrophobic interactions, gradually forming an oil film around the media particles. The driving force behind this process is physical adsorption rather than mechanical entrapment; hence, high-precision oil removal can be achieved under a relatively low pressure drop.

Simultaneously, the KFM Filter adopts a deep-bed filtration mode rather than surface filtration—the entire depth of the filter bed participates in interception, providing a dirt-holding capacity far higher than that of surface-filtration cartridges. This results in long backwash cycles and simple operation and maintenance, making it highly suitable for continuous industrial on-site operations.

5. Overall Design Philosophy of the Process Chain

Reviewing the entire process chain: Primary CDFU (Coarse Separation, dominated by cyclonic centrifugation) → Secondary CDFU (Polishing Treatment, dominated by ultra-micro bubbles + coalescence demulsification) → KFM Filter (Advanced Adsorption, intercepted by Activated Filter Media).

The design philosophy of this chain can be summarized as "segmented force application, step-by-step load reduction, and physical methods first." Rather than attempting to resolve all issues within a single piece of equipment, each stage of equipment is allowed to do what it does best. Prioritizing physical mechanisms over chemical agents lowers both operational costs and environmental burdens.

The actual operational data of the Bangladesh project—influent at 2,000 mg/L, effluent < 10 mg/L, and stable operation to date—serves as the ultimate endorsement for this design philosophy.