Two-Stage CDFU in Series Plus KFM Deep Filtration: Inside the Core Process Chain of SINOKLE's Bangladesh Heavy Fuel Oil Power Plant Wastewater Project
Industrial oily wastewater treatment has one universally acknowledged"hard nut to crack"——oily wastewater from heavy fuel oil power stations. This wastewater is high in oil, heavily emulsified and complex in composition, and conventional processes struggle to meet discharge limits consistently at a reasonable cost. The project SINOKLE delivered at a heavy fuel oil power station in BangladeshBR powergen limitedis a textbook sample for observing how a high-concentration oily wastewater treatment process moves from principle-level design all the way to engineering implementation.
This article dissects the technical core of the project from two angles: equipment principles and the way the process chain fits together.
1. Why Two Stages in Series Are Essential——Reading the process logic from the removal curve
The influent oil content of the Bangladesh project is2000mg/L, and the effluent target is no more than10mg/L. A removal requirement spanning this range makes it virtually impossible for any single-stage unit to do the job on its own.
The reason lies in the size distribution of the oil droplets.2000mg/LIn high-concentration oily wastewater at this level, free oil (droplet size>100μm) usually accounts for less than30%, while a large share of the oil exists as dispersed oil (10-100μm) and emulsified oil (<10μm). The design parameters of a single-stage flotation unit,——whether bubble size, residence time or surface loading,——can only be optimized for a relatively narrow droplet size band. If bubbles are made extremely fine in order to capture emulsified oil, free oil removal efficiency drops; conversely, if free oil is prioritized, large amounts of emulsified oil escape.
Two stages in series are essentially a way of treating the droplet size distribution in segments: stage one targets large droplets at high throughput; stage two targets small droplets at high precision.
2.CDFUStage One: Cyclonic Centrifugation+and Large Bubbles for"Coarse Separation"
The first-stageCDFUhas one core task: to bring the oil content down rapidly from the2000mg/Llevel to300-500mg/L。
The dominant mechanism at this stage is cyclonic centrifugal separation. The wastewater enters theCDFUtangentially at a certain velocity, creating a high-speed swirling field inside the unit. The difference in the forces acting on oil and water within the centrifugal field is amplified:——oil is slightly less dense than water, and under centrifugal force its tendency to converge toward the center is several orders of magnitude stronger than under quiescent conditions. This process needs no bubbles at all and relies purely on fluid mechanics, making it highly effective on free oil and on larger dispersed oil droplets.
At the same time, the first-stageCDFUalso releases bubbles, but these tend to be medium to large; combined with the turbulent mixing of the cyclonic field, they greatly increase the probability of collision between bubbles andoil droplets. Note that what this stage pursues is not"precision", but"throughput"——: in the high oil concentration range, remove the bulk of the oil first and free up concentration headroom for the downstream polishing steps.
3.CDFUStage Two: Ultra-Fine Bubbles+and Coalescence Demulsification for"Polishing"
The second-stageCDFUworks in a completely different range:——it faces the residual finely dispersed oil and emulsified oil left at concentrations of a few hundred milligrams per liter.
The technical focus of this stage shifts to two mechanisms: ultra-fine bubble generation and coalescence demulsification.
Ultra-fine bubbles are tens of microns in diameter or even smaller. According to Stokes' law and bubble-to--oil droplet collision models, the smaller the bubble, the larger its specific surface area and the longer it remains in the water, and the higher its probability of contacting fine oil droplets. ConventionalDAFunits typically produce bubbles on the order of50-100μm, whereas theCDFUultra-fine bubbles reach a considerably smaller order of magnitude, which is decisive for capturing emulsified oil droplets below10μmin size.
The coalescence demulsification mechanism, meanwhile, addresses situations where"the oil droplets are too stable for bubbles to adhere to". Emulsified oil is stable because a layer of surface-active substances or a charge layer on the droplet surface forms a barrier. Coalescence demulsification alters the surface properties of the droplets so that fine droplets can merge and grow when they collide;——once they reach a certain size, bubbles can capture them effectively.
It is worth pointing out that coalescence demulsification is a physical process and does not rely on chemical agents. Its significance for process economics and hazardous waste reduction has already been mentioned repeatedly and needs no further elaboration.
4.KFMActive Filter Media Filter: the Last Line of Defense From Tens to Single Digits
The second-stageCDFUeffluent oil content can normally be held to a few tens of milligrams per liter. But to go from tens down to below, deep filtration is required.
KFMThe core of the active filter media filter is not simple mechanical sieving.——If it relied on sieving, achieving micron-level filtration precision would produce a very large pressure drop and require very frequent backwashing.——Instead it relies on the surface adsorption and depth retention offered by the active filter media.
The active filter media has a high specific surface area and a particular surface oleophilicity. As water carrying trace oil droplets flows through the media bed, the droplets are adsorbed onto the media surface by van der Waals forces and hydrophobic interaction, gradually forming an oil film around the media particles. Because the driving force here is physical adsorption rather than mechanical interception, high-precision oil removal is achieved at a relatively low pressure drop.
At the same time, theKFMuses a depth filtration mode rather than surface filtration:——the entire depth of the filter bed takes part in retention, giving it a far higher dirt-holding capacity than surface filtration cartridges. Backwash intervals are long and operation and maintenance are simple, which suits continuous operation on industrial sites.
5. The Overall Design Philosophy of the Process Chain
Looking back over the whole chain: stage oneCDFU(coarse separation, dominated by cyclonic centrifugation),→ stage twoCDFU(polishing, dominated by ultra-fine bubbles+and coalescence demulsification),→ KFM(deep adsorption, retention by active filter media).
The design philosophy of this chain can be summed up as"apply force in stages, reduce the load step by step, and give priority to physical methods"——: do not try to solve every problem inside a single piece of equipment, but let each stage do what it does best; and favor physical mechanisms over chemical agents to lower operating costs and environmental burden.
The actual operating data from the Bangladesh project,——influent2000mg/L, effluent<10mg/L, and stable operation to this day,——is the best endorsement of this design philosophy.
