Emulsified Oil in Oily Wastewater: Why Ultrafine-Bubble CDFU Outperforms Conventional DAF

Walk onto most oily-wastewater sites and the conversation starts with the visible layer: the free oil that can be skimmed off a tank or scooped out of a channel. That job is tedious, not hard. The real difficulty sits underneath the surface, in the emulsified oil - droplets so finely dispersed that the water looks clear while the laboratory numbers say something else entirely.

Emulsified oil has been shattered into the micron and sub-micron range and is held apart by a film of surfactant or solid particles. What results is a stable dispersion that gravity settling and a simple oil-separation tank will not touch.

Three reasons emulsified oil resists separation

Size. Free oil droplets run from tens to hundreds of microns and rise readily in an oil-separation tank or a basic flotation cell. Emulsified oil is typically 1-5 microns, and sometimes below 1 micron. At that scale Brownian motion outruns the buoyancy-driven rise velocity, so the droplet has almost no tendency to travel upward at all.

Stability. The active substances coating each droplet act like a protective shell that keeps droplets from colliding and coalescing. The traditional remedy is to dose demulsifiers and coagulants to chemically break that shell. The trouble is the dosing window: too much agent and you produce large volumes of water-laden waste oil with high disposal costs; too little and demulsification is incomplete, so the effluent misses its target anyway.

Operating conditions. Take electric desalting wastewater from a refinery. Influent oil content can reach tens of thousands of milligrams per litre, and the stream carries dispersed oil, emulsified oil, entrained catalyst fines and other organics together. Conventional dissolved air flotation (DAF) needs a residence time of 15 to 40 minutes, occupies a large equipment footprint and draws significant energy, yet its removal efficiency for emulsified oil is often only 60% to 85%. What it leaves behind becomes the next stage's burden.

Where the usual answers stop short

Three families of solution dominate the market, and each has a ceiling.

• Chemical treatment. Coagulants such as PAC and PAM are dosed ahead of flotation or sedimentation. The route is well proven, but chemicals are consumed continuously, chemical sludge is produced in quantity, and performance on heavily emulsified water stays limited.
• Conventional DAF. Bubbles of 100-300 microns attach to oil droplets and carry them up. Large bubbles collide poorly with micron-scale droplets, so chemicals usually have to assist; without them, oil removal falls away sharply.
• Membrane filtration. Precision is genuinely high, but membrane fouling in oily service is severe, flux declines quickly, cleaning is frequent, and operation and maintenance costs are steep.

For years the industry has compromised between precision and cost. Want high precision? Accept chemicals, membranes and extra process stages. Want low cost? Accept a weaker effluent target. The question worth asking is whether both can be had at once.

CDFU: replacing large bubbles with ultrafine ones

SINOKLE has spent more than a decade on oily wastewater and answers that question by changing two variables: replace large bubbles with ultrafine bubbles, and replace chemical agents with purely physical separation. The core product, the CDFU cyclonic dissolved-air flotation unit, combines cyclonic centrifugal separation with dissolved-air flotation. It generates microbubbles only 5-30 microns in diameter, with a median around 25 microns - a scale that happens to collide efficiently with, and adhere to, 1-10 micron emulsified oil droplets. Conventional flotation bubbles, often over one hundred microns, tend to drift past emulsified oil without capturing it at all.

The second change matters as much as the first. The CDFU unit achieves demulsification and oil removal by purely physical means, with no chemical agent introduced at any point in the flow. No chemicals means no chemical sludge, and the recovered oil is water-free enough to be reused directly. Single-stage oil removal exceeds 90%, the residence time is only 1 to 5 minutes, and the equipment footprint is a fraction of that of a conventional DAF unit.

KHC coalescer: a first physical barrier

Where emulsification is especially stubborn, SINOKLE places the KHC high-efficiency coalescing oil separator upstream as an outpost. Its special fibrous packing captures tiny oil droplets so that they grow at the fibre junctions into large droplets and separate on their own, with coalescing precision down to 0.1 microns. The combined coalescing plus CDFU train applies two physical barriers in series - coalesce first, then float - stripping emulsified oil away layer by layer.

What tightening limits mean for the choice

Discharge standards for oily wastewater have tightened steadily, and in some key river basins and coastal areas the limits now sit close to surface-water requirements. The margin for error has gone; even a small shortfall is no longer acceptable. Against that backdrop, purely physical treatment has been winning acceptance with operators. From several refining and petrochemical projects of PetroChina and Sinopec in China to overseas oilfields in Congo, Africa, and across duties from electric desalting wastewater to fracturing flowback fluid, the approach has been demonstrated in service rather than only on a specification sheet.

Oily wastewater treatment does not have to rest on dosing chemicals. Ultrafine bubbles, a coalescing bed and a compact vessel can deliver high efficiency, low operating cost and zero secondary pollution at the same time - which, for most plants, is exactly the combination they have been waiting for.