Why Heavy-Oil Tank Draining Resists Conventional Flotation, and How a Two-Stage CDFU Train Fixes It
Heavy-oil depots rarely produce a predictable effluent. The stream that causes most of the trouble comes from tank draining, and it behaves nothing like the free-oil runoff a simple separator was designed for. Heavy oil is viscous and chemically messy; every transfer, pump pass and drain cycle shears it again, breaking it into fine droplets that quickly acquire a stable interfacial film. What leaves the tank is an oil-in-water emulsion that will sit there indefinitely. Quiescent gravity separation does almost nothing to it, and conventional flotation sized for free oil barely improves the picture.
The load makes it worse. Incoming oil content swings with the tank and with the operating schedule, so any plant designed around an average figure will drift out of specification the moment the feed spikes. Depots that responded by adding more tanks, more dosing points and more chemicals usually ended up with a larger operating bill and the same uncertainty. The discharge requirement that governs most of these projects is oil content ≤10 mg/L, and holding that figure through a fluctuating feed is the actual test.
Where the separation step was redesigned
SINOKLE approached the problem through core equipment rather than through chemistry. Sustained independent R&D produced the CDFU, a patented compact enclosed pressurized flotation unit. Compact is the operative word: the idea was never to build a larger flotation tank, but to put several separation mechanisms into one pressure vessel and let them reinforce each other.
Three of those mechanisms carry the work. Cyclonic centrifugal separation establishes a swirl field inside the closed chamber, so the density difference between oil, water and solids starts doing useful work before any bubble is involved, and the stream begins to stratify immediately. Ultrafine bubble generation, working with dissolved air flotation, floods the chamber with micro-bubbles that serve as carriers for oil droplets on their way to the surface. Coalescence and demulsification go after the emulsion itself: coalescence lets fine droplets merge into something heavy enough to rise, while demulsification destroys the interfacial film that was keeping them apart. Any one mechanism leaves one of those jobs undone; the combination closes the gap.
The practical consequence is that waste oil, emulsified oil and suspended solids are separated out of the same wastewater stream quickly, in one vessel, without a chemical programme wrapped around it.
Why two stages beat one large unit
A single CDFU removes a great deal of oil. Splitting the duty across two units in series is what makes the result repeatable. The first CDFU carries the bulk of the load, removing large quantities of free waste oil and suspended solids and dropping the system load before it reaches anything sensitive. The second CDFU sees a much thinner stream and can spend its whole capacity on emulsified oil and the fine droplets that escaped the first pass. By the time water leaves the second unit there is margin left over, and margin is exactly what a fluctuating feed demands.
Downstream sits an activated-media filter. Its job is unglamorous: intercept the fine residual pollutants the flotation stages missed and give the plant a final line of defence before the outfall. Together with the two-stage split, it turns a marginal design into one that absorbs surprises rather than reporting them.
The part that pays for itself
Waste oil from both CDFU stages is not a disposal problem. It is collected in a dedicated waste-oil collection tank and recovered for reuse. Because nothing in the route requires a demulsifier or a flocculant, the recovered oil is not contaminated with treatment chemicals and keeps its value. And because the separation is entirely physical, the plant never generates oily sludge, the hazardous-waste stream that keeps chemical routes expensive long after the capital cost has been forgotten.
Set against a conventional chemical train, four things change at once. Effluent holds the oil content ≤10 mg/L requirement through feed swings. Chemical purchasing and dosing disappear from the operating routine. Oily-sludge hazardous waste never appears. Separated oil becomes a recovered product instead of a disposal line item.
Reading the results on site
Commissioning tells you quickly whether the emulsion was the real constraint. Plants that were previously chasing the limit with chemical dosing usually see two things change at once: the effluent stops tracking the feed so closely, and the operating routine loses its most annoying task. Where the improvement is smaller than expected, the cause is almost always upstream rather than in the flotation train, whether that is a drain cycle that dumps far more oil than the design assumed, or a solids load that was never measured during sampling.
It is also worth being honest about what physical separation cannot do. Removing oil and suspended solids does not reduce dissolved organic load, and a depot that needs COD removal as well will need an oxidation step in front of or behind the flotation train. The two-stage CDFU design is a strong answer to one clearly defined problem, and it works best when it is specified for that problem rather than as a general-purpose upgrade.
Where the boundaries sit
This route is built for highly emulsified, oil-rich draining water. It is not a substitute for biological treatment where dissolved organics dominate, and a depot with a large sanitary or stormwater component will still need those streams handled separately. Water-quality assessment has to come first. If the emulsion is the binding constraint, the two-stage CDFU design addresses it directly; if the binding constraint is something else, more flotation capacity will not help. That judgement, not the equipment catalogue, is what decides whether the project works.