Inside the Dushanzi Case: How Two-Stage CDFU Cut Oil From 1013 mg/L to 17.4 mg/L

Electric desalting cut water is one of those streams that looks manageable on paper and turns difficult in practice. Its composition shifts, its emulsification is severe, and the oil droplets are small enough that conventional equipment cannot pull them out reliably. In 2019, PetroChina Dushanzi Petrochemical Company worked with Shenzhen SINOKLE Technology Co., Ltd. to install a two-stage CDFU (cyclonic dissolved gas flotation) train on this stream. The retrofit cleared a bottleneck that had persisted for years. Below is what the project involved and what the operating data showed.

The Water Quality Problem in Plain Terms

Dushanzi's electric desalting wastewater carries the classic combination of high oil content and high emulsification. Alongside free oil, vigorous mixing has generated a large amount of emulsified oil. Oil droplet size is mostly concentrated in the 10-20 μm range, and the droplet density sits extremely close to that of water, which is exactly the condition that defeats gravity separation.

Relying on gravity settling alone meant an extremely long retention time of 50-70 hours, and even then effluent quality fluctuated enough to threaten the strict requirements of the downstream biological treatment units. The traditional large-tank settling layout brought additional problems: a wide footprint, a high risk of volatile organic compound (VOCs) escape, and heavy operation and maintenance intensity. The project needed an efficient, intensive pretreatment step with no chemical addition.

Why the Chemical Route Was Dropped

Rather than refine the conventional "chemical demulsification plus settling" sequence, the project adopted two-stage CDFU cyclonic dissolved gas flotation as the core solution. Three mechanisms do the work.

Micro-nano bubble demulsification

Through an efficient dissolved gas system, CDFU generates a large number of micro-nano bubbles with a diameter of only 5-30 μm. These bubbles carry extremely high specific surface area and surface energy, so they actively capture and wrap small oil droplets and suspended solids in the water, forming bubble-oil droplet aggregates whose overall density is much lower than water. Float-up separation then happens at millisecond level.

Cyclonic centrifugal strengthening

A purpose-built cyclone field inside the vessel gives the fluid a powerful centrifugal force. That field sharply amplifies the density differences among oil, water and solids, which lifts separation efficiency and makes the unit particularly suitable for highly stable emulsions.

Full-process pure physical treatment

No demulsifier and no flocculant enter the process at any point. Secondary pollution from chemical agents is eliminated at the source, and the high reagent costs and hazardous waste sludge disposal problems that go with them disappear as well.

The Process Configuration

On the basis of those principles, SINOKLE tailored a two-stage series process: the first-stage CDFU removes most of the oil, and the second-stage CDFU performs deep fine treatment. The whole system uses a modular skid-mounted design with a footprint of only 11 m × 3 m × 5.2 m, which greatly reduces land use and allows rapid installation and commissioning.

Measured Results After Long-Term Operation

The project was officially put into operation in 2019 with a designed treatment capacity of 100 m³/h. After sustained stable operation, all system indicators performed excellently. The core monitoring data are as follows.

• Oil content: inlet 1013 mg/L, first-stage CDFU outlet 118 mg/L, second-stage CDFU outlet 17.4 mg/L
• Suspended solids SS: inlet 502 mg/L, first-stage CDFU outlet 97.5 mg/L, second-stage CDFU outlet 28.7 mg/L

After two-stage CDFU treatment the effluent is clear and transparent, and both oil content and suspended solids sit far below the design standards. That fully satisfies the inlet requirements of the subsequent treatment units and resolves the long-standing problem of cut water exceeding standards.

What the Case Demonstrates

The Dushanzi result is a strong verification of SINOKLE CDFU technology under extreme operating conditions. It shows that pure physical separation can handle highly emulsified oily wastewater, and it provides an efficient, green and replicable pretreatment model for the refining industry.

Reading the Numbers as a Design Engineer

Three things stand out in the data. The first is the size of the step between stages. The first-stage CDFU already removes roughly nine tenths of the incoming oil, and suspended solids fall by a similar proportion, which means the second stage is polishing a much easier stream rather than repeating the hard work. That is what keeps the two-stage configuration from doubling the operating burden.

The second is the absolute level reached. An outlet oil content of 17.4 mg/L is not merely compliant; it leaves margin. Downstream units see a stable feed even when the upstream desalting operation changes crude slate or throughput, and that margin is what prevents the recurring excursion events that make a biological train expensive to run.

The third is the footprint. A skid measuring 11 m × 3 m × 5.2 m replaced a settling arrangement with a retention time of 50-70 hours. For brownfield refineries, the land and the civil works that are no longer needed are often worth as much as the treatment performance itself.

What the Case Suggests for Similar Plants

For plants facing similar cut water, the useful lesson is not the specific equipment layout but the sequence: remove the bulk of the oil first, then polish to a low residual, and let physical mechanisms do the work that chemistry used to be paid for. That order keeps reagent consumption at zero, keeps hazardous sludge out of the flow sheet, and leaves the operator with a process that responds predictably when the feed changes.