Oily Wastewater at Heavy Oil Power Plants: A Two-Stage Physical Route Worth Costing

Where the Water Comes From

At a heavy oil power station, oily wastewater is rarely a single stream. It is oil depot drainage, floor washing water, oil pump house drainage and assorted equipment leaks, arriving intermittently and often together. The resulting mixture carries high oil content in the 50-500 mg/L range, a high proportion of emulsified oil, large amounts of suspended solids with SS in the 50-300 mg/L band, high viscosity and wide quality swings between one event and the next.

Untreated, that water costs three ways: environmental non-compliance, corrosion of downstream equipment, and lost recoverable oil. Gravity settling and chemical flocculation, the conventional answers, are inefficient on the emulsified fraction, generate sludge that has to be disposed of, and occupy land a power plant usually does not have.

The Physical Route

Two unit operations, run in series, produce a purely physical train.

CDFU: cyclonic dissolved gas flotation

The CDFU unit integrates cyclonic centrifugal separation with dissolved gas flotation. High-pressure dissolved gas generates 5-30 μm ultrafine bubbles that mix with the wastewater. Inside the swirl field those bubbles collide with and attach to oil droplets and suspended solids, forming a float that rises and is discharged. Treatment time is 1-5 minutes. Single-stage oil removal is above 90%, emulsified oil removal is above 90%, and no chemical is added anywhere in the stage.

KFM: active filter media filter

The KFM unit uses a modified silicate active filter media with a negatively charged surface and a molecular sieve structure, arranged in a graded particle size design: stage III coarse filtration followed by stage II fine filtration. Residual oil and fine particles are adsorbed in depth. The media resists fouling and holds its performance over a long service life. Backwash cycle is 8-12 hours, effluent oil ≤6 mg/L, SS ≤2 mg/L, and median particle size ≤1.5 μm.

The Two-Stage Flow

Stage one puts the wastewater through two-stage CDFU. The swirl field drives bubble-to-droplet attachment and separates dispersed oil, emulsified oil and SS quickly, with effluent oil ≤30 mg/L and SS ≤30 mg/L, no chemicals and a much smaller sludge stream. Stage two sends that effluent to the KFM filter, which removes what is left until the water meets discharge limits or reuse requirements such as boiler make-up.

System integration is skid-mounted with fully automatic control over IoT and AI monitoring. Capacity is flexible up to 500 m3/h per unit, and the footprint is about one-fifth of a conventional train, the number that usually decides whether a retrofit is possible at an operating station.

What the Numbers Buy

• Efficiency: CDFU short retention time plus KFM fine filtration delivers overall COD removal above 85%, at a treatment speed 3-8 times that of conventional methods, including on high-viscosity, highly emulsified water.
• Cost: zero chemicals, CNY 0.375 per ton, about 35% lower operating cost, and low maintenance because there are no wearing parts in the separation path.
• Compliance: no secondary pollution, no sludge stream, and effluent aligned with GB 8978-1996.
• Stability: tolerance of pH 3-10, fully enclosed operation with no odour leakage.
• Footprint: modular skid mounting, easy installation, suited to space-constrained plants.

Sierra Leone: 43.65 MW, 120 m3 a Day

At the 43.65 MW heavy oil power station built for the New Tonkolili iron ore mine in Sierra Leone, SINOKLE supplied a CDFU + KFM system treating 120 m3 of oily wastewater per day. The plant connected its first unit to the grid in June 2025. The commissioning window overlapped with the rainy season, and logistics into the site were the harder constraint. The system still delivered clear effluent and compliant discharge, and operating costs came in 40% lower.

Oil depot drainage at other heavy oil power plants is the same problem in a smaller package, which is why the same configuration travels well between sites.

Where the Train Needs Adjustment

Two conditions change the design. Free oil slugs, such as a tank drain released all at once, should be caught by an upstream interceptor, because a flotation cell is a separation device and not a storage buffer. And where the receiving water limit for ammonia or dissolved organics is tight, the physical train is not the last stage; it is the front end of a biological or oxidative one.

A third condition is seasonal. Intake temperature and oil viscosity move together, and a unit commissioned in warm weather will show a different retention behaviour in winter, so the operating envelope should be written against the coldest month rather than the design day.

Costing the Route Honestly

A two-stage physical train is not the cheapest thing to buy. It is usually the cheapest thing to run, and the difference matters over a station life measured in decades. Chemical dosing disappears from the consumables budget, sludge haulage disappears from the waste contract, and the maintenance scope narrows to pumps, valves and a backwash sequence. Against that, the capital cost sits in two vessels and a control system rather than in a single flocculation tank, and the footprint saved is land a power station can use for something else.

The comparison only becomes clear when the whole operating cost is counted: reagent, sludge disposal, downtime during reagent shortages, and the labour attached to a manual flocculation regime. Plants that compare purchase price alone almost always choose the conventional route and then spend the next ten years paying for it.