Oily Wastewater Treatment Solution for Cogeneration Power Plants

This question touches on a genuine pain point in wastewater treatment for the thermal power industry. To state the conclusion first: yes, there is. For low- to medium-concentration oily wastewater, coalescing separation technology has already proven, through real-world cases, its combined advantages in footprint, energy consumption, operation and maintenance, and effluent stability. A project at a certain back-pressure cogeneration power plant in Shaanxi Province is a worthwhile case to dissect.

Why is the oily wastewater from thermal power plants so difficult to treat?

The oily wastewater of a cogeneration power plant mainly comes from cooling water for the steam turbines and generators, drainage from the lubricating oil system, and workshop floor wash water. Oil exists in water in three forms: free oil (diameter >100 um), dispersed oil (10-100 um) and emulsified oil (<10 um). Traditional oil separators can only handle free oil; it is the dispersed and emulsified oil that makes up the bulk and is genuinely troublesome.

So what exactly differentiates the three mainstream processes?

At present, industrial oily wastewater treatment mainly follows three technical routes:

The first is dissolved air flotation (DAF). The principle is to release large numbers of micro-bubbles into the water; the bubbles adsorb oil droplets and float them to the surface to be skimmed off. The advantages are mature technology and high treatment capacity; the drawbacks are equally clear - chemicals must be added (coagulants, flocculants), operating costs are not low, the resulting scum is hazardous waste requiring secondary disposal, and the equipment footprint is large. In space-constrained power plants this is often a serious weakness.

The second is the biochemical method. It uses microorganisms to degrade organic matter and is suitable for high-strength organic wastewater. However, the oily wastewater of a thermal power plant consists mainly of mineral oil, which has poor biodegradability, and oil inhibits the microorganisms. To adopt biochemical treatment, demulsification and oil-separation pre-treatment must first be added upstream, making the whole process long and complex to manage.

The third is coalescing separation, and it is the technology we focus on today.

So how exactly does coalescing separation work?

SINOKLE's KHC coalescing oil-water separator is a representative product of this approach. The core principle is simple: the equipment is filled internally with specially modified coalescing fibre media that are oleophilic and hydrophobic. As oily wastewater flows through the coalescing layer, micron-scale dispersed oil droplets are captured, attached to and gathered by the fibres - multiple small droplets merge into larger ones on the material surface, and once the droplet diameter grows to the millimetre scale, buoyancy is enough to detach them from the media and rapidly float them up to the oil-collection zone at the top of the separator, while the purified water is discharged from the bottom.

Schematic diagram of the internal structure of the coalescing oil-water separator

Throughout the entire process there is no chemical dosing, no heating and no secondary hazardous waste - it is purely physical separation. A single unit integrates coalescing separation and automatic oil discharge, occupying a footprint equivalent to only a small horizontal storage tank.

Returning to the Shaanxi case, the data speak for themselves.

This project has a treatment capacity of 5 m3/h, and the influent is typical cogeneration oily wastewater - an emulsion mixture of lubricating oil and turbine oil, with the oil concentration fluctuating in the low-to-medium range. A primary filter intercepts large suspended solids upstream for pre-treatment, and the KHC coalescing oil-water separator performs the core separation downstream.

Oily wastewater treatment unit for cogeneration power plants

Effluent data after commissioning: the oil content is stably <5 mg/L. For comparison, the national first-class discharge limit is 10 mg/L, and the strictest local standard is 5 mg/L. This unit meets the tightest limit directly. Since its commissioning in September 2022, it has run continuously for three years with no coalescing-media compaction or efficiency decline.

Comparison of influent and effluent treatment performance

Which scenarios are suitable for choosing coalescing separation?

Frankly, no process is universal. Based on this case and experience from similar projects, coalescing separation is best suited to conditions where: the treatment volume is small to moderate (1-20 m3/h), the oil concentration is low to medium (<=500 mg/L), the oil is mainly mineral oil such as lubricating/turbine oil, the requirement for effluent stability is high, and the site is constrained. If your project meets three or more of these, coalescing separation is very likely the option with the best overall cost-performance.

For scenarios with very high oil concentrations or containing large amounts of heavy oil/tar, strengthened pre-treatment is needed ahead of coalescing separation, or a different process combination should be considered directly. Technology selection is always a matter of analysing the specific scenario - there is no one-size-fits-all solution.