Refining Wastewater That Resists Biology: How CDOF Restores Biodegradability

A refinery wastewater stream is easy to describe and hard to treat. It carries hydrocarbons at concentrations that overwhelm biology, it carries salt that suppresses the same biology, it arrives warm and alkaline, and its flow and strength swing with every change in crude slate and operating mode. Retrofitting another biological stage rarely fixes this. The more productive question is how to break the refractory fraction first, so that what reaches the biology is something the biology can actually handle.

Six properties that define the problem

• Organic richness. High concentrations of hydrocarbon and hydrocarbon derivatives resist natural biodegradation, which is why chemical oxygen demand (COD) stays high.
• Mixed composition. Organic compounds, inorganic substances and heavy metals appear together, and each needs attention of its own if ecosystem and health risks are to be avoided.
• Elevated oxygen demand indicators. Both biochemical oxygen demand (BOD) and COD run high, which is simply a statement of heavy organic load.
• Salt. Solvents and additives used in processing raise the salt concentration, and that inhibits the microorganisms in the biological units.
• Heat and alkalinity. Warm discharge plus process chemicals leave the stream typically alkaline, mostly in the range of pH 7.5 to 8.5.
• Fluctuation. Quality and quantity move a great deal, so whatever is installed has to absorb those swings without losing effluent quality.

What those properties turn into inside a treatment train

Translate the six characteristics into operating failures and the picture is familiar. COD from many chemical processes can reach tens of thousands of mg/L, far past what direct biological treatment will accept. The combination and concentration of organics, inorganics and heavy metals varies so widely that a single recipe of residence time and nutrient dosing cannot cover it. Heavy metals and organic solvents are toxic or inhibitory to microorganisms, so even a well-designed biological system loses metabolic activity. Deep colour and high turbidity survive conventional biological treatment and call for dedicated process steps.

A process built to do the breaking, not the polishing

The response from SINOKLE was to run oxidation and separation in one vessel rather than in two stages of a train. Drawing on its independently developed CDOFCDOF technology, the company proposed a process scheme aimed at degrading organic matter efficiently and lifting biodegradability substantially. The unit is a CDOF- Cyclonic Dissolved Ozone Flotation unit and it is patented. Inside, three mechanisms work at once: multi-stage catalytic ozone oxidation, hydrodynamic cavitation and cyclonic dissolved gas flotation.

The claim of being first of its kind in China rests on that integration — an oxidation reactor and a flotation separator that share the same hydraulic path, so the oxidant does not spend its strength before the bubbles have done their work. What it removes in one pass includes petroleum substances, suspended solids, colloids, chemical oxygen demand (COD) and bacteria.

What sits inside the battery limit

• An oxygen generation unit, which sets the oxygen purity and therefore the ozone yield downstream.
• An ozone generation unit.
• A high-efficiency ozone dosing unit, responsible for getting the gas into the liquid as fine bubbles rather than as slugs.
• A multi-stage catalytic reaction unit, where the refractory fraction is attacked.
• A cyclonic dissolved gas flotation unit, where the oxidised material and the suspended solids are lifted out.

What a refinery sample showed

Laboratory testing on a refining wastewater sample gave a concrete curve rather than a claim. At an ozone dose of 150 mg/L, effluent COD fell to 200.8 mg/L, with a total COD removal of 649.2 mg/L and an ozone dosing ratio of 0.23. Colour tracked the same direction: it dropped gradually as the dose went up, and at 150 mg/L the sample was essentially decolourised.

Two things in that data matter more than the headline removal. First, the ozone dosing ratio of 0.23 says how much oxidant is consumed per unit of COD removed, and that ratio is what determines the operating cost per cubic metre. Second, and more important for the plant, the process removed COD while significantly improving biodegradability — the effluent is now something a downstream biological train can finish, which is the whole point of putting oxidation in front.

Comparison against conventional ozone oxidation

Simple ozone oxidation puts gas into a tank and hopes for contact. The integrated route differs in three practical ways: contact intensity, because cavitation and cyclonic flow keep breaking bubbles; oxidant economy, because dosing is engineered rather than assumed; and footprint, because oxidation and flotation occupy one skid instead of two structures. For a revamp, that last point often decides the project before the chemistry does.

Boundaries worth stating

This is a front-end process, not a discharge guarantee on its own. It is designed to be followed by biological treatment, and the numbers it produces are about making that downstream step viable. Where the constraint is inorganic salinity rather than organics, the oxidation duty shrinks and the economics change. Where flow is very large and COD moderate, a cheaper conventional step may be sufficient, and the integrated unit is best reserved for the streams that actually resist biology.

For a refinery weighing options, the useful test is simple: run the sample, plot COD and colour against ozone dose, and read off the point where biodegradability stops improving. That point, not the maximum dose the oxidiser can deliver, is where the design should land.