Refinery Wastewater: Cutting COD and Protecting Membranes with CDOF Ozone Catalysis

Two Problems Refinery Operators Recognise Immediately

Refinery wastewater is a hard stream to run. Aromatic hydrocarbons, phenols and petroleum substances make up a refractory organic load, and chemical oxygen demand (COD) has a habit of drifting out of compliance just when the shift changes. Downstream, ultrafiltration and reverse osmosis elements foul earlier than the design case predicted, cleaning frequency climbs, water production falls, and O&M spend follows. Membrane cost and reagent cost are the two lines operators most want to cut.

The older answer, biological treatment followed by Fenton oxidation and then membrane filtration, attacks both problems but pays for it. Fenton chemistry needs iron salts and hydrogen peroxide in quantity, and the iron-bearing sludge it produces is hazardous waste. Ozone is gentler, yet a traditional contact tower needs a residence time usually above 90 minutes, ozone utilisation is low, and the O3/COD weight ratio sits somewhere between 2 and 4. The energy bill follows the dosing ratio.

What CDOF Does Differently

CDOF is not a single piece of equipment with a better impeller. SINOKLE designed it as a sealed, pressurised reactor that carries multi-stage ozone catalytic oxidation, swirl dissolved air flotation and high-efficiency coagulation inside one shell, so that oxidation and separation happen in the same volume of water at the same time.

On the oxidation side the unit runs homogeneous and heterogeneous catalysis together: a liquid catalyst is dosed while a patented reactor stage holds a solid catalyst. Ozone breaks down on the catalyst surface into hydroxyl radicals, and it is their oxidation potential of 2.80 V that does the work. Measured against conventional ozone oxidation, oxidation efficiency comes out 2-5 times higher.

Mass transfer is handled by patented jet aeration that shears ozone gas into ultra-fine bubbles. The gas-liquid contact area grows sharply, ozone dissolves quickly and reacts with pollutants instead of venting, and the gas-liquid interface is continuously renewed. Inside the reactor a strong swirl keeps the catalyst fluidised, so catalyst, pollutant and micro-bubble collide continuously rather than settling into channels. That combination pulls effective residence time down to within 15 minutes, and the unit volume is about 70% smaller than a traditional ozone tower for the same duty.

The Membrane Protection Half of the Story

Oxidation that leaves its intermediates suspended in the water has not solved the operator's problem. In CDOF the flotation stage runs concurrently with the reaction. Demulsified oil droplets, suspended solids and colloids, together with reaction intermediates, are lifted to the surface by swirl centrifugal force and bubble buoyancy, and are removed as scum. Water leaving the unit carries very low suspended solids and colloid content, which removes membrane fouling risk at its source rather than managing it downstream. Membrane service life extends accordingly.

Control Loops and the Real Cost Curve

Automation is where operating cost is won. The unit runs a DCS/PLC fully automatic control system that monitors ozone concentration in the tail gas and effluent ORP online, then feeds the signal back to adjust ozone dosing in real time. Compliance stays stable while dosing is optimised rather than padded. Ozone utilisation reaches 99.98% or higher, and the O3/COD dosing ratio can be brought down to 0.5-1.2, a fraction of what a conventional ozone stage consumes.

The process also runs with essentially zero chemical dosing beyond a trace of catalyst. No flocculant means no chemical sludge, and sludge volume falls by 90% or more. For a refinery under dual-carbon pressure, that is pollution reduction and carbon reduction in the same move.

Where It Has Already Run

SINOKLE has deployed CDOF at a number of large refining and petrochemical sites, including Sinopec and PetroChina installations, on duties that include high-salinity and highly emulsified streams. The relevant point is that these are plant performance figures under feedstock conditions that change, not laboratory results under fixed conditions. That is the standard a refinery specification should hold any advanced oxidation supplier to.

Reading the Value Beyond Compliance

Judged narrowly, CDOF is an upgrade to a wastewater process. Judged as an investment, it moves three cost lines at once. Chemical and energy spend falls directly through utilisation and control. Membrane replacement and cleaning maintenance fall indirectly because the upstream barrier holds. Skid mounting and high automation reduce operating labour and the safety exposure that comes with manual intervention on a pressurised oxidation system. And the environmental line, near-zero reagent with hazardous waste minimised and secondary pollution avoided, is what aligns the project with corporate dual-carbon targets rather than merely with a discharge permit.

For a refinery where COD compliance is uncertain and membranes foul early, the sequence worth evaluating is oxidation and separation inside one reactor, with the control system paying for itself through dosing discipline. That is the specific claim SINOKLE makes with CDOF, and the installations above are the evidence base behind it. The practical next step for an operator is a pilot on the actual stream rather than a bench test on a synthetic sample, because the refractory fraction of refinery wastewater changes with crude slate and the ozone demand follows it. Ozone demand measured on the real feedstock, over a full week rather than a single shift, is the number that determines whether the lifecycle saving materialises.