Tank Farm Wastewater: Why Coalescing Plus CDFU Replaces Chemical Demulsification

In petroleum storage and transportation, the tank farm is the junction where refining output meets the market. It is also a reliable source of oily wastewater. Most of it comes from tank dewatering: whenever the water that has accumulated at the bottom of a tank is drained on schedule, some oil travels with it. For a medium-sized petrochemical enterprise, the annual volume from this one operation runs into tens of thousands of cubic metres. Discharged without treatment, that is both an environmental problem and a steady loss of product.

Four characteristics that shape the design

Oil concentration moves constantly. Different tanks, different batches and different seasons produce markedly different oil content, spanning a few hundred milligrams per litre up to tens of thousands of milligrams per litre. A process tuned to one point in that range will misbehave elsewhere.

Emulsification is significant. Crude oil and refined products sit in contact with water for long periods, and temperature change plus agitation does the rest. The result is a stable emulsion of fine droplets that ordinary gravity separation does not touch.

Composition is mixed. Besides oil, the water carries suspended solids, silt and assorted impurities, so a single technology rarely gets the stream to standard on its own.

The discharge limit keeps tightening. As standards have been revised, the permitted oil level has fallen below 10 mg/L, a level that conventional processes find increasingly hard to hold.

What the chemical route costs

The traditional answer was chemical demulsification plus flotation plus filtration. It works, but it comes with a bill. Demulsifiers, flocculants and similar chemicals have to be bought continuously, which drives operating cost. Chemical residues can create secondary pollution of their own. Adaptability to water quality swings is poor, so effluent quality moves around with the influent. And the recovered oil comes out degraded, which makes reuse difficult even when the separation itself succeeds.

The SINOKLE purely physical route

For tank farm oily wastewater, SINOKLE Technology (Shenzhen) offers a solution that relies on physics rather than chemistry, built around a coalescing pressure deoiler combined with CDFU cyclonic dissolved-air flotation.

The coalescing pressure deoiler uses specially designed coalescing media to make the tiny oil droplets dispersed in water gradually merge into larger beads, forming an oil layer that is far easier to separate afterwards. It is particularly effective on high-concentration oily wastewater, which is exactly where a tank farm starts.

Behind it, the CDFU cyclonic dissolved-air flotation unit generates ultra-fine bubbles of 5-30 microns that capture the remaining emulsified droplets for deep oil removal. Because the two stages work on different droplet populations, the combination reaches water that neither would handle alone.

What it delivers

• Purely physical treatment. No chemical is added at any point, so secondary pollution is avoided at the source.
• Recoverable waste oil. Separated oil is of good quality and can be returned to use rather than disposed of.
• High treatment efficiency. Oil removal above 95%, with effluent oil below 10 mg/L.
• Low operating cost. No continuous chemical purchasing, which lowers long-term expense rather than shifting it elsewhere.
• Strong adaptability. Insensitive to water quality fluctuation, running stably without constant adjustment.

Sequencing the two stages

Order matters more than it first appears. The coalescing pressure deoiler goes first because it thrives on concentration: the heavier the oil load, the more effectively the media can bring droplets together. Handing it pre-diluted water would waste that advantage. CDFU then takes over the fine fraction that coalescing cannot reach, using ultra-fine bubbles to lift droplets too small to rise on their own.

Before any equipment is specified, the streams have to be characterised. Oil content and droplet size distribution differ between tank dewatering, contaminated drainage and maintenance washdown, and a design built on an average figure rather than the worst-case stream will underperform exactly when it is needed. Sampling across seasons helps as well, since temperature shifts change both emulsion stability and separation speed.

Recovery economics deserve the same attention as effluent quality. Tank farm oil is often good enough to return to the refinery once separated cleanly, which means the separation method has to avoid degrading it. That is the second reason chemistry is left out: demulsifiers and flocculants do not only add operating cost, they lower the quality of the oil that comes back out.

A Sichuan tank farm project

A petrochemical enterprise in Sichuan operates a crude oil storage tank farm whose oily wastewater is treated at a design capacity of 50 m3/h. SINOKLE supplied a combined process of coalescing pressure deoiler plus CDFU cyclonic dissolved-air flotation, and moved from contract signature to equipment shipment in two months.

Once in operation, the results held: effluent quality consistently meets standard, and waste oil recovery and refining were achieved alongside it, which makes the project pay on both the environmental and the economic side. Tank farm wastewater treatment is an unavoidable obligation for the storage and transportation sector; the practical question is whether it is treated as a cost or as a recovery opportunity.