Depot Drain Water Carries Gasoline: Recovering Product While Meeting Reuse Limits

At a light-oil storage depot, draining bottom water is routine maintenance with an awkward side effect: product leaves with the water. Gasoline goes out with the drain, every time the tanks are bottoms-sampled or dewatered. That gives the depot two problems wearing one coat. The water has to be treated well enough to be reused as wash water, and the oil carried out with it is high-quality light product worth recovering. A process that solves only the first one is throwing money into a treatment tank.

Why this stream is not ordinary oily wastewater

Unlike general refinery effluent, depot drain water is genuinely dual in nature. Environmentally, it cannot be discharged directly and must reach a quality suitable for reuse; at a depot that treated water goes back into tank and equipment washing, cutting discharge volume and fresh-water intake at the same time. Economically, the oil in it is gasoline, and not the heavy end either. Light fractions are valuable. Losing them to a wastewater system is a real line item, not a rounding error.

So the ideal drain-water process has to do both: bring the water to a reusable standard and recover the oil in a form that can go back into the product stream. That requirement constrains the mechanism. Chemical demulsification would contaminate the recovered oil, so the separation has to be robust enough to break the emulsion physically while leaving the oil quality intact.

How the two units divide the work

SINOKLE's answer runs two units in relay. CDFU, the swirl dissolved-air flotation (CDFU) unit, is the primary separator. It is a proprietary technology independently developed by SINOKLE, at an internationally advanced level, and was the first domestic next-generation high-efficiency, compact, enclosed, pressurized flotation technology. Its strength is packing several functions into one vessel: swirl centrifugation stratifies oil and water rapidly, microbubbles generated in situ act like countless small grippers catching oil droplets, and dissolved-air flotation supplies a steady separating force. Working together, they remove sludge oil, emulsified oil and suspended solids quickly, and the free and emulsified oil that dominate drain water is largely eliminated at this stage.

KHC, the coalescing oil separator, is the precision checkpoint. After the primary unit has handled the bulk, a small quantity of fine droplets remains, and the separator's job is the fine work: it lets those droplets merge and grow on the coalescing medium and then rise out of the water. Oil-water separation is pushed closer to completion, which is what keeps effluent compliant for reuse on a bad day as well as a good one.

Enclosure is not a detail

Light oil is volatile and low in density, which adds a safety dimension that heavier streams do not have. SINOKLE designed the process at system level around that fact: the train runs fully enclosed. Enclosure pays twice. Oil vapour does not enter the working environment, so the safety risk is reduced at source rather than managed with gas detectors; and the waste gas is routed to the vapour-recovery system, where volatile light fractions are captured instead of lost. For light-oil wastewater, enclosure is a safety baseline and an economic measure at the same time.

What the depot gets back

The returns are straightforward. On water, effluent is reused for depot washing, cutting discharge and saving fresh water. On oil, the separated high-grade light oil goes straight back into use, converting a loss into recovery and creating measurable economic value. On operation, the entire process is physical separation with no chemicals added, so there is no chemical cost and no secondary pollution, and the fully enclosed system stays safe and controllable.

What to specify before you buy

Two details decide whether the recovered oil is worth anything, and both belong in the enquiry rather than the commissioning report. The first is materials and sealing on the recovered-oil side: gasoline will find every threaded joint and every non-metallic seal in a warm skid, so the enclosure rating and the vapour-recovery tie-in have to be set by the depot's own safety case. The second is the drain-water collection practice upstream. If bottom water is drained fast and in large slugs, even a good separator sees a feed it was not sized for, and the fix is usually an equalisation volume rather than a larger unit. Get those two right and the process chain does what it claims; get them wrong and no amount of downstream equipment compensates.

Endorsement has followed performance. According to public information, the process has been widely applied to drain-water treatment at crude-oil tank farms and at product-oil and heavy-oil power-plant tank farms belonging to CNPC, Sinopec, China Power and other operators. Leading energy companies apply unusually strict requirements to both environmental compliance and operational reliability; the fact that the train is deployed and continues to run in their tank-farm service is the strongest reference available.

Drain water does not have to sit permanently on the environmental to-do list. With CDFU swirl dissolved-air flotation plus the KHC coalescing oil separator, a purely physical chain brings compliant reuse and product recovery together: water meets the reuse standard, oil goes back to the product stream, and no chemicals enter either. Choose the right process and oily drain water stops being a cost centre and starts behaving like a resource.