Oil Depot Oily Wastewater: Matching Every Oil Form to the Right Separation Step

Storage, transfer and loading operations at an oil depot generate oily water continuously. It arrives from tank cleaning, floor washing, rainfall across the loading bays and equipment condensate, and it is not ordinary sewage. Floating oil, emulsified oil, suspended solids and trace organics all ride along. Discharged untreated, it smothers the surface of receiving waters, blocks oxygen transfer, damages soil and groundwater over the long term, and throws away oil that could have been recovered. Getting the process right is therefore both an environmental obligation and a cost question.

Start from the form the oil takes

Depot water behaves differently from refinery or oilfield effluent: the oil load fluctuates hard, the pollutant morphology is mixed, and the water quality is unstable. Those three characteristics should drive the design.

Difficulty tracks droplet size in a fairly clean progression. Floating oil is coarser than 50 µm, separates quickly on standing and usually accounts for more than 70% of the oil present, which makes it the cheapest fraction to recover. Dispersed oil sits in the 20-50 µm range, stays suspended and only stratifies after long retention. Emulsified oil is finer than 20 µm, held in suspension by surface tension with oil and water effectively locked together; no amount of standing will split it, and it is the fraction that decides whether a plant complies. Mixed in with all of this are sand, rust and colloidal matter, and the COD value moves around enough to punish any process that assumes a steady feed.

Graded physical removal instead of one big unit

For a clean, stable stream such as aviation-fuel depot water, the sensible approach is multi-stage physical oil removal with a small amount of demulsification assistance and a filtration polish, and no biological section at all. Each stage removes one fraction and hands a narrower problem to the next.

Pretreatment: keep debris out and recover the free oil

Water first passes a mechanical bar screen to catch leaves, packaging and coarse grit, then enters a sedimentation tank where gravity removes rust and heavy suspended solids. The core of this stage is coarse oil removal in an oil separator, most commonly the inclined-plate type (CPI). Working on the shallow-settling principle, the plate pack shortens the stratification distance dramatically, so large droplets reach the surface quickly and are scraped off to recovery while settled sludge is drawn from the bottom on a cycle.

This step typically strips out more than 90% of the floating oil and leaves only dispersed and emulsified material behind. It is cheap, needs no chemicals, is simple to run, and belongs in every depot flowsheet regardless of what follows.

Core removal: break the emulsion

Gravity cannot touch what leaves the separator, so the next stage combines coalescing with dissolved air flotation (DAF).

In the coalescing step, water flows through a specialised medium whose surface attracts fine droplets. Droplets adsorb, accumulate and merge into larger beads that rise fast enough to separate. No chemical is involved, which is why the step suits high-oil streams and stays predictable.

The CDFU unit then handles the fine fraction. It merges cyclone separation with dissolved air flotation and generates 5-30 µm bubbles, using nitrogen rather than air, to capture what coalescing missed. Bubble-droplet agglomerates rise as scum and are scraped away automatically; the removal efficiency reaches over 90%. Nitrogen flotation is deliberate, since it avoids oxidising the recovered oil and keeps the product saleable while removing a secondary-pollution pathway. Compared with conventional flotation, the unit is smaller, faster and consumes no chemicals.

Where a stream is severely emulsified, operators add a coagulation demulsification step with PAC and PAM to destabilise the emulsion and let the fines aggregate into settleable or floatable flocs. It is a fallback rather than a default.

Polish: filtration and adsorption

By this point oil and suspended solids are essentially compliant, and because the stream carries little dissolved organic load, biological treatment would add cost without benefit. A quartz-sand filter followed by fine activated carbon is enough. Sand holds back residual fine solids and colloids; the carbon micropore structure adsorbs trace emulsified oil, residual hydrocarbons and off-colour or odour compounds, smoothing out the fluctuations that would otherwise cause the occasional exceedance.

Treated water meets the discharge requirements of the petroleum refining industry wastewater engineering code and can be sent to drain or reused for floor washing, equipment cooling and irrigation. Recovered oil, sludge and grit are collected, dewatered and disposed of through licensed channels.

What the combination buys

Four things, in practice. The flow is short and starts up fast, with no biomass to cultivate and no sludge to acclimate. Effluent is stable because each oil form is addressed by the stage best suited to it, and modest feed variation is absorbed. O&M cost stays low, since the train is essentially physical, chemicals appear only in extreme emulsification events, and there is no biological sludge. Safety and environmental performance improve as well: no odour, no risk of a crashed biological population, high-purity recovered oil and no secondary pollution, which suits the control standards an aviation-fuel depot has to meet.

The lesson generalises beyond aviation fuel. Depot water rewards a stripped-back physical train matched to droplet size. Adding a biological section because a refinery next door has one is the expensive mistake.