Electrodesalting Draw-Off Water: The Small Refinery Stream That Sets the Tone for the Whole WWTP
In petroleum refining, the salt and water entrained in crude oil — existing in dissolved or emulsified form — must be washed out by the electrical desalting process before the oil reaches the distillation units. The water drained off that step, the electrodesalting draw-off water, accounts for only about 3% of a refinery's total wastewater volume, but its quality characteristics have a significant impact on the downstream wastewater treatment system, and it must undergo professional pretreatment before being incorporated into the comprehensive wastewater plant.
Why a stream this small matters this much
Draw-off water carries what the desalter just pulled out of the crude: emulsified oil, fine solids and dissolved salts, all in a matrix that resists ordinary gravity separation. Sent straight into the comprehensive system, it coats primary treatment equipment, poisons biological stages and shifts the whole plant's operating point. Pretreating it at the source is cheaper than apologising for it downstream — a principle every refinery rediscovers the expensive way.
The desalting step itself explains the stream's character. Fresh water washes the crude, the electrostatic field coalesces the dispersed droplets, and the heavy phase — now carrying salt, emulsified droplets and the solids that the field rejected — drains off continuously. Because the wash water is recirculated to conserve fresh water and chemicals, whatever emulsifies in the first pass concentrates in the draw-off, which is why the stream's oil is so finely divided and why simple holding tanks do so little for it.
Why the stream keeps getting harder
Two pressures are working against the treatment field at once. First, crude oil quality shows a trend of becoming heavier and inferior; to enhance recovery, water injection operations and the amount of chemical aids continue to rise, significantly increasing the complexity of the wastewater composition. Second, to improve the comprehensive utilisation rate of crude oil, refineries re-refine preliminarily treated heavy dirty oil, which further aggravates crude oil emulsification and causes the electrodesalting draw-off water quality to deteriorate continuously. The stream that was once an inconvenience is now the hardest emulsion on the site.
The consequences show up downstream in a fixed order: first the oil separators lose efficiency as droplets never coalesce, then the coagulation chemistry consumes itself on the emulsion, and finally the biological stage sees residual organics it was never designed to handle. By the time the symptoms reach the effluent outfall, the cause is three units and one budget cycle upstream — which is why source treatment of the draw-off stream pays for itself faster than almost any other wastewater upgrade on a refinery site.
The breakthrough: cyclonic dissolved air flotation, compacted
One refining enterprise faced the worst of it — high residue content up to 40%, frequent shock conditions, and a traditional process that simply failed. The technical response was the compact cyclonic dissolved air flotation unit: the Cyclonic Dissolved Gas Flotation Unit, abbreviated as CDFU. This equipment integrates three core technologies — centrifugal separation, dissolved air flotation and ultrafine bubble generation — into one vessel, forming a synergistic treatment effect and achieving efficient separation of oily wastewater and suspended solids under exactly the shock conditions that break conventional trains.
The Xinjiang configuration
A petrochemical enterprise in Xinjiang built an "oil remover + two-stage CDFU" combined process. The oil remover and the first-stage CDFU unit apply physical demulsification technology to remove the vast majority of the floating oil, and the recovered oil is recycled for resource utilisation rather than entering the waste stream. The second-stage CDFU unit doses only a trace amount of flocculant at its end and achieves deep purification through ultrafine bubble adsorption. The treatment results: effluent quality significantly improved, meeting the influent standards of the wastewater plant, with the operational stability of the system greatly enhanced — and the recirculated oil turned a disposal cost back into product.
What the innovation buys
- Significantly enhanced shock resistance: the process system was designed for extreme conditions with high residue content (40%), ensuring stable operation where the traditional process failed.
- Improved resource utilisation: oil-water separation technology recovers the dirty oil, reducing the amount of hazardous waste generated.
- Optimised operating costs: reduced chemical usage, elimination of the oil drying device, and lower land occupation and treatment costs.
For refining and chemical enterprises facing large crude oil quality fluctuations and high treatment loads, this configuration offers a new answer for oily wastewater treatment — and a demonstration case for the industry's green transformation. The broader lesson is worth stating plainly: stabilising a wastewater system rarely comes from adding another treatment unit downstream. It comes from choosing pretreatment that can absorb the feed's worst day, which is what the two-stage CDFU arrangement does for the stream that used to destabilise everything behind it.
The Xinjiang train has since become the configuration others copy when their own desalter effluent starts dictating terms to the rest of the plant — proof that source treatment beats downstream apology.