Desalting Wastewater: The Refinery Stream That Overwhelms Conventional Treatment Trains

Crude oil arrives at the gate carrying salts it picked up underground or in transport - sodium chloride, magnesium chloride, calcium chloride among them. Left in place, those salts scale heat-exchanger and heating-furnace tubes as water evaporates, degrade heat transfer, raise flow resistance and, at the extreme, block tubes or burn through tube walls. Metal elements that slip into heavy distillates or residual oil poison catalysts and drag down the quality of downstream feedstocks and finished products. Desalting is therefore mandatory, and desalting produces a wastewater stream with a personality of its own.

How the stream is generated

Spec limits drive the design: salt content must be controlled below 5 mg/L in general service, and refineries equipped with residue hydrotreating or heavy-oil catalytic cracking need crude below 3 mg/L. Those numbers explain why every refinery treats desalting as a core process step rather than an optional one - a few points of salt decide catalyst life downstream. To hit those numbers, about 5% clean fresh water is injected into the crude along with a demulsifier that breaks the emulsified film around suspended salt particles. Inside the electric desalting unit, the field coalesces fine water droplets into larger drops, gravity settling completes the oil-water split, and the wastewater drains off the unit. Depending on crude properties and wash-water rate, that discharge carries roughly 200 mg/L of petroleum pollutants at a conductivity near 2000 μS/cm.

What makes it hard to treat

• COD swings across 3000-25000 mg/L - high and unstable;
• Oil content of 200-10000 mg/L under normal operation, exceeding 10% and reaching 50% in abnormal cases;
• Suspended solids (SS) held at 800-4500 mg/L - discharged raw into a treatment system, it would shock its operation severely;
• Continuous discharge with flow fluctuations of 2%-15%, requiring uninterrupted treatment to keep the desalting unit running stably;
• Weekly unit backwashing spikes COD, petroleum and suspended solids, hitting downstream facilities with periodic shocks.

The combination is what bites. Any single parameter - high COD, high oil, high SS - has an established answer. Desalting wastewater presents all of them at once, on a stream that never stops flowing and changes character with every crude slate change and every wash-water adjustment. A settling tank sized for Tuesday's average fails on Thursday's backwash spike; a cyclone tuned for a mid-range feed drifts out of spec when the oil emulsion tightens. Treatment design therefore has to start from the worst sustained case, not the design average.

The incumbent options, honestly assessed

Gravity settling pretreats the stream in a separation or settling tank before the main system, but tends to pass the shock downstream. Hydrocyclones apply centrifugal force yet struggle to hold stable effluent when oil content lurches. Coalescing separation enlarges oil droplets to accelerate separation and absorbs shock well, but weakens badly against severely emulsified feed. The mismatch is arithmetic: typical refinery and petrochemical wastewater plant inlet specifications call for COD below 1500 mg/L, petroleum below 200 mg/L and SS below 250 mg/L, while many actual desalting discharges exceed every one of those limits. With chemical additives proliferating in oil production and electric desalting units running continuously, conventional processes no longer guarantee continuous compliance.

The SINOKLE answer

Against these characteristics, SINOKLE paired a high-efficiency coalescing pressure oil remover with a cyclonic dissolved-air flotation unit, integrating coalescing separation with cyclonic technology. A double-flow-channel design and computational fluid dynamics optimize the flow-field distribution, so the train exploits both cyclonic centrifugal force and dissolved-air flotation. The result: single-stage oil removal above 90%, capture of oil droplets as small as 0.3 μm, and retention time of just 1-5 minutes. Equipment volume and weight run at only 1/8-1/3 of a conventional dissolved-air flotation system. Dual tangential feed boosts swirl intensity and separation efficiency; operation adapts to hydraulic load, oil characteristics and concentration fluctuation; no chemical dosing is needed and little flotation scum is produced; discharge of oil and sludge is fully enclosed, pressurized and automated.

Installation practicality counts too. The compact footprint means the train usually fits inside the battery limit of the existing desalting area, and the simple structure with low maintenance cost keeps the lifecycle bill modest. Intelligent, fully enclosed operation reduces staffing at the battery limit, and the green credentials - no chemicals, minimal scum - remove the secondary waste stream that chemical processes create. For plants planning wastewater reuse or capacity upgrades, the same train leaves room to grow without redesign.

Field record

The technology has been applied at Sinopec Cangzhou Refinery, PetroChina Sichuan Petrochemical, Changqing Petrochemical and Dushanzi Petrochemical, including a dedicated PetroChina Dushanzi desalting draw-off water treatment unit. Operating data show effluent petroleum consistently below 150 mg/L with strong shock resistance, continuous compliant discharge, and the kind of environmental benefit and operator acceptance that follow from numbers holding up over time. For refiners staring at inlet limits their desalter water cannot meet, that record is the argument.