DAF for Edible Oil Refining Effluent

Edible oil refining — degumming, caustic neutralisation, bleaching, and deodorisation — produces a wastewater stream that is fundamentally different from generic edible oil industry effluent, because nearly all of its pollutant load originates from one source: soap stock. Soap stock is the by-product of neutralising free fatty acids (FFA) in crude oil with caustic soda, and it is itself a stable oil-soap emulsion. When soap stock reaches the drain — through equipment washing, spills, tank decanting, or the acid-cracking step used to recover FFA — it does not behave like ordinary oily wastewater. The soap acts as its own emulsifying agent, holding oil droplets in a fine, persistent suspension that resists separation by gravity, retention time, or simple skimming.

This is the central engineering distinction for refining effluent versus other edible oil waste streams such as solvent extraction or vanaspati hydrogenation wash water: the contaminant is not just free oil floating on the surface, it is an actively emulsified system that must be chemically broken before any flotation or gravity process can remove it. Plants that install a generic API separator or static skimmer on refinery effluent typically see FOG removal well under 50%, because the soap-stabilised droplets are too fine and too buoyant-resistant to rise on their own. DAF is therefore not an optional upgrade for this waste stream; it is the technology that makes the rest of the treatment train viable.

Before DAF, soap stock destined for the effluent stream is normally acid-cracked — dosed with sulphuric acid or HCl to pH 2.5-4 — which protonates the fatty acid soap and liberates free fatty acid in its insoluble form. This cracked FFA separates and floats as a recoverable oil layer with genuine resale value to soap and oleochemical manufacturers, mirroring the fat-recovery economics seen in other high-FOG sectors like meat processing. Acid-cracking removes the bulk of the soap stock load as a product stream rather than a waste stream, which is why a well-designed refinery ETP treats soap-stock recovery as a process economics decision, not just a pollution control step.

What remains after acid-cracking and pH neutralisation is a lower-strength but still emulsified residual oil load — this is where DAF earns its place in the train. Alum or ferric chloride coagulation, dosed alongside an anionic or non-ionic polyelectrolyte, destabilises the remaining oil-in-water emulsion and aggregates it into a floc that microbubbles (generated from a pressurised recycle stream at 4-6 bar) can lift to the surface as a removable scum. Properly designed and dosed, this chemically-assisted DAF stage achieves 90-95%+ FOG removal from feed concentrations of 1,000-5,000 mg/L, dropping effluent FOG below 10-15 mg/L ahead of any biological treatment.

Because oil and fat carry an exceptionally high COD per unit mass, the FOG fraction in refinery effluent accounts for the large majority of total COD — often 2.0-3.0 g COD per gram of oil. This means FOG removal efficiency at the DAF stage is the dominant lever on overall COD reduction for this waste stream, not the downstream biological process. A DAF unit that underperforms by even 10-15 percentage points on FOG removal will pass through enough residual oil and COD to overload or inhibit the biological stage that follows, regardless of how well that stage is designed. This is the opposite emphasis from most food-sector ETPs, where the biological stage typically does the heavy lifting.

Spans Envirotech designs DAF systems for edible oil refineries with this sequencing in mind: acid-cracking and FFA recovery sized to capture maximum saleable by-product, neutralisation control to bring feed pH into the optimum coagulation range, and DAF units jar-tested against the specific crude oil source (palm, soybean, sunflower, rice bran, or mustard) since soap-stock composition and optimum coagulant dose shift meaningfully between feedstocks. Post-DAF effluent, with FOG and COD reduced to manageable levels, is then routed to MBBR or another biological polishing stage for residual soluble COD before discharge or reuse.