Oily Wastewater Treatment: Demulsification, DAF and Membrane Separation for Emulsified Oil
Emulsified oil is the single most common cause of upset in industrial biological treatment plants. A stable oil-in-water emulsion will not separate by gravity, will blind membranes within hours, and will coat the surfaces and biomass of any biological reactor it reaches. Effective treatment therefore follows a strict sequence: break the emulsion, separate the released oil, and only then proceed to biological or membrane polishing.
Oily wastewater originates in metalworking and machining, automotive manufacturing, rolling mills, oil refining, petrochemicals, food processing and ship bilge water. Oil is present in four forms: free oil, which rises readily to the surface; dispersed oil, in droplets of 20 to 150 microns; emulsified oil, in droplets of 0.1 to 20 microns stabilised by surfactants; and dissolved oil. Each form requires a different removal mechanism, and the design must account for all four. The related challenge of removing finely dispersed hydrocarbons from high-salinity streams is treated in oilfield produced water treatment with dissolved gas flotation.
Why Emulsions Resist Gravity Separation
Three mechanisms stabilise an oil-in-water emulsion.
Surfactant films: anionic, cationic and non-ionic surfactants align at the oil-water interface and create a mechanical barrier that prevents droplet coalescence.
Electrical double layer: charged droplet surfaces repel one another, so Brownian motion cannot bring them close enough to merge.
Fine solids: clay, metal fines and iron oxide particles accumulate at the interface and act as a rigid armour around each droplet.
Stokes' law shows why droplet size dominates: the rise velocity of an oil droplet is proportional to the square of its diameter. A 100-micron droplet rises roughly ten thousand times faster than a 1-micron droplet. Coagulation to increase effective droplet size is therefore the single highest-leverage intervention in any oily wastewater plant.
Demulsification Chemistry
Breaking the emulsion is the decisive step, and the choice of reagent depends on the emulsion type.
pH Adjustment
Most emulsifying surfactants used in metalworking fluids are anionic, and become less effective below their dissociation pH. Acidification to pH 2 to 3 with sulphuric acid collapses the surfactant film and permits coalescence. The technique is effective but consumes significant acid, produces a large sludge volume when the acid is neutralised, and must be followed by pH correction before downstream treatment. Alkaline cracking at pH 10 to 11 is used for some cationic emulsions.
Coagulants and Demulsifiers
Ferric chloride and ferric sulphate: 150 to 500 mg/L, effective across a wide pH range and also removes sulphide and phosphate
Polyaluminium chloride: 200 to 600 mg/L, less corrosive than ferric salts and produces less sludge, but less effective at low temperature
Cationic polyacrylamide: 5 to 30 mg/L as a coagulant aid, bridges the neutralised droplets into settleable flocs
Proprietary demulsifiers: blends of ethoxylated amines and resins, typically 100 to 400 mg/L, selected by jar testing
Thermal and Electrical Methods
Heating to 60 to 80 degrees Celsius reduces the interfacial viscosity and destabilises many emulsions, and is often combined with chemical dosing. Electrocoagulation, in which sacrificial aluminium or iron electrodes generate coagulant in situ, has proven effective for very stable machining emulsions, though electrode consumption and passivation remain the principal operating costs.
Dissolved Air Flotation
Dissolved air flotation is the standard separation step for oily wastewater once the emulsion has been broken. Microbubbles of 30 to 70 microns attach to oil droplets and flocs and lift them to the surface, where a mechanical skimmer removes them as sludge.
Design Parameters
Recycle ratio: 20 to 50 percent of influent flow pressurised at 4 to 6 bar
Air-to-solids ratio: 0.02 to 0.06, adjusted for incoming oil concentration
Surface loading rate: 4 to 8 cubic metres per square metre per hour for oily water
Hydraulic retention time: 20 to 40 minutes in the flotation cell
Reagent programme: coagulant, pH correction and polymer dosed upstream with 60 to 120 seconds of flash mixing for coagulant and 30 seconds gentle mixing for polymer
Performance and Limitations
A well-designed DAF unit reduces oil and grease from 500 to 2,000 mg/L down to 20 to 50 mg/L, and suspended solids to below 30 mg/L. It is sensitive to flow surges and to under-dosing, and the saturator requires periodic inspection for scale and for blockage of the pressure release valves. Where the water contains significant dissolved oil or very fine droplets below 5 microns, flotation alone will not be sufficient and a membrane stage is required.
Where floor space is at a premium, induced gas flotation using a mechanical impeller rather than a pressurised saturator offers a smaller footprint at slightly lower removal efficiency. The selection logic closely parallels that used for refinery wastewater oil, phenolic and sulphide removal, where API separators, flotation and biological treatment are staged in series.
Membrane Separation
Membranes produce the best effluent quality from oily water, but they are unforgiving of inadequate pretreatment.
Technology Choices
Ultrafiltration: removes emulsified oil and suspended solids completely, producing permeate below 5 mg/L oil; operating flux 30 to 70 litres per square metre per hour
Ceramic membranes: 0.05 to 0.2 micron pore size, withstand backwash, chemical cleaning and temperatures to 90 degrees Celsius; preferred where the oil is hot or abrasive
Polymeric PVDF and PES membranes: lower capital cost and higher packing density, but more sensitive to solvent attack and to high temperature
Reverse osmosis: applied downstream of ultrafiltration where dissolved organics or salinity must also be removed; requires oil to be below 1 mg/L to avoid catastrophic fouling
Membrane bioreactors: combine biological treatment with membrane filtration, achieving excellent oil and COD removal in a compact footprint
Fouling Control
Oil fouls membranes by adsorbing onto the surface and by forming a hydrophobic layer that resists hydraulic cleaning. Counter-measures include maintaining cross-flow velocity above 2 metres per second, backwashing at 2 to 3 times the operating flux every 20 to 30 minutes, and periodic chemical cleaning with alkaline detergent followed by acid. The cleaning protocols and dosing calculations follow the same logic as membrane fouling prevention and CIP system design. Where ceramic membranes are used, high-temperature caustic cleaning at 80 degrees Celsius restores permeability far more thoroughly than ambient cleaning.
Integrated Process Train
A typical robust configuration for metalworking or automotive oily wastewater follows five stages.
Equalisation and free oil removal: 8 to 24 hours retention with a surface skimmer and coarse coalescing plate separator
Chemical demulsification: pH adjustment, coagulant and polymer dosing with flash mixing
Dissolved air flotation: removal of released oil and floc to 20 to 50 mg/L residual oil
Membrane filtration: ultrafiltration or ceramic membrane to below 5 mg/L oil
Biological polishing: activated sludge or MBR for dissolved COD and residual organics
Where the site also generates concentrated liquid waste and the plant targets zero liquid discharge, the oil removal train becomes the front end of a much larger flowsheet, comparable in structure to chemical manufacturing wastewater treatment with solvent recovery.
Conclusion
Oily wastewater treatment succeeds or fails on demulsification. Gravity separation, flotation and membranes are all downstream of a chemical step that must be tuned to the specific emulsion in the plant. Jar testing with fresh wastewater at the start of every project is therefore not optional, and the reagent programme should be reviewed whenever the manufacturing process changes. With correct demulsification, a well-designed train can reliably deliver effluent below 5 mg/L oil, which is the threshold that protects both membranes and biological treatment from fouling.
Frequently Asked Questions
What oil concentration can a DAF unit achieve?
With proper demulsification, dissolved air flotation typically reduces oil and grease to 20 to 50 mg/L. Achieving below 20 mg/L requires longer retention, higher air-to-solids ratios, or a downstream membrane stage. Effluent oil above 50 mg/L after DAF almost always indicates that demulsification is incomplete rather than that the flotation cell is undersized.
Why is my ultrafiltration membrane fouling so quickly on oily water?
Three causes dominate: inadequate demulsification leaving free oil to coat the membrane, insufficient cross-flow velocity allowing a gel layer to form, and backwash intervals that are too long. Free oil above 20 mg/L in the UF feed is a reliable predictor of rapid fouling. The cleaning sequence described in membrane cleaning and chemical dosing guidance should be applied with an alkaline detergent first, since oil is best removed at high pH.
Can oily wastewater be treated biologically without oil separation?
Only for low concentrations. Biological systems tolerate oil up to roughly 50 mg/L, above which oil coats the floc and creates anaerobic conditions within the biomass. Where emulsified oil is present in the hundreds of milligrams per litre, a physical and chemical removal stage is mandatory before biological treatment, as in food processing plants combining fat removal with biological treatment.
