Industry Solutions
Oil and Grease in Industrial Wastewater: When to Use Skimming, Coagulation or DAF
Select oily-wastewater pretreatment by identifying free, dispersed, emulsified and soluble oil before choosing skimming, coagulation, DAF or biology.
"Oil and grease" is not one physical condition.
EPA troubleshooting guidance for oily industrial wastewater distinguishes free, emulsified and soluble forms, and the treatment options are not interchangeable.
The process-selection question is:
What form is the oil in, and what separation mechanism can actually remove that form?
Decision Path
Visible free oil layer? -> recover or skim first.
No free layer, but droplets/FOG float after quiescent settling? -> evaluate gravity/coalescing and DAF.
Stable cloudy emulsion that does not separate? -> test pH adjustment / emulsion breaking / coagulation before DAF.
Oil is largely dissolved or associated with soluble COD? -> physical flotation alone will not remove it; evaluate biological or another dissolved-phase treatment.
1. Free Oil
EPA describes free oil as oil that rises to the surface due to buoyancy during a short quiescent period.
Typical options include:
- gravity oil/water separation;
- skimming;
- coalescing.
This is the lowest-complexity oil fraction and should be recovered before it is emulsified by pumps or high-shear mixing where practical.
2. Dispersed Oil
Smaller oil droplets may remain suspended long enough that gravity separation becomes slow.
DAF can be evaluated because microbubbles can attach to low-density particles and oil droplets and float them to the surface.
Useful data include:
- oil and grease;
- TSS;
- droplet behavior;
- pH;
- temperature;
- surfactant use.
3. Emulsified Oil
EPA industrial guidance states that stable oil-in-water emulsions may require chemical treatment before separation.
Possible pretreatment includes:
- acidification where appropriate;
- coagulant;
- demulsifier;
- polymer;
- controlled mixing.
The actual chemical program should be established through representative testing.
4. Soluble Oil / Soluble Organics
EPA metal-finishing guidance notes that soluble oils are not removed by gravity separation, flotation or emulsion breaking in the same way as free and emulsified oils.
That fraction may require:
- biological oxidation;
- adsorption;
- membrane or other project-specific treatment.
Do not increase DAF chemical dose indefinitely to remove a truly dissolved fraction.
Worked Example: Separate the Load Before Selecting Equipment
Assume an industrial wastewater flow of:
- 200 m3/day;
- total oil and grease = 600 mg/L.
The incoming oil/grease mass is:
200 m3/day x 600 g/m3 = 120,000 g/day = 120 kg/day
A simple bottle test shows that 300 mg/L rises as free oil after quiescent separation.
That free-oil fraction represents:
200 m3/day x 300 g/m3 = 60 kg/day
This means approximately half of the measured oil/grease load is potentially recoverable before DAF in this illustrative case.
If the remaining 300 mg/L is a stable emulsion, the next test should be chemical destabilization plus flotation - not simply a larger gravity separator.
This example demonstrates mass-balance thinking; it is not a universal removal prediction.
EPA DAF Screening Envelope - Use as a Check, Not a Design Guarantee
EPA's historical DAF fact sheet reports broad screening ranges including:
- pressurization: 25-70 psig
- air-to-solids ratio: 0.01-0.1 lb/lb
- float detention: 20-60 min
- recycle where used: 5-120%
- surface hydraulic loading: 500-8,000 gpd/ft2
These ranges are intentionally broad and should not be copied directly into a proposal. They are useful as a review envelope: if a proposed operating point falls far outside historical guidance, verify it with vendor data and treatability testing.
Treatability Evidence Record
For each bottle/jar-test condition, record the following in a simple mobile-friendly list:
Condition ID - untreated / pH-adjusted / coagulant / coagulant+polymer pH - before and after treatment Dose - coagulant and polymer Observation after mixing - emulsion break, floc size, free oil layer Separation test - settling or flotation response Measured result - O&G, TSS and COD if available Decision - reject / repeat / advance to pilot
This record creates a traceable treatment basis rather than a visual guess.
Jar-Test / Treatability Sequence
For an emulsion, a useful bench sequence is:
- characterize untreated pH and conductivity;
- test whether pH adjustment destabilizes the emulsion;
- compare coagulant types;
- optimize coagulant dose;
- add polymer only after coagulation is established;
- evaluate floc and flotation behavior;
- measure treated oil/grease, TSS and COD.
Selection Boundaries
Free oil
Evaluate gravity separation, skimming or coalescing. Verify whether the oil rises and can be recovered before high-shear pumping.
Dispersed oil
Evaluate DAF. Verify bubble attachment, hydraulic loading and solids load.
Stable emulsion
Evaluate chemical destabilization followed by DAF. Require a representative jar/bottle test before equipment sizing.
Oil attached to solids
Evaluate coagulation/flocculation plus DAF. Track TSS and floc behavior.
Mostly soluble organic load
Do not expect DAF to solve the main COD duty. Evaluate biodegradability or another dissolved-phase process.
Credibility Checks Before Equipment Selection
Request:
- total oil and grease;
- TSS;
- total COD;
- soluble COD where available;
- surfactants/cleaners;
- pH;
- temperature;
- representative photos;
- simple settling/bottle test;
- jar-test results.
These data make the process decision inspectable.
Limits of This Guidance
This article does not define a universal:
- pH for emulsion breaking;
- coagulant dose;
- polymer dose;
- DAF removal percentage;
- gravity-separator efficiency.
EPA industry documents show that the required chemistry and sequence depend on the specific emulsion.
Technical Basis and References
- U.S. EPA, Field Manual - Performance Evaluation and Troubleshooting at Metal Finishing Wastewater Treatment Facilities, Section 5. It distinguishes free, emulsified and soluble oil and describes gravity separation, emulsion breaking and flotation.
- U.S. EPA, Iron and Steel Effluent Guidelines Development Document, treatment-technology discussion of emulsion breaking followed by DAF.
- U.S. EPA Industrial Wastewater Treatment Technology Database describes DAF as a technology for suspended solids, oil and grease.
Need Help Reviewing Oily Wastewater?
- flow;
- oil/grease;
- TSS;
- total and soluble COD if available;
- pH;
- temperature;
- surfactants/cleaners;
- bottle-test observations;
- jar-test results.
HydroNovus can provide a preliminary process-selection review.