Summary

Here we will dive into how we typically treat emulsified oils in wastewater, emphasizing chemical demulsification strategies integrated with dissolved air flotation (DAF) for removal. We cover coagulants, flocculants, reaction kinetics, and physical separation principles, supported by mathematical models, to achieve low levels of fats, oils, and greases (FOG) and total suspended solids (TSS) in effluents from various industries.

Introduction

Emulsified oils in wastewater present complex treatment challenges owing to their fine droplet dispersion, stabilized by surfactants, mechanical shear, or natural emulsifiers. These emulsions frequently originate from industrial operations including petroleum refining, metalworking, dairy processing (where milk fats contribute to very high FOG loads), and food and beverage processing (involving vegetable oils and animal fats). Effective remediation entails chemical demulsification to destabilize the oil-water interface, followed by physical separation to extract aggregated contaminants. This methodology reduces FOG and TSS to compliant levels, often below 15 mg/L for oil content [1]. Critical components will include either chemical reaction tanks or flocculation tubes, preparing the stream for removal via dissolved air flotation.

*A skidded chemical reaction tank showing top mounted mixers, ladders and mezzanine, along with chemical dosing pumps - Ecologix*

Process Overview

Treatment of emulsified oils proceeds through sequential stages: chemical demulsification, flocculation, and physical separation. In the chemical phase, demulsifiers and coagulants are introduced to neutralize surface charges on oil droplets, promoting coalescence. Flocculation ensues, where polymers facilitate the bridging of destabilized particles into macro-flocs. Physical separation, via DAF, leverages microbubbles to buoy flocs containing FOG suspended solids for skimming.

A foundational principle is Stokes' Law for particle settling or rising velocity:

v = \frac{2}{9} \cdot \frac{(ρ_{p} - ρ_{f}) \cdot g \cdot r^{2}}{μ}

Where $v$ is terminal velocity (m/s), $ρ_{p}$ and $ρ_{f}$ are particle and fluid densities (kg/m³), $g$ is gravity (9.81 m/s²), $r$ is particle radius ( $m$ ), and $μ$ is fluid viscosity (Pa⋅s). Chemical treatment enlarges $r$ , dramatically accelerating separation [2].

Demulsification efficiency can be quantified using the demulsification ratio:

Demulsification Ratio = (\frac{oil volume + water volume}{original emulsion volume + added demulsifier volume}) \times 100

This metric evaluates phase separation post-treatment [3].

Chemical Treatment Details

Demulsification relies on agents that disrupt the oil-water interface by adsorption, charge neutralization, or hydrophile-lipophile balance alteration. Inorganic coagulants include polyaluminum ferric chloride (PAFC), polyaluminum chloride (PAC; often formulated as [Al2(OH)nCl6-n]m [25]), polysilicate aluminum ferric sulfate (PSAFS), polyferric sulfate (PFS; [Fe2(OH)n(SO4)3-n/2]m [27]), aluminum sulfate (alum; Al2(SO4)3·14H2O), ferric chloride (FeCl3·6H2O), ferric sulfate (Fe2(SO4)3·9H2O), ferrous sulfate (FeSO4·7H2O), sodium aluminate (Na2Al2O4), and calcium hydroxide (lime; Ca(OH)2) [14]. Organic flocculants encompass polyacrylamide (PAM; (C3H5NO)n), natural alternatives like chitosan ((C6H11NO4)n), anionic polymers (e.g., polyacrylic acid derivatives), cationic polymers (e.g., polydiallyldimethylammonium chloride, PDADMAC; (C8H16ClN)n), and nonionic polymers (e.g., polyethylene oxide, PEO; (C2H4O)n) [15]. For pH adjustment, sulfuric acid (H2SO4) lowers pH to destabilize acidic emulsions, while sodium hydroxide (NaOH) elevates pH to precipitate solids in alkaline conditions [5].

Specialized demulsifiers for stable emulsions include quaternary ammonium compounds such as cetyltrimethylammonium bromide (CTAB; C19H42BrN or C16H33N(CH3)3Br), trioctylmethylammonium chloride (TOMAC; [CH3(CH2)7]3N(CH3)Cl), and ionic liquids like 1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C12mim][NTf2]; C16H23F6N3O4S2) or 1-octyl-3-methylimidazolium hexafluorophosphate ([C8mim][PF6]; C12H23F6N2P) [16]. These amphiphilic agents reduce interfacial tension, with CTAB achieving up to 90% demulsification efficiency in water-in-oil emulsions at 300-700 mg/L dosages [16]. Ionic liquids, often with hydrophobic tails, offer tunable properties for FOG-heavy streams, yielding 90-100% separation at low concentrations (100-3500 ppm) but may incur higher costs [16].

Coagulation mechanics involve charge neutralization, where cationic metal hydroxides from coagulants (e.g., Al3+ from PAC or Fe3+ from FeCl3) adsorb onto anionic oil droplets. For instance, PAFC at 300 mg/L, paired with PAM at 30 mg/L, reduces oil from 3000-5000 mg/L to <15 mg/L, with optimal pH 6.5-6.9 to avoid restabilization [1]. Kinetics follow second-order aggregation models, approximated by:

\frac{d N}{d t} = - k N^{2}

where $N$ is particle concentration, $t$ is time, and $k$ is the aggregation rate constant, influenced by mixing intensity and zeta potential [6].

Advanced optimization employs the Hydrophilic-Lipophilic Deviation - Net Average Curvature (HLD-NAC) model for surfactant-based systems. The HLD equation is:

HLD = F (s) - k (EACN) - f (A) - α_{t} (Δ T) + c_{c}

with terms for salinity ( $F$ (s)), oil hydrophobicity (EACN), alcohol effects ( $f$ (A)), temperature deviation (Δ $T$ ), and surfactant characteristic (Cc). Optimal demulsification occurs at HLD ≈ 0 (Winsor III phase), minimizing interfacial tension [7]. NAC extends this with net curvature:

H_{n} = \frac{1}{R_{o}} - \frac{1}{R_{w}} = - \frac{H L D}{L}

and average curvature bounded by characteristic length ξ, aiding in demulsifier formulation for dairy or food emulsions rich in FOG [7].

Flocculation in serpentine tubes involves the equivalent of rapid mixing (350-800 rpm, 1-5 min) for dispersion, then slow mixing (50-150 rpm, 10 min) for floc growth. Thermal enhancement (45-120°F) accelerates kinetics, particularly for viscous FOG-laden streams from food and beverage sources [6]. Organoclays, modified bentonites, serve as adsorbents for residual emulsions, outperforming activated carbon by a factor of seven in polishing steps [8].

*A skidded configuration of serpentine tubes - Ecologix*

The table below compares coagulants at 500 mg/L dosage:

(Table: Comparison of coagulants for emulsified oil treatment in wastewater, emphasizing DAF compatibility and FOG reduction [1][20][21][22][23][24].)
Coagulant	Demulsification Order	Advantages	Disadvantages
PAFC	Highest	Rapid dissolution, large dense flocs, short floating time, wide pH range, effective at low dosages.	Less effective at very low doses, potential for fine flocs at high doses, requires higher dosages than PAC in some applications.
PAC	High	Rapid dissolution, stable performance, better than simple inorganic salts, large dense flocs, lower optimal dosages.	High dosages can be costly, deliquescent and hard to preserve, slow flotation speed.
PSAFS	Medium	Rapid dissolution, large dense flocs, fast floating speed, low dosage effective.	Difficult to store, deliquescent, solution may blacken, leaves small flocs.
Alum	Medium	Inexpensive, widely available, dense flocs, easy to use if dosed properly.	Limited pH range, requires large amounts for dirty water, produces significant gelatinous sludge, slow flotation.
PFS	Low	Suitable for low temperatures, rapid flocculation, easy separation, wide applicable range.	Leaves fine flocs in effluent, less effective overall, poor removal of oil.
FeCl3	Variable (high in some oily wastes)	No pH requirement, broad range effectiveness, high removal for TSS/FOG (e.g., 97% oil/grease in combinations).	Highly corrosive (needs special equipment), price fluctuations, significant pH drop.
Chitosan	Variable	Eco-friendly/natural, low dosage needed, biodegradable sludge, high efficiency for turbidity/TSS, synergistic with PAC.	pH/solubility dependent, dosage sensitive (overdosing causes restabilization), scalability challenges.

Physical Removal Techniques

After chemical treatment, physical removal employs dissolved air flotation (DAF) systems, where pressurized air is dissolved in water and released to form microbubbles (30-120 μm) that attach to flocs, increasing buoyancy [10]. In a typical DAF unit, wastewater from the flocculation tubes enters the flotation tank, with bubble flow rates optimized for efficient collision [11].

A typical system includes the wastewater inlet, air saturation tank (pressurized at 4-6 atm), release into the flotation basin where bubbles form and lift flocs to the surface for skimming and clarified effluent outlet. Sludge is removed via flights [12].

*DAF system top view showing the counter current flight scrapper system and effluent clearwell - Ecologix*

Other methods include organoclay filtration for polishing, where modified bentonite adsorbs residual oil at rates seven times faster than activated carbon [13]. Gravity separators like API or CPI are used for preliminary free oil removal but are less effective for emulsions without prior chemical treatment. DAF excels in removing FOG and TSS, achieving >95% efficiency even in exceedingly tough to treat streams such as dairy wastewater [10].

Challenges in Treatment

Variability in emulsion stability from surfactants or FOG content leads to inconsistent removal. Overdosing causes charge reversal and restabilization, while pH fluctuations demand tight control. High viscosity in food and beverage wastes increases energy for mixing, and TSS buildup complicates sludge handling [18].

Solutions and Optimizations

Jar testing simulates conditions to refine dosages, incorporating HLD-NAC for demulsifier design. Multi-coagulant blends (e.g., PAFC + chitosan) enhance floc robustness for FOG-heavy streams. Real-time monitoring of TSS, FOG, and turbidity via sensors help ensure adaptive control [19].

Conclusion

Advanced chemical demulsification, bolstered by models like HLD-NAC and integrated with dissolved air flotation treatment, offers a sophisticated approach to emulsified oil removal in wastewater. By leveraging detailed chemistry, mathematics, and engineering, this framework addresses FOG and TSS in diverse industries like dairy and food and beverage, enabling efficient, scalable treatment.

Glossary of Terms

Alum: Aluminum Sulfate - Al2(SO4)3·14H2O, a primary coagulant for charge neutralization.
API: American Petroleum Institute gravity separator - A tank designed for oil-water separation based on density differences.
BOD: Biological Oxygen Demand - A measure of the amount of oxygen required by microorganisms to decompose organic matter in water.
Ca(OH)2: Calcium Hydroxide (Lime) - A coagulant used for pH adjustment and precipitation.
CaO: Calcium Oxide (Quicklime) - A coagulant aid for alkalinity control.
COD: Chemical Oxygen Demand - A measure of the oxygen required to chemically oxidize organic and inorganic matter in water.
Coagulant: A chemical agent that neutralizes charges on particles to promote aggregation.
CPI: Corrugated Plate Interceptor - An enhanced gravity separator using inclined plates to increase oil droplet coalescence.
CTAB: Cetyltrimethylammonium Bromide - C19H42BrN, a quaternary ammonium demulsifier for emulsion breaking.
DAF: Dissolved Air Flotation - A physical separation process using microbubbles to float contaminants.
Demulsification: The process of breaking stable oil-water emulsions.
Demulsifier: A substance that destabilizes emulsions by disrupting the interface between oil and water.
Effluent: The treated wastewater discharged from a treatment process.
Emulsion: A stable mixture of two immiscible liquids, such as oil droplets and water.
FeCl3·6H2O: Ferric Chloride - A primary coagulant for high FOG wastewater.
FeSO4·7H2O: Ferrous Sulfate - A primary coagulant used in oxidation-coagulation processes.
Fe2(SO4)3·9H2O: Ferric Sulfate - A primary coagulant for turbidity and oil removal.
Flocculant: A polymer that bridges aggregated particles to form larger flocs.
Flocculation Tubes: Pipe-like structures for controlled mixing to form flocs.
FOG: Fats, Oils, and Greases - Hydrophobic contaminants common in food-related wastewater.
H2SO4: Sulfuric Acid - Used for pH adjustment in demulsification.
[C12mim][NTf2]: 1-Dodecyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide - C16H23F6N3O4S2, an ionic liquid demulsifier.
Na2Al2O4: Sodium Aluminate - A coagulant for alkalinity and precipitation.
Na2SiO3: Sodium Silicate - A coagulant aid for floc strengthening.
NaOH: Sodium Hydroxide - Used for pH elevation in chemical treatment.
PAC: Polyaluminum Chloride - [Al2(OH)nCl6-n]m, a coagulant for emulsion destabilization [25].
PAM: Polyacrylamide - (C3H5NO)n, a flocculant that bridges particles into larger aggregates.
PAFC: Polyaluminum Ferric Chloride - A coagulant used for charge neutralization in emulsions.
PDADMAC: Polydiallyldimethylammonium Chloride - (C8H16ClN)n, a cationic polymer flocculant.
PEO: Polyethylene Oxide - (C2H4O)n, a nonionic polymer flocculant.
pH: A measure of the acidity or alkalinity of a solution on a scale from 0 to 14.
PFS: Polyferric Sulfate - [Fe2(OH)n(SO4)3-n/2]m, an iron-based coagulant for wastewater treatment [27].
PSAFS: Polysilicate Aluminum Ferric Sulfate - A composite coagulant effective at low temperatures.
TOMAC: Trioctylmethylammonium Chloride - [CH3(CH2)7]3N(CH3)Cl, a demulsifier for heavy oil emulsions.
TSS: Total Suspended Solids - The dry-weight of particles trapped by a filter, including organic and inorganic matter.
Turbidity: A measure of the cloudiness or haziness of water caused by suspended particles.

FAQ

What is the role of pH in chemical treatment? pH influences coagulant hydrolysis and emulsion stability; optimal ranges prevent restabilization.
How does DAF improve over gravity separation? Microbubbles enhance buoyancy for finer particles, effectively removing FOG and TSS.
What are common demulsifiers? Inorganic coagulants like PAFC or FeCl3, organic polymers like PAM, quaternary salts like CTAB, and ionic liquids like [C12mim][NTf2].
Why use flocculation tubes? They are used for a multi-stage mixing process that includes both rapid mixing for chemical dispersion and slower mixing for floc formation. The purpose is to build large, easily removable flocs. Floc tubes are also cheaper than chemical reaction tanks, so if the chemical reaction time is relatively low, we can design in floc tubes.
How do I test for treatment efficacy? Jar tests and demulsification ratio calculations will help you optimize your chemical dosages.

*Jar testing before and after demulsification and DAF treatment - Ecologix*

Advanced Chemical Demulsification and DAF Techniques for Emulsified Oil Removal in Wastewater

Summary

Table of Contents

Introduction

Process Overview

Chemical Treatment Details

Physical Removal Techniques

Challenges in Treatment

Solutions and Optimizations

Conclusion

Glossary of Terms

FAQ

Bibliography

Have a project you would like to discuss?

Summary

Table of Contents

Introduction

Process Overview

Chemical Treatment Details

Physical Removal Techniques

Challenges in Treatment

Solutions and Optimizations

Conclusion

Glossary of Terms

FAQ

Bibliography

Have a project you would like to discuss?

Related Products

Dissolved Air Flotation (DAF) Systems

Chemical Reaction Tanks

Activated Carbon