Heating Beats Stirring in New Model for Breaking Down Turpentine Wastewater Oil

In an industrial corner of East Java, Indonesia, a turpentine factory has been quietly discharging one of the most stubborn wastewater streams in the chemical processing world. The effluent leaving the plant carries oil and grease concentrations of up to 14,000 milligrams per liter, chemical oxygen demand approaching 8,200 milligrams per liter, and a pH as low as 1. Against national discharge limits of just 10 milligrams per liter for oil and grease and 150 milligrams per liter for COD, the scale of the challenge is stark. A new study published in Case Studies in Chemical and Environmental Engineering now offers a detailed, mathematically grounded answer to how this toxic brew can be tamed, and its central finding is delightfully counterintuitive: when it comes to breaking stubborn oil emulsions by heating, doing nothing, at least mechanically, works best.

The research team, led by Prayitno with Sri Rulianah, Wahyuni Ningsih, and Wahyu Widyananda, all affiliated with Indonesian institutions collaborating through the State Polytechnic of Malang, set out to build a quantitative model of thermal demulsification, the process by which emulsified oil droplets are coaxed into merging and separating from water. Their wastewater came directly from a turpentine plant in Trenggalek, where the production of turpentine oil and gum rosin leaves behind a cocktail of cellulose fibers, resin acids, carboxylic acids, and dissolved turpentine oil. These natural surfactants form rigid interfacial films around oil droplets, creating emulsions so stable that conventional treatment methods like flotation, neutralization, and sedimentation struggle to meet regulatory thresholds, particularly for oil and grease.

The experimental setup was elegantly simple. Two liters of filtered wastewater, held at its naturally acidic pH of 1 to 2, was placed in a beaker inside a temperature-controlled water bath. A commercial polymeric demulsifier, Nalco 14750, was dosed at 50, 100, or 150 milligrams per liter, while heating temperatures ranged from 30 to 50 degrees Celsius. Half the experiments ran with vigorous agitation at 5,000 revolutions per minute; the other half proceeded in complete stillness. Each five-minute treatment was then assessed for four regulated parameters: turbidity, total suspended solids, biochemical oxygen demand, and oil and grease. The removal data were fitted to second-order polynomial regression models, producing response surfaces that map exactly how temperature and dose interact to drive pollutant removal.

The mechanism underlying the process reads like a microscopic tug-of-war at the oil-water interface. The non-ionic polymeric demulsifier carries both hydrophilic and hydrophobic segments, allowing it to migrate to the droplet surface, penetrate the rigid film formed by gum rosin and carboxylic acids, and displace the natural emulsifiers stabilizing the emulsion. Meanwhile, moderate heat thins the interfacial film, lowers the zeta potential of the droplets, compresses the electrical double layer, and reduces the viscosity of the surrounding water, all effects that encourage droplets to collide, coalesce, and rise. The wastewater’s extreme acidity adds another lever: at pH 1 to 2, weakly acidic groups on the natural emulsifiers become protonated, reducing the negative surface charge on droplets and weakening electrostatic repulsion before the demulsifier even arrives.

Under agitation, the best single result came at 100 milligrams per liter of demulsifier, where turbidity fell by 92.21 percent, total suspended solids by 96.36 percent, biochemical oxygen demand by 99.84 percent, and oil and grease by an impressive 99.29 percent. But the response surfaces revealed a clear ceiling: temperatures above roughly 40 degrees Celsius combined with high-speed stirring actively sabotaged the process. The researchers traced this deterioration to well-established fluid dynamics. At 5,000 rpm, raising the temperature lowers the water’s viscosity, pushing the impeller Reynolds number higher and intensifying turbulence. According to Kolmogorov-Hinze theory, greater turbulent energy dissipation shrinks the maximum stable droplet diameter, meaning the stirrer begins slicing coalesced oil droplets back into tiny, newly stabilized fragments faster than the demulsifier can disrupt their regenerated interfacial films. In effect, the mixer re-emulsifies what chemistry has just separated.

The quiescent experiments told a strikingly different story. Without agitation, oil and grease removal ranged from 98.05 to 99.49 percent, consistently outperforming the stirred condition at comparable temperature and dose combinations, with a strong model fit of R-squared equal to 0.9312. Once the demulsifier and moderate heat had destabilized the emulsion, undisturbed conditions allowed the coalesced droplets to rise continuously to the surface, exactly as Stokes’ law predicts for creaming in a low-viscosity, quiescent fluid. Total suspended solids removal still reached up to 96.11 percent, aided by a slightly higher optimum temperature of around 43 degrees Celsius that compensated for the absence of mechanical collision energy. Turbidity removal peaked near 32 degrees Celsius at doses of 100 to 113 milligrams per liter.

The models also exposed a subtle danger in overdosing. Beyond roughly 100 milligrams per liter, removal efficiencies for oil and grease and biochemical oxygen demand began to falter. The explanation lies in surfactant physics: when the demulsifier concentration approaches the critical micelle concentration, excess molecules spontaneously assemble into micelles, spherical structures with oily interiors that encapsulate hydrocarbons and keep them dispersed in the water phase. Simultaneously, surplus surfactant adsorbing onto droplet surfaces enhances steric stabilization, blocking the very collisions the treatment is meant to promote. The result is a secondary, self-inflicted emulsion that resists settling, flotation, and even analytical detection, since standard methods count micelle-trapped oil as part of the oil and grease load.

To distill all four responses into a single operating recipe, the team applied the Derringer-Suich composite desirability method, treating every parameter as a larger-the-better characteristic and weighting them equally because each is independently regulated. Evaluated over a grid of 90,000 points across the experimental domain, the optimization delivered an unambiguous verdict. Without agitation, the overall desirability reached 0.8258 at approximately 35.2 degrees Celsius and 150 milligrams per liter of demulsifier, with the predicted model delivering oil and grease removal above 99.4 percent, total suspended solids removal above 96 percent, turbidity removal above 93 percent, and near-complete biochemical oxygen demand removal above 99.7 percent. The stirred condition managed only 0.7264, with a sharper, more fragile optimum that would make industrial operation far less forgiving of small deviations in temperature or dosing.

The practical implications extend well beyond one factory in Trenggalek. Skipping agitation eliminates the energy cost of continuous mixing, removes the capital expense of mixer installations, and cuts long-term maintenance for full-scale treatment plants. Compared with the electrocoagulation-Fenton process the same group previously tested, which achieved 99 percent oil and grease removal but demanded constant electrical input, consumable electrodes, and complex sludge handling, moderate thermal-chemical demulsification in a still tank is dramatically simpler. The authors note that the optimal dose of 150 milligrams per liter sits at the boundary of their tested range, meaning the true global optimum may lie beyond it, and they call for future work on demulsifier type, heating duration, and agitation speed. For now, the message to engineers battling oily industrial wastewater is refreshingly concise: dose it, warm it to about 35 degrees Celsius, and then let physics do the quiet work of separation.

Subject of Research: Thermal demulsification modeling of turpentine industrial wastewater using a polymeric demulsifier with multi-response optimization

Article Title: Analysis model of the demulsification process by heating in turpentine industrial wastewater

Article References: Prayitno, Rulianah, S., Ningsih, W., & Widyananda, W. (2026). Analysis model of the demulsification process by heating in turpentine industrial wastewater. Case Studies in Chemical and Environmental Engineering, 14, Article 101477. https://doi.org/10.1016/j.cscee.2026.101477

Image Credits: AI Generated

DOI: 10.1016/j.cscee.2026.101477

Keywords: turpentine wastewater, demulsification, oil and grease removal, wastewater treatment, polymeric demulsifier, response surface methodology, composite desirability, thermal separation, industrial effluent, emulsion breaking, polynomial regression, BOD removal