[Chinese Mainland] Research Achievement by Ocean University of China Team Published in Nature Water: Heterogeneous Wettability Membranes Enable Simultaneous and Parallel Separation of Both Oil-in-Water and Water-in-Oil Emulsions, with Separation Efficienc

The team led by Professor Li Yiming from the Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, has published its findings in the internationally top-tier journal Nature Water: by combining a hydrophilic/hydrophobic heterogeneously wetted membrane with a bidirectionally graded flow channel membrane cell, simultaneous parallel separation of two opposite types of emulsions—oil-in-water (O/W) and water-in-oil (W/O)—is achieved, with two-phase separation efficiency exceeding 99%, and anti-fouling performance and long-term stability significantly enhanced.

There has long been an awkward dilemma in the field of oily wastewater treatment: oil-in-water emulsions require hydrophilic membranes, while water-in-oil emulsions require hydrophobic membranes. Once the feed properties change, or when both types of emulsions appear simultaneously, one has no choice but to shut down and switch membrane materials, or connect multiple treatment units in series. This achievement from Ocean University of China attempts to solve this "either-or" dilemma in one go.

The Pain Points of the Old Approach: The Triple Cost of the Serial Mode

To understand the significance of this achievement, let us first look at the problems faced by traditional membrane processes when treating emulsified oil-water mixtures:

• Membrane materials must match the emulsion type—treating oil-in-water (O/W) emulsions relies on hydrophilic membranes, while treating water-in-oil (W/O) emulsions requires hydrophobic membranes; the surface wettability requirements of the two membranes are exactly opposite;
• Feed changes require switching—this is a "serial mode," which typically can only target one type of emulsion and obtain one main purified phase. When the feed composition changes, or even when the two types of emulsions coexist, it is necessary to identify the system in advance, replace membrane modules, or add multiple treatment units;
• Costs rise with complexity—a longer process means increased pumping energy consumption, higher cleaning frequency, and membrane module inventory and switching downtime being amortized into operating costs.

Such problems are quite common in scenarios involving petrochemicals, marine oil spill emergency response, food processing, and various types of industrial wastewater. Take a typical situation: after the wastewater from a refinery's turnaround maintenance, floor washing water, and initial rainwater are collected together, the emulsified state often drifts with changes in surfactant concentration, pH, and salinity, making it very difficult to lock in "treating only one type" at the design stage.

The New Approach: Heterogeneously Wetted Membrane + Bidirectionally Graded Flow Channel

The team's solution is not simply to improve the membrane material, but to redesign three levels together: membrane material, flow channel structure, and interfacial transport mechanism:

• Hydrophilic/hydrophobic heterogeneously wetted membrane—hydrophilic and hydrophobic regions are constructed on the same membrane, enabling the membrane to possess selective transport capability for emulsions of different properties simultaneously;
• Bidirectionally graded flow channel membrane cell—the flow channel geometry gradually changes along the path, causing the fluid to generate controllable shear and pressure gradients during movement;
• Closed-loop mechanism—establishing a complete pathway of "shear-induced demulsification—wetting-selective transport—Laplace interfacial anchoring": first, shear action destabilizes the emulsion, then the wettability difference of the membrane selectively transports different phases, and finally interfacial tension achieves anchoring and separation of the phase interface.

In this mechanism, CFD simulation is used as a tool to reveal the mechanical processes within the graded flow channel, rather than merely as a verification method. This means designers can first adjust flow channel parameters on the model and then guide experiments, which has direct value for subsequent scale-up.

The final performance metrics presented are: two-phase separation efficiency exceeding 99%, while anti-fouling performance and long-term stability are significantly enhanced. The latter is particularly critical for oily wastewater—the adsorption, spreading, and pore blocking of oily pollutants on the membrane surface have always been the main killers of membrane process lifespan.

Why "Parallel Separation" Deserves More Attention Than "Higher Flux"

In recent years, there have been many achievements in the field of membrane materials, with metric competition mostly focused on the trade-off between flux and rejection rate. The difference of this work lies in that it addresses a process-level problem rather than the performance limit of a single component:

• From serial filtration to parallel separation — a single unit, a single pass, simultaneously treating two types of emulsions with completely opposite properties, no longer relying on pretreatment identification and mode switching;
• Reduced system complexity — theoretically, the variety of membrane modules, switching operations, and intermediate storage tanks can be reduced, with footprint and automation control difficulty decreasing accordingly;
• More friendly to complex multiphase systems — oily wastewater under real operating conditions is rarely a "pure" single emulsion, and a compatible unit offers stronger engineering adaptability.

For the industry, the application of this pathway can initially focus on three directions: First, resource recovery from oily wastewater — that is, recovering oil products beyond merely meeting discharge standards, which already has an economic basis in the refining and machining industries; Second, oil spill emergency response — offshore or shoreline emergency equipment has higher requirements for small size, strong adaptability, and rapid deployment; Third, resource recovery from complex multiphase systems, such as industrial cleaning wastewater containing surfactants.

Of course, from laboratory apparatus to industrial sites, several verification thresholds remain: the cost and consistency of scaled-up fabrication of heterowettability membranes, the hydraulic stability of gradient flow channels under high flow rates, the applicability of the mechanism under high-viscosity or high-salinity systems, and the degradation patterns of membrane surface properties after long-term operation. The answers to these questions determine whether it ultimately remains an elegant mechanistic study or becomes an engineerable technological pathway.

What It Means for Us: TIANYI TECH's Assessment

TIANYI TECH has long served municipal and industrial park wastewater treatment and reclaimed water reuse clients. Regarding the oily wastewater niche, our assessment is: what truly bottlenecks the industry is often not "what level to treat to," but "inability to treat once the influent changes". The value of this achievement lies precisely in bringing the latter problem into the designable range.

Based on our project experience, we offer three recommendations:

• Write disturbance resistance into technical requirements — for oily wastewater with large influent property fluctuations (maintenance wastewater, floor wash water, mixed collection of initial rainwater), it is recommended to list "adaptability when emulsion type changes" as an evaluation indicator during bidding and solution review, rather than only assessing effluent oil limits under a single operating condition;
• Emphasize front-end segregated collection — the economics of any membrane-based oil-water separation is built on stable concentration and water volume, and segregated collection and segregated storage are often the steps with the highest return on investment;
• Make membrane fouling monitoring a routine item — flux decline curves, transmembrane pressure trends, and cleaning cycle changes should be systematically recorded to schedule maintenance before performance inflection points appear, rather than waiting until flux drops below the threshold.

It should be noted that the achievements described in this article are still at the journal publication stage, with no publicly available engineering application cases or long-term field data. When following such frontier achievements, we adhere to one principle: treat the performance metrics in papers as signal lights, not as construction drawings.

About TIANYI LIMITED: TIANYI LIMITED is deeply engaged in the field of wastewater treatment and reclaimed water reuse, committed to providing efficient, low-carbon, and sustainable water treatment solutions for municipal and industrial clients.