Produced water and fuels

Nanobubble flotation of produced water

In laboratory flotation of oily saline water, adding micro- and nanobubbles removed more than 99% of the emulsified oil Etchepare et al., 2017. Produced water, the brine that comes up with oil and gas, has to be cleaned before it is discharged or reinjected, and fine oil droplets are the hardest part to separate.

The mechanism is floc aeration. Nanobubbles form or become trapped inside flocculated oil droplets, creating "aerated flocs" that are much less dense than the surrounding water and rise quickly. Their high surface area and hydrophobic surfaces make them nucleate preferentially on oil droplets, which raises the chance of collision and attachment compared with larger bubbles Shen et al., 2022.

In studies on saline produced water, oil content fell from starting concentrations of 334–484 mg/L to less than 1 mg/L, below the offshore discharge standard of 29 mg/L Etchepare et al., 2017. The bubbles in these studies were generated by depressurization, as in conventional flotation, or with a multiphase pump, and they improved adhesion between bubbles and oily flocs, which sped up separation Oliveira et al., 2017.

Nanobubbles alongside dissolved air flotation

Nanobubbles reached droplets that conventional dissolved air flotation tends to miss Oliveira et al., 2017. DAF releases air from pressurized water as microbubbles of roughly 30–100 µm. These work well on larger oil globules, but emulsified droplets smaller than about 20 µm tend to follow the water streamlines around the bubbles and escape capture.

Nanobubbles below 200 nm have little buoyancy and stay suspended for a long time, so they remain in the water column long enough to meet these fine droplets. Oliveira et al. showed that flocculation-column flotation with nanobubbles from a multiphase pump improved the separation of emulsified crude oil in saline water compared with conventional methods. The nanobubbles acted as "active fillers" inside the oil flocs, increasing their apparent size and buoyancy and reducing the hydraulic retention time required Oliveira et al., 2017.

Chemistry remained part of the process. In later trials with electric-field-generated air nanobubbles, oil removal rose to 98.5% with an optimized polymer dose of 0.2 wt%, against 62% for air sparging, a result reported as allowing reduced chemical use English, 2025. The bubbles worked with the flocculant, not in place of it.

Hydrodynamic cavitation in biodiesel production

In the studies reviewed, hydrodynamic cavitation shortened biodiesel reaction times Cako et al., 2022. Conventional production with mechanical stirring is limited by mass transfer between vegetable oil and alcohol, which do not mix. Collapsing cavitation bubbles create local hot spots of high temperature and pressure and fast micro-jets. The resulting turbulence emulsifies the two liquids, enlarging the contact area between them and speeding up the reaction Sun et al., 2023.

Reaction time, yield and cost

Cako et al. reported that multi-hole orifice-plate reactors reached 99% biodiesel yield within 5 minutes of processing. Ultrasonic bath methods needed 90 minutes to reach 95% yield. The processing cost with cavitation was estimated at 4.80 USD/m³, lower than acoustic cavitation (6.7–10.8 USD/m³) and mechanical stirring, which the review attributed to lower energy use and faster throughput Cako et al., 2022.

The intense shear also allowed lower catalyst concentrations and alcohol-to-oil ratios while the fuel still met the ASTM D6751 and EN 14214 quality standards, according to a second review Sun et al., 2023. Both sources are reviews that compare published processes rather than a single controlled trial.

Nanobubbles in liquid fuels

One article reported that electric-field-generated air nanobubbles in diesel fuel raised combustion efficiency by about 16% English, 2025. The proposed explanation starts with surface tension, which dropped by about 7% with air nanobubbles. Lower surface tension lets the injector break the fuel into finer droplets, which burn more fully.

Nanobubbles were also reported to stabilize water-in-diesel emulsions, an approach studied for its effect on nitrogen oxide (NOₓ) formation. The bubbles acted as a buffer that kept water droplets from coalescing, which supports the "secondary atomization" of emulsion droplets in the combustion chamber English, 2025. This lesson cites no emission measurements and makes no emissions claim.