Nanobubbles provide little oxygen – but can improve water quality

“It is important to recognise that it is mainly the microbubbles that are responsible for oxygen transfer. The nanobubbles contribute to a very small extent,” said researcher Roman Netzer during the recent Future Smolt 2026 conference at Sunndalsøra, Norway.

Netzer, from research institute Sintef, presented the first results from the AQUAbubble innovation project, owned by OxySolutions. The company develops venturi systems for advanced oxygenation, and the project investigates, among other things, how the different bubble sizes produced affect oxygen transfer and water quality in recirculation systems.

An important message from Netzer was that the effect on oxygenation and the effect on water quality are not necessarily due to the same mechanisms.

“Oxygenation and water quality should be considered as two separate goals, and not mixed together,” he emphasised.

The microbubbles supply the oxygen

Netzer explained the difference between the different bubble sizes:

• Coarse bubbles, with diameters over 100 micrometres, rise rapidly towards the surface, allowing much of the oxygen to be lost to the atmosphere.

• Microbubbles are between 1 and 100 micrometres in diameter. They rise more slowly, giving the oxygen more time to dissolve in the water.

• Nanobubbles are smaller than one micrometre and are so small that they cannot be seen with the naked eye. They have almost neutral buoyancy and can remain suspended in water.

“Everything that looks like a cloud is therefore not nanobubbles, but microbubbles,” Netzer pointed out.

He explained that nanobubbles have a very large total surface area in relation to the gas volume. At the same time, they contain very little gas and are relatively stable.

“Large bubbles naturally contain a lot of gas, while nanobubbles have a very low gas content. This means that they cannot emit very much gas either,” he said.

He gave the example that a single microbubble of about 50 microns will contain as much oxygen as about 4.5 million nanobubbles of 0.1 microns. Nanobubbles are are also very stable and hardly give off gas. With a given concentration of 1 billion nanobubbles per litre, nanobubbles will contribute about 0.05 mg/L of oxygen – which, according to Netzer, accounts for less than 0.1% of dissolved oxygen.

“That is why microbubbles are most interesting when the purpose is to add oxygen. The potential advantages of nanobubbles lie primarily in their surface chemistry and how they interact with substances and particles in the water.”

Achieved 60% oxygen transfer

The researchers have conducted laboratory tests with OxySolutions' OBS2 venturi system, both in freshwater and seawater.

Using so-called Nanoparticle Tracking Analysis (NTA), they investigated the size, distribution, and concentration of nanobubbles.

In seawater, they achieved concentrations of around 60 million nanobubbles per millilitre, comparable to levels reported in the literature and by other suppliers.

The experiments also showed that salinity had a major impact on bubble formation. When the salinity passed around 11–12 parts per thousand, a finely distributed cloud of small bubbles formed relatively suddenly.

The researchers also investigated how efficiently the oxygen was transferred to the water.

“We found somewhat lower transfer efficiency in fresh water and higher in sea water, where we reached around 60%.”

This was lower than what some suppliers state, but Netzer emphasised that the measurement conditions are of great importance. Among other things, contact time and oxygen deficiency in the water will affect the results.

He believed that it should be possible to achieve up to 90% oxygen transfer with the venturi system under favourable conditions.

At the same time, the calculations showed that only a very small part of the amount of oxygen in the oxygenated water could be linked to the nanobubbles themselves.

Clearer water and less CO2

The project has also conducted field trials at commercial hatcheries. The researchers used a test module with two separate recirculating aquaculture systems, each with two tanks. One system was oxygenated with a venturi system from OxySolutions, while the reference system used conventional oxygen cones.

They examined water quality, bacteria, growth, and various fish welfare parameters throughout two production cycles.

One of the clearest results was lower carbon dioxide (CO2) concentration in the system that was oxygenated with a venturi.

“We had about a 10-13% difference in biomass. Even when we normalised the raw data against the biomass, there was still a pretty clear difference in favour of the venturi oxygenated system.”

Netzer believed this provided a strong indication that venturi oxygenation could contribute to lower CO₂ levels.

Another notable difference was the clarity of the water.

“We also saw that the water in the venturi oxygenated tanks was significantly clearer. The turbidity was about 30% lower.”

The difference increased as the production cycle progressed.

“In total, we followed two production cycles and found that the results were quite reproducible. We saw the same trends in both cycles.”

The researchers also recorded favourable differences in other water quality parameters, including redox potential, total ammonia nitrogen (TAN), and nitrate.

Sporadic measurements also indicated lower total bacterial counts in the venturi oxygenated system. However, the data base was limited.

Black sludge surprised scientists

A more unexpected result was that the sludge from the venturi oxygenation system took on a distinctly darker colour.

“One difference was very clear, and it came as a bit of a shock at first: the system that was oxygenated with a venturi had black or dark grey sludge.”

In the reference RAS that used oxygen cones, the sludge had a normal appearance.

The researchers hypothesise that the colour difference is due to oxidation of dissolved manganese, where manganese (II) ions are converted to manganese dioxide.

“This is most likely due to more oxidising conditions in the venturi oxygenation system.”

The measurements of redox potential also indicated more oxidising conditions.

Netzer highlighted the surface chemistry of nanobubbles as a possible explanation for several of the water quality differences. The very small bubbles have a large total interfacial area and can thus affect the interaction between water, particles, biofilm, and microorganisms.

At the same time, he said it is worth noting that the venturi system produces bubbles of various sizes. He believed that the field trials therefore show the effect of the oxygenation system as a whole, not the isolated effect of the nanobubbles.

No clear effect on fish growth

Despite the differences in water quality, the researchers found no clear differences in the biological performance of the fish.

Survival was very good in both systems, and venturi oxygenation had no detectable negative effect on this parameter.

There were some weak indications of better growth and feed factor in the system with venturi oxygenation, but the differences were small.

“I want to emphasise that these are only weak indications, not clear differences,” said Netzer.

The researchers also investigated whether the different oxygenation affected how the fish were distributed in the tanks, but found no difference.

During the question and answer session, Netzer was asked whether the surface activity of the nanobubbles and the more oxidising conditions could cause oxidative stress and affect the fish’s mucus layer and gills.

He replied that the researchers had not investigated this in detail, but that so far they had found no signs of negative effects on the fish.

“My assessment is that our system creates a good number of nanobubbles, but that the concentration is still so low that they do not affect the fish, at least not as far as we have been able to see.”

He also referred to experiments with other nanobubble generators, where researchers had not observed negative effects on the mucus layer or fish behaviour. He also emphasised that the mucus layer had not been studied in detail.

Must distinguish between oxygen and water chemistry

Netzer summarised that the venturi system produces a wide range of bubble sizes, with different properties and functions.

“Microbubbles are mainly responsible for oxygen transfer and can also help remove CO₂. Nanobubbles still have an important role. They are stable and contain very little gas, but have an enormous total surface area. This is important because of their surface chemistry. They contribute very little to the amount of dissolved oxygen. At the same time, they affect particles, biofilm, bacteria, and other water quality parameters, including turbidity,” he said.

“Nanobubbles contribute to a positive effect on water chemistry through surface chemistry, but not to a significant extent to oxygenation. It is important to distinguish between these effects. Oxygenation and water quality should therefore be considered as two separate goals, and not mixed together.”