Wastewater : Nanobubbles: Changing the Wastewater Treatment Game?

If wastewater is not properly treated, it can negatively impact the environment and human health. It’s as simple as that. But because of population growth, accelerated urbanisation and economic development, the quantity of wastewater being generated and its overall pollution load are increasing globally, UN Water states.

The problem is manifold: the ever-increasing use of chemical fertilisers and pesticides as well as the usage of untreated wastewater in irrigation pollutes groundwater and surface water alike. In many areas, industry still discharges waste directly into water courses, whereas in poorer urban areas, a large proportion of wastewater is discharged untreated directly into the closest drainage channel or water body.

Added to this is the fact that safely reused wastewater is grossly undervalued as a potentially affordable and sustainable source of water, energy, nutrients and other recoverable materials.

A study from 2021 estimated that globally, only about 52% of sewage is treated. But the sewage treatment rates vary enormously in the different countries around the world. While high-income countries treat approximately 74% of their sewage, developing countries treat an average of just 4.2%.

UN Water is monitoring the percentage of wastewater flows from households, services and industrial premises that are treated in compliance with national or local standards (see graph 1). Indicator 6.3.1 is available online. UN Water Target 6.3 seeks to halve the proportion of untreated wastewater discharged into water bodies worldwide.

• It is estimated that over 80% of wastewater is released to the environment without adequate treatment. (UN WWDR, 2017)
• 44% of household wastewater worldwide is not safely treated. (UN-Water 2021)
• 60% of water bodies assessed in 89 countries have good ambient water quality. (UN-Water 2021)
• Water quality data is not collected in the majority of countries, meaning that over three billion people are at risk because the health of their freshwater ecosystems is unknown. (UN-Water 2021)
• Of the 89 countries with water quality data, only 52 have information about groundwater. This is problematic because groundwater often represents the largest share of freshwater in a country. (UN-Water 2021)

UN Water stresses that the positive impacts on water quality and supply by increasing wastewater recycling and safe reuse will “drive progress in public health, environmental sustainability and economic development by providing new business opportunities and creating more ‘green’ jobs.”

Over the years, different methods to treat wastewater have been developed using basic physics and high technology alike to reclaim the water we use. Most often a wastewater treatment plant performs the two main stages, primary and secondary treatment, while advanced treatment also incorporates a tertiary treatment phase with polishing processes and nutrient removal.

In the last couple of years, a new method has emerged using nanobubbles. “Like a bubble, a nanobubble is a gas entity in – or at the border of – a continuous medium, usually a liquid. What differentiates a nanobubble from a regular bubble is its size,” says Marie Jehannin, an expert in surface science at 3A Composites, who has researched nanobubbles for many years. “To be considered as a nanobubble, the diameter of the bubble must be between 1 and 1,000 nm.” There are mainly two kinds of nanobubbles: bulk nanobubbles, which are fully surrounded by liquid, and surface nanobubbles, which sit on a solid at the solid/liquid interface.

Another big difference between regular bubbles and nanobubbles is their buoyancy. “A regular bubble in a liquid usually goes up due to buoyancy and will burst once it has arrived at the surface,” Jehannin says. On their way up, they diffuse whatever oxygen they can before they reach the top of the surface. Typically, these bubbles are used in aerating wastewater streams.

Nanobubbles, on the other hand, are subjected to a much smaller buoyant force due to their small volume. In the case of nanobubbles, the Brownian motion, which refers to the random motion of small particles that are suspended in liquids, would in theory overcome the buoyant force and the nanobubbles would stay in solution longer than a regular bubble, as the expert explains: “To take a very simple image, the situation is similar to the one of dust particles. Whereas big dust particles inevitably land on the floor, smaller dust particles can remain in the air much longer because their Brownian motion overcomes gravity.”
The longer contact of the nanobubbles and the water enables them to transfer gas more efficiently. In addition, nanobubbles have a strong negative surface charge.

“However, the main interesting point about nanobubbles is that, according to the well-accepted Epstein−Plesset model, they should not be stable for more than a few milliseconds: a timespan so short that it is almost impossible to detect them. So, one could say that in theory nanobubbles should not exist.”

And yet they do. In 2008 researchers published the first evidence of the long-term stability of surface nanobubbles. Seven years later, scientists finally produced the undeniable proof of the long-term stability of surface nanobubbles. They were observed to be stable for up to five days. To date, however, there is no scientific consensus on the stability of bulk nanobubbles.

Nanobubbles are only present on hydrophobic surfaces. Even though surface nanobubbles are probably ubiquitous in nature, they remain mainly unnoticed. “They were imaged by atomic force microscopy in water, both saturated with dissolved gas and undersaturated, and in different solvents such as formaldehyde, ethylammonium nitrate and propylammonium nitrate,” says the scientist. “An interesting aspect is also in which solvent they could not be found, namely DMSO and propylene carbonate.”
Where bulk nanobubbles can or cannot be found remains unknown up to now.

Nanobubbles are now the smallest bubble size known, 500 times smaller than a microbubble and about the size of a virus. Their unique characteristics are directly related to their miniature size:

• Neutral buoyancy – they lack enough buoyancy to reach the surface and instead follow Brownian motion and so stay much longer in a solution. This results in:
• High oxygen transfer efficiency.
• Strong negative surface charge – the integrity of the bubble is preserved at any depth for extended periods of time. Additionally, it improves separation efficiency in flotation processes by increasing collision probability, so more suspended matter can be floated.
• Surface area – they have more than 400 times the surface area of a typical microbubble. The high surface-over-volume ratio means that the gas transfer from the nanobubble to the liquid could be much more efficient.

These characteristics make them extremely interesting for wastewater treatment. There are a few companies worldwide that produce nanobubble generators used to improve water quality in aquaculture, agriculture, and lakes and ponds as well as to reduce chemical usage in mining and oil recovery. And for use in wastewater treatment plants.

“Their properties enable them to enhance biological, chemical and physical processes whether it be enabling separation – so removing fats, oils and grease – from waste streams that are emulsified or improving the kinetics and the aeration efficiency of existing biological systems as well as reducing the use of chemicals,” says Andrea White, Application Engineering Leader at industrial-scale nanobubble technology company Moleaer. Nanobubble research showed that there is some selectivity with what contaminants can be removed.

Good results are seen with fats, oils and grease as well as with the removal of surfactants. “Surfactants are not talked about enough within wastewater treatment, although they are ubiquitous in beauty products, household cleaners, soaps and industrial cleaners,” White adds, stressing that the presence of surfactants prohibits what is known as primary and secondary wastewater treatment since they emulsify the waste streams and so make it very hard to either float or settle out the solids during primary treatment.

When bubbles become coated with surfactants, they become larger due to the reduction of their surface tension, which in turn makes their transfer efficiency poor. As a result, they rise to the surface faster and are not able to transfer their oxygen as efficiently. Studies have shown that removing the surfactants upstream enables the efficiency of the existing aeration system to be improved. The generator can easily be installed within the existing pumping system.

“What we have seen with the process improvements translates to about a 25% increase in treatment capacity,” Andrea White says. Facilities that are overloaded and in need of somewhere between 10 and 25% increase capacity have the option to use nanobubble pretreatment to make their existing infrastructure more efficient instead of having to do a capital improvements project. According to their data, if the nanobubble generator goes in line with an existing pump system and there are primary clarifiers, the secondary treatment aeration energy alone can be reduced by up to 40%. “Across the primary clarifier we are also seeing an improvement of 10% in total suspended solids removal efficiency.”
Looking at the ratio of BOD (biochemical oxygen demand) to TSS (total suspended solids), it can be improved from 2:1 to 1:1. “So you are able to capture more of the solids and those solids contain more of the BOD,” says White. At the disinfection system with chlorine, Moleaer observes a decrease in chlorine demand which is related to the reduction in BOD demand across the secondary treatment. Additionally, nanobubbles seem to have an inhibitory effect on biofilms.

There are different methods to generate nanobubbles such as electrochemical reaction, gas diffusion through a porous membrane and cavitation. “Our patented technology does not create nanobubbles in batch, as a lot of others do, but continuously,” says Andrea White. “With our method you are able to pass more and larger soft solids through the technology without having to worry about clogging. Thus you are able to utilise nanobubbles for treating highly contaminated wastewater.” Even though there are generators that go down to 10 gallons (approx. 38 litres) per minute, the most commonly used generators range from 1,000 gallons (approx. 1,700 litres) to 4,000 gallons (approx. 15,000 litres) per minute. The maintenance requirements vary depending on the waste stream.

The pandemic also has its effect on our wastewater. With everybody cleaning and disinfecting more, there is a noticeable increase in surfactants coming into the waste streams.
An interesting observation is that with the use of high-efficiency fixtures within homes, there is less water being sent to the collection systems. This also results in contaminants concentrating in the waste stream. Now the facilities that were designed to treat a much more dilute waste stream are struggling to treat these more concentrated waste streams. “Those effects are particularly true for the US. When we educate people on how to use the nanobubble generator to address those challenges, it really seems to resonate with the US market,” White says. In Europe, on the other hand, there is a big push towards saving energy and chemicals. “Nanobubbles can also be utilised to address those issues as well,” she adds.

For the future, Andrea White hopes to basically change the whole wastewater collection system: “As I said, nanobubbles should be used as far upstream as possible,” she says with a smile.
In the collection system the wastewater sits for a very long time, it ferments, it goes septic. “So these solids that are very easy to remove start to resolve. And then it takes a lot of energy to remove the soluble contaminants. But if you can inject nanobubbles into the collection system, you can start to collect these solids intact, so that you fundamentally shift the amount of energy that’s required to treat these waste streams.”
To be able to do that, the collection system needs to be screened before it can be sent through the nanobubble generator. “I think eventually it will take people rethinking the way that they design the collection.”