Nanobubbles in Water

Hello, welcome back to the fifth episode in our Nanobubbles 101 self-led audio course. And once again, I am your host, Niall English, CTO of AquaB, and also a professor in chemical engineering at UCD in Ireland. Now let's talk about a particularly important application area today, which is water treatment and wastewater and the gasification of water, in particular the aeration, but other gases too.

I suppose the status quo is that we, and by we I mean mankind, we're putting several percent of our electricity supply all around the world in each country into the aeration of water in water treatment plants, and to some extent in irrigation and to some extent in aquaculture as well. And we are using these vast blowers, for example, in activated sludge tanks, occasionally sometimes aerators based on movement in lakes and so forth to combat algae. And essentially we're just generating in most of these cases very large bubbles that rise up into the atmosphere, the great yonder of the atmosphere within a minute or so or less.

So being able to make bubbles and the air and gas that we're introducing actually stay in the water for much longer is in essence some type of holy grail of aeration or gasification, whether we're adding air or CO2 or oxygen for whatever purpose. The typical level of transfer efficiency of the oxygen in established bubbles into the surrounding Henry's Law state driven by Fick's Law, that's typically of the order of no more than a few percent. If we're using the latest and greatest microbubble generator, that's substantial energy expense because of small pores and membranes, then that level of transfer efficiency might get up towards of the order of 12, 15 percent, but at a substantially higher energy cost.

Anyway, the energy cost of this aeration is ruinously high. So in order to be able to deal with this, we have to see if we can generate much longer-lived bubbles at a reasonable energy cost or a lower energy cost. And if we could do that, then can we simply because those bubbles are in the water for much longer, by default, can they just transfer a much greater proportion of their sacred cargo or their sacred treasure of oxygen or carbon dioxide or whatever gas is desired into the regularly-solvated phase, in other words, the Henry's Law phase of regular isolated molecules?

And that's very important. So whenever I discuss conventional dissolution, I'm talking about a single solitary molecule, whether it's CO2 or oxygen or nitrogen or hydrogen, surrounded by many solvent molecules, typically water or it could be petroleum or whatever the other mother liquid is. Whereas a nanobubble, of course, is a massive clump of thousands or tens or hundreds of thousands of gas molecules all gathered together.

And they, of course, release their cargo of oxygen or carbon dioxide or their gas molecules from reservoirs, which effectively they are by Fick's Law, which posits that there's a driving force from a large supply of abundant gas molecules into the wider water at large or liquid at large where there's very few gas molecules. And each gas molecule is surrounded just by water or solvent molecules. So effectively, these nanobubbles serve as an efficient reservoir to release slowly or quickly, depending on how voracious the appetite of the water is, as defined by the biological and chemical oxygen demand, for example, or demand for other gas, for example, if we're sparging carbon dioxide in to grow out certain strains of algae, for example.

We can discuss that a bit later. So in essence, nanobubbles, if generated properly and if they're inherently metastable, could potentially deliver gas transfer efficiencies of the order of perhaps 80 to 90%, instead of a few percent with regular bubbles, macrobubbles that is, regular macrobubbles, or perhaps 12, 15% with small major scale bubbles or micro scale bubbles. So simply because they're in the water for so long, because they don't rise.

And if you can evade buoyancy and use their Stokes law, then that means you get to contribute to the Henry's law level, or in other words, regular dissolution from the nanodesolved phase or nanobubbles themselves. So that really very much is the goal in water treatment and just general aeration or gasification of water, whether it's in aquaculture, oil and gas, fermentation, irrigation, general water treatment would activate the sludge, industrial water treatment, enhanced flotation, dissolved air flotation, etc. It really is about boosting the level of gas transfer efficiency and that excellent surface area to volume ratio that we have with nanobubbles, as opposed to their larger macro bubble or even mesobubble counterparts.

So for example, one important thing if we look at flotation operations, for example, in DAF, dissolved air flotation, is that the small natured nanobubbles has a high surface area per unit volume. And then because of the electrostatically attractive and active surface skin, if you like, of the nanobubbles as measured by their larger magnitudes, this is as a potential. They can attract a greater variety and they can attract more strongly the adsorbates and micro particulates, salts and all these charged molecules.

An analogy might be say, a magnetic analogy might be having like a magnet attracting all of these iron filings, except with nanobubbles we're talking about an electrostatic analog of that, if you like. But anyway, soon these kind of initially naked virgin nanobubbles are clothed and cloaked in this entourage, if you like, or an electrostatic orbit of hangers-on that adsorb electrostatically. And then the nanobubble, I suppose, winds up being at the center of what becomes every time a micron scale colony.

Now, if something grows into the micron scale, you can't evade buoyancy and Stokes law, and then you begin to rise up, but much more quickly. So using nanobubble-enhanced DAF is like having regular flotation on steroids. And of course, this can also be used for treating of tailings, etc.

in downstream mining operations, for water treatment in mining operations. So this is an important area. Also in the case of activated sludge, simply being able to reduce the energy demand by having the blowers switch on less often.

You can have legacy blowers on a relay switch linked up to your level of desired dissolved oxygen.

That means we can keep the level of DO in terms of close to its Goldilocks zone target set point of, say, for example, 3 mg per litre. But having the blowers, which are energy expensive, switch on a lot less because we're able to create a population of nanobubbles, and we only need to switch on the blower again once those nanobubbles are consumed by the hungry, activated sludge microbes with their voracious appetite for dissolved oxygen. But then again, that's the whole point.

To feed these little critters the DO they need to get on with the job of cleaning the water biologically, which they do so well, if only they can get enough DO to keep them going. It's their fuel, you see, the DO. Similar with fish.

I mean, fish need oxygen just as much as us. They just take it in dissolved form from the water through their gills. And by having Nanobubbles there, we can reduce the aeration energy requirement.

And also the reactive oxygen species can help with some of the problems that we have in aquaculture, such as lice and so forth as well, the reactive oxygen species. So that's another secondary benefit. Equally with ozone in water treatment, we can use that for disinfecting water, for example, in hospitals or situations where we might be, say, cleaning lines or piping in chemical plants as well.

So in terms of other areas, we've all heard of PFAS or the forever chemicals in water. With all of these fluorinations and perfluoroalcanated active groups, and in reality, there's thousands of these, because of these are electrostatically active, in many cases, they can also be adsorbed onto nanobubbles, as well as some residual levels of salt. So we can achieve preliminary levels of, to some extent, of desalting at a modest level, but also we can remove a plurality, a sizable minority of PFAS molecules as well.

So essentially, we can think of nanobubbles as effectively giving the water a molecular head start, if you like, or a molecular head will start to say more advanced downstream membrane processes. So in other words, if we can do some heavy lifting by using nanobubbles to accumulate some salt or some PFAS or other electrostatic impurities that are unwelcome, then if we can have those float up to the top using DAF methods and skim them off, we can then take the residual water and then perhaps give that over to more conventional membrane approaches for say desalting, desalination or treatment of PFAS as well. So I'd like to think of Nanobubbles as a molecular level head start to do some of that heavy lifting as well.

In terms of the wider environment, I was alarmed to learn about two years ago, I found out that well over a third, perhaps even almost 40% of methane going to atmosphere was originating from undererated waterways. And I was utterly shocked. I hadn't known that until about two years ago.

And I know that much methane is generated from bovine sources, from cow cattle and other agricultural sources. But such a vast amount coming from undererated waters, I didn't know about. And this is leading, of course, to terrible problems with algae blooms and cyanobacteria, blue-green cyanobacteria, from the blue-green algae.

And that arises from water that's effectively dead or stagnant and is undererated and underoxygenated. So if we can get some long-lasting, long-lived Nanobubbles in there with some of the reactive oxygen species that can also improve some of the diversity and get the benthic zone aerated to some extent, that would really suppress eutrophication, which is the production of methane down in the deep anoxic zone of the water body. So it's using Nanobubbles for that purpose, especially linked up with solar power and floating photovoltaics or FPV, whether on or offshore, is a great idea, especially if the energy front print is modest.

And that's something we're working on at AquaB. Not that I'm particularly here to talk too much about what we're doing at AquaB, but that's something we're working on. So this can also really, really help to try to decouple, if you like, bad air from bad water, if you want to talk about it simplistically.

So if we clean the world's water to suppress eutrophication, in many ways, we're cleaning the world's air. So Nanobubbles really have a lot to contribute to the world's atmosphere as well by reducing eutrophication. One final area that I did mention earlier, I promised to come back to, is say production of algae.

So the only gas in water is not just air or oxygen. Let's talk about carbon dioxide for a minute. So we may deliberately and willfully wish to grow and cultivate algae.

Certain strains of algae respond very well to being fed with carbon dioxide. So if we can introduce dissolved carbon dioxide in the form of nanobubbles, it may allow us to reduce the level of nutrient we need to feed to produce algae, and it may reduce the total need of CO2 we need. And if less CO2 is going up to atmosphere, because most of it's staying in the water in the form of nanobubbles now and being used by the water itself, then that's a good thing, that reduces carbon dioxide emissions substantially.

I have alluded to one thing here by talking about nutrients and carbon dioxide or any nanobubble gas being able to reduce nutrients. And that's an important point that I want to talk about here in water treatment, even in broad terms. And the important point is that nanobubbles, because they're electrostatically active, for example, as evidenced by their zeta potential, they will attract a series of electrostatically active molecules, which can include a nutrient or a fertilizer or a surfactant, for example.

Say we're trying to add fertilizer to water for the purposes of irrigation or in farming. Something that we certainly found that could be is that we can use less fertilizer. Because a lot of the fertilizer molecules are attracted to the nanobubbles, then they will have a much higher delivery efficiency, because since the nanobubbles themselves can penetrate through the plant cell wall matrix, they can do so with their cargo of adsorbed fertilizer molecule to get in much more easily into the plant.

And the same is the case with, for example, perhaps some type of adsorbed fish feed molecules or adsorbed molecules for nutrients for algae growth. And also, I would say, for surfactants that would be added, for example, as phase segregation agents for oil-water separation, or for example, in upstream oil production, or gas production, where we're adding surfactants to nanobubbles that can penetrate the rock pores more easily to get to the intercalated hydrocarbon that's stuck in the rock-bound pores. So being able to use less additives is clearly a very attractive proposition, whether it's less fertilizer, less nutrient, less surfactant, because many of these fertilizers, surfactants, etc.

are very expensive. And of course, and I know that the agriculture industry, oil industry, fish farm industry don't want to talk about this, but I certainly will, excessive use of fertilizer or nutrients, excessive use of surfactants, or chemicals in general, ultimately makes it into the groundwater. In other words, ultimately makes it into drainage water, draining from fields and into lakes and reservoirs and estuaries, which means that these go into overdrive, creating blue-green algae, and really destroys the water quality.

So if we can use nanobubbles for the upstream, more environmentally friendly production of oil, gas, water, water treatment, aquaculture, irrigation, fermentation, et cetera, et cetera. So we're using less nutrient, less of these chemicals, less of these forever chemicals, less surfactants, less fertilizer, et cetera, et cetera. If we're able to use less, because nanobubbles deliver it so much more efficiently, then there's less to go in the water that ultimately ends up by drainage into these groundwater supplies.

So therefore, that's solving it from an upstream level. And then at the downstream level, at the estuaries, at the rivers and reservoirs, we can, of course, use nanobubbles with or without solar power to take a second bite of that cherry and to really try to tackle this problem again by having bountiful and plentiful and inexpensive aeration to really prevent that water from eutrophying and effectively turning dead and destroying the planet with gigatons of methane. My god, the prospect is so alarming.

I really think that nanobubbles can and should help. Anyway, on that rather sober and perhaps even somber note, I wanted to thank you for listening to this particular episode about how nanobubbles can help in water treatment and water aeration and water gasification in general, what the implications are, all benign, all positive. Briefly today, to reflect on what we've just discussed, we discussed the so-called adsorption of gas molecules onto nanobubbles.

For example, fertilizers, forever chemicals, PFAS. We discussed what manipulation of dissolved gas level means, what that can mean for water treatment. We went through important trends in agriculture, aquaculture.

We discussed fermentation briefly. And of course, we spoke about some important applications in oil and gas, all of which hinder and water treatment. So once again, thank you very much for your time.

In our next episode, we're going to be examining nanobubbles in a very important industry that affects every one of us, and that is the energy sector. So I look forward to that episode, and please do join us next time. Thank you very much.