By: Pål Mugaas Jensen, editor at Landbased AQ. Article originally published in Norwegian.
What happens to the water quality in a RAS facility if you add nanobubbles? And do you see other effects on the system efficiency and fish health? These were some of the answers the technology company Moleaer wanted to answer when they took a scientific approach at Lødingen Fisk.
The testing Moleaer did at Lødingen Fisk has resulted in a report called "Effect of oxygen nanobubbles on performance and water quality in recirculating aquaculture systems (RAS)". In this article, Federico Pasini, senior R&D scientist at Moleaer, explains the results they got.
Pasini says the goal of the study was to quantify how the introduction of oxygen nanobubbles can improve water quality, reduce energy and water consumption through improved oxygenation, better particle removal, increased nitrification rates and prevention of biofilm. The study was carried out in two stages to evaluate the short- and long-term effects on RAS.
The first was performed to characterize the effect of nanobubbles on the process within 48 hours of starting up the nanobubble generator (NBG). The second round of measurements was after 50 days of continuous operation of the NBG, with fish much larger fish and with a significantly higher oxygen requirement, during a period when the facility was at 60% of full capacity.
"We also studied what happened and how the process responded to the presence of nanobubbles. Finally, we also tried to look at what happened to the fish, although we still have questions about that. Nevertheless, we identified some interesting points for future studies," he says.
PROPERTIES OF Nanobubbles
- Nanobubbles are typically between 80-200 nanometres in size and are thus the size of bacteria and viruses.
- Nanobubbles behave differently when it comes to stability, surface charge, neutral buoyancy, oxidation, etc. because they are nanoscopic.
- Nanobubbles have neutral buoyancy and can remain suspended in liquid (most often water) for several weeks without rising to the surface and disappearing.
- With their nanoscopic size, nanobubbles ensure that a huge total bubble surface remains in contact with the water – altering and enhancing many physical, chemical and biological processes.
- Nanobubbles have a strong negative surface charge that prevents them from merging and enables them to physically break up emulsions and coagulate small particles – such as emulsified fats, oils and lubricants – from water.
Results from the short-term study
Determine the presence of nanobubbles
"So what happened immediately after you started the nanobubble generator?"
"As expected, we were able to detect oxygen nanobubbles in the process. Measuring nanobubbles or nanoparticles is not entirely simple; There are many variables, and precision and a strict protocol are required. We measure nanoparticles usually before the experiment, as a baseline without nanobubbles, i.e. before the generators were started. We then continued to collect samples after the nanobubble generators were started, and we measured a net increase in each step of the process, averaging 60 million per ml," he says.
"And we know from our experiments in the laboratory and other experiences in the field that this concentration is definitely sufficient to see an effect on water, biology and to some extent biochemistry. We therefore know that in general, regardless of which water system we treat, we expect a reaction at that concentration," says Pasini.
He says they saw that in about three hours, there was a net increase of about 140 million nanobubbles per milliliter, with an average size of 170 nanometers. These nanobubbles disperse and dilute throughout the system, resulting in this average concentration in the process.
Highly EFFICIENT oxygen DISSOLUTION
He explains that by creating such small and stable bubbles, we can ensure a very high oxygen release efficiency, which was shown immediately through the metrics in the four sampling points. "We had an immediate increase in oxygen at all stages of the process."
But a very interesting and clear effect was also seen on scrubbing and removing biofilm (see also fact box below).
The scouring properties of nanobubbles
- Nanobubbles have a very hard surface that can mechanically tear off biofilm.
- Think of a nanobubble with oxygen like a balloon. A balloon is filled with gas covered by a shell of elastic plastic that holds the gas in. In a nanobubble, the shell is replaced with a small layer of OH ions around the gas. They are electrically charged and specifically attract the water around them through hydrogen bonds. This gives the bubbles a very hard shell because they are effectively made of ions and electrical charge that holds the bubble together.
- And since you are at the nano level, the forces are greater in relation to the size. The scale of the curvature around the bubbles is small enough for the water structure itself to be interesting.
- One of the things that happens is that the surface tension in the water changes locally so that you get a reorientation of the water molecule. From the outside, you will see this bubble as usually negatively charged, because you have more OH- facing outwards in the water molecule. Therefore, they bind to different ions, they participate in reactions, both chemical and biological, and one can think of them as carriers. In other cases, since they are charged, they are very interactive with surfaces, such as biofilm.
"We know nanobubbles will be involved in one way or another in everything that has to do with surface interactions. In this case, this causes some of the biofilm and some of the old coating on the surfaces of the tanks and pipes to loosen. Within 24 to 48 hours, we see an increase in solids, which then dissolve over time, which in turn results in less need for ozone.
Nanobubbles therefore create a scouring effect where certain types of biofilm detach from surfaces, which then go into the water and can be filtered out.
The nanobubbles themselves have a very hard surface, and with the speed they have, they are actually able to scour loose biological material, so that it is suspended in the water stream and can then go to the drum filters where it is taken out.
"This is why we see an increase in TSS (total suspended solids) at almost every step of the process, especially from the fish tank. We can see that during 48 hours there were more solids, indicating the release of more particles. The feeding rate was the same, so it wasn't because of any other conditions in the tank.
"We also tested the oxygen release efficiency, and we measured an efficiency of over 90% at that level of gas injection, which was low at the beginning of the process. We also saw indications of improved disinfection, which was later confirmed by the UV and ozone data," Pasini says.
The effect on nitrification
"The most interesting thing, in my opinion, was the beneficial effect on the biofilter, especially on nitrification," he says (See Figure 2). Because in the end, it is nitrification that determines how much load the plant can withstand.
"Even though you can add more oxygen, increase feeding and increase fish density, you will still need a stronger and more efficient biofilter to handle all the ammonia, so you don't create any risk to fish welfare.
Figure 2: Nitrite effluent from biofilter
They observed that the presence of nanobubbles had an immediate positive effect on nitrification performance.
Biological nitrification is a process that involves specific types of bacteria, divided into two categories that perform two different processes. First, AOB bacteria (aerobic bacteria) that use oxygen to break down ammonia into NO₂ (nitrite). Then, another group of bacteria, called NOB (nitrite-oxidizing bacteria), takes this nitrite and converts it into NO₃ (nitrate), which is the end product of biological nitrification.
Nitrate is the least toxic form of fish, while nitrite is highly toxic even at low concentrations.
"It is therefore very important that we prevent the accumulation of nitrite, which is a symptom of a partial nitrification process. We show that with nanobubbles, we help to complete the step that was previously limiting, and overall we have a positive effect on water quality, which is very important since nitrite is so toxic to fish," he says.
Results from the long-term study
Clearer water
The first effect achieved from the start, where biofilm was removed from the pipes and surfaces, gradually resulted in clearer water at the plant. See also Figure 3.
The nanobubbles adhere to surfaces and create a gas coating on them.
"We can see this as a beneficial effect on turbidity. After 50 days, when the fish were much larger and they were given much more feed, we had a much clearer water than when we started," says Pasini.
Economically beneficial
Another valuable observation that is highlighted is that the dissolved oxygen increased at each stage of the process.
The water quality in the tanks was significantly improved after nanobubbles had been allowed to work for 50 days.
"And what is more relevant is that we increased the utilization of the oxygen by 60 percent compared to just constant oxygenation. We were thus able to save a lot of oxygen compared to what they would normally use in the plant. This is a very strong economic argument for this type of technology," he says.
Improved nitrification
Federico Pasini says they also confirmed that nitrification was improved with nanobubbles in the water. See also Figure 2.
"The most important source of ammonia in the process is the feed, and the feeding increased throughout the experiment. But we saw that nitrite levels were getting lower and lower, which means that the performance of the biofilter had improved.
Fish Tanks
"We also saw that CO₂ levels followed roughly the same trend, so it may appear that it also has a positive effect on degassing, probably as a result of a better carbon conversion in the biofilter. So overall, we improved the water quality and reduced oxygen consumption," he says.
The efficiency of the biofilter, measured as nitrification rate, was found to increase by over 60%.
As a result of the cleaning and scouring effect and the reduced turbidity, the overall specific consumption of ozone per tonne of biomass also decreased by almost 70%.
"This confirmed that we had a positive effect on biofilm prevention, surface cleaning, disinfection and the overall turbidity of the water," says Pasini.
Improved fish welfare?
The effects on water quality also led to some interesting observations, which he adds they will continue to dig deeper into.
"More studies will be required to really understand what is happening when it comes to fish welfare. But in general, we saw indications of positive effects on fish welfare in terms of feed factor and relative growth index. Despite the observations we made and the data we collected, we absolutely need to investigate further before we draw any final conclusions," he emphasizes.
"What we can conclude from the studies is that we generally had higher stability in dissolved oxygen (DO), as the oxygen content was higher overall and less variable throughout the process. This can certainly have a positive effect on fish welfare.
The fact that the turbidity in the fish tank was reduced can also have a positive effect on fish welfare, as well as prevent fouling and bacterial formation to a certain extent.
"The improved biofilter efficiency, generally lower concentration of ammonia and nitrite, as well as lower CO₂ concentration are all parameters that mean a healthier environment for the fish.
Pasini says when it comes to further research efforts related to fish health, they will collaborate with other research institutions that have the expertise to study the medical and more metabolic aspects of fish growth in general.
"We will do our best to give them as much knowledge about nanobubbles as we can offer, to try to solve this puzzle," he concludes.
