A Recirculating Aquaculture System (RAS). Image courtesy of Aquacare
A Recirculating Aquaculture System (RAS) is an onshore farming technology, which reuses (or recirculates) the same water many times over to farm aquaculture on land. This indoor and tank based system, is a highly technical and advanced method of aquaculture that is growing in popularity thanks to its environmental, sustainable and highly predictable production credentials.
One of the key reasons that it is considered to be such an environmentally friendly and sustainable farming method, is due to the limited amount of water used by this technique. Furthermore, as the aquaculturist is better able to control the normally external factors such as daylight, oxygen levels, water temperature and disease in the water, more stable conditions are created for the fish being grown. This creates less stress on the fish which means higher animal welfare and therefore yields better - and more predictable - growth potential.
However for any given RAS, achieving such outcomes will rely almost solely on the correct choice and use of technology - as well as its sound management. Here, compressed air plays a pivotal role.
Water quality is probably the most important factor in operating a successful recirculating system. To keep the fish alive and healthy, the water (which is constantly reused in a recirculating aquaculture system) must be treated continuously. It is necessary to treat the water to remove waste and to introduce oxygen. Two of the most important applications of compressed air in a RAS will therefore be biofilter aeration and CO2 stripping, which we discuss in more details below.
Fish need feed and oxygen to thrive in aquaculture. Along with clean water, these are the two major aspects which determine success in this field. However, in digesting feed - and the proteins that make up for a large proportion of the feed - nitrogenous byproducts are created. In most fish the main byproducts created will be ammonia and CO2. As ammonia is toxic for fish, it needs to be removed from the water, by either flushing it out of the system (by means of a flow through / semi recirculating system), or it needs to be converted into a non toxic nitrogen form (nitrate).
Proper filtration is therefore of paramount importance, which is why the aerobic biofilter is often considered to be at the heart of a recirculating aquaculture system. Once the water leaves the fish tanks it goes through a drumfilter where solid organic waste is trapped and removed. This initial filtration stage is important. A poorly functioning drumfilter will mean high suspended solids (a high concentration of bacteria and other organic substances) are carried over into the biofilter. And this will ultimately impact how efficiently and effectively a biofilter can operate. For example, excessive solid loads can cause plugging within aeration columns, screens and spray nozzle orifices, which could eventually result in system failure1. In addition, particles can potentially clog biofilters and reduce their efficiency2. Furthermore, solids support the growth of heterotrophic bacteria which can outgrow and compete with nitrifyers. The nitrification process is strongly inhibited by heterotrophic processes when high amounts of organic carbon are present3.
Biofilters are effectively large tanks filled with bio media. The job of the biofilter is to convert the dissolved nitrogen byproducts from the toxic form of ammonia via nitrite, to the non toxic form of nitrate through a process called nitrification. They can either be designed as moving bed bioreactors (MBBR) or fixed bed bioreactors (FBB). In either case bacteria goes to work in the tanks to break down the ammonia. Both rely on a dependable and consistent supply of compressed air to function correctly.
In the MBBR biofilters, a constant air flow is required in order to keep the biomedia moving. The movement is needed to bring the bacteria in contact with high volumes of water, so that they always have access to nutrient rich water and can perform best. One advantage of the movement of the media is that by rubbing on other bio media particles, excess bacterial growth is rubbed off.
And, in the case of an FBB biofilter, compressed air is often used to create turbulence in order to flush any settled debris out of the filters.
Most Recirculating Aquaculture Systems that fail, do so due to insufficient filter design. Amongst a number of causes this may be because either the biofilters are too small to reach the calculated capacity, or because the drumfilter can’t take out small particles which leads to high turbidity.
Another important step in filtering the water comes after the biofilter stage. Before the water goes back into the fish tanks, the carbon dioxide which has now accumulated from the fish as well as the bacteria in the biofilter must be removed. A method called CO2 Stripping is therefore employed.
There are two main methods of CO2 stripping - the aeration well system and the trickling system. In the aeration well system, heavy aeration diffuses air into the water. By diffusing the air into the water, the dissolved gases in the water strive for an equilibrium between the molecules in the water and the diffused air. As the CO2 concentration in the water is initially much higher compared to the air, the CO2 diffuses into the air and is finally stripped from the water. Furthermore mixing the water in the chamber will give it more contact with the surface where more CO2 is leaving to the atmosphere. CO2 stripping should always be implemented after the biofilter, as a biofilter in a Salmonid system accounts for roughly 37% of the total CO2 emissions into the system4.
With a trickling system, the water hits multiple obstacles that diffuse the water drop into many small drops with a much larger surface area. This makes it easier for gas molecules to exchange and reach an equilibrium - if enough fresh air is supplied. Is it the opposite of aeration but with the same end result i.e stripping the gases off the water.
Both biofiltration and CO2 stripping require aeration 24/7, 365 days a year. If these processes failed, the aquaculturist would only have a maximum of 30 minutes until mass mortalities occur. Failure of this process can therefore result in losses as high as 100%. You can therefore start to understand why investing in a reliable aeration system that has enough redundancy is critical in designing a RAS.
Be sure to join us next month for Part 2 of this blog post, where we will discuss how and why compressed air is essential to the oxygenation process required in a Recirculating Aquaculture System.