In the previous post, we touched on the pros and cons of utilizing recirculating aquaculture systems (RAS) for mud crab aquaculture.
An efficient recirculating aquaculture system relies on several technologies working in tandem to achieve optimal water chemistry.
In this article, we will discuss each of the unit operations involved in most recirculating aquaculture system (RAS).
The most important design criteria for recirculating aquaculture systems (RAS) is to determine the optimal pathway to remove solids. The general idea of husbandry revolves on the idea of caring and cultivation of animals which in most cases involves providing the animals with nutritious feed. Over time, leftover feed tends to accumulate in the form of solid waste. The by-product of the metabolic process also generates solid fecal matter. Accumulation of these solids in the culture tanks will often result in pollution of the water resulting in stress and mortality. Hence, it is important for farm operations to periodically remove these solids from the recirculating aquaculture systems (RAS). Typical technologies involved in removing solids involves mechanical filtration and foam fractionation. Mechanical filtration involves using some means of constriction to sieve out bigger solids while allowing water to pass thru the sieve. As a result, the solids are trapped in these mechanical filters and removed from the recirculating aquaculture systems (RAS). Hence, it is typical for recirculating aquaculture systems (RAS) to employ media filters or drum filters to remove solids. Foam fractionators are also used in tandem with media or drum filters to remove finer solids.
The bulk of the solids will be removed from the recirculating aquaculture systems(RAS) by the means of solid removal. However, some of them might accumulate in no-flow zones while decomposing in the water. The decomposing organic matter often results in high traces of ammonia in the culture water. The fecal matter which is high in ammonia will also contribute to the ammonia levels further polluting the cultured water. Unlike leftover feed or fecal matter, ammonia exists in a solution form prohibiting the removal via mechanical filtration. It is also noteworthy to mention that ammonia is toxic to animals, and keeping low levels is one of the key challenges for many recirculating aquaculture systems (RAS). Hence, biofiltration was incorporated into the recirculating aquaculture systems (RAS) to convert ammonia into other compounds. Biofiltration employs the help of the microbes known as nitrifying bacteria to convert ammonia to nitrite and subsequently nitrate. These microbes keep the ammonia levels low in the culture water enabling a toxic-free environment for the culture animals. Designing and maintaining an optimal environment for these microbes is no easy feat, and the success of many farms is dependent on the competency in biofiltration. Typical technologies include the moving bed bioreactors, tricking filters and fluidized bed filters which are known as bioreactors. The key concept for bioreactors is to provide surface area and good living conditions for the bacteria to efficiently convert ammonia into a less toxic form.
Once water is stripped off solids and dissolved ammonia, the next step is to remove pathogens that might cause diseases. Bacteria, viruses, and parasites are destroyed during the process prohibiting the build-up that might cause an outbreak. Diseases control technologies are mainly achieved by incorporating an ultraviolet (UV) reactor or ozonation.
For some super intensity recirculating aquaculture systems (RAS) that have a high consumption of oxygen, it is crucial to add in oxygen to ensure the availability of oxygen for the culture species. It is also noteworthy to mention that the biofiltration process requires oxygen to convert ammonia. Hence it is to ensure ample supply of oxygen is available, typical technologies like oxygen cone, venturi aerators or air blowers are used in recirculating aquaculture systems (RAS).
Successful recirculating aquaculture systems (RAS)depend on the efficiency of these individual unit operations. A chain is as strong as its weakest link, failures in any of the unit operations are likely to result in a catastrophic event.