Foam Fractionation Technology in RAS for Mud Crab Aquaculture | Aquaculture Technology
It is relatively popular to utilize foam fractionators in saltwater aquariums for enhanced solid removal. Despite its popularity, many are still confused about how foam fractionator removes waste from the cultured water. If you have not read our post on the unit operations for recirculating aquaculture, please check out this post here. In this post, we will be discussing on the theory of operations for foam fractionators and their application in our vertical mud crab farm in Malaysia.
To understand the need for foam fractionators, we need to first understand how foams are created in the culture systems. Foaming is a result of excessive proteins in the cultured water either from the feed or a dead crabs. Most of the time, excessive feed and faecal matter will be filtered out from the culture water with the usage of drums or sand filtration. Sometimes, some of these solid will escape the filters as they disintegrate into fine pieces as they flow through the pumps. This causes these solid, usually less than 30 microns to dissolve into the water creating a nasty layer of foam. Hence, here comes the need to remove these dissolve protein to reduce the amount of ammonia treatment in the subsequent unit operations. Note that the adoption of foam fractionator technology cannot replace the primary method of solid removal via drum filtration or sand filters. The general idea is for these primary methods to remove 70-90% of the solids in the culture systems, while foam fractionators will remove the remaining 10-20%. It is also important that different feeds will create different amounts of foams, and this is particularly observed in a low amount of foam created with formulated feed. Note that foam fractionators are typically used in saltwater conditions, and they are not used in freshwater culture systems.
Foam fractionators are the unit process involved to remove dissolved proteins or organics in the culture systems. The commercial equipment that performs foam fractionation is called protein skimmers. Protein skimmers are typically found in saltwater aquarium tanks used in the hobbyist industry. All protein skimmers rely on the same fundamental principle of creating fine air-bubbles to foam out the dissolved proteins. The typical difference between the efficiency of these protein skimmers lies in how these air bubbles are created. High efficiencies protein skimmer relies on creating very fine air bubbles that act on the culture water. Typical methods to make air bubbles are using an air-pump with diffuser, venturi or a needle wheel pump. Needle wheel pumps are often able to create very fine air bubbles by using special pump impellers that chops up the incoming air-water mixture. However, needle wheel pumps are commonly used in the hobbyist industry and not in the commercial recirculating aquaculture systems. Due to the standardization of equipment used in recirculating aquaculture systems, the venturi systems are usually the preferred method to generate air bubbles in a protein skimmer. Some commercial skimmers also use a 2-pumps system, where the former is responsible for bringing in water for treatment, and the latter is used to generate air bubbles.
Choosing your protein skimmers can be a daunting task given the wide selection in the market. The first criteria are to look at the volume of air bubbles generated. However, foam fractionators are usually sold by the volume of cultured water flowing through the protein skimmer. In theory, the treatment efficiency is only dependent on the amount of airflow and independent on the water flow. Apart from looking to purchase this equipment, you can also opt to attempt to build your protein skimmer based on the concepts shared in this post. Ideally, you want to 1) maximize the volume of air bubbles created, 2) minimize the air bubbles created, 3) increase the retention time of the bubbles spent in the column by increasing the height of the column. Note that all skimmers come with a collection cup in which the waste is collected and disposed daily. The operation of the protein skimmers requires daily intervention to adjust the water height by using the valve, and fine adjustment is needed after feeding.
There are pros and cons when using a protein skimmer. The pros include cleaner culture water that is free of these dissolve proteins. Lower protein levels in water often reduce the biofiltration equipment and reduce the overall oxygen requirement in the biofilters (freeing up more oxygen for the crabs) more on biofilters here. Furthermore, cleaner water also increases the Ultraviolet transmittance levels and reduces the need for high power UV lights. The cons include higher power usage to generate air-bubbles and pumping water through the skimmer. Note that you will also need skilled operators to adjust the foaming rates in the skimmer based on different feeds.
In conclusion, foam fractionators are an excellent piece of technology that should be adopted in a recirculating aquaculture system. We only decided to incorporate the protein skimmer one year into operations and decided that the pros outweigh the cons.