Recirculating Systems 101

Recirculating systems are aquaculture systems in which the majority of water is reused following the removal of solids and the biological conversion of dissolved ammonia to nitrate. Water quality and chemistry is often modified before the water is returned to the fish. For instance, oxygen may be injected directly into the water, carbon dioxide may be off-gassed, or water may be ozonated or passed though UV light banks to control pathogens and dissolved organic compounds.

Recirculation rates are typically above 80% (20% of the water volume is replaced daily) and commercial recirculation rates often exceed 90%. Recirculating systems are gaining in popularity because they are very space and resource efficient, can be located in close proximity to target markets, and they afford a high level of control over the rearing environment of the aquatic animals. They are used in the United States primarily because the species grown in them (predominantly tilapia, but also hybrid striped bass, yellow perch, large-mouth bass, and catfish) require warmer-than-ambient water to grow optimally.

Solids Removal

The first step in recirculating water is the removal of solids. Inadequate solids removal can result in elevated ammonia levels, low oxygen levels, lower system carrying capacity, reduced water clarity, higher aeration/oxygenation costs, and a poorer quality fish product. Solids removal is very important because it dramatically reduces the organic content of the water. The bacteria that break down organic matter compete for biofilter space with the bacteria that convert ammonia to nitrate. Moreover, the bacteria that break down the organic matter compete with the fish for oxygen. As a result, excessive organic material may lead to an accumulation of ammonia and depletion of oxygen. Solids removal is generally accomplished by the use of settlement, screen filters, bead filters, or sand filters.

Biofiltration

The next step is biofiltration. Biofiltration is the use of naturally occurring bacteria to convert toxic ammonia to non-toxic nitrate through the toxic intermediate nitrite. This process is often called "nitrification" or "ammonia oxidation." Two genera (or groups) of bacteria accomplish this process. The group of bacteria that convert ammonia to nitrite is named Nitrosomonas and the group of bacteria that convert nitrite to nitrate is named Nitrobacter. These bacteria are generally classified as nitrifying bacteria because they recover energy from the oxidation of inorganic nitrogen compounds much like animals derive energy from burning sugars. This process consumes oxygen, carbon, and other inorganic nutrients and generates nitrate and lowers pH as the nitrifying bacteria multiply.

Nitrifying bacteria require constant exposure to water (they die immediately when desiccated), either ammonia or nitrite, appropriate pH (6.5-8.0, optimal 7.0-7.5), a carbon source (bicarbonate is ideal), and a surface to grow on. Nitrifying bacteria work at a wide range of temperatures, but in fish culture systems, the warmer the water, the better they do their job. If you maintain total carbonate alkalinity above 100 mg/l, the carbon requirements of the bacteria will be met and the pH will generally be appropriate.

The biofilter designs used most often in commercial systems are RBC’s (rotating biological contactors), trickling filters, or filters that perform both solids removal and biofiltration, such as bead filters. Some filters are completely submerged, some are partially submerged, while others are not submerged at all. When a filter is not submerged, the oxygen needed for nitrification comes directly from the air and is free. These filters are however at greater risk for becoming dry. System configuration, budget, objectives, and biofiltration requirements all come into play when deciding the best type of biofilter for your particular project.

Additional Water Treatment

Following biofiltration, the water may require further water quality or chemistry modification before returning to the fish. As previously mentioned, nitrifying bacteria remove dissolved oxygen and lower pH. Therefore, water may have to be aerated or oxygenated before returning to production tanks and in systems using poorly buffered water sources, the alkalinity of the water may have to by modified by metering a base into the water. Ozonation may also be used to remove dissolved organic compounds from system water and both ozone and ultraviolet radiation can be used to destroy pathogenic organisms.

The Fish Tank

The final component of recirculating systems is the fish tank. Tanks come in all shapes and sizes from round to octagonal to square to rectangular and every where in between. We recommend round tanks with sloped floors to center drains. This configuration maximizes water quality uniformity, is self-cleaning, results in the highest water clarity, is highly compatible with tilapia behavior, and construction is simple and cost-effective. When selecting a tank, the tank needs to achieve your volume requirements within the allotted space, needs to be easy to work in, has to result in immediate solids removal before the solids break up into smaller particles, and has to be harvestable.

The final component configurations, component size ratios, and component types differ for every commercial system. If you are a newcomer to recirculating aquaculture, be sure to take your time in research. Remember also that there are very few "new ideas" that haven’t already been tried and few mistakes that haven’t already been made. If you have an engineering or mechanical background and you think you’ve figured out a better way to raise fish than anyone else, WATCH OUT!! - you probably don’t. Besides, you probably haven’t considered the fish’s perspective enough. If you know about fish, but not engineering, by all means, get help. If you are considering making aquaculture a business, it is all about management - managing money, personnel, fish, water quality, and product quality. Don’t be in a rush, do your homework, and we wish you the greatest success.