Recirculating aquaculture systems (RAS) have been developed to tackle environmental issues linked with conventional aquaculture, such as water consumption and waste management. Nonetheless, waste remains an unavoidable byproduct even in these advanced systems. Hence, the question arises: How can an industry committed to sustainability effectively manage waste without compromising its core principles?
Let’s start by considering the different types of waste generated in RAS:
Solids
As part of the waste stream in aquaculture, solids are primarily made up of uneaten feed and feces from the fish. Solid waste can clog the system and degrade water quality if not correctly managed. To address this, technologies like drum filters, belt filters, and swirl separators are commonly used to remove solids from the tank water, but once the solids have been separated, what happens to them?
Nutrients
Over time, fish excrete dissolved nutrients such as ammonia, nitrite, and nitrate. If these nutrients accumulate, they can become toxic to the fish. Biofilters convert these harmful compounds into something less toxic for fish. However, it is essential to note that even these less harmful nutrients, if released into the environment, can lead to detrimental effects such as algal blooms, oxygen depletion in surface waters, and potential harm to aquatic life. So how is this waste managed?
Toxins
Apart from waste products related to nutrition, fish can also generate toxins as metabolic byproducts. For instance, fish respiration produces carbon dioxide, which can lower the water’s pH and induce stress in fish when levels become excessively high.
Carbon dioxide produced by fish respiration in recirculating aquaculture is addressed through a two-step process. First, gas exchange occurs naturally in tanks due to the surface area of the water, where carbon dioxide dissolves into the air above the water. This process, however, is not typically sufficient in high-density systems. Therefore, the primary method of carbon dioxide removal in these cases is through degassing or “stripping,” where water is exposed to air to facilitate the rapid exchange of gasses, such as in a packed column or spray bar. The expelled carbon dioxide is then disposed of into the atmosphere.
Well, that one, at least, was easy.
What is the Nutrient Cycle, and Why Does it Matter in RAS?
The nutrient cycle is vital in maintaining water quality in natural aquatic environments. Fish excrete waste in the form of ammonia, which is converted by bacteria colonies into nitrite and then nitrate through nitrification. Nitrate serves as a nutrient for plants in the environment. The plants can photosynthesize and grow by consuming nitrates while also acting as a natural filter.
The water, free of harmful excess nutrients, can provide a healthy fish environment.
In traditional open-water aquaculture systems, ammonia and nitrates generated by fish waste flow into large bodies of water. This process can cause significant damage to local ecosystems when concentrated wastes fail to disperse. However, this laissez-faire approach is not possible in recirculating systems, as the water must be filtered and continuously recirculated. In RAS, then, it’s vital to remove dissolved nutrients from the system before they have a chance to accumulate in large concentrations.
In recirculating aquaculture, biofilters are often used the same way as in your hobby aquarium, breaking ammonia and nitrogen compounds down into something less harmful. In this case, the beneficial bacteria that grow on biofilter media consume toxic ammonia and mitigate its damaging effect through nitrification. However, even though the resulting nitrites are less harmful than ammonia, high concentrations can still cause damage to aquatic organisms. In denitrification, a different type of bacteria in the biofilter breaks nitrites down even further into nitrates. Nitrates are not actually toxic for fish, but high concentrations can still stress them and make them susceptible to disease.
Fortunately, nitrate is the primary source of nitrogen for aquatic plants. In a natural environment, plants will happily take it up.
Dealing With the Solid Waste
In a closed-loop aquaponics system (RAS), the inputs mainly consist of fish and feed. Consequently, the waste solids produced are primarily composed of uneaten food, fish feces, and other metabolic byproducts. When these solids are mechanically filtered and removed from the tanks during routine cleaning, they are pretty wet. This slurry can be collected and used directly as a nutrient-rich fertilizer for houseplants, gardens, or trees. Alternatively, it can be dried, pelletized, and applied to cropland using a fertilizer spreader.
Another intriguing use for aquaponics waste solids is converting them into a concentrated gas mixture of 70% methane. This conversion process, known as anaerobic digestion, is facilitated by bacteria that thrive in oxygen-deprived environments. The methane generated from fish waste holds the potential to be harnessed as a power source for the RAS operation itself, completing the closed-loop sustainability cycle.
Dealing With the Dissolved Waste
As in nature, dissolved fish waste in a recirculating system is a mixture of ammonia and other nitrogen compounds and is dealt with through biofiltration. The result is concentrated, nutrient-rich water. This water can be highly damaging to natural ecosystems, which are not prepared to handle a sudden influx of so much food. Instead, those ecosystems are often overrun with opportunistic algae blooms, which trigger eutrophication and can lead to untold environmental damage.
Fortunately, this nutrient-rich water can be used in several beneficial ways, including as feed for plant crops in an aquaponics system or as a concentrated feed for houseplants, gardens, or even farm crops. Alternatively, the nitrates can be denitrified further into nitrogen gas by a different set of bacteria, leaving behind pure water that can be safely reintroduced into the RAS.
Waste Management in RAS: A Sustainability Challenge?
Waste management in RAS is a critical consideration when pursuing sustainable seafood production. While RAS offers significant advantages in reducing waste discharge and improving resource efficiency compared to traditional aquaculture systems, effective waste management practices must be in place to maintain environmental sustainability. Continuous research and technological advancements are addressing the challenges associated with nutrient dynamics and waste management, making RAS an increasingly viable solution for sustainable seafood production.
Nutrient dynamics and waste management are essential components of successful recirculating aquaculture systems. Through proper management of the nutrient cycle, efficient waste removal techniques, optimization of nutrient utilization, and adopting strategies to minimize environmental impact, RAS can become an even more sustainable and environmentally friendly method of seafood production. As technology advances to address waste management challenges, RAS takes on an even more significant role in contributing to the overall goal of sustainable seafood production.
