Introduction to RAS (Recirculating Aquaculture Systems)

What is RAS?

Recirculating Aquaculture Systems (commonly referred to as RAS) are a type of land-based aquaculture production structure for a variety of aquatic species (both freshwater and saltwater). This highly productive, intensive farming method focuses on reusing water resources after continuous treatment while also providing a highly controllable environment for the reared species.

Advantages & Disadvantages of RAS

RAS facilities offer numerous advantages besides reusing water supplies (low carbon footprint) and high-density production (recommended for more than 40 kg/m³ density). These systems can provide an optimal living environment for the reared species (temperature, water quality, saturation, feed, etc.) with high biosecurity standards (pathogens & predators) while at the same time preventing them from escaping (loss of stock plus invading into the wild ecosystems). In addition to the eco-friendly nature of RAS facilities, the water treatment to collect and control the waste can really minimize water pollution. Last but not least, the space required for these facilities is significantly less than the sea cages, which can also be constructed anywhere, giving investors a huge advantage.

On the other hand, RAS farms are operating at very high costs (equipment, staff, electricity, etc.). Well-trained and highly educated staff with technological and rearing knowledge are needed for the operation. RAS facilities run at high risk, and the staff needs to confront malfunctions, power shutdowns, or even diseases if the biosecurity level is low. That is why it is also important to have backup support from external consultants 24/7. Moreover, continuous monitoring of water parameters is a must for the operation, meaning sensors and alarms have to be set up in the correct order and place, with correct values, working the whole day, and updated constantly.

Reared species

All fish species reared in sea cages can also be produced in RAS facilities. The most common species are:

Atlantic Salmon

Arctic Char

Trout

Catfish

Halibut

Turbot

Sole

Tuna

Shrimps

Sturgeon

Yellowtail Kingfish

Barramundi

Cobia

Sea bream

Sea bass

Red porgy

Carp

Operation and Construction in RAS facilities

In RAS facilities, the water is pumped into the system only at the beginning of the procedure. The amount of input water the facility needs daily depends on the type of facility. A semi-flow-through farm might need up to 30% new water of the total water volume, while a completely closed RAS facility only requires 5% to compensate for water loss and evaporation (Klaoudatos S., 2010). The input water has to go through UV light to be sterilized so that no pathogens can enter the system and infect the reared species. If that happens, the infection can be instant and catastrophic due to the high density. A well-designed facility exploits gravity to its maximum to transfer the water from one stage to the next, reducing the energy cost as much as possible. Otherwise, more than one pump is needed, so they have to be synchronized to achieve the intended water flow per hour at each stage.

The water in RAS does not follow a single linear direction but splits frequently into different paths, creating more than one loop. For example, after UV light filtration, some water can go through the degasser, and the rest can go straight to the fish tanks and/or the biofilters. But let's take the stages one by one.

Biofilters

The most essential part of a RAS farm is its biological filtration. Biofilters can convert the toxic ammonia (NH₃) that comes from fish as a waste product into less toxic nitrogen compounds (nitrites NO₂ and nitrates NO₃). Following the Nitrogen Cycle, bacteria of the genus Nitrosomonas that grow in the biofilters will convert NH₃ into NO₂. The next step is for the NO₂ to be oxidized into NO₃ by bacteria of the genus Nitrobacter. The process of converting NH₃ into NO₃ is called nitrification and requires oxygen. However, a very high concentration of NO₃ can also be toxic for our fish, so beneficial bacteria of the genus Pseudomonas can convert it into non-toxic nitrogen gas (N₂). This process is called denitrification and occurs in an anaerobic environment and low organic matter (Meade, 1974), following the nitrification process.

NH₃ + O₂ → NO₂- + 3H+ + 2e-

NO₂- + H2O → NO₃- + 2H+ + 2e-

The bacteria grow in the biofilters on biomedia. There are different types of biomedia, depending on the type of biofilter being used. The shape, volume, and surface area of the biomedia provided to bacteria can determine the amount of NH₃ that can be oxidized, thus the effectiveness of the biofilter and, therefore the production.

Mechanical Filtration

Drum filters are the most common mechanical filters used in RAS farms. The aim of drum filters and any other mechanical filter is to physically remove solid waste (mainly feces and/or uneaten food) from the water before it enters the biofilters. Despite their small size, drum filters offer a bigger surface area to be cleaned with less energy and water consumption than other mechanical filters. Their effectiveness depends on the mesh size (25 to 150 μm).

Foam fractionation is the most efficient method for fine particle removal in a range between 0.5 μm and up to 50 μm. Protein skimmers are designed so that the water from the fish tanks (including all the solids and fine particles that the water contains) flows through with high pressure. At the same time, an air pump supplies the protein skimmer with compressed air and creates fine bubbles at the bottom of the skimmer's cylindrical body. In this way, by moving upwards, the bubbles adsorb the fine particles dissolved in the water, which moves downwards on their surface. Additionally, foam fractionation also removes organic compounds such as enzymes and proteins, among others (Dissolved Organic Compounds – DOCs). The bubbles and adsorbed compounds reach the water's surface and are removed from the skimmate collection cup. The effectiveness of a protein skimmer depends on the size of the fine bubbles and the exposure time of the bubbles to the organic compounds.

ras-system-2

Degassing and Oxygenation

Different fish species have different needs for oxygen levels. An oxygen cone or diffuser can supply oxygen to the fish. As mentioned above, biofilters need oxygen to oxidize the toxic NH₃, which an air pump can easily supply. Fish also produce CO₂, which has to be removed before the water reaches the fish tanks again. It is worth mentioning that saturated water high in CO₂ levels can harm fish.

Fish Tanks

A fish tank can be constructed using different sizes, shapes, and materials. Feed distributors and mort collectors are installed in each tank.

Monitoring and Control

Monitoring water quality in RAS farms is essential for successful production. Sensors are installed in every stage of the facility, monitoring the living environment of the reared fish (pH, temperature, O₂ level, water level, etc.). Moreover, water levels and quantities are vital elements for a stable system. Feed distribution systems can also be installed to facilitate easier and better control of feed intake.

Most RAS farms fail to succeed due to poor initial design, lack of technical and scientific support, and financial reasons (Timmons, 1997).