Recirculating aquaculture system (RAS) technology
A recirculating aquaculture system (RAS) consists of fish tanks and several filtration units which clean the water. In a classic RAS the water is thereby in constant flow from the fish tanks through the filtration system and then back to the fish tanks (Figure 4). Due to the metabolism of the fish, the water that leaves the tanks contains high concentrations of solids, nutrients, and carbon dioxide, whilst it is oxygen-poor compared to inflowing water. The goal of the filtration units is to decrease the solids, nutrients, toxins, and carbon dioxide concentrations, and increase the levels of dissolved oxygen in the water before it is returned to the fish tank.
The filtration system consists of several stages (Figure 4). The first treatment step after the outflow is the solids separation (Figure 4, Point 2) where the solids (feed remains, faeces and bacteria assemblages) are removed from the water. After this, the water is disinfected with UV (Figure 4, Point 6). This step is not always implemented in fish farms and can also be placed after the biofilter. The water then enters the biofilter (Figure 4, Point 3), where bacteria metabolise part of the organic load, and oxidize ammonia to nitrite and then to nitrate. All these bacterial metabolic reactions use dissolved oxygen (O2) and, like the fish, release carbon dioxide (CO2) into the water. Therefore, the CO2 in water have to be lowered after biofiltration. This is done in the degassing unit in which the water to air surface area is increased so that the CO2 the air phase (Figure 4, Point 4). As a last step, the oxygen concentration in the water has to be increased to a suitable level for the fish. This is done in the oxygenation unit (Figure 4, Point 5). The following sections describe these system components in more detail.
Figure 4: Main components of a recirculating aquaculture system (RAS)
The fish tank
The fish tank is the grow out area for the fish and therefore a core component of a RAS. The ‘classic’ tank designs are round tanks and square flow channels. One of the main aspects that makes round tanks favourable over square flow channels is the self-cleaning effect that can be achieved through a circular hydraulic pattern (Figure 5). The flow in the fish tanks has two functions: (i) uniform distribution of inflow water and fish feed; and (ii) transport of particles to the centre of the tank. Primary rotating flow is the flow from the inlet and then clockwise/anticlockwise around the tank. It transports settleable solids to the bottom. The primary rotating flow creates secondary radial flow and together they generate a self-cleaning tank.
Figure 5: Role of primary and secondary flow patterns: the primary flow ensures good water distribution of the inlet water and the secondary flow contributes to effective solids removal (adapted after Timmons et al. 1999)
Although round tanks have numerous advantages over square tanks, their main disadvantage (low area efficiency) often makes them a suboptimal solution for a RAS farm. Therefore, numerous other forms of tanks have been developed and tested in the past decades (more details are presented in Chapter 12).
Since RAS has gained popularity and these systems are also planned as large-scale ventures (e.g. Nordic Aquafarms is planning to invest in a 500 Million USD RAS farm in Belfast, Maine, USA), large tank designs have become increasingly important. These large tanks are often (at least in theory) much more cost-efficient than the traditional smaller tanks (Figure 6).
Figure 6: A large round tank (6 m deep, 32.5 m in diameter) as part of a salmon RAS (Swiss Alpine Fish)
Flow conditions have an important impact on fish health. One can establish different water flows and thus structure the basins hydraulically by using panels. In this way the fish stay in the optimal part of the tank (Figure 7). It is important to know that swimmers need to swim, in other words they need a current. The speed of the current must be adapted to the fish species. Generally, smaller fish require a lower current speed, though it must be high enough to ensure that the solids separation still works. All this also has an impact on the quality of the fish flesh.
Figure 7: Flow system specially developed for farming salmon, Swiss Alpine Fish AG, Lostallo, Switzerland
Solids separation
There are several reasons for the removal of solids. Firstly, water quality is improved by reducing the organic solids which reduces mineralisation (aerobic respiration) and therefore also helps to stabilize the oxygen content. Secondly, the preservation of the water quality also benefits feed uptake and stock control. Furthermore, solids removal reduces the bacterial load, because it removes the food source for microorganisms. High bacterial activity in the water column leads to unnecessary consumption of oxygen.
Another benefit of solids removal is the prevention of clogging of the fish gills which may lead to slow growth or even fish death. However, this depends on the fish species. Filter feeding fish, like many carp species, may even rely on a certain amount of suspended compounds in their natural habitat and can therefore also withstand a higher amount of suspended solids in RAS than, for example, salmonids (Avnimelech 2014).
One of the most important technical reasons why solids need to be removed is the potential clogging of the biofilter (c.f. Chapter 9). Moreover, the effectiveness of germ reduction through disinfection (c.f. Chapter 9) is increased through the removal of solids. The solids in fish water have different sizes, and treatments to remove these solids vary mostly according to their size (Figure 8).
Wastewater treatment and sludge disposal are important cost factors of intensive RAS. A RAS requires 300-1000 l water exchange per kg of fish produced, and yields 100-200 g dry weight sludge. To minimize the volume of wastewater it is feasible to treat the sludge water that results from the solid separation. In this way even a low-tech filtration system can achieve a significant reduction of the final wastewater volume.
Figure 8: Solid removal processes and the particle size range (in μm) over which the processes are most effective (adapted after Timmons and Ebeling 2007)
Disinfection
Bacterial as well as viral diseases can pose serious problems in intensive RAS. Disinfection of the water using ozone or UV-irradiation are the most common methods. UV light at a certain intensity can destroy the DNA of bio-organisms such as pathogens and one-celled organisms. In RAS the UV light (Figure 9) is mostly encompassed in a short piece of pipe between the mechanical filtration unit (e.g. drum filter) and the biofilter. The intensity or dose of UV light can be expressed in µWs/cm2 (energy per area). In RAS the UV dose needed to kill (deactivate) around 90% of the organisms ranges between 2000-10,000 µWs/cm2. However, to kill all fungi and small parasites a dose of up to 200,000 µWs/cm2 be necessary. For maximum efficiency it is important to place the UV light after the mechanical filtration system so that it is not blocked by the suspended solids.
Figure 9: UV reactor (AKR UV Systems)
The addition of ozone (O3) is another efficient method for reducing pathogens and other unwanted organisms in a RAS. In contact with water it splits into O2 a free oxygen radical O. This radical ‘attacks’ and oxidizes organic substances. This results in the degradation of suspended particles or some substances (clarification of water turbidity, colour formation by humic acids). Likewise, the biological cell walls of the organisms are also attacked by the radical O of the ozone molecule, killing off bacteria, floating, and filamentous algae. However, ozone is very reactive and can also harm the nitrifying bacteria in the biofilter and attack the fish gills if applied in too high amounts. The dosage therefore has to be monitored permanently. Chemical agents can be used for punctual treatments to reduce germ concentrations in the water. Hydrogen peroxide (H2O2) is commonly used, sometimes stabilized by peracetic acid (CH3CO3H). Overdosing can have severe effects on fish health and can damage the filter bacteria.
Table 1: Advantages and disadvantages of disinfection with UV, ozone, and hydrogen peroxide (H2O2) in RAS
| Disinfection agent | |||
|---|---|---|---|
| UV | Ozone | H2O2 | |
| Advantages | Works only locally in the UV reactor Can be applied without harming fish Simple management Cheap | Very effective in killing unwanted organisms like pathogens Breaks down complex molecules into small, biodegradable compounds Oxidizes nitrite to nitrate | Very effective in killing unwanted organisms like pathogens |
| Disadvantages | Sensitive to water turbidity, ineffective in water with high solids loading Bulbs need to be replaced (every year) If the radiation period is too short (i.e. the system has a too high flow rate) the UV- disinfection is ineffective | Complicated dosing Can harm fish and biofilter On-off of ozone system may lead to varying nitrite levels and decrease the amount of nitrifying bacteria in the biofilter Relatively expensive | Limited application, like disinfection of empty tanks and equipment or reduction of bacterial load in the fish tank Overdose is likely to severely damage the fish! Also damages the filter |
Biofiltration
The nitrification process takes place in the biofilter to oxidise the toxic free ammonium into toxic nitrite and eventually to non-toxic nitrate. The nitrifying bacteria are the heart of the biofilter. These bacteria grow on the filter media surface. The media can be fixed (e.g. trickling filter) or moving (e.g. moving bed filter). The nitrifying bacteria are sensitive to water quality changes in the system (especially pH and temperature), and rapid changes should therefore be avoided or done in slow steps as otherwise large amounts of nitrifying bacteria may die off which would lead to ammonia and nitrite spikes in the system. Moreover, as the nitrifying bacteria are aerobic, the dissolved oxygen content in the biofilter should always be kept at a certain threshold (depending also on the water temperature). The chemical reactions taking place in the biofilter are explained in Chapter 5. More details about choosing the right biofiltration are provided in Chapter 12.
Degassing and aeration
Gas transfer between the liquid and the gas phase occurs when there is sub-saturation in one phase. Gas solubility is dependent on pressure, temperature, salinity, and gas partial pressure. The transfer takes place over the contact surfaces between gas and liquid. Aeration increases the oxygen content in the water. Degassing removes gases such as carbon dioxide from the water.
Degassing
Gases, especially carbon dioxide resulting from respiration of the fish and bacteria, accumulate in the system water. These can have harmful effects on the fish if concentrations become too high. Therefore, a degassing unit is usually added to intensive RAS. Gas outtake (degassing) is achieved by increasing the contact surface area between the water and the air, either by aeration of the water column, or by sprinkling water through the air. Different biofilters already have a high degassing effect: in a trickle filter the water passes through the air, while in a moving bed filter the air passes through the water. This may therefore make an additional degassing unit redundant.
Oxygenation
Dissolved oxygen (O2) content is one of the most important water quality parameters in RAS and often the first constraint in emergency situations (e.g. in case of power cuts, pump failure etc.). There are numerous techniques to enrich dissolved oxygen in the water. Gas intake of water (aeration) can be enhanced by: (i) maximizing the oxygen/water contact area by using whirls or small bubbles; (ii) maximizing the oxygen/water contact period by using small bubble diameter and/or by a slow water flow; (iii) increasing the pressure (increases solubility) – water level, pressure vessel; and (iv) increasing partial pressure of O2(increases solubility) – pure oxygen.
High Efficiency Oxygen Input
In intensive RAS the oxygenation technologies depend on using pure oxygen rather than simple aeration which becomes impractical at certain fish densities. The oxygen is either produced on site with an oxygen generator or supplied by an external firm and stored in liquid oxygen tanks outside the aquaculture facility.
Low Efficiency Oxygen Input
In extensive fish ponds low efficiency oxygen input is usually sufficient. This is achieved by (i) keeping the water cool, as this dissolves more oxygen, and (ii) increasing the water movement. Different modes of aeration can support this (see Chapter 12).
Pumps and pumping pits
A pump is to RAS what the heart is to the human body. If it fails, then the result can be catastrophic. Therefore no expense should be spared when buying a pump. One can use speed controlled pumps to reduce the flow if needed. By using a parallel series of pumps with check valves, the chances of system failure can be reduced. Before buying a pump, the pressure losses in the pipes should be calculated, for example with the help of this online calculator: http://www.pressure- drop.com/Online-Calculator/.
Copyright © Partners of the Aqu@teach Project. Aqu@teach is an Erasmus+ Strategic Partnership in Higher Education (2017-2020) led by the University of Greenwich, in collaboration with the Zurich University of Applied Sciences (Switzerland), the Technical University of Madrid (Spain), the University of Ljubljana and the Biotechnical Centre Naklo (Slovenia).