Principle drawing of a recirculation system. The basic water treatment system consists of mechanical filtration, biological treatment and aeration/stripping. Further installations, such as oxygen enrichment or UV disinfection, can be added depending on the requirements.
Chapter 2: The recirculation system step by step
In a recirculation system it is necessary to treat the water continuously to remove the waste products excreted by the fish, and to add oxygen to keep the fish alive and well. A recirculation system is in fact quite simple. From the outlet of the fish tanks the water flows to a mechanical filter and further on to a biological filter before it is aerated and stripped of carbon dioxide and returned to the fish tanks. This is the basic principle of recirculation.
Several other facilities can be added, such as oxygenation with pure oxygen, ultraviolet light or ozone disinfection, automatic pH regulation, heat exchanging, denitrification system etc. depending on the exact requirements.
Fish in a fish farm require feeding several times a day. The feed is eaten and digested by the fish and is used in the fish metabolism supplying energy and nourishment for growth and other physiological processes. Oxygen (O2) enters through the gills, and is needed to produce energy and to break down protein, whereby carbon dioxide (CO2) and ammonia (NH3) are produced as waste products. Undigested feed is excreted into the water as faeces, termed suspended solids (SS) and organic matter. Carbon dioxide and ammonia are excreted from the gills into the water. Thus fish consume oxygen and feed, and as a result the water in the system is polluted with faeces, carbon dioxide and ammonia.
Only dry feed can be recommended for use in a recirculation system. The use of trash fish in any form must be avoided as it will pollute the system heavily and infection with diseases is very likely. The use of dry feed is safe and also has the advantage of being constituted to meet the exact biological needs of the fish. Dry feed is delivered in different pellet sizes suitable for any fish stage, and the ingredients in dry fish feed can be combined to develop special feeds for fry, brood stock, grow-out etc.
In a recirculation system, a high utilization rate of the feed is beneficial as this will minimise the amount of excretion products thus lowering the impact on the water treatment system. In a professionally managed system, all the feed added will be eaten keeping the amount of uneaten feed to a minimum. The conversion rate (FCR), describing how many kilos of feed you use for every kilo of fish you produce, is improved, and the farmer gets a higher production yield and a lower impact on the filter system. Uneaten feed is a waste of money and results in an unnecessary load on the filter system. It should be noted that feeds especially suitable for use in recirculation systems are available. The composition of such feeds aims at maximising the uptake of protein in the fish thus minimising the excretion of ammonia into the water.
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Components in a recirculation system
Fish tanks
The environment in the fish rearing tank must meet the needs of the fish, both in respect of water quality and tank design. Choosing the right tank design, such as size and shape, water depth, self-cleaning ability etc. can have a considerable impact on the performance of the species reared.
If the fish is bottom dwelling, the
need for surface area is most important and the depth of water and the speed of the water current can be lowered (turbot, sole or other flatfish), whereas pelagic living species such as salmonids will benefit from larger water volumes and show improved performance at higher speeds of water.
In a circular tank, or in a square tank with cut corners, the organic particles have a relatively short residence time of a few minutes, depending on tank size, due to the hydraulic pattern and gravitational forces. The whole water column in the tank is moving around the centre. A vertical inlet with horizontal adjustment is an efficient way of controlling the current in such tanks.
In a raceway, no gravitational forces can be created to make a current, and the hydraulics have no positive effect on the removal of the particles. On the other hand, if a fish tank is stocked efficiently with fish, the self-cleaning effect of the tank design will depend more on the fish activity than on the tank design. For all types of tanks, the inclination of the bottom has no importance with regards to the self-cleaning effect, but it will make complete draining easier when the tank is emptied.
| Tank properties | Circular tank | D-ended raceway | Raceway type |
| Self-cleaning effect | 5 | 4 | 3 |
| Low residence time of particles | 5 | 4 | 3 |
| Oxygen control and regulation | 5 | 5 | 4 |
| Space utilization | 2 | 4 | 5 |
Circular tanks take up a lot of space compared to raceways, which adds to the cost of constructing a building. By cutting off the corners of a square tank an octagonal tank design appears that will give better space utilization than circular tanks, and at the same time the positive hydraulic effects of the circular tank are achieved. It is important to note that construction of large tanks will always favour the circular tank as this is the strongest design and the cheapest way of erecting containments.
A hybrid tank type between the circular tank and the raceway called a “D-ended raceway” also combines the self-cleaning effect of the circular tank with the efficient space utilization of the raceway. However, in practice this type of tank is seldom used, presumably because the installation of the tank requires extra work and new routines in management.
AKVA GroupControl and regulation of oxygen levels in circular tanks or similar is relatively easy because the water column is constantly mixed making the oxygen content almost the same anywhere in the tank. This means that it is quite easy to adjust the oxygen level up or down depending on the situation as the effect of more oxygen added will be detected almost immediately by the oxygen probe in the tank. In a raceway, however, the oxygen content will always be higher at the inlet and lower at the outlet, which also gives a different environment depending on where each fish is swimming. The oxygen probe for measuring the oxygen content of the water should always be placed in the area with the lowest oxygen content, which in a raceway is near the outlet. This downstream oxygen gradient will make the regulation of oxygen more difficult as the time lag from adjusting the oxygen up or down at the inlet to the time this is measured at the outlet can be up to an hour. This situation may cause the oxygen to go up and down all the time instead of fluctuating around the selected level.
Tank outlets must be constructed for optimal removal of waste particles, and fitted with screens with suitable mesh sizes. Also, it must be easy to collect dead fish during the daily work routines.
Tanks can be fitted with water level alarms, oxygen probes for oxygen control and alarms, and emergency oxygen diffusers.