Advantages and disadvantages of re-use systems
Re-use of water will reduce the amount of new water required for the fish farm. Therefore farms can be established on sites where the amount of water is a limiting factor, or established farms can increase production without increasing the amount of new water required.
Limited resourses of freshwater are today a serious problem in the world. Water consumption has shown exponential growth during the past few years. Warning signs are evident, such as lowered groundwater tables, reduced size of lakes and dis-appearance of marshland. This is indicative of more competition for freshwater resources in the future, which will of course affect the fish farming industry because of its huge consumption of freshwater. All methods that reduce water consumption in fish farming, such as re-using water, are therefore of general interest.
By using a water re-use system, it is possible to farm fish species that have higher temperature requirements than the natural temperature in the area, for instance to grow warm-water species at high latitudes in the northern hemisphere.
If there are stringent requirements for cleaning the effluent water, re-use systems will assist the process because the amount of water to be treated is reduced.
If the water supply to the farm is of poor quality, the requirements for improvement will be increased. Re-use systems will be of interest in such cases because the amount of new incoming water, where the quality must be improved, is reduced together with the treatment costs.
If the inlet water has to be pumped to a higher level to get it to the farm, the costs can be consider-able; this may favour a re-use system. The same is the case if new water supplied to the farm is metered.
Although re-use systems have advantages, they also have several disadvantages; these must therefore be weighed against each other. In most cases the dis-advantages are greater than the advantages. It is best therefore, to have a site with enough good quality water of the correct temperature to suit the species grown, and low costs associated with transferring the inlet water from the source to the farm.
The two main disadvantages of re-use systems are the investment and operating costs. Because the number and size of the components for water treatment is higher than for a flow-through farm, the investment costs are also higher. In systems with a high degree of re-use (>95%) the investments can be significantly greater than for traditional flow-through farms, several times as high per unit farming volume.
In a normal re-use system there is continuous transport of water performed by some kind of pump, which results in constant running costs for the pump(s). Since so much technology is usually involved in purification systems, a re-use system will also be more exposed to technical faults. To ensure a functioning system, the requirements for monitoring water quality and water flow are greater than in traditional flow-through systems which translates to larger monitoring systems and more back-up systems. Furthermore, the time limit/reaction time is reduced when a fault occurs, e.g. pump failure, filter blockage, which increases the requirements for having operators on stand-by.
Some of the equipment used in the re-use system also requires a high level of technological and bio-logical knowledge to operate; this imposes extra requirements for the competence of farm opera-tors. The need for maintenance of the equipment is much higher, which also represents a significant cost.
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