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Characterization of the water
Good characterization of the water to be filtered is necessary so that the correct filter can be chosen. The characteristics of the inlet water will vary from site to site, whether it is lake water, river water, groundwater or seawater. Before choosing a filter it is therefore necessary to take samples to be able to characterize the water.
The volume of wastewater coming from fish farms is normally much higher and the concentration of the discharged substances much lower than those entering a municipal wastewater treatment plant; they are, however, and comparable to those in the water discharged from municipal wastewater treatment plants, i.e. water that has been purified. Requirements for the design and construction of wastewater plants for fish farming are therefore dif-ferent to those used to treat muni-cipal wastewater. Hence, the purification equipment and technology used in municipal wastewater treatment cannot be transferred directly to fish farming conditions, even if the basic principles are the same.
The composition of the outlet water from a fish farm depends upon a number of factors, including species, growth rate, feed composition and utilization, feed conversion rate and water amount (see for example, ref. 8). The first step in reducing the discharge from the fish farm, without using any filter at all, is therefore to have an optimal feed that is fully utilized and consumed by the aquatic organ-isms. This also includes optimal management of the farm, having correct water quality and quantity, and feeding in an optimal way.
Experiments have shown that the predominant particle size in the outlet water from fish farming is less than 30–40 μm.9,10 The large number of small particles account for only a limited part of the total volume of discharged particles. Since the volume of particles is much more important than the number of particles when talking about the load on the recipient water body, it is of great importance to remove the few large particles. However, in re-use systems the small particles will normally dominate, since it is easy to remove the larger particles. This can also been seen with water that goes brown in high re-use systems, because the small particles remain in the water.
The density of faeces from fish farming varies. Reported densities are above 1, from 1.005 to 1.2, which means that the faeces will settle in water. A study of intact faeces from rainbow trout showed an average sinking velocity of the faeces of 1–2.5 m/min depending on fish size.
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