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
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.