Mechanical filters remove the solid wastes produced in the recirculation system, which consist mainly of faeces, uneaten feed and bacterial debris. Mechanical filtration may be done in conjunction with the biofilter (e.g. bead filters). Solids are also removed daily during the siphoning of the tank.
The use of sedimentation tanks and mechanical filters in recirculating hatcheries was discussed in Valenti et al. (1998). Sedimentation tanks are useful when a single biofilter is used for all tanks. Different types of mechanical filters may be used, but they should be properly sized to match the desired flow rates and the size of solids to be removed. Examples of mechanical filters include sand filters, drum screen filters and expanded media filters (e.g. bead filters). These may be upflow or downflow designs and can be easily cleaned. Because of the relatively low biomass associated with hatchery systems, sand or bead filters are adequate. Sand filters usually contain a sand particle size of 850 ^m. Mechanical filters are typically placed prior to UV light units and biofilters to maximise their efficiencies.
These filters require flushing on a regular basis (once to several times daily) to prevent any accumulation of organic material which causes clogging, channelling and growth of potentially pathogenic bacteria. Sand filters may be back-washed with freshwater and air to minimise seawater use. The size of the filter is usually small, relative to the total volume of the system; therefore the introduction of clean freshwater is normally not a problem.
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