PURPOSE: To prevent large animal planktons represented by Tigriops from getting into a filter layer by forming the filter layer using a plurality of materials which have different specific gravities and which exhibit approximately equal distribution ratio in the case of back-washing.
CONSTITUTION: A filter layer 6 is formed using a plurality of filter materials 4, 5 which have different specific gravities and which exhibit approximately equal distribution ratio. For example, as filter materials, silica sands 4 (specific gravity 2.65) with an effective diameter of 0.6mm and garnet 5 (specific gravity 3.95) with an effective diameter of 0.28mm are selected to form the filter layer at volume ratio of 50 to 50%. In this manner, filtration accuracy of sea water filters can be improved, whereby large animal planktons represented by Tigriops (size is about 1mm) can be prevented from getting into the filter layer, thereby leakage of the layer can be avoided. As a result, yield rates of cultured seeds and saplings in fish farming can be raised, leading to an improved productivity.
DOMOTO SHINICHI
KITAMURA TAKAO
SEKI TETSUO
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