PURPOSE: To sufficiently agitate and wash filtering sand by using backwash water in the time of a backwash and not to discharge the filtering sand from an overflow port.
CONSTITUTION: In a sand filter, a fixed quantity of filtering sand is put, and the sample water poured from the position over the filtering sand is flowed through this filtering sand and then is discharged from a filtered water outlet 1b. A slope part 70 to have a fixed slope set up at or around the position over the filtered water outlet 1b, a filter 30 set up at the base position of the slope part and at the position over the filtered water outlet, and a structure 60, in the base, set up at a certain fixed position just above the filtered water outlet and to form a fixed narrow path between its base corner and the slope part so that the backwash water supplied from the filtered water outlet in the backwash time flows only just upward make up the filter. In the upper part, a space part of convective agitation becoming gradually and enough wider against the narrow path, between the structure 60 and the outer wall of the sand filter, is formed, and the backwash water and filtering sand to pass through the narrow path along the slope part in the backwash time can be made convective agitation in the space part of convective agitation.
TANAKA TOSHIO
KITAMOTO TAKASHI
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