PURPOSE: In a treatment system incorporating in a treatment box a plurality of filter elements having plural axially spaced fins thereon, with the fins intermeshed, to improve dehydration efficiency by lightly predehydrating the raw liquid.

CONSTITUTION: Each filter element F has an axis 10 on which plural fins 12 are fitted with spaces 13. Plural elements F are placed in a treatment box 1, with fins 12 intermeshed. While a raw liquid is flowed from a feed port 4 through the box 1 toward a sludge removal port 6, the elements F are rotated in the flow direction so that the liquid passes through the spaces 13 or filtration. The raw liquid is fed to the system after dehydrated by mechanical dehydrator means such as endless filter type dehydrator, centrifuge type dehydrator, and roll press type dehydrator.