PURPOSE: In a treatment system incorporating in a treatment box a plurality of filter elements having plural axially spaced fins thereon, with fins intermeshed, to conduct high-efficiency dehydration without pressurizing 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 rotatably placed in a treatment box 1, with the fins 12 intermeshed. While a raw liquid is flowed from a feed port 3 through the box 1 toward a sludge removal port 5, the elements F are rotated in the flow direction so that the liquid passes through the spaces 13 for filtration. The supply of the raw liquid from flocculation agitation tank (used as a raw liquid reservoir) to the feed port 3 is effected no by liquid head but by a constant- volume feed means such as a constant delivery pump 22.