PURPOSE: To dehydrate the sludge produced in high load microbe treatment at high speeds and produce cake of low moisture contents by adding sawdust and/or fiber dust to the sludge discharged from a waste water treating device utilizing microbes then adding a high polymer flocculant thereto.

CONSTITUTION: Sawdust and/or fiber dust of 10W200 mesh grain sizes are added as a dehydration assitant to the excess sludge discharged from a waste water treating device utilizing microbes, and further a high polymer flocculant is added thereto to allow flocs to form. Thence, these flocs are dehydrated with a filter press, a vacuum dehydrator or centrifugal dehydrator, whereby cake of low moisture contents is obtained. The amount of addition of the sawdust and fiber dust is 20W 100% based on the weight of sludge solids, more preferably around 50%. The amount of addition of the high molecular flocculant is satisfactory if it is 0.2W0.5% based on the weight of sludge solids. Thereby, the sludge is solid-liquid separated continuously, efficiently and effectively.