PURPOSE: To subject sludge water to solid-liquid separation to reduce the vol. thereof and to utilize a separated liquid as washing water by a method wherein a flocculant is injected into sludge water while sludge water is drawn into a sludge water storage tank from a purifying tank under vacuum and the separated liquid of the sludge water is returned to the purifying tank.

CONSTITUTION: When a number of valves 1-4 are selectively opened and closed and a vacuum pump 5 is driven, the pressure in a sludge water storage tank 6 becomes negative. A water supply and discharge hose 7 is delivered and the sludge water in a purifying tank is drawn into a sludge water storage tank 6. At the same time, a grinder 10 and an injection pump 11 are driven and a flocculant is injected into sludge water from a flocculant storage tank 12 through a nozzle 13. In order to return a separated liquid to the purifying tank, a three-way changeover valve 9 is changed over to a separated liquid pressure feed pump 14. Then, a pressure pump 15 is driven and sludge water is separated by a filter 18 while the separated liquid is stored in a separated liquid storage tank 20. Thereafter, a high pressure pump 23 is driven and the separated liquid is ejected from the washing nozzle 24 of a washing high pressure hose 22 to wash the purifying tank.