PURPOSE: To enable to maintain mean residence time of sludge at fixed value by operating result of operation of a measuring device of sludge volume index, amt. of feed water, amt. of returned sludge, and amt. of excess sludge, etc., and drawing controlled amt. of excess sludge from the final settling basin.

CONSTITUTION: A sludge settling curve is prepd. using a sludge volume meter (SV meter) 10a, and the volume of sludge STt after (t) min from the beginning of measurement and the volume of sludge V30 after 30min from the beginning of measurement are determined. Both values SVt and SV30 are inputted to an operating device 10b of sludge volume index (SVI). SVI is determined from the value of WLSS of a WLSS meter 6 inputted from the device 10b. The output of the SVI measuring device 10 is inputted to a device 12 for operating the amt. of returned sludge and SV1 is used for the control of the amt. of operated results. The value of (t) is 20W60min. Furthermore, sludge is supplied from an aeration tank by a pump 11 to the above described SV meter 10a.