Abstract: In an intermittent aeration activated sludge process in which aeration and agitation is repeated alternately, a first aeration tank and a second aeration tank are connected to each other and an ORP meter is applied to each tank. In the first aeration tank, the sum of an aeration period and a denitrification period of an agitation step is controlled to a predetermined period T.sub.gs based on a time when a bending point appeared on an ORP curve in a previous cycle. In the second aeration tank, the sum of an aeration period and an agitation period is controlled to a predetermined period T.sub.ds longer than T.sub.gs based on a time when an ORP measured value reached a predetermined value in a previous cycle. Operations of the first and second aeration tanks are simultaneously transferred from the agitation to the aeration based on detection of the predetermined ORP value. As a result, a necessary anaerobic condition period is secured in the first aeration tank.

Abstract: The image of microorganisms contained in a sludge of an aeration tank is picked up by an image pickup device having the function of magnifying and picking up microorganism images. Binarizing processing is then made to obtain a binarized image from the resulting gray image, and the binarized image is subjected to thinning processing. The number of pixels of the image left after often the thinning processing is counted to measure the amount of filamentous microorganisms. In this manner, the amount of the filamentous microorganisms can be measured accurately irrespective of their thickness.

Abstract: A system for controlling a process utilizing filamentous microorganisms is disclosed. The amount of the microorganisms in a medium for growing the filamentous microorganisms is measured, and the control factors associated with the growth of the microorganisms in the medium are controlled on the basis of the measurement of the amount of microorganisms. In the process of measuring the amount of microorganisms, an optical image of a specimen of the medium is formed and scanned two-dimensionally to produce a series of brightness signals corresponding to the brightness distribution of the optical image. The level of the brightness signal is used to determine the amount of the microorganisms contained in the medium.