Efficient wastewater treatment requires monitoring of the biological activity of activated sludge. Its metabolic heat can be a critical indicator. Thus, a new method, measuring the temperature gradient between sludge sediment and the supernatant, is proposed for analyzing sludge. A prototype of the device was developed using an insulated cylinder with a series of thermistor sensors attached in parallel. Sedimentation of sludge in the lower part of the cylinder concentrated microorganisms, which produced more metabolic heat than the supernatant. The heat produced correlated well with the sludge concentration. The temperature profiles of each probe also enable real-time monitoring of sludge settling. Extended operation of the sensor evaluates the sludge quality, such as metabolic sustainability and the time required for the sludge to float under anaerobic conditions. A temperature-gradient sensor was implemented and its utility was confirmed by measuring the heat produced in sludge from a local wastewater plant. (C) 2007 Society of Chemical Industry.