The purpose of the study was (1) to develop a respiration meter capable of continuously measuring, using different procedures, the oxygen uptake rate of activated sludge and (2) to expand knowledge about respiration related characteristics of wastewater and activated sludge.A newly-developed respiration meter is described. The meter consists of a closed, completely mixed respiration chamber of 0.5 to 1 litre through which activated sludge is continuously pumped. The characteristic feature of this meter is that the dissolved oxygen concentration in the sludge entering the chamber and in the sludge leaving the chamber is measured with one single probe, located at one opening. This is realised by changing the direction of the flow through the chamber. The respiration rate is calculated from the dissolved oxygen mass balance over the respiration chamber. Because the derivative of the mass balance is included in this calculation, the respiration rate can also be calculated during dynamic conditions. An improved method for calculating the respiration rate is described, which accounts for the time lag of the DO probe. An additional result of this improvement is that it yields the time constant of the probe response, which provides a diagnosis of the probe condition.Experimental research was performed using a continuous pilot activated sludge plant with a completely mixed aeration tank of 0.475 m 3, fed with domestic wastewater, and a batch reactor with an aeration tank of 1.5 to 2 litres.A strategy is described for measuring four types of respiration rate of the same sludge under different conditions: endogenous, instantaneous, actual and maximum respiration rate. Emphasis is given to the actual respiration rate. The actual respiration rate is defined as the oxygen uptake rate of the sludge in the aeration tank. It is demonstrated that this rate is measured if the respiration chamber and the aeration tank are equally loaded with wastewater. An improved method is described which does not involve addition of wastewater to the respiration chamber. Instead, the transient respiration rate during two modes of operation which are alternately executed is used to calculate the actual respiration rate.The measurement of the maximum respiration rate is discussed in some detail with emphasis on the partition of readily biodegradable matter into two components. The maximum respiration rate is measured if wastewater is continuously fed into the respiration chamber so that the loading exceeds a certain critical loading. Batch respirometric tests are used to verify the continuous measurement of this maximum rate. An application of the developed measurement is described in which the effect of the influent flow rate on the maximum respiration rate of nitrifying sludge was investigated.Methods are described for continuous estimation of the short-term biochemical oxygen demand (BOD st ) of influent and effluent by using respirometry. BOD st values of the influent are verified with batch measurements. The BOD st of the examined wastewater appears to be mainly caused by ammonium being oxidized by nitrifiers.Batch respiration measurements have been used for identifying a mathematical nitrification model. The investigation was focused on finding an optimal experimental design and a good model validation method.