A respirographic biosensor is presented that is capable of monitoring the waste load and potential toxicity of wastewaters, both off‐line in a laboratory or on‐line at the wastewater treatment plant. The principles of the sensors' operation have been developed and implications of the design choices evaluated. Short term BOD values were obtained every 30 min. The linear dynamic range spanned concentrations differing by a factor of 5000. This range could be expanded by a factor of 10 by adjusting the aeration rate of the bioreactor in the sensor. The response time for toxicity detection was approximately 1 h. The use in the sensor of activated sludge from the plant concerned ensured relevant toxicity information was obtained. To check the condition of the sludge, an independent respiration measurement is proposed. When a siginificant activity change is observed, the sludge in the sensor must be replaced. The presence of oxidoreduction chemicals can cause interferences that may lead to measurement errors. Based on a difference in reaction kinetics, their presence can be assessed and the effect eliminated. Both on‐line and laboratory applications in the chemical industry are presented. Special emphasis is given to the usefulness of the sensor data for waste management of production divisions. On‐line assessment of load variations and hydrogen peroxide spills are given as illustrations of the implementation of the sensor on the treatment plant. Attention is drawn to the potential application of the data for process control and improved performance of the treatment plant.