An electronic nose consisting of twelve polypyrrole conducting polymer sensors was evaluated for its ability to monitor quiescent sewage liquors at three wastewater treatment works. Samples collected from the works inlet. the settlement rank and the final effluent outlet over an 8-month period, showed that a non-specific sensor array can distinguish between sewage samples of different type and from different treatment works. The correlation of the electronic nose responses with the 5-day biochemical oxygen demand (BOD5) using canonical correlation (a statistical reduction technique), showed that no generalised relationship can be found between these parameters for raw sewage, settled sewage and final effluent samples from three treatment works. However. when samples From a single works are compared separately, strong correlations can be produced between the electronic nose responses and BOD5 for time intervals of approximately 4 weeks. These relationships show that it is possible to fit single lines between the response patterns of an electronic nose and BOD5 for sewage samples at a single treatment works. The results suggest that it may bt:feasible to use an electronic nose to monitor and/or control the biochemical activities of a wastewater treatment process. (C) 1998 Elsevier Science Ltd. All rights reserved.