The most common cause of complaints about sewage works is odour nuisance, but it's also the most controversial, in the sense that it's the most difficult to define and quantify. Depending on wind dispersion it is by no means unusual for an odour that can be detected at the site boundary to go unnoticed by the plant operators in the control room. In addition, it is only recently that reliable sensors have been developed to detect and measure odours in a meaningful way. But now Veolia Water Solutions & Technologies is about to install the UK's first sewage works odour monitoring system for Southern Water.

Measuring odour is not like doing a chemical analysis of atmospheric pollutants. Indeed, you may know perfectly well what pollutants are being released and at what rate, but this will not necessarily tell you anything at all about the odour. Odour is caused by volatile organic compounds - usually in a mixture - and the "perceived odour" is the result of their combined interaction with the huge number of sensors in the human nose. Atmospheric emissions from sewage works are generally specified in terms of maximum allowable concentrations of hydrogen sulphide, mercaptans, and ammonia but, quite often, odours can be detected even when individual parameters are within consented limits.

The significant breakthrough in odour monitoring was the development of "electronic nose" technology by Odotech, a spin-off company of the Engineering School of the École Polytechnique de Montréal. It uses a matrix of sensors to analyse an air sample for a range of parameters, and can be calibrated using olfactometry to respond in exactly the same way as the human nose to identify a perceived odour. This means that it can distinguish between odours generated by a particular works and those originating from other sources.