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Municipal wastewater treatment plants (WTP) emit odorous compounds that produce nuisance to the workers and nearby residents. Several chemical compounds contribute to odour problems, among them, sulphurous organic compounds, hydrogen sulphide, phenols and indoles, ammonia, volatile amines and volatile fatty acids. In the current study, hydrogen sulphide (HS) concentrations were measured during the summer period of 2007 by a portable handheld device at the WTP of Chania City (Greece). Measurements were taken in several places within the facility. The highest hydrogen sulphide levels were measured close to the primary sedimentation tanks and the tanks where the recycled activated sludge is mixed, the sludge from the primary sedimentation tanks reaching 30 ppm. In conjunction with the measurements, the Gaussian dispersion model AERMOD code was modified in order to estimate the maximum odour concentration for very short time steps using peak-to-mean ratios. The probability of detection of HS exceeds 50% at 400 m distance from the main emission sources (time interval of 5 s) with a relative high degree of annoyance (3.2 AU) under typical summer period conditions. Furthermore, relations between odour annoyance and odour exposure concentrations have been embedded in the model, in order to express the odour impacts in terms of probability of detection and degree of annoyance of the population near the WTP of Chania. [ABSTRACT FROM AUTHOR]