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Odours from wastewater treatment plants are comprised of a mixture of various gases, of which hydrogen sulphide (H2S) is the main constituent. Sulphurous compounds can be degraded by microorganisms commonly found in wastewater. The use of activated sludge (AS) diffusion as a dual-role system, for the treatment of wastewater and for odour control, offers an alternative to traditional sulphurous waste gas treatment processes, such as biofilters, bioscrubbers and biotrickling filters, both in practical terms (use of existing facilities) and economically (minimal capital cost). Activated sludge diffusion avoids the common problems associated with these processes such as media plugging, excess biomass accumulation, gas short-circuiting, and moisture control and maintaining the correct biofilm thickness. The design issues to be considered when using AS diffusion for odour abatement, comprise odourous air pre-treatment, blowers and diffuser types, corrosion protection and increase in odour emission intensity. Nitrification inhibition depends on the composition and acclimation of the biomass, the concentration of H2S and other components of the wastewater. Hydrogen sulphide removal rates of >98% were consistently achieved for loads of 3–34 mg H2S/g MLSS/h, in two case studies, which also showed that sludge type has an impact on the ability of the sludge to degrade H2S. Wastewater process performance measured as five-day biological oxygen demand (BOD5), chemical oxygen demand (COD) and effluent suspended solids removal was not affected by H2S diffusion at 5 ppm. A change in the microorganism population dynamics of an activated sludge was observed when it was exposed to H2S for a period of more than 21 days.