Biological water treatment has been shown to effectively remove biodegradable organic matter, chlorinated by-products and ozonation by-products from drinking water during a large pilot-scale study for the American Water Works Association Research Foundation using the North Saskatchewan River, at Edmonton. In addition to studying total organic carbon, assimilable organic carbon, chlorine demand, haloacetic acid formation potential, trihalomethane formation potential, adsorbable organic halide formation potential, chloral hydrate and aldehydes, this study used a flavour profile panel to follow the removal of odour through different process trains involving biological treatment during the annual spring runoff which has historically caused odour incidents in the water supply. Over the 5-week period of the study, the raw water was found to develop from a very mild grassy odour to a strong odour, variously characterized as septic, manure, musty, earthy and hay-like. The odour persisted and changed character to varying degrees through the various process trains under study. The results verified the futility of relying on a strictly oxidative treatment like ozone for odour removal as well as showing that biological treatment using granular activated carbon could produce an essentially odour-free effluent during a transient raw-water odour event.