The City of Toronto, Ontario, operates four wastewater treatment plants (WWTPs) in an urban environment of 2.5 million people. At the 820-megalitres per day (ML/d) Ashbridges Bay (Main) Treatment Plant, Canada's largest, the impending discontinuation of sludge incineration meant the plant would lose its efficient and successful odourous air treatment system. An alternative method of odour control is required to treat emissions from existing and new sludge management facilities. The odour control system is mandated by both regulatory and public pressure to achieve non-detectable off-site odours. The challenge lay in finding an economic odour control system that would meet a stringent regulatory limit and survive intense public scrutiny.