Wastewater treatment works in Scotland have evolved substantially in the past 20 years.  During the 1990s, major investments were made to comply with the Urban Wastewater Treatment directive.  These investments were designed to protect the environment from the adverse effects of urban waste water discharges, and were not specifically focused on odour control.   Odours continued to present issues at some sites, and in 2005, a statutory Code of Practice (CoP) for odour control at sewage works in Scotland was published by the Scottish Executive.