New simple methodology to estimate degree of dilution in CSO discharges from sewer systems

Performance measures relating to combined sewer overflow (CSO) operation are widely known to have limitations. In the UK they have evolved from simplistic settings based on six times dry weather flow, to Formula A (Government, 1970) and eventually to more complex consent conditions of spill volumes, spill frequency and water quality. Performance measures related to the quality of the receiving waters have also been developed in an attempt to better represent the impact of CSO spills. However, difficulties associated with measuring CSO spills and associated river flows (including the forecasting of events, taking a representative number of water quality samples, and the general costs of this type of activity) make it very difficult to accurately quantify CSO performances (Blanksby, 2002).