Over 100 Years of CSO Retention Treatment Basin Performance Data – What It Tells Us

Multiple wet weather treatment facilities were constructed in the Rouge River watershed in southeast Michigan in the early 1990s. These facilities have been operating in excess of 10 years and have generated extensive data on the characteristics of combined sewage and the effectiveness of retention treatment basins to control and treat wet weather discharges. The long duration of data collection has provided additional insights into the hydrologic response of combined sewer systems during this period of record. This paper evaluates data from nine of the Rouge CSO facilities as well as selected other southeast Michigan CSO basins in order to gain a better understanding of the performance of wet weather facilities. The data set used for this paper represents more than 100 collective years of data. As a result of the extent of the data set a robust representation of a wide range of event conditions are included that provides a historical basis for comparing various management approaches.Most long term control planning efforts are based on the predictions of hydrologic and hydraulic models, which are developed from monitoring data. Monitoring data used to support modeling efforts is often limited in duration, which requires projections for larger unrecorded storm events, different seasonality, and changes in annual conditions. In modeling activities, various estimates are given to represent variability of system responses from year to year or season to season. Rarely is the planner provided with an extended monitoring period to gain insight into long term system behavior. The goal of this paper is to present an evaluation of combined sewer facility performance that is strictly based on an assessment of monitoring data reflective of long term monitoring.The data evaluation presented focuses on hydrology, pollutant characterization and treatment effectiveness. The hydrologic response of the collection systems was evaluated in terms of overall flow generation (volume), distribution of events by size and seasonality of response. Pollutant evaluation included influent and effluent loads and concentrations. Treatment evaluation focuses on the effectiveness of disinfection in these facilities.