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  Advanced Topics in Wet-Weather Discharge Control
September 2009


Image: Advances Topics in Wet-Weather Discharge Control Cover

This report discusses four related but generally independent wet-weather flow (WWF) topic areas, namely: i) opportunities for advanced practices in WWF control technology, particularly as it applies to sewered systems; ii) tradeoffs between storage facilities (tanks) and enlarged trunk sewers (tunnels) in CSO control; iii) disinfection/sedimentation tradeoffs in primary treatment; and iv) routing methods for indicator bacteria analysis in stormwater. The literature surrounding these areas is reviewed, and then each of the four areas is developed as a separate sub-theme of the report. An evaluation of advanced practices identifies seven areas where knowledge gaps are likely to frustrate current practices, and also explores some potential development and innovation areas that offer promise in the near future for improvements in practice. Some key areas that are candidates for development include evaluation of BMP placement and performance, real time control, information management, decision support systems, and control system theory and application in the watershed context. The analysis of oversized tunnels as alternatives to storage tanks in the CSO control context is made, and conclusions are drawn as to the relative merits and potential cost tradeoffs between these options, based on available data. The report also evaluates the relationships and tradeoffs between disinfection and sedimentation in primary treatment of wastewater. The combination of these two methods is explored, and the inverse relationship between sedimentation efficacy and the need to add disinfectant is assessed. A systems model was developed to represent these two processes. Based on typical data, the model was applied to a sedimentation system and a disinfectant system individually, and then in various combinations. The model was found to be a useful and simple way to simulate sedimentation and disinfection design alternatives. The complexities of bacterial behavior in the environment, including substrate adhesion and clumping, are discussed. The results of modeling and analysis are assessed and the potential for improvements in practice is addressed. The scale dependence of BMPs and relevance to indicator bacteria controls is assessed by means of a case study. From this effort, conclusions were drawn that certain end-of-pipe BMP treatment and control may be of limited value for control of indicator organisms, and should be avoided in favor of site-specific studies of cause, effect and in-stream water quality. Also significant is the conclusion that ponds are sensitive to mixing and antecedent conditions, and that drawing down ponds via filtration or exfiltration between events may be a factor in enhancing pond performance, particularly in small events. Recommendations for future research are made.


Thomas O'Connor

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