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Donohue was retained by the City of Waukesha to develop a Master Plan of its sanitary sewer collection system to identify capital improvement projects to provide reliable wastewater collection and conveyance for at least the next 5 years. The City serves a population of approximately 68,000. The City’s treatment plant has a design capacity of 14 mgd; however, during wet weather the plant may treat in excess of 50 mgd. The sanitary sewer system consists of 250 miles of gravity sewer, 23 miles of force mains, and 43 lift stations, several of which present operational challenges during wet weather.
The first phase focused primarily on evaluating flows and conveyance capacities, which was accomplished by developing a MikeUrban/MOUSE hydraulic model. This model was calibrated to pump station and flow monitoring data collected in the spring and summer of 2009. Flow monitoring data was also used to complete a comprehensive Inflow and Infiltration (I&I) Study. This study determined that I&I in the majority of the service area is not excessive; however, in select areas, I&I creates operational challenges with the pump stations.
Donohue also completed an analysis to determine if certain pumping stations within the system could be eliminated. and determined that two bypass sewer projects under consideration could eliminate up to eleven pump stations. Replacing these stations with gravity sewers would improve system reliability by reducing the number of stations that would have to be maintained, would eliminate these stations’ force mains, some of which have been problematic, and would reduce energy and O&M costs.
A desktop force main risk assessment was completed in order to prioritize force mains for inspection and repair. To complete the risk assessment related to failure of the City’s force mains, a point system was developed to rank force mains for repair, replacement, or monitoring. This system considered the consequences of a force mains failure and the likelihood of failure based on factors such as pipe material, remaining useful life, soil corrosivity, history of pipe leaks.
Phase II will include an alternative analysis, construction cost estimates for capital improvements, CMOM program planning, and an ongoing SSES study.
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