Using Long-Term Simulation for Improving a Sewer System Overflow Control Strategy

The Milwaukee Metropolitan Sewerage District (MMSD) is implementing a comprehensive program to reduce the frequency and volume of overflows. Under this program, MMSD strategies have included construction of additional flow storage and conveyance facilities, increased wet weather treatment capacity and upgraded the real-time control (RTC) in the operation of gates and diversions in the collection system. After constructing nearly one billion dollars in storage and conveyance system improvements, overflow frequencies have been reduced by more than 90 percent since 1993. Further improvements can be obtained through optimized operation of these facilities. Strategy improvements have been difficult to identify in practice because of ongoing changes to system facilities and the relative infrequency of the storm events that cause overflows. A simulation modeling tool known as MACRO has been developed to permit the analysis of MMSD storage and conveyance facilities under a fixed set of system conditions over long periods of time. Various management options can be modeled to investigate potential strategies to minimize the risk of overflows. A key policy objective is minimizing the frequency and volume of separate sewer overflows (SSO) under the constraint of avoiding discharge permit violations. This objective results in several different trade-offs between SSO frequency and other undesirable outcomes including CSO or excessive plant flow. In this study, MACRO runs are used to determine tradeoff curves based on varying the degree of certain control inputs. These curves are demonstrated to be useful in identifying the optimal operating procedure to meet a specified set of objectives.