CONTROL STRATEGY FOR A NEW MILLENIUM – SIMPLER IS BETTER

The Milwaukee Metropolitan Sewerage District (MMSD) retained RUST⊘HARZA to evaluate a portion of its interceptor sewer system. As part of this evaluation, RUST⊘HARZA has proposed a new strategy to regulate flows from tributary combined sewers into the interceptor sewer system. This regulation strategy provides a system that is both self-acting and responsive to a variety of flow scenarios and rainfall distribution patterns. Rather than installing conventional regulating devices which provide a set flowrate into an interceptor sewer, the proposed system will incorporate devices which provide a constant hydraulic grade line in the interceptor. As such, the amount of flow that enters the interceptor sewers depends upon available downstream capacity. Regulating hydraulic grade line rather than flowrate allows the maximum amount of flow to enter the interceptor sewers while providing protection from potential basement flooding caused by excessive surcharge levels.In addition to more efficient use of intercepting sewer capacity, the regulation strategy proposed by RUST⊘HARZA also provides a significant cost savings when compared to conventional flowrate regulation. Most of the devices chosen for the proposed strategy require minimal modification of existing structures to install, resulting in a construction cost savings of approximately 20 Million. None of the regulating devices chosen by RUST⊘HARZA require a centralized control system or highly trained operators. In addition, 96% of the regulating devices chosen do not require an outside power source to function. These two advantages provide significant savings in operating costs over the lifetime of the system.