OPTIMIZING MIXED LIQUOR SETTLEABILITY AT THE JONES ISLAND WWTP

The Milwaukee Metropolitan Sewerage District (MMSD) provides wastewater collection and treatment services to a population of approximately 1.2 million people in the greater Milwaukee, Wisconsin metropolitan area. The MMSD service area includes both separate and combined sewer systems, from which wastewater is collected and conveyed to MMSD's two large wastewater treatment plants (WWTPs), the Jones Island (JI) and South Shore (SS) WWTPs. Both plants achieve advanced secondary treatment levels before discharging their treated effluent to Lake Michigan.During the 1980's and 1990's the MMSD executed a $2.2 billion Water Pollution Abatement Program (WPAP) that included extensive upgrades at both treatment plants and significant collection system improvements including construction of the Deep Tunnel Storage System. The Deep Tunnel Storage System was constructed with the intent of storing the majority of peak wet weather flows, that otherwise would overflow from the system, for treatment at a later time after the peak flows subsided. Since completion of the WPAP the number of overflow events from the MMSD system has decreased significantly, but have not been totally eliminated. With goals of continuing to improve the water quality within the Milwaukee area watershed, the MMSD continues to develop and implement plans that improve the performance of its facilities and minimize the occurrence of overflow events.One of the key elements of this ongoing work has involved study and implementation of improvements to optimize performance and maximize peak flow capacity at the District's JI WWTP. The JI WWTP has a nominal peak capacity of 14.8 m3/s (330 MGD) but has historically suffered from highly variable performance that has often limited peak flow capacity to 11.2 m3/s (250 MGD) or less. A study performed in 2001 identified secondary clarifier capacity as being most limiting on peak flow capacity, due to a combination of clarifier hydraulic inefficiencies, return activated sludge (RAS) withdrawal constraints, and frequent occurrences of poor mixed liquor settleability. A follow-up study and design project was performed to investigate and remedy each of these problem areas. This paper focuses on the efforts to optimize mixed liquor settleability at the JI WWTP.