CHALLENGES ASSOCIATED WITH DESIGN OF THE MWRA UNION PARK DETENTION/TREATMENT FACILITY

The Massachusetts Water Resources Authority (MWRA) is continuing to implement projects to reduce and eliminate the impacts of combined sewer overflows (CSOs) that discharge to Boston Harbor and its tributaries. Implementation of these CSO control projects is mandated under MWRA's federal court order to clean up Boston Harbor, and in accordance with MWRA's Long- Term CSO Control Plan.One of the projects currently under design is the Union Park Detention/Treatment Facility. This facility will provide treatment for wet weather flow discharged from the Boston Water and Sewer Commission (BWSC) Union Park Pumping Station (UPPS). The UPPS provides flood protection for the South End neighborhood of Boston during approximately 25 storm events per year. During these events, urban wet weather flow (containing a mixture of combined sewage and stormwater) is pumped to the Roxbury Canal Conduit for conveyance to the Fort Point Channel in Boston Inner Harbor.The Union Park Detention/Treatment Facility will intercept wet weather flow upstream of the UPPS, provide coarse and fine screening, disinfection using sodium hypochlorite, storage/treatment in underground concrete detention basins, and dechlorination using sodium bisulfite prior to re-directing the flow to the UPPS wetwell for pumping. During smaller storm events, the entire volume of flow tributary to the detention/treatment facility will be held in the detention basins. During larger storm events, the detention basins will provide for chlorine contact and sedimentation to reduce the discharge of pathogens, total suspended solids (TSS), and biochemical oxygen demand (BOD). Dechlorination will occur downstream of the detention basins. Following each storm event, a combination of gravity and pumped dewatering will occur to return detention basin contents to the combined sewer system for transport to MWRA/s Deer Island Wastewater Treatment Plant. Once dewatered, the detention basins will be cleaned by means of flushing gates. These gates will hold back a portion of the storm volume and rapidly release it to create a wave to scour the bottom of the detention basins. An odor control system, using activated carbon technology, will be provided to treat all ventilation air from portions of the facility that are exposed to combined sewage.Several challenges must be faced and overcome during the design and implementation of the Union Park Detention/Treatment Facility. Some of the more significant challenges include: siting the facility in a densely developed, urban residential/commercial setting; institutional challenges associated with a facility to be jointly owned and operated by two public agencies; maintaining the flood control function provided by the existing UPPS; and other technical challenges. This paper presents the design features and approaches developed to address project challenges, many of which may be faced by others tasked with implementing similar wet weather pollution control projects in urban areas.