Integrated Water Management

North Dorchester Bay CSO Facilities


North Dorchester Bay CSO Facilities

North Dorchester Bay, MA


About this Project

The MWRA’s North Dorchester Bay (NDB) CSO Project is designed to eliminate CSO discharges to South Boston beaches in up to a 25-year storm event and to eliminate separate stormwater discharges in up to a five-year storm event. FST provided MWRA with design and construction phase services for the pumping station, ventilation building, force main, and new gravity sewers.
The 15 mgd station dewaters the storage tunnel during and after storm events. The facility is a wet pit/dry pit type station, with submersible pumps in a dry pit application. Design includes local SCADA control at the pump station, with diversion structures at the outfalls connected to the pump station via dedicated telephones lines. Remote monitoring and control of the SCADA system is provided to the MWRA Operations Control Center in Chelsea. Normal station operation calls for two raw wastewater pumps to operate and deliver the design flow rate of 15 mgd. The flow control valve limits the discharge rate to 15 mgd with the variation in pump output resulting from the changes in static head in the wet well. One raw wastewater pump is provided as standby to meet DEP design criteria; the dewatering pump is provided for final wet well dewatering, and for pumping of infiltration entering the storage tunnel that flows to the pumping station.
The bottom of the pumping station substructure extends 68’ below ground. Geotechnical investigations determined that a slurry wall was required for support of excavation. In addition, to mitigate differential settlement issues, 49 minipiles were constructed to support the building substructure and superstructure. Minipiles extended 15 to 25 feet into bedrock from existing grade.
The ventilation building provides treatment for the air in the CSO storage tunnel. This facility also includes local and remote SCADA control. This structure is a below ground, cast-in-place concrete structure with two 9-foot diameter activated carbon adsorber vessels; two 75 hp, 19,500 cfm fans; electrical room; toilet room; and a mechanical room with fire suppression equipment.

The pumping station connects to the terminal shaft of the storage tunnel with about 100’ of 36” diameter sewer, constructed by jacking between the storage tunnel and the pumping station wet well. The pumps discharge to a 24” diameter force main sewer that delivers the pumped flow from the pumping station to the local gravity sewer system.

More Information

Owner: Massachusetts Water Resources Authority (MWRA)

Location: Boston, MA

Highlights: FST provided MWRA with design and construction-phase services for the pumping station, ventilation building, force main, and new gravity sewers.