Sewage sources to Boston Harbor after the Bay outfall

Pollution in Boston Harbor

A turning point in the cleanup of Boston Harbor was in July 1998 when the Nut Island Treatment Plant (NITP) was decommissioned and its flow re-routed to the Deer Island Treatment Plant (DITP) for secondary treatment. After this, most of the wastewater discharged to the harbor received secondary treatment, except during the heaviest rainstorms. MWRA discharges to the southern harbor ended.

The maps below illustrate simply how, for three time periods, MWRA discharges changed, while discharges from other sources remained constant. The large arrows represent treatment plants and the smaller arrows indicate rivers, combined sewer overflows, and stormwater runoff. After July 1998, DITP was the main pollution source in the harbor, discharging up to 350 million gallons of treated wastewater per day. After September 2000, when effluent discharges were moved from the harbor to the bay, rivers, runoff, and other “non-point” sources became the main sources of pollution to the harbor.

(Click on each map to enlarge)
A. Before July 1998: The treatment plants discharged poorly treated wastewater. Until 1991, after sewage had undergone primary treatment, the treated sewage solids (sludge) were discharged into the harbor. In 1997, the first battery of secondary treatment was in place at DITP. In 1998, up to 65% of sewage was receiving secondary treatment.
B. July 1998 to September 2000: With the completion of the inter-island tunnel from Nut Island to Deer Island, South System sewage was sent to DITP for secondary treatment. NITP was closed, ending discharges to the southern harbor. By September 2000, 85% of the sewage was receiving secondary treatment.
C. After Sep. 2000: The third and final battery of secondary treatment was complete, better removing solids and contaminants. The outfall transported cleaner effluent out of the harbor and into the bay for greater dilution. Pollution sources were now from rivers, CSOs, and stormwater runoff.


Sewage sources to Boston Harbor after the Bay outfall
Sewage sources to Boston Harbor after the Bay outfall