A. Before July 1998, poorly treated effluent was discharged from DITP and NITP (the Deer Island and Nut Island Treatment Plants) into Boston Harbor. The emptying of sewage solids (sludge) in the northern harbor ended in 1991. In 1998, up to 65% of sewage was undergoing secondary treatment.

B. From July 1998 to September 2000, the tunnel from Nut Island to Deer Island allowed south system sewage to be sent to DITP for secondary treatment, while NITP was closed. By September 2000, when the discharge was moved to Massachusetts Bay, 85% of sewage was receiving secondary treatment.