The Boston Harbor Project was a $3.8 billion, 11-year program to construct new wastewater treatment facilities at Deer Island. Completed in 2001, the project has ensured that wastewater discharged from 43 communities in greater Boston is treated to meet all state and federal standards for protecting public health and the marine environment. The project included primary and secondary treatment facilities on Deer Island; a headworks on Nut Island that replaced an outdated treatment plant; a five-mile-long tunnel connecting the new Nut Island Headworks with Deer Island; a 24-foot-diameter, 9.5 mile-long tunnel that discharges treated wastewater through diffusers into the deeper waters of Massachusetts Bay; and the beneficial re-use of treated sludge by turning it into a dried fertilizer product.