Though most of the wastewater flows by gravity some low-lying areas require pumping. At present, MWRA provides full primary and partial secondary treatment to its wastewater flows. The first phase of secondary treatment began operating in July 1997. A secondary treatment plant capable of handling all flows will come on line in phases ending in 1999. Sewage is piped from communities to several headworks where bricks, logs and other large objects are screened out.
Pumps draw the sewage through deep-rock tunnels under the harbor to Deer Island.
Mud and sand settle in a tank called a grit chamber. Later, this material, known as grit and screenings, is taken to a landfill for environmentally safe disposal. This primary treatment removes very few toxic chemicals. In the secondary treatment plant oxygen is added to the wastewater to speed up the growth of micro-organisms. These microbes then consume the wastes and settle to the bottom of the secondary settling tanks.