Digestion readies sludge for conversion to fertiliser. Microorganisms in the digesters grow by consuming sludge and breaking down the organic matter while producing methane gas. This gas is used for heat and power. The new egg shape is ideal for mixing and has so far yielded greater quantities of methane per volume of sludge processed to offset fuel costs, while producing a consistently well-digested sludge. Before 1991, digestion was essentially the process that readied sludge for release onto the harbour's outgoing tide.
Untreated sewage carries large numbers of disease-causing microbes.
Because the same toxicity that makes chlorine a good disinfectant makes it dangerous to marine organisms, the less used the better. The residents of Eastern Massachusetts in the USA have all their water needs met by the Massachusetts Water Resources Authority water system. Around 2.2 millions residents, and 5,500 industrial users receive their water from the Quabbin and Wachusett watersheds and reservoirs. All the water starts as rainwater or snowmelt, which flows into a reservoir from surface streams and rivers. The Quabbin Reservoir in western Massachusetts is fed by the Swift River and by flood flows diverted from the Ware River between October and June.