The Deer Island sewage treatment is one of the crown jewels of New England's public infrastructure. Its construction was mandated by a Federal Judge in the 80s due to chronic violations of the Clean Water Act by Boston Area municipalities in the disposal of their sewage. Constructed in the early 90s, it was built under budget and on schedule which is a rarity in these parts.
The large egg shaped things are sludge digesters which generate a significant amount of methane. This methane is burned in a combined heat and power system to provide about a third of the plant's energy needs. There are also two diesel fired combustion turbines that can produce 26 MWs as well as smaller emergency back-up generators. Deer Island also has significant renewable energy facilities including hydroelectric generators in the outfall shaft, ~ 700 kW of solar PV, two 600 kW wind turbines, and a small 100 kW FloDesign wind turbine. Read more about the renewables at Deer Island here.
For those sophisticated ISO-NE market watchers, you may ask why those two diesel fired combustion turbines are not listed as resources in the ISO-NE FCM. The reason is due to the cross-harbor cable that connects Deer Island to the NStar system. The adjacent town of Winthrop is served by National Grid while the Deer Island plant is connected to NStar. NStar and the MWRA have been in negotiations to upgrade the underwater cable to allow Deer Island to push power back into the system. The costs to do this are substantial and involve moving the underwater cable. Based on the minutes of MWRA board meetings (go to p. 5), it appears that NStar is not enthusiastic about the project. An aerial photo of the plant is shown below, but if you want a good up close look at this beauty, go to the public park that surrounds it. There are awesome views of the harbor and it doesn't smell nearly as bad as you think it would.