The sludge from the primary and secondary treatment is sent to centrifuge thickeners where the sludge is thickened to reduce its water content. Afterwards, the sludge is sent to 12 egg-shaped digesters where eight of the digesters process sludge from the primary treatment plant and the remaining four process sludge produced from the secondary treatment plant. These digesters use a process called anaerobic digestion. This process involves creating a warm, well-circulated, oxygen-free environment which encourages the growth of bacteria. The bacteria breaks down the raw sludge, destroying most pathogens and producing methane gas. The methane gas is then used in the treatment plant at its power generating facility. The sludge is then sent to a sludge-to-fertilizer plant where it is recycled into fertilizer pellets.
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| Sludge
digesters Each digester is 140 ft tall and holds 3 million gallons of sludge. Image credit: MWRA |
The sludge is transported on a barge to a sludge-to-fertilizer
plant where it is processed in rotating, high temperature dryers. The
sludge is then transformed into granular fertilizing pellets used for
landscaping, gardening, and large scale agriculture.
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| Wastewater
sludge from the Deer Island Treatment Plant after thickening and
dewatering. Image credit: MWRA |
Sludge pellets ready for market as fertilizer. Image credit: MWRA |
| Until 1991, sludge was a major pollutant of Boston Harbor. Up to 70 dry tons of partially treated sludge was poured into the Harbor on the outgoing tide. |