Over one hundred tons of pure oxygen are manufactured each day at Deer Island's cryogenic facility to support the biological treatment process. Sludge and scum from secondary treatment are thickened in centrifuges. Polymer is added in the secondary thickening process to increase its efficiency. Digestion then occurs in 12 distinctive egg-shaped anaerobic digestors, each 90 feet in diameter and approximately 130 feet tall. Mimicking the stomach's natural digestion process, microorganisms naturally present in the sludge work to break sludge and scum down into methane gas, carbon dioxide, solid organic byproducts, and water.
Digestion significantly reduces sludge quantity.
The methane gas produced in the digestors is used in the plant's on-site power generating facility to save operating costs by reducing consumption of purchased energy. Digested sludge leaves Deer Island by barge for MWRA's Pelletizing Facility at Fore River, where it is further processed into a fertilizer product. Odor control is used for primary and secondary treatment process facilities, as well as the sludge processing, plant pumping, and grit removal facilities. Odor control performance is constantly monitored and is governed by a special DEP air quality permit. After passing through primary and secondary treatment, wastewater is disinfected with sodium hypochlorite to kill bacteria.