And this is the competition, in my neck of the woods--Deer Island Waste Management sewage treatment plant (from their website--I find the egg-shaped digesters particularly fascinating. unclear how much energy
it consumes, it produces 3 megawatts, that would be 72 megawatt hours per day assuming it runs all night).
Wastewater "influent" from MWRA customer communities arrives at the plant through four underground
tunnels. Pumps then lift the influent about 150 feet to the head of the plant. There are three main pump
stations. The North System is served by the North Main Pump Station and the Winthrop Terminal Headworks, containing ten 3,500 hp pumps and six 600 hp pumps. The capacity for the North System is 910 mgd. The Lydia Goodhue Pump Station for the South System can handle an additional 360 mgd of flow, and contains eight 1,250 hp pumps. The pumping capacity at the new Deer
Island plant has dramatically increased the volume of wastewater that can be taken into the plant from the conveyance tunnels. This reduces back-ups and overflows throughout the system when wet weather causes peaking of system flows.
After pumping, flows pass through grit chambers that remove grit for disposal in an off-island landfill. Next, flows are routed to primary treatment clarifiers that remove about half of the pollutants brought to the plant in typical wastewater (50-60% of total suspended solids and up to 50% of pathogens and toxic contaminants are removed). In this step, gravity separates sludge and scum from the wastewater. The plant uses 48 primary clarifiers that are 186 feet long by 41 feet wide by 24 feet deep. The clarifiers have a "stacked" settling surface at mid-depth to double the settling capacity of the tanks that are squeezed into the tight space confines of Deer Island.
Secondary treatment mixers, reactors and clarifiers remove non-settleable solids through biological and gravity treatment. The biological process is a pure oxygen-activated sludge system, using microorganisms to consume organic matter that remains in the wastewater flow. Secondary treatment raises the level of pollution removal to over 85%.
Three "batteries" of secondary treatment were completed in 1997, 1998 and 2001, respectively. Over one hundred tons of pure oxygen are manufactured each day at Deer Island's cryogenic facility to support the biological treatment process. The Deer Island Treatment Plant generates 130 - 220 tons of pure oxygen per day to support the secondary treatment process.
Sludge and scum from primary treatment are thickened in gravity thickeners. 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 digesters, 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 byproduct of the digestion process is 70 percent methane gas, which is captured and piped to boilers that generate enough heat to warm the buildings on the site as well as for the heat-dependent treatment processes. The steam from those boilers is sent through a steam turbine generator (STG) producing an average of approximately 3 megawatts of electricity. Digested sludge leaves Deer Island is transported through the Inter-Island Tunnel to MWRA's pelletizing facility at Fore River, where it is further processed into a fertilizer product.
Air scrubbers and carbon adsorbers remove odors and volatile organic compounds from treatment process "off-gases". 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. There are two disinfection basins, each approximately 500 feet long with a capacity of 4 million gallons, in which the effluent is mixed with sodium hypochlorite. Finally, sodium bisulfite is added to dechlorinate the water, so that chlorine levels in the ultimate discharge will not threaten marine organisms. After disinfection and dechlorination, the effluent is ready to be discharged.