Massachusetts Water Resources Authority Deer Island Treatment Plant Boston

FACILITY BASIC INFORMATION

• Plant Name: Deer Island Treatment Plant (DITP)
• Location: Boston (Winthrop), Suffolk County, Massachusetts
• Operating Authority: Massachusetts Water Resources Authority (MWRA)
• Design Peak Capacity: 1,270 MGD
• Current Average Flow: ~360 MGD
• Population Served: ~2.3 million residents
• Service Area: 43 Communities in Greater Boston and Metro West
• Receiving Water Body: Massachusetts Bay (via Ocean Outfall)
• NPDES Permit Number: MA0103284
• Year Commissioned: 1995 (Current facility; fully operational 2001)

TARGET AUDIENCE

• Municipal consulting engineers evaluating coastal high-flow facilities
• Wastewater treatment plant operators and asset managers
• Regulators studying successful large-scale harbor cleanups
• Environmental engineers interested in renewable energy at WWTPs
• University researchers focusing on pure oxygen activated sludge

Read more4D-Printed Smart Materials For Water Treatment

The Deer Island Treatment Plant (DITP) is the second-largest wastewater treatment plant in the United States and the operational cornerstone of the Massachusetts Water Resources Authority (MWRA). Located on a 150-acre peninsula at the entrance to Boston Harbor, this critical infrastructure asset treats an average of 360 million gallons daily (MGD), with a peak hydraulic capacity of 1.27 billion gallons daily. Serving 43 communities and approximately 2.3 million people (34% of the state’s population), Deer Island is widely recognized as an engineering marvel that facilitated one of the most successful environmental recoveries in American history: the cleanup of Boston Harbor.

Commissioned in phases starting in 1995 and fully operational by 2001, the $3.8 billion facility replaced aging primary treatment systems that historically discharged near the shore. Today, DITP operates as a model of sustainability, generating roughly 28% of its own electricity through onsite renewable sources, including methane cogeneration, wind, hydro, and solar. It stands as a benchmark for high-volume coastal treatment, utilizing pure oxygen activated sludge processes and a 9.5-mile deep-ocean outfall tunnel to ensure environmental compliance and the protection of Massachusetts Bay.

Facility Overview

A. Service Area & Coverage

The MWRA sewerage system covers a 500-square-mile area encompassing the Greater Boston region. The collection system is a hybrid of separated sanitary sewers and combined sewer systems (CSO) in older municipalities like Boston, Cambridge, Somerville, and Chelsea. The system relies on a vast network of 226 miles of interceptor sewers and 11 pumping stations to convey flow to Deer Island. The flow arrives via two distinct deep-rock tunnels: the North System (serving communities north of the Charles River) and the South System (serving communities south of the river).

B. Operational Capacity

Deer Island is designed to handle massive fluctuations in flow due to the combined sewer systems in its catchment area.

• Average Daily Flow: 350–360 MGD
• Peak Design Flow: 1,270 MGD (Primary Treatment)
• Peak Secondary Capacity: 700 MGD

During significant wet weather events, flows exceeding the 700 MGD secondary capacity receive primary treatment and disinfection before being blended with secondary effluent for discharge, a strategy permitted to prevent hydraulic washouts of the biological biomass.

C. Discharge & Compliance

Read moreA Comparison Between Aerobic And Anaerobic Wastewater Treatment Technology

Treated effluent is discharged into the Atlantic Ocean (Massachusetts Bay) rather than the confined Boston Harbor. This is achieved via the Inter-Island Tunnel and the famous 9.5-mile Outfall Tunnel, which is 24 feet in diameter and terminates in 55 diffusers at a depth of 100 feet. The location ensures rapid dilution and prevents nutrient accumulation near the coast. The plant operates under a strict NPDES permit monitored by the EPA and MassDEP, consistently earning Platinum Peak Performance Awards from NACWA for years of perfect compliance.

Treatment Process

The Deer Island facility utilizes a sophisticated treatment train designed to handle high organic loads within a constrained physical footprint. The process flow moves from the North and South Main Pump Stations through preliminary, primary, and secondary treatment, followed by disinfection.

A. Preliminary Treatment

Raw wastewater enters via the North Main Pump Station (NMPS) and South System Pump Station (SSPS).

• Pumping: The NMPS houses ten 3,500-HP motors driving pumps capable of 350 MGD. The SSPS (Lydia Goodhue Pump Station) has eight pumps with a capacity of 660 MGD.
• Grit & Screening: Flows pass through grit removal chambers to settle out sand and heavy inorganics. Subsequently, mechanical bar screens remove rags, plastics, and large debris to protect downstream equipment.

B. Primary Treatment

Given the limited acreage of Deer Island, the facility utilizes 48 stacked primary clarifiers. This innovative two-tiered design (essentially one tank built on top of another) doubles the settling area without increasing the land footprint.

• Mechanism: Gravity settling removes approximately 50-60% of Total Suspended Solids (TSS) and 30-40% of BOD.
• Scum Removal: Floating grease and oils are skimmed from the surface and sent to the digestion complex.
• Sludge Removal: Primary sludge is pumped to gravity thickeners.

C. Secondary Treatment (Pure Oxygen Activated Sludge)

Deer Island employs a high-rate Pure Oxygen Activated Sludge process. This method was selected over conventional aeration because pure oxygen allows for higher biomass concentrations and faster treatment rates, reducing the required tank volume—a necessity for the island site.

• Cryogenic Oxygen Plant: An onsite cryogenic facility separates nitrogen from air to produce roughly 130-150 tons of pure liquid oxygen daily.
• Reactor Trains: The oxygen is fed into covered, sealed biological reactor trains where microorganisms consume dissolved organic matter.
• Secondary Clarifiers: Following aeration, the mixed liquor flows to 54 secondary clarifiers (also stacked/multi-level configuration) where the biomass settles.
• Efficiency: This stage removes over 85-90% of remaining TSS and BOD.

D. Disinfection & Dechlorination

The clarified effluent enters the disinfection basin, which measures nearly 800 feet long.

• Chlorination: Sodium hypochlorite is added to kill pathogens (bacteria and viruses).
• Dechlorination: Before discharge, sodium bisulfite is added to remove residual chlorine, preventing toxicity to marine life in Massachusetts Bay.

E. Solids Handling (The “Egg” Digesters)

Deer Island is visually defined by its 12 egg-shaped anaerobic digesters, each 140 feet tall.

• Thickening: Primary sludge is gravity thickened; waste activated sludge (WAS) is thickened via dissolved air flotation (DAF) and centrifuges.
• Digestion: The thickened sludge enters the anaerobic digesters, kept at 98°F. The “egg” shape facilitates better mixing and eliminates dead zones where grit could accumulate (a common issue in cylindrical tanks).
• Residence Time: approximately 18-20 days.
• Resource Recovery: The process reduces sludge volume by ~55% and generates significant methane gas used for plant power.
• Final Disposal: Digested sludge is piped 7 miles via the Inter-Island Tunnel to the Fore River Pelletizing Plant in Quincy, MA. There, it is heat-dried into fertilizer pellets (branded as “Bay State Fertilizer”) for beneficial reuse.

Infrastructure & Facilities

A. Physical Plant

The facility occupies roughly 150 acres of the island, with the remaining 60 acres dedicated to public parkland (Deer Island Public Access Area). The site layout is a masterclass in hydraulic engineering, managing gravity flow where possible and utilizing vertical construction (stacked clarifiers, tall digesters) to maximize capacity.

B. Energy Systems & Neutrality

MWRA Deer Island is a leader in energy resilience and self-generation.

• Biogas Cogeneration: Methane from the digesters fuels a Combined Heat and Power (CHP) plant, providing heat for the process and electricity for operations.
• Wind Energy: Two 600-kW wind turbines are prominent features on the site.
• Solar: Photovoltaic arrays are installed on building roofs and ground mounts.
• Hydroelectric: Two hydro turbines capture energy from the treated effluent as it drops down the outfall shaft into the tunnel.

Collectively, these sources generate between 25% and 30% of the plant’s total electricity needs and nearly 100% of its heating needs.

C. Odor Control

Because the plant sits near the town of Winthrop and Logan International Airport, odor control is paramount. The facility uses extensive air scrubbing systems, including wet chemical scrubbers (using bleach and caustic soda) and carbon adsorbers, treating nearly all process air from the headworks, primary tanks, and solids handling areas.

Recent Upgrades & Major Projects

Regulatory Compliance & Environmental Performance

Deer Island operates under NPDES Permit #MA0103284. The permit is complex, requiring monitoring not just of the effluent, but of the receiving water column, sediment, and marine life in Massachusetts Bay.

• BOD/TSS Limits: The plant consistently achieves BOD and TSS removal rates well above the required 85%, typically operating in the 90-95% removal range for secondary treated flow.
• Nutrients: While not subject to a strict nitrogen limit like plants discharging into the Chesapeake or Long Island Sound (due to the high flushing rate of the Atlantic Ocean), the plant monitors nitrogen loads carefully.
• Outfall Monitoring: An independent science advisory panel oversees the monitoring of the outfall to ensure no adverse impacts on the Stellwagen Bank National Marine Sanctuary.

Operational Excellence

Staffing: The facility employs several hundred staff, including Grade 7 (highest level) wastewater operators, skilled tradespeople, and engineers. The MWRA maintains a rigorous training program to manage the complex cryogenic and high-voltage electrical systems onsite.

Automation: A centralized SCADA system monitors thousands of data points, allowing operators to automate flow balancing between the North and South systems and manage the blending of primary and secondary flows during peak wet weather events.

Challenges & Future Planning

Aging Infrastructure

With the plant now over 25 years old, the MWRA is shifting from a construction phase to a heavy asset maintenance phase. Corrosion from the salt air environment and the aggressive nature of wastewater requires constant rehabilitation of concrete and steel structures.

Climate Resilience

As an island facility, DITP is vulnerable to sea-level rise and storm surges. MWRA has conducted vulnerability assessments and is implementing hardening measures, such as elevating critical electrical gear and installing flood barriers, to ensure the plant remains operational during “100-year” storm events which are becoming more frequent.

Emerging Contaminants

While current regulations focus on conventional pollutants, MWRA is monitoring regulatory discussions regarding PFAS (forever chemicals). Future capital plans may need to address new removal requirements if EPA standards evolve.

Community & Regional Impact

The cleanup of Boston Harbor is widely cited as one of the greatest environmental success stories in the nation. The “dirty water” era of the 1980s has been replaced by a harbor that supports swimming, fishing, and a booming waterfront economy. The Deer Island facility itself is a destination, featuring 60 acres of public access parkland with 2.6 miles of trails offering views of the city and ocean, integrating industrial infrastructure with public recreation.

Technical Specifications Summary

Related Facilities

• Fore River Pelletizing Plant (Quincy, MA): Receives digested sludge from Deer Island via tunnel; processes it into fertilizer pellets.
• Headworks Facilities: Chelsea Creek, Ward Street, and Columbus Park headworks screen and pump flows from the remote collection system to the island.
• Clinton Wastewater Treatment Plant: A separate MWRA facility serving the Clinton/Lancaster area.

Frequently Asked Questions

Q: What are the “eggs” on Deer Island?
A: They are 140-foot tall anaerobic digesters. Their egg shape aids in mixing sludge and prevents grit accumulation, optimizing the breakdown of solids into methane gas and sludge cake.

Q: Does the plant smell?
A: Generally, no. Deer Island has one of the most extensive odor control systems in the world, scrubbing air from all major process areas before releasing it through carbon filters and stacks to protect neighbors in Winthrop.

Q: Is the effluent safe for the ocean?
A: Yes. The effluent meets strict EPA standards. It is discharged 9.5 miles offshore in deep water to ensure rapid dilution. Decades of monitoring have shown no adverse impact on the marine ecosystem of Massachusetts Bay.

Q: How does the plant handle hurricane flows?
A: The plant can ramp up to 1.27 billion gallons per day. Flows exceeding the secondary capacity (700 MGD) receive primary treatment and disinfection before being blended with the secondary effluent, ensuring all water is treated before discharge.

Q: Can the public visit Deer Island?
A: Yes. While the treatment plant interior is restricted for security, the perimeter features a public walkway/park open from dawn to dusk. MWRA also offers scheduled tours for school and professional groups.