Mattabassett District Water Pollution Control Facility Cromwell

1. INTRODUCTION

The Mattabassett District Water Pollution Control Facility (WPCF) stands as the third-largest wastewater treatment plant in Connecticut and a critical environmental asset for the Connecticut River watershed. Serving a highly industrialized and densely populated region comprising New Britain, Middletown, Berlin, and Cromwell, the facility treats an average of 20 to 35 million gallons daily (MGD), with hydraulic peaks reaching 110 MGD during wet weather events.

Originally commissioned in 1968, the facility has evolved from a conventional secondary treatment plant into a state-of-the-art advanced treatment facility. Following a comprehensive $100+ million upgrade completed in 2015, the plant now features advanced nitrogen removal capabilities and a high-efficiency Fluidized Bed Incinerator (FBI) for solids handling. The Mattabassett District is recognized regionally for its sophisticated solids management program and its pivotal role in reducing nitrogen loading to Long Island Sound, consistently meeting stringent limits imposed by the Connecticut Department of Energy and Environmental Protection (CT DEEP).

2. FACILITY OVERVIEW

A. Service Area & Governance

Unlike municipal plants operated by a single city, this facility is governed by The Mattabassett District, a regional quasi-municipal entity created by the State Legislature in 1961. The District serves four constituent communities:

• New Britain: A dense urban center with significant industrial discharge.
• Middletown: A mixed urban/suburban municipality.
• Berlin & Cromwell: Growing suburban communities with residential and commercial bases.
• Contract Communities: The District also accepts flows from portions of Farmington, Newington, and Rocky Hill.

The collection system is unique in that the District owns and operates the majestic trunk sewers and interceptors that convey flow from the town borders to the plant, while the municipalities retain ownership of their local collection systems.

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B. Operational Capacity

The facility is designed to handle significant flow variations due to the combined sewer systems present in parts of the service area (particularly New Britain).

• Average Daily Design Flow: 35 MGD
• Peak Hourly Flow: 110 MGD
• Current Average Loading: ~21 MGD (varies by season/rainfall)

The plant utilizes 100% of its secondary treatment capacity year-round, with excess wet weather flows receiving primary treatment and disinfection before blending, in strict accordance with wet weather flow management protocols.

C. Discharge & Compliance

Treated effluent is discharged into the Connecticut River via a submerged multi-port diffuser located approximately 25 miles north of Long Island Sound. The facility operates under a National Pollutant Discharge Elimination System (NPDES) permit administered by CT DEEP. The plant is a critical participant in the Long Island Sound Nitrogen Credit Exchange Program, having reduced total nitrogen discharge by nearly 70% from historical baselines to combat hypoxia in the Sound.

3. TREATMENT PROCESS

The Mattabassett District WPCF utilizes an advanced modification of the activated sludge process, specifically designed for nitrogen removal, followed by tertiary filtration and chlorination.

A. PRELIMINARY TREATMENT

Raw wastewater enters the headworks where it undergoes rigorous physical separation to protect downstream equipment:

• Mechanical Screening: Three mechanically cleaned bar screens remove rags, plastics, and large debris.
• Grit Removal: Aerated grit chambers decrease the velocity of the flow, allowing heavy inorganic materials (sand, gravel, coffee grounds) to settle while organic matter remains suspended.

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• Septage Receiving: A dedicated automated station accepts septage from licensed haulers, integrating it into the headworks flow.

B. PRIMARY TREATMENT

Flow enters four rectangular primary settling tanks. These tanks reduce the velocity of the wastewater to allow settleable solids to drop to the bottom as primary sludge, while oils and grease (scum) float to the surface. Chain-and-flight mechanisms scrape sludge to hoppers and skim scum from the surface.
Removal Efficiency: Typically 30-35% of BOD and 50-60% of TSS.

C. SECONDARY TREATMENT (Nitrogen Removal)

The heart of the facility is the biological treatment stage, which was significantly reconfigured during the 2012-2015 upgrade to achieve biological nutrient removal (BNR).

• Process Type: Modified Ludzack-Ettinger (MLE) / Four-Stage Bardenpho configuration capabilities.
• Aeration Tanks: The facility utilizes four aeration trains. Each train is divided into anoxic zones (where no oxygen is added) and aerobic zones (aerated).
• Aeration System: Fine bubble diffusion provides high-efficiency oxygen transfer.
• Biological Mechanism:
  • Nitrification: In aerobic zones, bacteria convert ammonia to nitrate.
  • Denitrification: Nitrate-rich mixed liquor is recycled to the anoxic zones, where bacteria strip the oxygen molecules from the nitrate, releasing harmless nitrogen gas into the atmosphere.

• Secondary Clarifiers: Four circular clarifiers separate the biological mass (RAS) from the treated water. The RAS is returned to the aeration tanks to maintain the biological population.

D. TERTIARY TREATMENT

To meet stringent nitrogen limits that biological treatment alone cannot always achieve, specifically during cold weather months, the plant employs tertiary denitrification.

• Technology: Deep Bed Denitrification Filters.
• Operation: Methanol is added as a carbon source to drive the conversion of remaining nitrates to nitrogen gas within the filter media. The filters also serve as a final polishing step for Total Suspended Solids (TSS).

E. DISINFECTION

The facility uses a chemical disinfection process:

• Chlorination: Sodium hypochlorite is injected into the contact tanks to eliminate pathogenic bacteria and viruses.
• Dechlorination: Before discharge into the Connecticut River, sodium bisulfite is added to remove residual chlorine, ensuring the effluent is non-toxic to aquatic life.
• Disinfection Season: Required from May 1st through September 30th (per CT DEEP standard), though the facility may operate year-round depending on specific permit conditions.

F. SOLIDS HANDLING (Thermal Oxidation)

Mattabassett is a regional leader in solids disposal, processing not only its own sludge but also receiving cake from merchant facilities.

• Thickening: Gravity belt thickeners concentrate waste activated sludge.
• Dewatering: Centrifuges dewater the sludge to approximately 24-28% solids.
• Incineration: The facility operates a Fluidized Bed Incinerator (FBI).
  • Unlike older multiple-hearth furnaces, the FBI suspends the sludge in a bed of hot sand kept in motion by air jets.
  • This ensures complete combustion, lower emissions, and higher energy efficiency.
  • The ash byproduct is stable and disposed of at approved landfills.

G. PROCESS CONTROL

The entire facility is monitored via a centralized SCADA (Supervisory Control and Data Acquisition) system. This allows operators to monitor dissolved oxygen levels, return sludge rates, and chemical dosages in real-time, optimizing the plant for both compliance and energy efficiency.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The site spans roughly 30 acres adjacent to the Connecticut River. The architecture reflects a functional industrial aesthetic, with the incineration building dominating the skyline. Recent upgrades included a new Administration Building and Laboratory that facilitate modern operational management.

B. Energy Systems & Heat Recovery

The Fluidized Bed Incinerator includes a sophisticated Waste Heat Recovery System. The thermal energy generated during sludge combustion is captured via a heat exchanger. This recovered energy is used to:

1. Pre-heat the combustion air (reducing fuel usage).
2. Provide building heat for the facility during winter.
3. Maintain process loop temperatures.

This system significantly reduces the facility’s reliance on natural gas and lowers the overall carbon footprint.

C. Odor Control

Given the proximity to residential areas and Route 9, odor control is a top priority. The facility employs a multi-stage approach:

• Source Capture: Covers on primary tanks and sludge storage areas.
• Biofilters: Organic media beds that use bacteria to oxidize odorous compounds.
• Carbon Adsorption: Activated carbon scrubbers polish the air from the most odorous process areas (headworks and sludge processing) before release.

5. RECENT UPGRADES & MAJOR PROJECTS

Nitrogen Removal & Facility Upgrade Project (2012-2015)

This was the most significant capital project in the District’s history, aimed at meeting the requirements of the Long Island Sound Total Maximum Daily Load (TMDL) for nitrogen.

• Project Budget: ~$105 Million
• Funding Sources:
  • CT Clean Water Fund (State Grants & Low-Interest Loans)
  • District Reserves

• Key Contractors:
  • Design Engineer: Wright-Pierce
  • Construction Contractor: C.H. Nickerson & Co.
  • Construction Admin: CH2M (now Jacobs)

• Technical Highlights:
  • Construction of new concrete aeration tanks configured for BNR.
  • Installation of a new Fluidized Bed Incinerator to replace aging Multiple Hearth Incinerators, meeting new EPA MACT (Maximum Achievable Control Technology) sewage sludge incineration standards.
  • New 72-inch outfall pipe and multi-port diffuser to improve mixing in the Connecticut River.
  • Installation of denitrification filters.

• Results: The project successfully reduced nitrogen discharge to below permit limits (often achieving < 4 mg/L Total Nitrogen) and drastically reduced air emissions from the solids handling process.

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. Permit Requirements

The facility operates under a stringent NPDES permit that regulates:

• CBOD & TSS: Traditional secondary treatment standards (typically < 30 mg/L average monthly).
• Nitrogen: The facility has a General Permit for Nitrogen Discharges. It is allocated a specific mass loading (lbs/day) of nitrogen. If the plant discharges less than its allocation, it sells credits to the state exchange; if it discharges more, it must purchase credits.
• Metals: Due to the industrial user base in New Britain and Middletown, the plant strictly monitors heavy metals (Copper, Zinc, Lead) in the effluent.

B. Air Compliance

The Fluidized Bed Incinerator operates under a Title V Air Permit. It is equipped with advanced emissions control, including a venturi scrubber (for particulate), a wet electrostatic precipitator (WESP), and a regenerative thermal oxidizer (RTO) or carbon polishing to handle mercury and total hydrocarbons.

7. OPERATIONAL EXCELLENCE

A. Staffing

The Mattabassett District employs approximately 50-60 personnel, including administrative staff, laboratory technicians, maintenance mechanics, and process operators. Operations staff maintain 24/7 coverage. Given the complexity of the FBI and BNR processes, the facility requires high-level certifications (CT DEEP Class IV Wastewater Operator certification for supervisors).

B. Laboratory Capabilities

The on-site laboratory is certified by the Connecticut Department of Public Health. It performs daily analysis for process control (microscopy, settleability) and compliance reporting (BOD, TSS, Ammonia, Nitrogen series, pH, Fecal Coliform).

8. CHALLENGES & FUTURE PLANNING

A. Emerging Contaminants (PFAS)

Like all wastewater facilities, Mattabassett is monitoring regulatory developments regarding PFAS (Per- and polyfluoroalkyl substances). As a receiver of industrial wastewater and landfill leachate (if applicable), the District is evaluating source control strategies to minimize PFAS entering the plant.

B. Infrastructure Aging

While the main process trains were upgraded in 2015, the underground conveyance infrastructure (interceptors and trunk sewers) requires ongoing rehabilitation to prevent inflow and infiltration (I/I), which consumes hydraulic capacity during storms.

C. Energy Neutrality Goals

Future planning includes looking at ways to further optimize energy generation from sludge incineration or potential co-digestion opportunities, although the current FBI setup prioritizes volume reduction and clean emissions over biogas generation.

9. TECHNICAL SPECIFICATIONS SUMMARY

10. FAQ SECTION

Technical Questions

1. Does the Mattabassett facility generate electricity?
While the facility does not use anaerobic digesters to create biogas for electricity (like cogeneration plants), it utilizes a sophisticated waste heat recovery system on its Fluidized Bed Incinerator. This recovers thermal energy to heat the facility and process streams, significantly offsetting natural gas costs.

2. How does the facility handle wet weather flows?
The plant is designed for a peak of 110 MGD. Flows up to a certain threshold receive full tertiary treatment. Extreme peak flows may receive primary treatment and disinfection before blending with the fully treated effluent, ensuring all discharge meets bacterial limits.

3. What technology is used for Nitrogen removal?
The plant uses a two-step approach: Biological Nutrient Removal (BNR) in the aeration tanks using anoxic zones, followed by deep bed denitrification filters with methanol addition to polish the effluent to low nitrogen levels required for Long Island Sound.

4. Why was a Fluidized Bed Incinerator chosen over digestion?
Due to the limited land area and the high volume of solids (including industrial contributions), incineration offers maximum volume reduction (reducing sludge to sterile ash). The Fluidized Bed technology was chosen for its superior fuel efficiency and lower emissions compared to the previous multiple hearth furnaces.

General Interest Questions

5. Does the plant smell?
Historically, odors were a challenge, but the recent $100M upgrade included extensive odor control systems. Biofilters and carbon scrubbers now treat air from the headworks and sludge processing areas, significantly reducing off-site odors.

6. Is the discharge safe for the Connecticut River?
Yes. The effluent is treated to meet strict state and federal standards. It is disinfected to kill bacteria and dechlorinated to protect fish and aquatic life before entering the river.

7. Who runs the facility?
It is operated by The Mattabassett District, a quasi-municipal government entity with a Board of Directors appointed by the constituent towns (New Britain, Middletown, Berlin, and Cromwell).