City Of Stamford Wastewater Treatment Plant
The authoritative technical resource for the Stamford Wastewater Treatment Plant.
1. INTRODUCTION
The Stamford Water Pollution Control Authority (SWPCA) operates one of the most critical wastewater infrastructure assets on the Connecticut coastline. Serving the state’s second-largest city and the neighboring Town of Darien, this **24-MGD** Class IV facility plays a pivotal role in the environmental health of Western Long Island Sound. The plant is distinguished not only by its advanced biological nutrient removal (BNR) capabilities—essential for mitigating hypoxia in the Sound—but also for its innovative approach to biosolids management, featuring a specialized sludge drying facility aimed at energy recovery and volume reduction.
Located in the ecologically sensitive and residential Harbor View neighborhood, the facility operates under strict regulatory scrutiny regarding effluent quality and odor control. With recent capital investments exceeding **$40 million** in the last decade, the SWPCA continues to modernize its treatment train, transitioning from legacy systems to high-efficiency technologies including UV disinfection and upgraded headworks, ensuring compliance with Connecticut Department of Energy and Environmental Protection (CT DEEP) standards.
2. FACILITY OVERVIEW
A. Service Area & Coverage
The SWPCA serves a dense urban and suburban service area covering approximately 38 square miles. The collection system manages wastewater for the City of Stamford (population ~136,000) and accepts flow from the Town of Darien through a long-standing inter-municipal agreement. The system is comprised of over **300 miles of sanitary sewers** and **24 pump stations**, navigating a topography that requires significant lift capacity to convey flow to the Harbor View plant.
B. Operational Capacity
The facility is designed for an average daily flow of **24 MGD** with a peak hydraulic capacity reaching significantly higher levels during wet weather events. Historical trends indicate an average daily flow fluctuating between 17 and 19 MGD, providing a capacity utilization of approximately 75%. However, like many New England coastal cities, the system faces challenges with Inflow and Infiltration (I/I), which can cause instantaneous peak flows to exceed 50 MGD during severe storm events, necessitating robust hydraulic management strategies.
C. Discharge & Compliance
Treated effluent is discharged into the East Branch of Stamford Harbor, which feeds directly into Long Island Sound. Consequently, the facility operates under a stringent NPDES permit focused heavily on **Nitrogen removal** to combat eutrophication. The plant participates in the Long Island Sound Nitrogen Credit Exchange Program, a general permit requiring continuous optimization of denitrification processes. The facility has consistently maintained compliance with BOD and TSS removal efficiency requirements generally exceeding 90%.
3. TREATMENT PROCESS
A. PRELIMINARY TREATMENT
Raw influent enters the headworks where it undergoes aggressive physical screening. The facility utilizes mechanical **bar screens** with fine openings to remove rags, plastics, and debris essential for protecting downstream pumps. Following screening, flow enters **aerated grit chambers** where velocity is reduced to allow inorganic solids (sand, gravel) to settle while keeping organic matter in suspension. The grit is mechanically removed, washed, and dewatered for landfill disposal. A robust chemical odor control system (scrubbers) operates at the headworks to mitigate impacts on the surrounding residential zone.
B. PRIMARY TREATMENT
Wastewater flows into **three rectangular primary clarifiers**. These tanks reduce flow velocity, allowing settleable solids to drop to the bottom as primary sludge, while grease and oils float to the surface for skimming. The primary treatment stage typically removes 50-60% of Total Suspended Solids (TSS) and 30-35% of Biochemical Oxygen Demand (BOD). Primary sludge is pumped directly to the solids handling train for thickening.
C. SECONDARY TREATMENT (Activated Sludge)
The core biological treatment utilizes a **conventional activated sludge process** modified for Biological Nutrient Removal (BNR). The system consists of aeration tanks equipped with fine-bubble diffusers to maximize oxygen transfer efficiency. To meet the Total Nitrogen limits for Long Island Sound, the facility employs anoxic zones within the aeration train to facilitate denitrification (conversion of nitrate to nitrogen gas).
Following aeration, the mixed liquor flows to **three secondary clarifiers**. Here, the biological floc settles, producing a clear supernatant. A portion of the settled solids is returned to the aeration tanks as Return Activated Sludge (RAS) to maintain the biological population, while the excess is removed as Waste Activated Sludge (WAS).
D. DISINFECTION
Historically a chlorination facility, the SWPCA converted to **Ultraviolet (UV) Disinfection**. The effluent passes through banks of UV lamps which alter the DNA of pathogenic microorganisms (bacteria and viruses), rendering them unable to reproduce. This modernization eliminates the safety hazards associated with storing liquid chlorine gas and removes the need for dechlorination chemicals (sulfur dioxide) prior to discharge, resulting in a cleaner effluent profile for the marine environment.
E. SOLIDS HANDLING & DRYING
Stamford employs a sophisticated solids handling process. Primary sludge and WAS are thickened (using gravity thickeners and rotary drum thickeners, respectively) before being dewatered via **centrifuges**.
The dewatered cake is processed in an on-site **sludge drying facility**. This thermal drying process produces Class A Biosolids in the form of dried pellets (approx. 90% solids). These pellets are pathogen-free and significantly reduced in volume compared to traditional dewatered cake, lowering disposal transport costs. The pellets can be utilized as a soil amendment/fertilizer or as a renewable fuel source due to their caloric value.
4. INFRASTRUCTURE & FACILITIES
A. Physical Plant
The plant occupies a constrained footprint on Harbor View Avenue, surrounded by the West Branch and East Branch of Stamford Harbor. The site includes the Administration Building (housing SCADA control and laboratory), the Process Building, the Drying Facility, and maintenance garages. The architecture is functional industrial, though efforts have been made to screen the facility from neighbors using vegetation and fencing.
B. Energy & Sustainability
The SWPCA is a significant energy consumer, with major loads from aeration blowers, influent pumps, and the thermal drying system. Energy efficiency measures have included the installation of Variable Frequency Drives (VFDs) on major pumps and blowers to match energy use with hydraulic demand. The facility has investigated waste-to-energy technologies, including gasification of the dried biosolids pellets to generate electricity onsite, aiming to create a circular energy loop.
C. Odor Control
Given its location in a high-value residential area, odor control is a primary operational objective. The facility utilizes dual-stage odor control towers (wet scrubbers) and carbon adsorption units treating air from the headworks, primary clarifiers, and sludge processing areas. Negative pressure is maintained in key process buildings to prevent fugitive emissions.
5. RECENT UPGRADES & MAJOR PROJECTS
UV Disinfection System Upgrade (2018-2020)
- Project Scope: Replacement of the aging UV disinfection system with a modern, high-efficiency TrojanUV system. Included channel modifications and electrical upgrades.
- Cost: Approx. $2.5 Million
- Drivers: Equipment obsolescence, energy efficiency, and tighter reliability requirements.
- Result: Improved pathogen kill rates with reduced energy consumption and lower maintenance hours.
Electrical Distribution & Standby Power (2019-2021)
- Project Scope: Comprehensive upgrade of the plant’s electrical switchgear and installation of new emergency generators.
- Cost: Approx. $4-5 Million
- Drivers: Aging electrical infrastructure and the need for resiliency against grid failures during coastal storms.
- Result: Enhanced reliability and ability to maintain full treatment capacity during utility power outages.
Upcoming/Ongoing: Stormwater & Sewer Separation
While not strictly “inside the fence,” the City of Stamford is engaged in long-term sanitary sewer and stormwater separation projects. These capital improvements are vital for the plant’s operation as they reduce peak hydraulic loads during storm events, minimizing the risk of washouts in the biological process and ensuring stable treatment performance.
6. REGULATORY COMPLIANCE
A. Permit Requirements
The facility operates under NPDES Permit No. CT0101214. Key discharge parameters include:
- BOD5: 30 mg/L (Monthly Average)
- TSS: 30 mg/L (Monthly Average)
- Total Nitrogen: Subject to the General Permit for Nitrogen Discharges (mass loading limits vary by year based on the Trading Program).
- Bacteria (Enterococci): Geometric mean limits for salt water discharge.
B. Nitrogen Trading Program
Stamford is a key participant in Connecticut’s Nitrogen Credit Exchange. When the facility discharges less nitrogen than its assigned limit, it generates credits that can be sold to the state; conversely, exceeding the limit requires purchasing credits. The SWPCA has generally performed well, optimizing its anoxic zones to maintain Total Nitrogen levels below the aggressive targets set to protect Long Island Sound water quality.
7. OPERATIONAL EXCELLENCE
The SWPCA is staffed by a team of approximately 30-40 professionals, including state-certified wastewater operators (Class I through IV), maintenance mechanics, laboratory technicians, and administrative staff. The facility utilizes a comprehensive SCADA (Supervisory Control and Data Acquisition) system allowing for real-time monitoring of DO (Dissolved Oxygen) levels, tank levels, and motor status. The on-site laboratory conducts daily process control testing, while compliance testing is often verified by third-party certified labs.
8. CHALLENGES & FUTURE PLANNING
A. Coastal Resilience
Located immediately on the harbor, the SWPCA is vulnerable to sea-level rise and storm surge. Following Superstorm Sandy, resilience became a top priority. Future planning involves hardening critical infrastructure, elevating electrical substations, and installing flood barriers to protect process tanks from saltwater intrusion.
B. Aging Infrastructure
While the drying facility and UV systems are relatively modern, structural components of the primary and secondary tanks date back decades. A long-term Capital Improvement Plan (CIP) is in place to address concrete rehabilitation and the replacement of underground piping networks that are reaching the end of their useful life.
C. PFAS and Emerging Contaminants
Like all wastewater facilities, Stamford is monitoring the evolving regulatory landscape regarding PFAS (Per- and polyfluoroalkyl substances). Future upgrades may require advanced tertiary treatment technologies if strict effluent limits for these “forever chemicals” are mandated by state or federal EPA.
9. TECHNICAL SPECIFICATIONS SUMMARY
| Parameter | Specification |
|---|---|
| Facility Type | Advanced Secondary with BNR |
| Design Capacity | 24.0 MGD |
| Peak Hydraulic Capacity | > 50 MGD |
| Treatment Process | Screening, Grit Removal, Primary Clarification, Activated Sludge (Anoxic/Aerobic), Secondary Clarification |
| Disinfection | Ultraviolet (UV) Irradiation |
| Nutrient Removal | Biological Nitrogen Removal (Denitrification) |
| Biosolids Processing | Gravity Thickening, Centrifuge Dewatering, Thermal Drying (Pelletization) |
| Biosolids Product | Class A Dried Pellets |
| Service Area | Stamford, CT and Darien, CT |
| Receiving Water | Stamford Harbor (Long Island Sound) |
| NPDES Permit | CT0101214 |
10. FAQ SECTION
Technical Questions
Q: What is the specific Nitrogen removal method used?
A: The plant utilizes modified activated sludge with anoxic zones to facilitate biological denitrification, converting nitrates to nitrogen gas before secondary clarification.
Q: Does the facility generate its own power?
A: The facility has explored waste-to-energy gasification using dried sludge pellets. While the drying infrastructure is operational, full energy self-sufficiency remains an ongoing optimization goal involving complex thermal systems.
Q: How is the Stamford plant rated by CT DEEP?
A: It is rated as a Class IV facility, the highest classification in Connecticut, requiring the most experienced level of certified operators due to its size and process complexity.
Public Interest Questions
Q: Does the plant smell?
A: While wastewater treatment inherently generates odors, the SWPCA employs advanced chemical scrubbers and carbon filters. Most processes are covered or enclosed to capture and treat foul air before release.
Q: Where does the treated water go?
A: After thorough treatment and disinfection, the water is discharged via an outfall into the East Branch of Stamford Harbor, eventually mixing with Long Island Sound.
Q: Does Stamford treat water from other towns?
A: Yes, the SWPCA accepts wastewater from the neighboring Town of Darien via a piped connection, governed by an inter-municipal agreement.