Philadelphia Water Department Southeast Wastewater Treatment Plant

1. INTRODUCTION

The Southeast Water Pollution Control Plant (SE WPCP) is one of three major water pollution control facilities operated by the Philadelphia Water Department (PWD). Situated near the strategic confluence of the Schuylkill and Delaware Rivers, the facility is critical to the environmental health of the Delaware Estuary. Treating an average of 100 to 120 million gallons daily (MGD), the plant serves the densely populated Center City and South Philadelphia corridors, as well as surrounding suburban municipalities.

Distinguished by its use of High Purity Oxygen (HPO) activated sludge technology and its proximity to Philadelphia’s major sports complex, the Southeast plant has become a model for energy resilience. Following the completion of a massive cogeneration facility in recent years, the plant now generates a significant portion of its own electricity and heat requirements through biogas utilization. This facility represents a successful case study in converting a legacy Combined Sewer Overflow (CSO) impacted facility into a modern resource recovery center.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The SE WPCP serves a diverse 21-square-mile drainage basin. The service area is characterized by high-density residential zones, significant commercial districts (including downtown Philadelphia), and industrial zones along the waterfront. Crucially, the plant serves a Combined Sewer System (CSS) area. During wet weather events, the collection system conveys a mixture of sanitary sewage and stormwater runoff to the facility. The system includes major interceptors such as the Lower Delaware Low Level Interceptor.

B. Operational Capacity

The facility is designed for an average annual daily flow of 121 MGD with a peak hydraulic capacity of approximately 224 MGD. Due to the CSS nature of the collection system, flow rates are highly variable. Dry weather flows typically range between 95-105 MGD, while storm events challenge the hydraulic maximums. The plant handles a high organic loading due to the density of the service area, with BOD₅ and TSS influent concentrations typical of urban combined wastewater.

C. Discharge & Compliance

Treated effluent is discharged into the Delaware River via a submerged outfall. The receiving water is classified as Zone 3 of the Delaware Estuary. The facility operates under NPDES Permit No. PA0026662, administered by the Pennsylvania Department of Environmental Protection (PADEP). Stringent limits are placed on Carbonaceous Biochemical Oxygen Demand (CBOD₅), Total Suspended Solids (TSS), pH, and Fecal Coliform. Recent regulatory focus has intensified regarding dissolved oxygen requirements in the Delaware River to support the propagation of sensitive aquatic species, specifically the Atlantic Sturgeon.

3. TREATMENT PROCESS

The Southeast WPCP utilizes a High Purity Oxygen (HPO) activated sludge process, distinct from the conventional aeration used at other PWD facilities. This configuration allows for a smaller physical footprint—essential given the plant’s urban location—and higher reaction rates.

A. Preliminary Treatment

Influent wastewater enters the headworks where it passes through mechanically cleaned bar screens to remove large debris, rags, and plastics. This is followed by grit removal in aerated grit chambers. The grit is settled, washed, and dewatered before being hauled off-site to sanitary landfills. Effective preliminary treatment is critical at this site due to the combined sewer system, which transports significant grit loads during storm events.

B. Primary Treatment

Wastewater flows into rectangular primary sedimentation tanks. Here, the velocity of the flow is reduced to allow settlable solids to drop to the bottom as primary sludge, while grease and oils float to the surface for skimming. The mechanical chain-and-flight collectors scrape sludge to hoppers. This stage typically removes 50-60% of suspended solids and 30-40% of BOD. The primary sludge is pumped directly to the gravity thickeners.

C. Secondary Treatment (High Purity Oxygen)

The biological heart of the SE WPCP is the HPO activated sludge system. Unlike conventional aeration tanks that pump ambient air (21% oxygen), this facility utilizes covered, staged aeration tanks where high-purity oxygen (>90%) is introduced.

• Oxygen Generation: The plant utilizes a Cryogenic Air Separation plant (or Pressure Swing Adsorption depending on current operational mode) to generate pure oxygen on-site.
• Bioreactors: The covered tanks maintain a high partial pressure of oxygen, driving mass transfer into the mixed liquor. This supports a higher Mixed Liquor Suspended Solids (MLSS) concentration and allows for shorter hydraulic retention times compared to conventional air systems.
• Secondary Clarification: The mixed liquor flows to final clarifiers where the biological floc settles. The clear supernatant overflows the weirs to disinfection.

D. Disinfection

Disinfection is achieved through chlorination using sodium hypochlorite (NaOCl). The effluent passes through contact tanks to ensure adequate pathogen inactivation (kill log). Following the contact period, the water undergoes dechlorination using sodium bisulfite to neutralize residual chlorine, preventing toxicity to aquatic life in the Delaware River.

E. Solids Handling & Biosolids

PWD operates a centralized biosolids approach, with the Biosolids Recycling Center (BRC) located adjacent to the Southeast plant.

• Thickening: Primary sludge is gravity thickened; Waste Activated Sludge (WAS) is typically thickened via centrifugation.
• Digestion: Thickened sludge undergoes Anaerobic Digestion. The digesters are heated to mesophilic temperatures (~98°F). This process stabilizes the sludge, reduces volatile solids, and generates methane-rich biogas.
• Dewatering: Digested sludge is dewatered using high-speed centrifuges to produce a “cake” with 25-30% solids.
• Disposal/Reuse: The biosolids are utilized in various beneficial reuse programs, including land application and composting, marketed as “Earthlife.”

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The facility occupies a constrained urban footprint south of the Walt Whitman Bridge. The site includes the Process Control Building, the Oxygen Generation Facility, the Headworks building, and the expansive digestion tank farms. The architecture is strictly industrial, characterized by the large, covered oxygenation decks and circular clarifiers.

B. Energy Systems & Cogeneration

The Southeast WPCP is the crown jewel of PWD’s energy program. The facility features a sophisticated Biogas Cogeneration Facility.

• Fuel Source: Methane gas produced during the anaerobic digestion process.
• Technology: Combined Heat and Power (CHP) engines.
• Capacity: Approximately 5.6 MW.
• Performance: The CHP system provides electricity to power plant equipment (such as the energy-intensive cryogenic oxygen plant) and recovers waste heat to maintain digester temperatures and heat buildings. This system provides over 85% of the plant’s electrical needs, significantly reducing the carbon footprint and operational costs.

C. Odor Control

Due to its location immediately adjacent to the Citizens Bank Park and Lincoln Financial Field, odor control is a paramount operational priority. The plant utilizes chemical scrubbers and biofilters to treat foul air extracted from the headworks, thickeners, and sludge loading areas.

5. RECENT UPGRADES & MAJOR PROJECTS

Headworks and Infrastructure Renewal (Ongoing)

PWD has invested heavily in the replacement of mechanical bar screens and grit handling equipment. Given the age of the facility, ongoing capital improvements focus on replacing electrical switchgear, rehabilitating concrete in primary tanks, and upgrading the SCADA (Supervisory Control and Data Acquisition) network for better automation.

Future: Act 537 & Green City, Clean Waters

While not a direct plant expansion, the “Green City, Clean Waters” program aims to reduce the hydraulic load on the Southeast plant by managing stormwater at the source using green infrastructure. However, traditional infrastructure upgrades at the plant are planned to increase wet weather treatment reliability.

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

The SE WPCP operates in strict compliance with the Clean Water Act. The Delaware River Basin Commission (DRBC) plays a significant role in setting water quality standards for the estuary.

• Nutrient Removal: Unlike plants discharging to the Chesapeake Bay, the Delaware regulatory environment is currently shifting. While the plant performs conventional secondary treatment, PWD is actively engaged in studies regarding dissolved oxygen restoration in the Delaware Estuary, which may necessitate future upgrades for enhanced ammonia-nitrogen removal (nitrification).
• CSO Management: The facility is integral to PWD’s Long Term Control Plan (LTCP) to manage combined sewer overflows. By maximizing flow through the plant during storms, the utility minimizes untreated discharges at regulation points.

7. CHALLENGES & FUTURE PLANNING

Aging Infrastructure

Maintaining the structural integrity of concrete tanks and process piping installed in the mid-20th century remains a primary challenge. PWD employs a robust asset management program to prioritize rehabilitation projects.

Climate Resilience

Situated on the tidal Delaware River, the Southeast plant is vulnerable to storm surge and sea-level rise. PWD actively models climate scenarios to plan for flood protection measures, elevating critical electrical gear, and ensuring the outfall structure functions correctly under higher tidal conditions.

Regulatory Shifts

The potential for stricter ammonia or total nitrogen limits from the DRBC/EPA represents the most significant future driver for process changes. Retrofitting an HPO plant for biological nutrient removal (BNR) is technically complex and capital intensive.

8. TECHNICAL SPECIFICATIONS SUMMARY

9. FAQ

Q: Why does the Southeast plant use High Purity Oxygen?
A: HPO systems are used to treat high-strength wastewater in a smaller physical footprint. Given the limited land availability in South Philadelphia, HPO allows for higher biomass concentrations and faster treatment than conventional air systems.

Q: Does the plant smell?
A: Wastewater treatment naturally generates odors, but PWD employs extensive odor control scrubbers. The proximity to the Sports Complex makes odor control a high priority, and complaints are generally minimal during normal operations.

Q: Is the plant energy independent?
A: While not 100% independent under all conditions, the cogeneration facility allows the plant to produce approx. 85% of its required electricity using biogas, making it a leader in renewable energy for wastewater utilities.

Q: What happens to the “sludge”?
A: The solids are digested to reduce volume and pathogens, then dewatered. The resulting biosolids are often used for land reclamation, mine reclamation, or composting operations, turning a waste product into a resource.