Allegheny County Sanitary Authority Woods Run Wastewater Treatment Plant Pittsburgh

FACILITY BASIC INFORMATION

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1. INTRODUCTION

The Allegheny County Sanitary Authority (ALCOSAN) Wastewater Treatment Plant serves as the cornerstone of environmental health for the Greater Pittsburgh region. Situated along the Ohio River, this facility is one of the largest wastewater treatment plants in the Ohio River Valley. Originally commissioned in 1959 to curb the discharge of raw sewage into local waterways, the plant currently provides treatment for 83 municipalities, covering a service area of approximately 310 square miles and a population of roughly 900,000.

The facility is currently at the center of one of the nation’s most ambitious infrastructure initiatives: the Clean Water Plan. Operating under a federal Consent Decree, ALCOSAN is executing a multi-billion dollar capital program to address Combined Sewer Overflows (CSOs) inherent to the region’s topography and aging collection system. With a current rated capacity of 250 MGD and wet weather expansion plans targeting 600 MGD, the plant utilizes High-Purity Oxygen Activated Sludge technology and advanced incineration for solids handling, setting a benchmark for large-scale urban wet weather management.

2. FACILITY OVERVIEW

A. Service Area & Coverage

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ALCOSAN serves a unique and challenging topography defined by steep hills and three major rivers (Allegheny, Monongahela, and Ohio). The service area encompasses the City of Pittsburgh and 82 surrounding suburban municipalities. The collection system is a hybrid of separate sanitary sewers and combined sewers, with the latter comprising a significant portion of the older infrastructure in Pittsburgh proper. The authority maintains approximately 90 miles of large-diameter interceptor sewers (ranging from 10 inches to 10 feet in diameter) that convey flow by gravity to the central plant.

B. Operational Capacity

The plant operates with a hydraulic design capacity of 250 million gallons per day (MGD). However, during dry weather, the average daily flow typically ranges between 180 and 200 MGD. The region’s combined sewer system creates extreme hydraulic peaking; during significant precipitation, flows can rapidly exceed the plant’s treatment capacity, necessitating the current massive expansion projects. Historically, the plant has managed wet weather through regulated bypasses, a practice now being aggressively minimized through the Clean Water Plan.

C. Discharge & Compliance

Treated effluent is discharged into the Ohio River. As a major tributary to the Mississippi River, water quality standards are strictly enforced by the Pennsylvania Department of Environmental Protection (PA DEP) and the U.S. EPA. The facility operates under NPDES Permit No. PA0025984. Recent compliance efforts focus heavily on reducing CSO events and meeting stringent limits for solids and biological oxygen demand, with future scrutiny likely increasing on nutrient loading.

3. TREATMENT PROCESS

The ALCOSAN facility employs a secondary treatment train utilizing high-purity oxygen activated sludge, a technology chosen for its ability to treat high-strength waste within a constrained physical footprint.

A. Preliminary Treatment & Pumping

Raw wastewater enters the facility via the deep tunnel interceptor system, arriving at the massive Main Pump Station. This station is one of the deepest and largest of its kind, lifting flow from the interceptors (approx. 100 feet below grade) to the surface treatment units.

• Screening: Mechanically cleaned coarse bar screens remove large debris (rags, wood, plastics) to protect downstream pumps.
• Grit Removal: Following screening, flow passes through aerated grit chambers where inorganic solids (sand, gravel, cinders) settle out.

B. Primary Treatment

Flow is distributed to a battery of rectangular primary sedimentation tanks.

• Clarification: Settleable solids drop to the bottom as primary sludge, while grease and floatables are skimmed from the surface.
• Efficiency: The primary system is designed to remove approximately 60-65% of Total Suspended Solids (TSS) and 30-35% of Biochemical Oxygen Demand (BOD).

C. Secondary Treatment (High-Purity Oxygen)

The biological heart of the plant is the High-Purity Oxygen (HPO) Activated Sludge system.

• Process: Unlike conventional aeration that uses ambient air, ALCOSAN uses a cryogenic oxygen generation plant on-site to produce pure oxygen. This oxygen is injected into covered aeration basins.
• Benefits: The HPO process allows for a higher biomass concentration (MLSS), faster reaction rates, and better settling characteristics, essentially allowing the plant to treat more volume in smaller tanks—critical for the land-locked site.
• Clarification: Mixed liquor flows to secondary clarifiers (circular) where the biological floc settles. Return Activated Sludge (RAS) is recycled to the aeration tanks, while clarified effluent proceeds to disinfection.

D. Disinfection

Disinfection is achieved using liquid sodium hypochlorite (chlorination).

• Contact: Effluent flows through chlorine contact tanks to ensure pathogen inactivation.
• Dechlorination: Before discharge to the Ohio River, sodium bisulfite is added to remove residual chlorine, protecting aquatic life in the receiving water.

E. Solids Handling

ALCOSAN utilizes thermal processing for biosolids reduction.

• Thickening & Dewatering: Primary and waste activated sludge are blended and dewatered using high-performance centrifuges to achieve a solid cake (typically 25-30% solids).
• Incineration: The dewatered cake is incinerated in Fluidized Bed Incinerators. This reduces the volume of solids by over 90%, resulting in a sterile ash.
• Disposal: The residual ash is landfilled. This method destroys pathogens and organic pollutants, including emerging contaminants.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The facility occupies a roughly 59-acre site on the North Side of Pittsburgh, tightly constrained by the Ohio River to the west and Highway 65/residential neighborhoods to the east. This constraint dictates vertical construction and high-rate treatment technologies. The site includes the iconic Operations Building, massive cryogenic oxygen towers, and the distinct stack of the incineration complex.

B. Energy Systems

ALCOSAN is a significant energy consumer but has implemented recovery systems. The incineration process includes waste heat recovery boilers that generate steam. This steam drives turbines to generate electricity or drive process blowers, offsetting a portion of the grid energy requirement. The implementation of variable frequency drives (VFDs) on major pumps has further optimized consumption.

C. Odor Control

Given the proximity to the Brightwood and Marshall-Shadeland neighborhoods, odor control is paramount. The plant utilizes chemical scrubbers (packed towers) and activated carbon systems, particularly at the headworks, primary clarifiers, and sludge processing areas. The covered nature of the HPO aeration basins also inherently contains process odors.

5. RECENT UPGRADES & MAJOR PROJECTS

ALCOSAN is currently executing the Clean Water Plan, a multi-decade, multi-billion dollar initiative to comply with the federal Consent Decree.

North End Plant Expansion (Ongoing)

• Project Scope: This is the centerpiece of the current CIP. It involves expanding the plant’s wet weather capacity from 250 MGD to 480 MGD (with potential for 600 MGD peak).
• Components: Construction of new primary sedimentation tanks, reconfiguration of the disinfection system, and massive hydraulic improvements.
• Budget: Estimated >$300 Million.
• Technical Highlight: The project requires building new treatment infrastructure while keeping the existing plant fully operational—a complex “open heart surgery” engineering challenge.

Main Pump Station Upgrade (Completed/Ongoing Phases)

• Scope: Rehabilitation and replacement of the massive raw sewage pumps and motors to ensure reliability and handle increased wet weather flows.
• Drivers: Aging infrastructure (original 1950s equipment) and hydraulic capacity needs.
• Results: Increased reliability and hydraulic lift capacity to feed the expanded treatment train.

Regional Tunnel System (Planning/Design Phase)

• Scope: Construction of a deep rock tunnel system (Ohio River Tunnel) to store and convey combined sewer overflows to the plant.
• Timeline: Construction expected to ramp up mid-to-late 2020s.
• Function: Similar to projects in DC and Chicago, this tunnel will act as an underground reservoir, capturing peak flows during storms and pumping them to the plant for treatment after the storm subsides.

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. Permit Requirements

The facility operates under a strict NPDES permit focusing on CBOD5, TSS, pH, Fecal Coliform, and Total Residual Chlorine. The defining regulatory instrument, however, is the Consent Decree with the EPA, DOJ, and PA DEP. This decree mandates the elimination of sanitary sewer overflows (SSOs) and the dramatic reduction of combined sewer overflows (CSOs).

B. Compliance Strategy

ALCOSAN’s strategy has shifted from “gray infrastructure only” to a balanced approach. While the plant expansion and deep tunnels constitute the gray infrastructure, the authority is also funding “green infrastructure” (Green Revitalization of Our Waterways – GROW program) within customer municipalities to reduce stormwater inflow at the source.

7. OPERATIONAL EXCELLENCE

A. Staffing

The facility is staffed 24/7/365 by a workforce of over 400 employees, including PA DEP certified operators, maintenance mechanics, electricians, and engineers. The on-site laboratory is fully accredited and performs thousands of analyses annually for process control and regulatory reporting.

B. Technology

The transition to a highly automated SCADA system allows for precise control of the High-Purity Oxygen system and incineration temperatures. Real-time monitoring of the interceptor system via level sensors helps operators manage incoming flows and maximize tunnel storage utilization.

8. CHALLENGES & FUTURE PLANNING

A. Wet Weather Management

The primary engineering challenge remains the sheer volume of water entering the system during storms. The “flashy” nature of Pittsburgh’s hydrology means flow can jump from 150 MGD to over 800 MGD in hours. Managing these hydraulics without washing out the biological mass is a constant operational focus.

B. Site Constraints

With no room to expand horizontally, future technologies must be compact. This may necessitate moving toward Membrane Bioreactors (MBR) or Ballasted Flocculation (e.g., ACTIFLO) for future tertiary treatment or peak flow management.

C. Funding and Rates

The Clean Water Plan represents a cost of over $2 billion. Balancing the necessary rate increases for customers while maintaining affordability programs is a major administrative challenge.

9. COMMUNITY & REGIONAL IMPACT

ALCOSAN is more than a utility; it is a regional partner. The GROW Program provides matching grants to municipalities for source reduction projects (rain gardens, sewer separation), fostering regional collaboration. The annual “Open House” is the largest environmental festival in the region, drawing thousands of residents for tours, demystifying the treatment process and building public support for infrastructure investment.

10. TECHNICAL SPECIFICATIONS SUMMARY

11. RELATED FACILITIES

ALCOSAN maintains an extensive network of diversion structures and regulator chambers throughout the collection system. While the Woods Run plant is the sole treatment facility, the planned Regional Tunnel System will essentially act as a massive linear extension of the plant’s headworks, providing storage and conveyance. Additionally, ALCOSAN coordinates with municipal pump stations that lift flow into the regional interceptors.

12. FAQ SECTION

Technical Questions

1. Why does ALCOSAN use High-Purity Oxygen (HPO)?
HPO was selected to treat high organic loads in a smaller tank volume compared to conventional air activated sludge. This is critical given the facility’s constrained footprint between the river and the highway.

2. What is the status of the Clean Water Plan?
The plan is in active implementation. The plant expansion is under construction, and design work is progressing for the regional tunnel system. The goal is to complete major components by the late 2020s/early 2030s.

3. How are biosolids handled?
ALCOSAN incinerates dewatered sludge. The resulting ash is disposed of in landfills. Incineration significantly reduces volume and eliminates pathogens.

4. Does the plant perform nutrient removal?
Currently, the plant is designed for carbonaceous BOD removal and TSS removal. While not currently designed for high-level Nitrogen or Phosphorus removal (BNR), future regulations on the Ohio River could necessitate upgrades.

Public Interest Questions

5. Can the public tour the plant?
Yes, ALCOSAN hosts an annual Open House (typically in September) which includes extensive tours. Group tours can also be arranged through their educational outreach department.

6. Does the plant smell?
While wastewater treatment naturally generates odors, ALCOSAN employs extensive odor control scrubbers. Most process tanks are covered to capture and treat foul air before release.

7. What happens during heavy rain?
During extreme rain, the volume of water in the combined sewers exceeds the plant’s capacity. To prevent plant washout and basement backups, excess diluted flow is discharged via Combined Sewer Overflows (CSOs) into the rivers. The current expansion projects aim to drastically reduce the frequency and volume of these events.

8. Is the water safe to drink after treatment?
The effluent is treated to be safe for the environment (aquatic life) but is not potable (drinking) water. However, downstream communities draw drinking water from the Ohio River, which is why ALCOSAN’s treatment standards are so critical.