City Of Pittsburgh ALCOSAN Wastewater Treatment Plant

FACILITY BASIC INFORMATION

• Plant Name: Allegheny County Sanitary Authority (ALCOSAN) Wastewater Treatment Plant
• Location: 3300 Preble Avenue, Pittsburgh, Allegheny County, Pennsylvania
• Operating Authority: Allegheny County Sanitary Authority (ALCOSAN)
• Design Capacity: 250 MGD (Average Daily), expanding to 480 MGD (Wet Weather)
• Current Average Flow: ~200 MGD
• Population Served: ~825,000 residents
• Service Area: 83 municipalities (City of Pittsburgh + 82 surrounding communities)
• Receiving Water Body: Ohio River
• NPDES Permit Number: PA0025984
• Year Commissioned: 1959

1. INTRODUCTION

The Allegheny County Sanitary Authority (ALCOSAN) Wastewater Treatment Plant represents the cornerstone of water quality management for the Greater Pittsburgh region. Situated along the Ohio River on the city’s North Side, this 59-acre facility serves as the terminus for a sprawling collection system covering approximately 310 square miles. Commissioned in 1959, ALCOSAN is one of the largest wastewater treatment facilities in the Ohio River Valley, providing vital sanitation services to over 825,000 residents across 83 diverse municipalities.

Currently, the facility is the focal point of a historic $2 billion “Clean Water Plan.” This massive infrastructure initiative aims to address chronic Combined Sewer Overflows (CSOs) mandated by a federal consent decree. Through a combination of plant expansion—increasing wet weather treatment capacity from 250 MGD to 480 MGD—and regional conveyance upgrades, ALCOSAN is transitioning from a mid-20th-century conventional plant into a modern, high-rate wet weather treatment complex. For engineering professionals, ALCOSAN offers a unique case study in retrofitting large-scale infrastructure within a highly constrained urban footprint while maintaining continuous operation.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The ALCOSAN service area is topographically complex, defined by the steep hills and valleys of the Allegheny Plateau. The system serves the City of Pittsburgh and 82 suburban municipalities. The demographics are a mix of dense urban residential, commercial districts, and light industrial zones. The collection system is notable for its deep tunnel interceptors; the authority maintains approximately 90 miles of interceptor sewers that convey flow by gravity along the Allegheny, Monongahela, and Ohio Rivers to the centralized treatment plant. A significant portion of the service area utilizes combined sewers, necessitating robust wet weather management strategies.

B. Operational Capacity

The facility is currently rated for a hydraulic design capacity of 250 Million Gallons per Day (MGD). Historically, flows fluctuate significantly due to precipitation events.

• Dry Weather Flow: Typically ranges between 160–180 MGD.
• Wet Weather Peak: The system is currently hydraulically limited, but upcoming expansions target a pump station capacity of 600 MGD with a full treatment capacity of 480 MGD.
• Capacity Utilization: During dry weather, the plant operates at approximately 60-70% capacity, but during storm events, hydraulic loading exceeds current secondary treatment capabilities, triggering permitted bypasses that are the target of current capital improvements.

C. Discharge & Compliance

Treated effluent is discharged directly into the Ohio River. The discharge is regulated under NPDES Permit No. PA0025984, administered by the Pennsylvania Department of Environmental Protection (PA DEP). The facility faces strict scrutiny regarding Combined Sewer Overflows (CSOs) and Sanitary Sewer Overflows (SSOs). The plant has maintained a strong record of compliance for dry-weather parameters (BOD, TSS), but the primary regulatory driver remains the mitigation of wet-weather overflow volume mandated by the EPA Consent Decree.

3. TREATMENT PROCESS

ALCOSAN utilizes a conventional activated sludge process tailored to handle high-volume fluctuations common in combined sewer systems. The treatment train is designed to maximize removal efficiencies within a land-locked 59-acre site.

A. PRELIMINARY TREATMENT

Raw wastewater enters the facility via deep tunnel interceptors at the Wet Well.

• Pumping: The main pump station lifts flow from the interceptors (approx. 40 feet deep) to the headworks. The pumps are large centrifugal units capable of handling massive variable flows.
• Screening: Heavy-duty mechanical bar screens remove large debris, plastics, and rags to protect downstream equipment. Screenings are washed, compacted, and sent to landfills.
• Grit Removal: Aerated grit chambers are utilized to settle out heavy inorganic materials (sand, gravel, coffee grounds) while keeping organic solids in suspension.

B. PRIMARY TREATMENT

Flow moves to rectangular primary sedimentation tanks.

• Clarification: The facility utilizes a battery of rectangular primary clarifiers equipped with chain-and-flight sludge collectors.
• Efficiency: These units typically remove 50-60% of Total Suspended Solids (TSS) and 30-40% of Biochemical Oxygen Demand (BOD).
• Scum Removal: Surface skimmers remove grease and oils, which are concentrated and handled separately (often incinerated).

C. SECONDARY TREATMENT

The biological core of the plant is the Conventional Activated Sludge system.

• Aeration Basins: The plant features parallel aeration tanks using plug-flow configurations.
• Aeration System: While historically relying on coarse bubble diffusion, recent optimizations have moved toward fine bubble diffusion and mechanical surface aeration in specific zones to improve Oxygen Transfer Efficiency (OTE).
• Secondary Clarifiers: Following aeration, the mixed liquor flows to final clarifiers. The return activated sludge (RAS) is pumped back to the aeration tanks to maintain the biomass, while waste activated sludge (WAS) is sent to solids handling.

D. DISINFECTION

ALCOSAN employs chemical disinfection prior to discharge.

• Chlorination: Sodium hypochlorite (liquid bleach) is added to the effluent in chlorine contact tanks to neutralize pathogens (fecal coliform/E. coli).
• Dechlorination: To protect aquatic life in the Ohio River, sodium bisulfite is added at the end of the contact period to remove residual chlorine before final discharge.
• Seasonality: Per PA DEP regulations, disinfection is typically required from May 1 through September 30 (recreational season), though monitoring occurs year-round.

E. SOLIDS HANDLING

ALCOSAN is notable for its thermal processing of biosolids, serving as a regional solids processing hub.

• Thickening: Primary sludge is gravity thickened; WAS is thickened using Dissolved Air Flotation (DAF) or centrifugal thickeners.
• Dewatering: High-solids centrifuges dewater the sludge to approximately 25-30% solids to prepare it for autogenous combustion.
• Incineration: The facility operates Fluidized Bed Incinerators (FBIs). This technology suspends the sludge in a hot sand bed, ensuring complete combustion and minimizing volume.
• Disposal: The resulting inert ash is collected via scrubbers and electrostatic precipitators and hauled to a sanitary landfill. Energy recovery from the incinerator flue gas is utilized for process heating.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The 59-acre site is densely developed. Major structures include the Main Pump Station (a historic landmark of industrial architecture), the distinctively shaped digester/holding tanks (though anaerobic digestion is not the primary reduction method, tanks are used for sludge storage), and the massive incinerator building. The administration building houses a fully accredited environmental laboratory.

B. Energy Systems

As a major industrial energy consumer, ALCOSAN draws significant power from the grid. However, the facility employs waste heat recovery from the fluidized bed incinerators to heat buildings and process streams during winter months. Recent capital projects have focused on upgrading VFDs (Variable Frequency Drives) on major pumps and aeration blowers to improve electrical efficiency.

C. Odor Control

Located near high-profile commercial zones (including Acrisure Stadium and the Carnegie Science Center), odor control is paramount. The facility employs a mix of chemical wet scrubbers and activated carbon adsorption systems, particularly at the headworks, sludge dewatering, and incineration complexes. The “Good Neighbor” policy drives strict monitoring of hydrogen sulfide levels at the fence line.

5. RECENT UPGRADES & MAJOR PROJECTS

ALCOSAN is currently executing its “Clean Water Plan,” a multi-decade, multi-billion dollar initiative. Below are the key components.

North End Plant Expansion (Ongoing 2020-2027)

• Project Scope: Construction of new primary and secondary treatment trains to increase wet weather capacity.
• Budget: Part of the larger ~$2 Billion program.
• Technical Highlights: The expansion requires building over existing structures and utilizing vertical space due to the site’s land constraints. It involves a new final settling tank complex and a new chlorine contact tank.
• Goal: Increase plant capacity from 250 MGD to 480 MGD (treated) and 600 MGD (pumping).

Main Pump Station Upgrade (2020-2025)

• Project Scope: Replacement of main raw sewage pumps and construction of a new wet well.
• Project Drivers: The need to convey higher wet weather flows to the plant rather than bypassing at interceptor overflow structures.
• Technical Specs: Installation of high-capacity submersible pumps capable of handling 600 MGD peak flow.

Ohio River Tunnel (Future Phase)

• Concept: A deep rock storage tunnel system to capture overflows from the combined sewer system along the Ohio River.
• Function: To store peak wet weather volume and convey it to the plant for treatment once storm flows subside.

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. Permit Requirements

The facility operates under a stringent NPDES permit that regulates BOD5, TSS, pH, Fecal Coliform, and various metals.

• BOD5 & TSS: Monthly average limit of 30 mg/L (standard secondary treatment).
• Removal Efficiency: Must achieve >85% removal of BOD and TSS.

B. Consent Decree

ALCOSAN is subject to a Consent Decree with the U.S. EPA, PA DEP, and the Allegheny County Health Department. Entered to address violations of the Clean Water Act regarding CSOs, this legal document dictates the timeline and scope of the Clean Water Plan. The primary goal is to eliminate SSOs and significantly reduce the volume and frequency of CSOs into the three rivers.

C. Environmental Stewardship

The authority has implemented a “Green Revitalization” grant program, funding green stormwater infrastructure (GSI) projects across its service municipalities (rain gardens, bioswales, permeable pavement) to reduce hydraulic loading at the source.

7. OPERATIONAL EXCELLENCE

A. Staffing

ALCOSAN employs approximately 400 staff members, including PA DEP certified wastewater operators (Class A), skilled maintenance tradespeople, and engineers. The facility maintains an internal training program to ensure operators meet continuing education requirements.

B. Technology & Innovation

The facility utilizes a comprehensive SCADA (Supervisory Control and Data Acquisition) system to monitor the vast interceptor network remotely. Real-time level sensors in the collection system provide data to optimize pump station operations during storm events, maximizing storage within the interceptor pipes (in-line storage) to minimize overflows.

8. CHALLENGES & FUTURE PLANNING

A. Topography and Combined Sewers

Pittsburgh’s steep topography causes rapid runoff concentration, leading to “flashy” flows at the plant. Managing the transition from 160 MGD to 600 MGD within hours is the primary operational challenge.

B. Regionalization

ALCOSAN is in the process of assuming ownership of certain multi-municipal trunk sewers that were previously owned by individual municipalities. This “regionalization” effort aims to streamline maintenance and reduce infiltration/inflow (I/I) across the system.

C. Future Planning

The Long-Term Control Plan continues through 2036. Future phases involve the construction of regional conveyance tunnels (Ohio, Allegheny, and Monongahela tunnels) which will act as massive underground storage tanks, similar to systems in Chicago and Milwaukee.

9. COMMUNITY & REGIONAL IMPACT

ALCOSAN is a vital economic enabler for the region; without expanded capacity, new development in Pittsburgh would be stalled by sewer tap-in restrictions. The facility hosts a popular annual “Open House” event, drawing thousands of residents for tours, emphasizing transparency and environmental education. The plant’s performance directly correlates to the recreational viability of the Three Rivers, supporting the region’s boating and fishing industries.

10. TECHNICAL SPECIFICATIONS SUMMARY

12. FAQ SECTION

Technical Questions

1. What is the main driver for the current plant expansion?
The primary driver is the EPA Consent Decree requiring a drastic reduction in Combined Sewer Overflows (CSOs) into the Ohio, Allegheny, and Monongahela Rivers.

2. Does ALCOSAN use anaerobic digestion?
No. While the facility has tanks that resemble digesters, they are used for sludge storage. The primary stabilization and reduction method is Fluidized Bed Incineration.

3. What is the target wet weather capacity?
The Clean Water Plan targets a treatment capacity of 480 MGD and a pumping capacity of 600 MGD.

4. How is the plant coping with the land constraints?
The North End expansion utilizes vertical construction and high-rate treatment technologies to fit increased capacity within the existing 59-acre footprint.

Public Interest Questions

5. Why does the river sometimes smell near the plant?
While ALCOSAN employs extensive odor scrubbers, atmospheric inversions or maintenance activities can occasionally allow odors to escape. However, many river odors are actually caused by CSOs upstream or industrial activities, not necessarily the plant itself.

6. Can I visit the plant?
Yes. ALCOSAN hosts an annual Open House (usually in September) which is the largest environmental festival in the region, offering full facility tours to the public.

7. Is the water discharged safe for drinking?
The effluent is treated to meet strict environmental standards for river health, but it is not potable (drinking) water. However, downstream communities do draw their drinking water from the Ohio River, which is why ALCOSAN’s treatment is so critical.