City Of Portland Columbia Boulevard Wastewater Treatment Plant

1. INTRODUCTION

The Columbia Boulevard Wastewater Treatment Plant (CBWTP) serves as the cornerstone of wastewater management for the City of Portland, Oregon. As the state’s largest wastewater treatment facility, CBWTP is a complex hydraulic system designed to manage the unique challenges of a combined sewer system (CSS). The facility treats an average dry-weather flow of approximately 70 million gallons per day (MGD) but is engineered to handle massive hydraulic spikes up to 450 MGD during storm events to prevent Combined Sewer Overflows (CSOs) into the Willamette and Columbia Rivers.

Commissioned in 1952 and significantly expanded in the 1970s and 1990s, the plant is currently undergoing the most significant capital improvement phase in its history. Operated by the Bureau of Environmental Services (BES), CBWTP is distinguished by its High Purity Oxygen (HPO) activated sludge process and its pioneering efforts in resource recovery, specifically the conversion of biogas into Renewable Natural Gas (RNG) for the commercial market.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The CBWTP services a population of approximately 665,000 across the Portland metropolitan area. The collection system feeding the plant is a hybrid infrastructure comprising both separated sanitary sewers and a legacy combined sewer system that collects stormwater and sanitary sewage in the same pipes. The service area spans roughly 100 square miles, capturing flow from nearly all of Portland, with intergovernmental agreements accepting flow from portions of Gresham and Lake Oswego. The conveyance system relies heavily on the “Big Pipe” projects—large-diameter tunnels completed in 2011—that store and transport effluent to CBWTP.

B. Operational Capacity

The plant exhibits a massive variance between dry and wet weather operations:

• Average Dry Weather Flow: 70 MGD
• Peak Wet Weather Capacity: 450 MGD

This 6:1 peaking factor drives the facility’s design philosophy. While secondary treatment capacity is currently limited to approximately 100-120 MGD, the plant utilizes chemically enhanced primary treatment (CEPT) and blending strategies during peak events to manage excess flows in compliance with state regulations.

C. Discharge & Compliance

Treated effluent is discharged into the Columbia River via two submerged outfalls located approximately 0.5 miles downstream from the plant. The discharge is regulated under a National Pollutant Discharge Elimination System (NPDES) permit issued by the Oregon Department of Environmental Quality (DEQ). The facility operates under strict constraints regarding Total Suspended Solids (TSS), Biochemical Oxygen Demand (BOD), and increasingly, temperature and nutrient loading, which are critical for the protection of Salmonid species in the Columbia River basin.

3. TREATMENT PROCESS

CBWTP utilizes a screen-to-solids treatment train designed for high-volume throughput and resource recovery.

A. PRELIMINARY TREATMENT

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The headworks facility is designed to protect downstream equipment from debris and grit. Raw influent is lifted via major pump stations (including the Portsmouth Pumping Station) into the headworks.

• Screening: Mechanically cleaned bar screens remove large debris (rags, plastics, wood).
• Grit Removal: Aerated grit chambers settle out heavy inorganic materials (sand, gravel) to prevent abrasion on pumps and accumulation in digesters.
• Compaction: Screenings are washed and compacted before landfill disposal.

B. PRIMARY TREATMENT

Following preliminary treatment, flow enters the primary clarification stage. The plant utilizes a battery of rectangular primary clarifiers. During wet weather events, the plant employs Chemically Enhanced Primary Treatment (CEPT), adding coagulants (typically ferric chloride and anionic polymers) to increase settling velocities and capture TSS/BOD at rates higher than gravity settling alone. This allows the plant to treat flows exceeding the secondary system’s capacity.

C. SECONDARY TREATMENT

The core biological treatment utilizes a High Purity Oxygen (HPO) Activated Sludge process. Unlike conventional aeration which uses ambient air (21% oxygen), the HPO system utilizes covered aeration basins and >90% pure oxygen.

• Oxygen Generation: The plant operates a Vacuum Swing Adsorption (VSA) system (replacing older cryogenic plants) to generate pure oxygen on-site.
• Reactors: The HPO process allows for higher mixed liquor suspended solids (MLSS) concentrations and smaller tank volumes compared to conventional air activated sludge. This is crucial for the footprint-constrained site.
• Secondary Clarifiers: Mixed liquor flows to secondary clarifiers (circular) where biomass settles and is returned (RAS) to the head of the aeration basins or wasted (WAS) to solids processing.

D. DISINFECTION

Disinfection is achieved through chlorination using sodium hypochlorite. The effluent passes through chlorine contact basins to ensure pathogen inactivation. Prior to discharge into the Columbia River, the effluent undergoes dechlorination using sodium bisulfite to protect aquatic life from chlorine toxicity.

E. SOLIDS HANDLING & RESOURCE RECOVERY

CBWTP is a regional leader in biosolids management and energy recovery.

• Thickening: Primary sludge is gravity thickened; Waste Activated Sludge (WAS) is thickened via gravity belt thickeners (GBT) or dissolved air flotation (DAF).
• Anaerobic Digestion: The plant operates a complex of anaerobic digesters (mesophilic and thermophilic capabilities) that stabilize the sludge and reduce volatile solids.
• Biogas Utilization: Methane produced during digestion is captured. Historically used for cogeneration (heat and power), the plant has transitioned to upgrading gas for the commercial pipeline (see Section 5).
• Dewatering: Digested sludge is dewatered using high-solids centrifuges.
• Biosolids Reuse: The final cake, Class B biosolids, is transported for land application on dry-land wheat farms in eastern Oregon, recycling nutrients back into agriculture.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The facility occupies a substantial industrial footprint in North Portland along the Columbia Slough. The site includes the wet-stream treatment trains, a massive solids handling complex, maintenance shops, and the administration/laboratory building. A dedicated lagoon system is used for sludge storage during winter months when land application is restricted.

B. Energy Systems & RNG

Energy management is a defining feature of CBWTP. The “Poop to Power” initiative successfully transitioned the plant to a Renewable Natural Gas (RNG) production facility. Instead of flaring excess methane or relying solely on aging cogeneration engines, the plant now scrubs and compresses biogas to natural gas pipeline standards. This RNG is injected into the NW Natural utility pipeline, generating revenue and displacing fossil fuels in the transportation sector.

C. Odor Control

Given the proximity to the growing North Portland neighborhoods, odor control is paramount. The plant utilizes covered primary headworks and foul air collection systems routed to chemical scrubbers (wet scrubbers using hypochlorite/caustic) and activated carbon filters to treat H2S and organic odors.

5. RECENT UPGRADES & MAJOR PROJECTS

The Bureau of Environmental Services is currently executing the largest capital improvement campaign in the plant’s history.

Secondary Treatment Expansion Program (STEP) – ~$600+ Million (Ongoing)

Project Scope: The STEP project is a massive overhaul designed to increase secondary treatment capacity and reliability. It addresses the bottleneck where peak wet weather flows must currently bypass secondary treatment.

• Timeline: Construction began in 2019; anticipated completion in 2024/2025.
• Key Contractors: Kiewit (CM/GC), Brown and Caldwell (Design).
• Technical Highlights:
  • Construction of two new 110-foot diameter secondary clarifiers (bringing the total to 10).
  • Modifications to the aeration basins to improve selector zones and filament control.
  • Expansion of the secondary solids handling system.
  • Installation of new blower systems for increased energy efficiency.

• Results: Upon completion, the project will increase secondary treatment capacity, significantly reducing the volume of blended effluent discharged during storms and improving total nitrogen/ammonia removal.

RNG Infrastructure Project – $16 Million (Completed 2019)

Project Scope: Installation of a biogas conditioning system to convert raw digester gas into pipeline-quality natural gas.

• Technology: Pressure Swing Adsorption (PSA) or membrane separation to remove CO2, siloxanes, and hydrogen sulfide.
• Impact: Reduces greenhouse gas emissions by approximately 21,000 tons per year and generates roughly $3 million to $5 million annually in revenue for the city through the sale of RNG and renewable fuel credits (RINs).

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. Permit Requirements

The facility operates under NPDES Permit No. 101131. Key parameters include:

• CBOD5: Monthly average limits typically around 25 mg/L (dry weather).
• TSS: Monthly average limits typically around 30 mg/L (dry weather).
• E. Coli: Strict limits requiring effective chlorination.
• Mixing Zone: The permit defines a mixing zone in the Columbia River where immediate dilution occurs; acute toxicity limits must be met at the edge of this zone.

B. Compliance History

CBWTP generally maintains a strong compliance record. The primary regulatory challenges historically involved CSO events. The completion of the West Side and East Side Big Pipe projects (ending in 2011) reduced CSOs to the Willamette River by 94% and to the Columbia Slough by 99%. The current focus is on maximizing wet weather treatment at the plant to minimize “blending” (where primary effluent bypasses secondary treatment), which is a driver for the STEP project.

7. OPERATIONAL EXCELLENCE

Staffing: The plant is staffed 24/7 by a team of Grade III and IV wastewater operators, industrial mechanics, and instrument technicians. The facility houses a fully accredited environmental laboratory that performs daily process control testing and compliance monitoring.

Technology: BES employs a robust SCADA system for plant automation. Recent integrations include advanced process control logic for the new oxygen generation system and automated polymer dosing for CEPT during rapid flow changes.

8. CHALLENGES & FUTURE PLANNING

A. Current Challenges

• Resiliency: The Pacific Northwest is in the Cascadia Subduction Zone. Retrofitting the massive concrete structures (digesters, basins) for seismic resilience is a massive, ongoing engineering challenge.
• Emerging Contaminants: Like all major WWTPs, CBWTP is monitoring regulatory horizons regarding PFAS and microplastics, which may require tertiary filtration in the future.
• Changing Hydrographs: Climate change is altering rainfall patterns in Portland, potentially leading to more intense, short-duration storms that stress the combined system.

B. Master Planning

The Integrated Plan approach by BES looks at stormwater, wastewater, and watershed health holistically. Future capital planning focuses on continued seismic hardening of the solids handling complex and potential nutrient removal upgrades should Columbia River standards tighten regarding nitrogen loading impacting downstream oxygen levels.

9. COMMUNITY & REGIONAL IMPACT

The CBWTP is vital to the ecological health of the Columbia River, a working river that supports international shipping, hydroelectric power, and critical salmon habitats. The plant’s effective operation ensures that the waterway remains safe for recreation and commerce. Economically, the facility’s shift to RNG production demonstrates how municipal infrastructure can become a revenue-generating asset rather than solely a cost center.

10. TECHNICAL SPECIFICATIONS SUMMARY

11. RELATED FACILITIES

Tryon Creek Wastewater Treatment Plant: A smaller secondary facility in Lake Oswego operated by Portland BES, treating flows from the southwest metro area.

Innsbruck Pump Station: One of the critical lift stations in the collection system network.

Swan Island Pump Station: The primary lift station for the Big Pipe system, pumping combined sewage to CBWTP.

12. FAQ SECTION

1. What is the peak capacity of the Columbia Boulevard Wastewater Treatment Plant?

The plant has a peak wet weather hydraulic capacity of 450 MGD. This high capacity is necessary to manage surges from Portland’s combined sewer system during heavy rains.

2. Does CBWTP use open air aeration basins?

No. CBWTP uses a High Purity Oxygen (HPO) system where aeration basins are covered. This maintains high oxygen partial pressure for efficient biological treatment and helps contain odors.

3. What is the STEP project at CBWTP?

The Secondary Treatment Expansion Program (STEP) is a major capital project (approx. $600M) adding two new clarifiers and upgrading aeration basins to increase the volume of wastewater receiving full secondary treatment during storms.

4. How is biogas used at the facility?

Biogas produced in the anaerobic digesters is conditioned to remove impurities and injected into the NW Natural commercial gas pipeline as Renewable Natural Gas (RNG), used primarily for transportation fuel.

5. Where are the biosolids disposed of?

After digestion and dewatering, the Class B biosolids are transported to eastern Oregon (Madison Farms) and applied to agricultural land as a soil amendment/fertilizer.

6. Does the plant treat stormwater?

Yes. Because much of Portland utilizes a Combined Sewer System, the plant treats a mixture of sanitary sewage and stormwater runoff.

7. Is the facility under a consent decree?

Portland operated under a consent decree regarding CSOs for roughly 20 years. The completion of the Big Pipe project in 2011 largely satisfied these requirements, though the facility continues to report strictly to DEQ regarding overflow events.

8. Can the public tour the facility?

Yes, BES offers guided tours for schools, community groups, and professional organizations, though availability may vary based on construction activity and staffing.