City Of Salem Wastewater Treatment Plant

FACILITY BASIC INFORMATION

• Plant Name: Willow Lake Water Pollution Control Facility (WPCF)
• Location: 5915 Windsor Island Road N, Salem, OR 97303
• Operating Authority: City of Salem Public Works Department
• Design Capacity (Peak Wet Weather): 155 MGD
• Current Average Dry Weather Flow: ~25 MGD
• Population Served: ~200,000 residents
• Service Area: City of Salem, City of Keizer, City of Turner, and unincorporated Marion County
• Receiving Water Body: Willamette River
• NPDES Permit Number: 101344
• Year Commissioned: Original 1960s (Major expansion 1990s)

1. INTRODUCTION

The Willow Lake Water Pollution Control Facility (WPCF) serves as the primary wastewater treatment asset for the state capital of Oregon. Operated by the City of Salem, this regional facility treats an average dry weather flow of approximately 25 million gallons per day (MGD) but is hydraulically designed to handle massive wet-weather peaks up to 155 MGD due to the region’s significant rainfall and inflow/infiltration (I/I) challenges. Serving a population of over 200,000 across Salem, Keizer, and Turner, the plant is a critical guardian of the Willamette River watershed.

Willow Lake is distinct among Pacific Northwest facilities for its “coupled” secondary treatment process—combining trickling filters with activated sludge—and its extensive resource recovery programs. The facility operates a robust cogeneration power plant fueled by digester gas and manages the innovative Biocycle Farm, where biosolids and reclaimed water are utilized to grow poplar trees. With recent multi-million dollar investments in energy efficiency and process optimization, Willow Lake WPCF remains a model of sustainable infrastructure management in the Willamette Valley.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The Willow Lake WPCF provides regional treatment services for the Urban Growth Boundary of Salem. The collection system spans approximately 450 miles of gravity mains and force mains, supported by over 20 pump stations. The user base is a mix of residential (low and high density), commercial, and significant food processing industries that contribute high-strength organic loads during harvest seasons. The facility operates under an Intergovernmental Agreement (IGA) to treat flows from the neighboring City of Keizer and the City of Turner.

B. Operational Capacity

The hydraulic profile of Willow Lake is defined by extreme seasonality.

• Average Dry Weather Flow: 20–25 MGD
• Peak Wet Weather Flow: Up to 155 MGD

During heavy winter precipitation events, the plant utilizes a specialized wet-weather treatment train to manage hydraulic surges that exceed the secondary treatment capacity, ensuring that excess flows receive primary treatment and disinfection before discharge, compliant with regulatory blending allowances.

C. Discharge & Compliance

Treated effluent is discharged into the Willamette River. The discharge is regulated by the Oregon Department of Environmental Quality (DEQ) under strict National Pollutant Discharge Elimination System (NPDES) standards. The facility faces stringent constraints regarding thermal load (temperature), ammonia, and nutrients, particularly during the low-flow summer months when the river’s assimilative capacity is reduced.

3. TREATMENT PROCESS

The Willow Lake WPCF utilizes a Coupled Trickling Filter / Activated Sludge (TF/AS) process. This hybrid configuration provides the shock-loading resilience of trickling filters with the high-quality effluent polishing of activated sludge.

A. PRELIMINARY TREATMENT

Raw influent enters the headworks where it passes through mechanically cleaned bar screens to remove large debris, rags, and plastics. Following screening, flow enters aerated grit chambers where velocity is reduced to allow inorganic solids (sand, gravel, coffee grounds) to settle while keeping organic matter in suspension. The removed grit and screenings are washed, compacted, and hauled to a landfill. Odor control at the headworks is critical due to nearby residential zones, utilizing chemical scrubbers to treat foul air.

B. PRIMARY TREATMENT

Flow proceeds to primary clarification. The facility utilizes large circular primary clarifiers where heavy organic solids settle to the bottom as raw sludge, and floatable grease/scum is skimmed from the surface. The hydraulic retention time allows for approximately 30-40% removal of BOD and 50-60% removal of Total Suspended Solids (TSS). The primary sludge is pumped directly to the anaerobic digesters.

C. SECONDARY TREATMENT (TF/AS Process)

The secondary system is the technological core of the plant:

• Stage 1: Trickling Filters (Roughing): Primary effluent is pumped over plastic-media trickling filters. As water trickles down the media, a biofilm of microorganisms consumes a significant portion of the soluble organic matter (BOD). This stage is highly energy-efficient and provides stability against shock loads from industrial dischargers.
• Stage 2: Activated Sludge (Polishing): The effluent from the trickling filters flows into aeration basins. Here, fine-bubble diffusion provides oxygen to a suspended biomass (mixed liquor). This stage polishes the remaining BOD and facilitates nitrification (conversion of ammonia to nitrate) during warmer months.
• Secondary Clarification: The mixed liquor settles in secondary clarifiers. The clean water overflows the weirs, while the settled biomass is either returned to the aeration basins (RAS) or wasted (WAS) to solids handling.

D. DISINFECTION

The clarified effluent undergoes disinfection to eliminate pathogens. Willow Lake currently utilizes gas chlorination followed by dechlorination using sodium bisulfite prior to discharge into the Willamette River. The contact basins ensure sufficient dwell time for effective kill rates. (Note: Future master planning evaluates the potential conversion to UV or Ozone to reduce chemical handling and disinfection byproducts).

E. SOLIDS HANDLING & COGENERATION

Solids handling at Willow Lake is a comprehensive resource recovery operation:

• Anaerobic Digestion: Primary sludge and thickened waste activated sludge are blended and pumped to mesophilic anaerobic digesters. The process reduces volatile solids and produces methane-rich biogas.
• Cogeneration (CHP): The produced biogas fuels a cogeneration facility with internal combustion engines, generating approximately 1.2 MW of electricity—offsetting nearly 50% of the plant’s electrical demand and providing heat for the digesters.
• Dewatering & Reuse: Digested sludge is dewatered using belt filter presses. The resulting Class B biosolids are transported to the Biocycle Farm (Poplar Tree Farm), a city-owned agroforestry site where biosolids are land-applied as fertilizer, and reclaimed water is used for irrigation.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The Willow Lake site encompasses over 55 active acres within a larger footprint. The architecture reflects its mid-century origins with significant modern industrial additions. The site includes a fully equipped environmental laboratory, administration building, and extensive maintenance shops capable of heavy equipment repair.

B. Energy Systems & Cogeneration

Willow Lake is a leader in energy independence. The Cogeneration Facility is a pivotal asset, utilizing two 600-kW engines fueled by digester gas. The facility employs a sophisticated heat recovery loop that captures thermal energy from the engines to maintain optimal temperatures in the anaerobic digesters and heat facility buildings. This “Closed Loop” energy system shields the utility from volatile energy market prices.

C. Biocycle Farm

Located distinct from the main plant, the Biocycle Farm is an integral infrastructure component. This site utilizes treated effluent and biosolids to cultivate hybrid poplar trees. It serves as a beneficial reuse site for nutrients that would otherwise burden the Willamette River, acting as a natural sink for nitrogen and phosphorus.

5. RECENT UPGRADES & MAJOR PROJECTS

Cogeneration Facility Replacement (2019-2021)

• Project Scope: Replacement of aging cogeneration engines and associated gas cleaning systems.
• Budget: ~$8 Million
• Technical Highlights: Installation of new high-efficiency lean-burn internal combustion engines capable of meeting stricter air emissions standards. The project included advanced gas conditioning skids (siloxane removal) to extend engine life.
• Results: Restored reliable power generation capacity of 1.2 MW, reducing the facility’s carbon footprint and grid dependency.

Willow Lake WPCF Master Plan (Ongoing)

• Scope: Comprehensive evaluation of facility assets, capacity constraints, and regulatory future.
• Key Drivers: Aging infrastructure (50+ years old), impending nutrient limits on the Willamette, and population growth in the Keizer/Salem corridor.
• Future Projects Identified:
  • Evaluation of Disinfection alternatives (UV/Ozone).
  • Headworks screening improvements.
  • Aeration basin diffuser upgrades for energy efficiency.

Upcoming: Nutrient Removal Upgrades

As Oregon DEQ tightens regulations regarding temperature and nutrients in the Willamette Basin, Willow Lake is in the planning phases for process modifications to enhance Biological Nutrient Removal (BNR), specifically targeting ammonia and phosphorus reduction.

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. Permit Requirements

Operating under NPDES Permit No. 101344, the facility must adhere to strict limits:

• BOD5 & TSS: Monthly average limits of 30 mg/L (May-Oct) and removal efficiency >85%.
• E. Coli: Geometric mean limits of 126 organisms/100 mL.
• Thermal Load: Strict heat load limits (kcal/day) to protect salmonid migration in the Willamette River.

B. Environmental Stewardship

The City of Salem has maintained a strong compliance record. The Biocycle Farm is a standout feature of their stewardship, effectively diverting hundreds of dry tons of biosolids from landfills annually and sequestering carbon through poplar tree growth.

7. OPERATIONAL EXCELLENCE

Willow Lake employs approximately 45-50 full-time staff, including Oregon-certified wastewater operators (Levels I-IV), industrial millwrights, instrument technicians, and lab analysts. The facility utilizes a robust SCADA (Supervisory Control and Data Acquisition) system that allows for real-time monitoring of dissolved oxygen profiles, digester gas production, and hydraulic loading.

The laboratory is ELAP-accredited, performing daily compliance testing for BOD, TSS, pH, Ammonia, and E. Coli, ensuring process upsets are identified and rectified immediately.

8. CHALLENGES & FUTURE PLANNING

A. Inflow and Infiltration (I/I)

The most significant operational challenge is the high peaking factor. During Pacific Northwest winter storms, older clay pipes in the collection system allow groundwater and stormwater to enter, causing flows to spike from 25 MGD to over 100 MGD in hours. Managing these hydraulic surges requires complex “wet weather” operating modes involving flow equalization and blending.

B. Thermal Load Management

The Willamette River is temperature-sensitive for endangered fish species. Willow Lake faces increasing pressure to lower effluent temperature. Future planning may involve effluent cooling technologies or increased reuse to divert warm water away from the river during critical migration periods.

10. TECHNICAL SPECIFICATIONS SUMMARY

11. FAQ SECTION

Technical Questions

Q: What is the “Coupled Process” used at Willow Lake?
A: It is a hybrid secondary treatment system where wastewater first passes through Trickling Filters (fixed film) to reduce the bulk of organic load, followed by Activated Sludge (suspended growth) basins for polishing. This offers energy efficiency and stability.

Q: Does Willow Lake generate its own power?
A: Yes. The facility captures methane from anaerobic digesters to fuel two internal combustion engines, generating approximately 1.2 Megawatts of electricity, covering roughly 50% of the plant’s power needs.

Q: How are biosolids managed?
A: Solids are anaerobically digested and dewatered. The resulting Class B biosolids are transported to the City’s Biocycle Farm, where they are applied as fertilizer for poplar trees.

Public Interest Questions

Q: What happens during heavy rain storms?
A: Flows can increase 5-6 times normal levels. The plant has specialized wet-weather processing gear to treat these high volumes, ensuring the water discharged remains safe and compliant with environmental permits.

Q: Can I tour the facility?
A: The City of Salem Public Works Department periodically offers tours for schools and community groups. Interested parties should contact the Public Works dispatch or administration office directly.