Clean Water Services Durham Advanced Wastewater Treatment Facility

FACILITY BASIC INFORMATION

• Plant Name: Durham Advanced Wastewater Treatment Facility
• Location: 16060 SW 85th Ave, Tigard, Washington County, Oregon
• Operating Authority: Clean Water Services (CWS)
• Design Capacity (Dry Weather): 25.7 MGD
• Peak Hydraulic Capacity: 113 MGD
• Current Average Flow: ~22-24 MGD
• Population Served: ~185,000 residents (estimate based on service area)
• Service Area: Tigard, Durham, King City, portions of Tualatin, Beaverton, and Portland
• Receiving Water Body: Tualatin River (Willamette River Basin)
• NPDES Permit Number: 101142 (Oregon DEQ)
• Year Commissioned: 1976 (Major consolidations and expansions subsequently)

1. INTRODUCTION

The Durham Advanced Wastewater Treatment Facility (AWTF) is one of two major resource recovery hubs operated by Clean Water Services in Washington County, Oregon. Treating an average dry weather flow of nearly 26 million gallons daily (MGD), it serves the dense urban corridor of Tigard, Tualatin, and Durham. As a facility discharging into the Tualatin River—a slow-moving, environmentally sensitive tributary of the Willamette River—the Durham plant operates under some of the strictest regulatory requirements in the Pacific Northwest, particularly regarding phosphorus and temperature.

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Distinguished by its early adoption of resource recovery technologies, Durham was the first plant in the United States to install the commercial Ostara Pearl® nutrient recovery process, turning nuisance struvite into fertilizer. The facility represents a hybrid of rigorous mechanical engineering and watershed-based environmental management, utilizing advanced tertiary treatment alongside river flow augmentation strategies to maintain regulatory compliance.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The Durham AWTF serves the southeastern portion of Clean Water Services’ district, covering a highly urbanized and commercialized zone. The collection system feeding the plant includes major trunk lines from Tigard and Tualatin. The service area is characterized by a mix of medium-density residential zones and significant commercial districts, including the Washington Square Mall area. The collection system relies on a network of lift stations to convey flow to the plant’s headworks located near the Tualatin River floodplain.

B. Operational Capacity

The facility is designed for an Average Dry Weather Flow (ADWF) of 25.7 MGD. However, the Pacific Northwest climate necessitates a robust wet-weather handling strategy. During significant storm events, the Durham AWTF is rated for a Peak Instantaneous Flow (PIF) of up to 113 MGD to handle inflow and infiltration (I/I). Historical data indicates that while average flows have remained steady due to conservation efforts despite population growth, peak events continue to challenge hydraulic throughput, driving recent headworks and secondary expansion projects.

C. Discharge & Compliance

Effluent is discharged directly into the Tualatin River. The Tualatin has a Total Maximum Daily Load (TMDL) restriction that dictates extremely low limits for ammonia and phosphorus to prevent algal blooms. Furthermore, thermal load is a critical parameter; CWS operates under a watershed-based permit that allows for water quality trading—planting riparian shade trees upstream to offset thermal discharge, rather than relying solely on mechanical chilling at the plant.

3. TREATMENT PROCESS

The Durham AWTF utilizes a tertiary treatment train designed for high-efficiency nutrient removal. The process flow is categorized into liquid stream treatment and solids handling/resource recovery.

A. Preliminary Treatment

Raw influent enters the headworks where it passes through mechanical bar screens to remove large debris, rags, and plastics. Following screening, the flow enters aerated grit chambers where heavier inorganic materials (sand, gravel, coffee grounds) settle out. The grit is washed, dewatered, and sent to a landfill. Recent upgrades have focused on increasing the capture ratio of screenings to protect downstream pumps and biosolids integrity.

B. Primary Treatment

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Flow moves to primary clarifiers where gravity settling removes approximately 60-70% of Total Suspended Solids (TSS) and 30-40% of Biochemical Oxygen Demand (BOD). Primary sludge is pumped to the gravity thickeners, while primary effluent flows to the aeration basins.

C. Secondary Treatment (Biological Nutrient Removal)

The core of the treatment process is the Activated Sludge system configured for Biological Nutrient Removal (BNR). The aeration basins utilize a selector zone configuration to promote the growth of specific bacteria that facilitate nitrification (converting ammonia to nitrate) and denitrification.
Key Components:

• Aeration Basins: Equipped with fine-bubble diffusers to maximize oxygen transfer efficiency.
• Secondary Clarifiers: Circular clarifiers separate the biomass (mixed liquor) from the treated water.
• RAS/WAS: Return Activated Sludge is recycled to the front of the aeration basins, while Waste Activated Sludge is removed to control the Mean Cell Residence Time (MCRT).

D. Tertiary Treatment

To meet the Tualatin River’s strict phosphorus limits (often as low as 0.1 mg/L in summer), Durham employs advanced tertiary treatment:

• Chemical Precipitation: Alum (Aluminum Sulfate) is added to precipitate remaining phosphorus and coagulate solids.
• Tertiary Filtration: The chemically treated water passes through deep-bed granular media filters (anthracite/sand) to polish the effluent, removing fine particulate matter and associated phosphorus.

E. Disinfection

Historically utilizing chlorine gas, the facility has transitioned to disinfection technologies that ensure safety and minimize byproducts. The current system utilizes Ultraviolet (UV) Light disinfection or Sodium Hypochlorite (depending on season and reuse application) to neutralize pathogens before the water is dechlorinated (if chlorine is used) and discharged to the river or diverted for water reuse irrigation in local parks and golf courses.

F. Solids Handling & Resource Recovery

Durham is a pioneer in solids processing:

• Thickening: Primary sludge is gravity thickened; WAS is thickened via dissolved air flotation (DAF) or rotary drum thickeners.
• Anaerobic Digestion: Thickened sludge is stabilized in anaerobic digesters, reducing volatile solids and producing biogas.
• Nutrient Recovery (Ostara Pearl®): Centrate (nutrient-rich liquid) from the dewatering process is treated in a fluidized bed reactor where magnesium is added. This causes phosphorus and ammonia to crystallize into high-purity struvite fertilizer (marketed as Crystal Green®). This prevents struvite scaling in plant pipes and recovers a valuable resource.
• Dewatering: Digested sludge is dewatered using high-solids centrifuges.

4. INFRASTRUCTURE & FACILITIES

A. Energy Systems & Cogeneration

Clean Water Services maximizes energy efficiency at Durham through a Combined Heat and Power (CHP) system. Methane gas produced during anaerobic digestion is captured and scrubbed to fuel two 1-megawatt internal combustion engines (co-generation). These engines generate approximately 50-60% of the facility’s electrical demand and provide heat for the digesters and buildings, significantly reducing the plant’s carbon footprint and reliance on the grid.

B. Odor Control

Given the facility’s proximity to residential neighborhoods and the high-traffic Bridgeport Village and Washington Square areas, odor control is paramount. The facility utilizes biological soil filters and chemical scrubbers at the headworks and solids handling buildings. Foul air is captured and forced through organic media where microorganisms digest odor-causing compounds (H2S).

5. RECENT UPGRADES & MAJOR PROJECTS

Durham Secondary Treatment Expansion (2018-2021)

Budget: ~$50 Million
Scope: This critical project addressed hydraulic bottlenecks in the secondary treatment process.

• New Secondary Clarifier: Construction of a new 140-foot diameter secondary clarifier to improve solids settling during high flow events.
• Aeration Basin Improvements: Modifications to existing basins to optimize BNR performance.
• Seismic Retrofits: Strengthening of existing structures to meet modern seismic codes for the Cascadia Subduction Zone.

Headworks Screening Improvements (2015-2017)

Budget: ~$15 Million
Scope: Replacement of aging bar screens with modern, fine-screening technology to protect downstream membrane and biological processes. The project included improved washer/compactors to reduce the organic content of the screenings sent to landfill.

Upcoming: Durham Facility Plan Implementation (2024-2030)

Clean Water Services is currently in the planning and early design phases for the next generation of upgrades, focusing on:

• Replacing aging electrical switchgear and MCCs.
• Rehabilitation of primary clarifiers.
• Potential expansion of tertiary filtration capacity to meet future reuse demands.

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. Permit Requirements

The Durham AWTF operates under an NPDES permit issued by the Oregon Department of Environmental Quality (DEQ). The permit is notable for its stringent seasonality:

• Summer (May-Oct): Ultra-low limits on Total Phosphorus (0.1 mg/L monthly avg) and Ammonia-N due to low river flow.
• Winter (Nov-Apr): Higher limits allowed due to higher river flows and lower temperatures.

B. Watershed-Based Permitting

Uniquely, CWS operates under a watershed-based permit that integrates the Durham and Rock Creek facilities. This allows for an offset program where CWS plants millions of native trees and shrubs in the Tualatin Basin to shade streams. This “thermal trading” creates a cooling effect equivalent to the thermal load discharged by the treatment plants, avoiding the need for energy-intensive mechanical refrigeration of effluent.

7. OPERATIONAL EXCELLENCE

A. Staffing & Certification

The facility is staffed 24/7 by a team of certified wastewater operators, mechanics, and instrument technicians. Operators maintain Oregon DEQ Grade III and IV certifications. CWS is known for a strong internal training program and high retention rates among technical staff.

B. Technology & Innovation

Durham is a reference site for the Ostara Pearl® nutrient recovery system. By recovering phosphorus as struvite, the plant reduces internal nutrient loops (where phosphorus is re-released in digestion and returned to the headworks), thereby improving the reliability of the main liquid treatment stream while creating a revenue-generating fertilizer product.

8. CHALLENGES & FUTURE PLANNING

A. Seismic Resilience

Located in the Pacific Northwest, the threat of a Cascadia Subduction Zone earthquake drives much of the capital planning. Recent and future projects heavily weight seismic hardening of critical process tanks and pipe galleries to ensure the facility can function or recover quickly after a major event.

B. Emerging Contaminants

Like all advanced facilities, Durham is preparing for future regulations regarding PFAS (Per- and polyfluoroalkyl substances). CWS is actively monitoring research on destruction and sequestration technologies, although no specific PFAS treatment train is currently online.

9. COMMUNITY & REGIONAL IMPACT

The Durham facility is not just a utility but a community partner. The plant provides Class A recycled water for irrigation at the nearby Tigard High School, Cook Park, and the Tualatin Country Club, conserving potable drinking water. Furthermore, CWS’s “Leaf the Place Better” program utilizes biosolids/compost generated from the treatment process for local landscaping and soil amendment.

10. TECHNICAL SPECIFICATIONS SUMMARY

11. FAQ

Technical Questions

1. What is the peak flow strategy for the Durham AWTF?
The plant utilizes equalization and robust hydraulic capacity in primary treatment to manage wet weather flows up to 113 MGD. Flows exceeding secondary capacity may receive primary treatment and disinfection (blending) in accordance with permit regulations during extreme storm events.

2. How does the Ostara system benefit plant operations?
By removing phosphorus and ammonia from the dewatering centrate (recycle stream), it reduces the nutrient load returned to the head of the plant by up to 20-30%. This prevents nuisance struvite scale in pipes and reduces the energy/chemical demand in the main BNR process.

3. What coagulants are used for phosphorus removal?
Alum (Aluminum Sulfate) is the primary coagulant used prior to tertiary filtration to achieve low effluent phosphorus limits.

Public Interest Questions

4. Does the plant smell?
The plant employs advanced odor control scrubbers. While occasional odors may occur during maintenance or extreme weather, the facility is designed to contain odors within the site boundary.

5. Is the water released into the river clean?
Yes. The effluent meets or exceeds Safe Drinking Water standards for many parameters and is often cleaner than the river water it enters. It is safe for aquatic life and human contact (swimming/boating).