King County Wastewater Treatment Division West Point Treatment Plant Seattle

FACILITY BASIC INFORMATION

Plant Name: West Point Treatment Plant

Location: 1400 Discovery Park Blvd, Seattle, King County, WA 98199

Operating Authority: King County Wastewater Treatment Division (WTD)

Design Capacity: 133 MGD (Average Wet Weather), 440 MGD (Peak Hydraulic)

Current Average Flow: ~90 MGD (Dry Weather)

Population Served: Approx. 1.8 million (Regional System Total)

Service Area: Seattle and northern King County suburbs (approx. 34 sq. miles)

Receiving Water Body: Puget Sound (Central Basin)

NPDES Permit Number: WA0029181

Read more4D-Printed Smart Materials For Water Treatment

Year Commissioned: 1966 (Primary), 1995 (Secondary Upgrade)

1. INTRODUCTION

The West Point Treatment Plant (WPTP) is the largest and most critical wastewater treatment asset in the Pacific Northwest, serving the metropolitan Seattle area. Operated by the King County Wastewater Treatment Division (WTD), this facility is unique in its engineering constraints; it is situated on a compact footprint within Discovery Park, Seattle’s largest public park. To accommodate high volumes within limited space, the plant utilizes a High Purity Oxygen (HPO) activated sludge process, allowing for higher mixed liquor suspended solids (MLSS) concentrations and smaller aeration basin volumes compared to conventional air activated sludge systems.

Commissioned originally as a primary treatment facility in 1966 and upgraded to secondary treatment in 1995 at a cost of $573 million, West Point treats wastewater from a combined sewer system. This necessitates a massive hydraulic peak capacity of 440 MGD to manage stormwater surges, despite an average dry weather flow of approximately 90 MGD. The facility is currently navigating a complex phase of capital improvements focused on power reliability, seismic resilience, and nutrient management in the sensitive Puget Sound ecosystem.

2. FACILITY OVERVIEW

A. Service Area & Coverage

West Point serves a 34-square-mile area encompassing the City of Seattle and districts to the north, including Shoreline, Lake Forest Park, and parts of Snohomish County. The collection system is a mix of separated sanitary sewers and combined sewers (stormwater and wastewater in the same pipe). This configuration presents significant operational challenges, requiring rapid ramp-up capabilities during precipitation events to mitigate Combined Sewer Overflows (CSOs).

B. Operational Capacity

The plant demonstrates extreme hydraulic variability.

• Average Dry Weather Flow: 90 MGD
• Average Wet Weather Flow: 133 MGD
• Peak Hydraulic Capacity: 440 MGD

During peak storm events, flows can escalate from 100 MGD to 440 MGD in under four hours. The facility utilizes an onsite emergency bypass system (as a last resort) and massive pumping capabilities to manage these surges. Capacity planning is currently focused on wet-weather management rather than dry-weather population growth, which is largely offset by water conservation efforts.

C. Discharge & Compliance

Read moreA Comparison Between Aerobic And Anaerobic Wastewater Treatment Technology

Treated effluent is discharged into Puget Sound via a deep-water outfall terminating approximately 3,600 feet offshore at a depth of 240 feet. The diffuser section disperses effluent to maximize mixing zone dilution. The plant operates under Washington State Department of Ecology NPDES Permit No. WA0029181. Recent regulatory focus has shifted toward the Puget Sound Nutrient General Permit, which may mandate future nitrogen reduction technologies.

3. TREATMENT PROCESS

A. PRELIMINARY TREATMENT

Influent enters via the Interbay and Fort Lawton tunnels.

• Screening: Six mechanically cleaned bar screens remove large debris (rags, plastics) to protect downstream pumps.
• Grit Removal: Following screening, flow enters aerated grit chambers where velocity reduction allows inorganic solids (sand, gravel) to settle while keeping organics suspended.
• Pumping: The Intermediate Pump Station lifts flow to the primary sedimentation tanks.

B. PRIMARY TREATMENT

West Point utilizes 12 rectangular primary sedimentation tanks.

• Mechanism: Chain-and-flight sludge collectors scrape settled solids to hoppers and skimmers remove floating grease/scum.
• Performance: Designed to remove approximately 50-60% of Total Suspended Solids (TSS) and 25-35% of Biochemical Oxygen Demand (BOD).
• High Flow Bypass: During flows exceeding 300 MGD (secondary capacity), primary effluent can be blended with secondary effluent prior to disinfection to protect the biological inventory from washout, in accordance with permit allowances.

C. SECONDARY TREATMENT (High Purity Oxygen)

The core of West Point’s secondary treatment is the High Purity Oxygen (HPO) activated sludge process.

• Oxygen Generation: Cryogenic oxygen generation plants onsite produce >95% pure oxygen.
• Aeration Basins: The plant features covered aeration basins organized in four trains. Pure oxygen is injected into the mixed liquor in staged, covered reactors.
• Advantage: HPO allows for higher biomass concentrations (MLSS 3,000–5,000 mg/L) and faster reaction rates, reducing the required tank volume by nearly 50% compared to conventional aeration—essential for the site’s constrained footprint.
• Secondary Clarifiers: 13 secondary clarifiers separate biomass from treated water. Gravity settles the solids; Return Activated Sludge (RAS) is pumped back to the aeration basins, while Waste Activated Sludge (WAS) is sent to thickening.

D. DISINFECTION

West Point utilizes chemical disinfection rather than UV, largely due to hydraulic constraints and existing infrastructure.

• Chlorination: Sodium hypochlorite (12.5% solution) is added to the secondary effluent.
• Dechlorination: Sodium bisulfite is added prior to discharge to neutralize residual chlorine, protecting marine life in Puget Sound.

E. SOLIDS HANDLING

Solids processing is a major operational component, handling loads from West Point and occasionally other regional facilities.

• Thickening: Primary sludge is thickened using gravity belt thickeners (GBT). WAS is thickened via centrifuges.
• Digestion: Six mesophilic anaerobic digesters stabilize the solids, reducing volume and pathogen content. The system operates at approximately 98°F.
• Dewatering: Digested sludge is dewatered using high-solids centrifuges to produce a “cake” (approx. 20-25% solids).
• Biosolids: The final product, branded as “Loop®,” is Class B biosolids used for forestry application in the Cascades and agriculture in eastern Washington.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant & Site Constraints

The facility occupies approximately 32 acres within the 534-acre Discovery Park. To mitigate visual and auditory impact, the plant is surrounded by landscaped berms, and significant portions of the facility (including the HPO tanks) are covered or low-profile. The site design incorporates extensive odor control scrubbing to prevent nuisance odors in the park.

B. Energy Systems & Cogeneration

West Point is a leader in energy recovery.

• Biogas Utilization: Methane gas produced in the digesters fuels Internal Combustion (IC) engines.
• Cogeneration: These engines drive generators to produce electricity and raw sewage pumps (direct drive). Heat recovered from the engine jackets and exhaust is used to heat the digesters and plant buildings.
• Energy Profile: The cogeneration system can provide a significant portion of the plant’s electrical demand, reducing reliance on the Seattle City Light grid.

5. RECENT UPGRADES & MAJOR PROJECTS

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. Permit Requirements

The facility operates under a stringent NPDES permit. Key parameters include:

• CBOD5: Monthly average limit of 25 mg/L (or 85% removal).
• TSS: Monthly average limit of 30 mg/L (or 85% removal).
• pH: Maintained between 6.0 and 9.0.
• Fecal Coliform: Geomean limit of 200 CFU/100 mL.
• Chlorine Residual: Maximum daily limit of roughly 139 µg/L (varies by dilution factor).

B. Compliance History & The 2017 Incident

Historically, West Point has been a Platinum Award winner from NACWA. However, on February 9, 2017, a catastrophic equipment failure triggered by a power disruption caused the plant to flood, resulting in $50M+ in damages and emergency bypasses. Since full restoration in mid-2017, the plant has returned to full compliance, implementing redundant safety systems to prevent recurrence.

7. OPERATIONAL EXCELLENCE

King County WTD employs over 100 staff at West Point, including operators, mechanics, electricians, and instrumentation specialists. Operations staff generally hold Washington State Group III or IV Wastewater Certifications. The facility utilizes a distributed control system (DCS) for process automation, with increasing integration of AI for predictive maintenance on critical rotating assemblies (centrifuges and pumps).

8. CHALLENGES & FUTURE PLANNING

A. Nutrient Removal (The “General Permit”)

The Washington Department of Ecology has issued the Puget Sound Nutrient General Permit (PSNGP) to address low dissolved oxygen levels in Puget Sound. West Point was not originally designed for biological nutrient removal (BNR). Retrofitting this constrained site for nitrogen removal (denitrification) presents a massive engineering challenge, potentially costing billions and requiring novel technologies like granular activated sludge or membrane aerated biofilm reactors (MABR) to fit within the existing footprint.

B. Climate Resiliency

Located at sea level, West Point is vulnerable to storm surge and sea-level rise. Hydraulic modeling is currently underway to assess the hydraulic grade line (HGL) impacts of rising tides on the outfall and gravity discharge capabilities.

9. COMMUNITY & REGIONAL IMPACT

West Point is critical to the economic engine of Seattle, enabling high-density development by managing wastewater infrastructure. The facility maintains a Community Advisory Committee and strictly manages construction traffic through the affluent Magnolia neighborhood. The “Loop” biosolids program contributes to a circular economy, returning carbon and nutrients to local forests and farms.

10. TECHNICAL SPECIFICATIONS SUMMARY

11. FAQ SECTION

Technical/Professional Questions

1. Why does West Point use High Purity Oxygen (HPO)?
HPO allows for a higher Mixed Liquor Suspended Solids (MLSS) concentration and faster reaction rates, significantly reducing the required aeration basin volume. This was necessary to fit secondary treatment within the limited acreage of Discovery Park.

2. How does the plant manage peak flows of 440 MGD?
The plant utilizes high-capacity raw sewage pumps. If flows exceed the biological treatment capacity (~300 MGD), the permit allows for “blending,” where excess primary effluent bypasses the aeration basins and is blended with secondary effluent before disinfection.

3. What is the status of nutrient removal at West Point?
The plant currently monitors nutrients but was not designed for Nitrogen removal. King County is currently evaluating technologies to meet the new Puget Sound Nutrient General Permit requirements, which poses significant space and cost challenges.

4. How is power reliability ensured?
Following the 2017 flood, King County installed advanced voltage support systems and UPS backups to bridge power sags, ensuring that pumps and control systems remain online during utility grid fluctuations.

Public Interest Questions

5. Can the public tour West Point?
Yes, King County WTD offers educational tours for schools and community groups. Tours must be scheduled in advance via their website.

6. Does the plant smell?
The plant uses extensive odor control systems, including chemical scrubbers and carbon filters, to treat air from the headworks, primary tanks, and solids handling areas. Complaints are rare despite the location inside a public park.