King County Wastewater Treatment Division South Treatment Plant Renton
FACILITY BASIC INFORMATION
- Plant Name: South Treatment Plant (South Plant)
- Location: Renton, King County, Washington
- Operating Authority: King County Wastewater Treatment Division (WTD)
- Design Capacity (AWWF): 115 MGD (Average Wet Weather Flow)
- Peak Hydraulic Capacity: 325 MGD
- Current Average Flow: ~90 MGD (Combined Annual Average)
- Population Served: Approx. 800,000 residents
- Service Area: South and East King County (Auburn, Kent, Renton, Bellevue, Issaquah)
- Receiving Water Body: Puget Sound (via Elliott Bay deep marine outfall)
- NPDES Permit Number: WA0029581
- Year Commissioned: 1965 (Major expansions in 1986, 2000s)
1. Introduction
The South Treatment Plant (South Plant), located in Renton, Washington, serves as a critical infrastructure cornerstone for the rapidly growing region east and south of Seattle. Operated by the King County Wastewater Treatment Division (WTD), this facility manages wastewater for approximately 800,000 people across the Green River and Lake Washington basins. Originally commissioned in 1965 and substantially expanded in the mid-1980s, South Plant is an activated sludge facility designed to handle an average wet weather flow of 115 million gallons per day (MGD) with a peak hydraulic capacity of 325 MGD.
Distinct from its counterpart at West Point, South Plant is noted for its pioneering resource recovery programs, particularly in biosolids production (Loop®) and Renewable Natural Gas (RNG) generation. In recent years, the facility has become a focal point for engineering studies regarding nutrient removal optimization in response to the Puget Sound Nutrient General Permit. With ongoing capital improvements exceeding $100 million aimed at energy sustainability and seismic resilience, South Plant continues to set benchmarks for large-scale municipal wastewater operations in the Pacific Northwest.
2. Facility Overview
A. Service Area & Coverage
South Plant serves a massive geographic footprint covering the suburban and semi-urban corridors of King County. The collection system feeds into the Renton facility via major interceptors from the Eastside (Bellevue, Redmond, Issaquah) and the South (Kent, Auburn, Renton). The service area is characterized by a mix of residential density and significant light industrial/commercial zones, including aerospace and technology sectors. The facility manages flows from approximately 18 local sewer agencies that contract with King County WTD.
B. Operational Capacity
The plant is designed to manage significant hydraulic variability due to Inflow and Infiltration (I/I) common in the Pacific Northwest.
- Average Dry Weather Flow (ADWF): ~60-70 MGD
- Average Wet Weather Flow (AWWF): 115 MGD
- Peak Hydraulic Capacity: 325 MGD
Flows are conveyed to the plant primarily through the Eastside Interceptor and the Auburn/Kent Interceptor. During extreme storm events, the plant utilizes on-site hydraulic controls to manage peak loads and prevent collection system overflows.
C. Discharge & Compliance
Treated effluent is not discharged into the nearby Green River. Instead, it is conveyed approximately 12 miles northwest via the Effluent Transfer System (ETS) to Elliott Bay in Puget Sound. The discharge occurs through a deep-water marine outfall off Duwamish Head, diffusing effluent at depths ensuring rapid mixing. The facility operates under NPDES Permit WA0029581, administered by the Washington State Department of Ecology, with strict limits on BOD, TSS, pH, and chlorine residual. Recent regulatory focus has shifted heavily toward nutrient loading (Nitrogen) impacts on the Puget Sound ecosystem.
3. Treatment Process
South Plant utilizes a conventional activated sludge process. The treatment train is designed for robust removal of suspended solids and organic load, followed by disinfection.
A. Preliminary Treatment
Raw influent enters the headworks where it passes through mechanical bar screens to remove large debris (rags, plastics, wood). Following screening, the flow enters aerated grit chambers where velocity is controlled to allow inorganic heavy solids (sand, gravel, coffee grounds) to settle while keeping organic material in suspension. The removed grit and screenings are dewatered and hauled to a landfill. Odor control at the headworks is a critical operational priority due to the plant’s location within a commercial/retail district.
B. Primary Treatment
Wastewater flows into rectangular primary sedimentation tanks. Here, flow velocity is reduced to approximately 2-3 feet per minute, allowing settleable solids to drop to the bottom as raw sludge, while oils and grease float to the surface for skimming.
- Configuration: 12 Rectangular Primary Sedimentation Tanks
- Removal Efficiency: Typically removes 50-60% of Total Suspended Solids (TSS) and 25-35% of Biological Oxygen Demand (BOD).
C. Secondary Treatment
The biological treatment stage utilizes a conventional activated sludge process (unlike the High Purity Oxygen system used at King County’s West Point plant).
- Aeration Basins: The plant operates multiple aeration basins equipped with fine-bubble diffusers to maximize oxygen transfer efficiency. The mixed liquor suspended solids (MLSS) are maintained to support a healthy biomass that consumes dissolved organic matter.
- Secondary Clarifiers: The mixed liquor flows to 12 circular secondary clarifiers where the biomass settles out. A portion of the settled solids is returned to the aeration basins as Return Activated Sludge (RAS) to maintain the biological population, while the excess is removed as Waste Activated Sludge (WAS).
- Performance: The secondary process consistently achieves removal rates exceeding 85% for both BOD and TSS, meeting secondary treatment standards.
D. Tertiary/Water Reuse
While the majority of flow undergoes secondary treatment, South Plant maintains a Class A Reclaimed Water facility. This system utilizes coagulation, filtration (sand/anthracite), and high-level disinfection to produce water for on-site irrigation, process uses, and off-site use by nearby customers (e.g., City of Renton). This reduces the facility’s demand on potable water systems.
E. Disinfection
Secondary effluent is disinfected using sodium hypochlorite (liquid bleach) in chlorine contact channels to eliminate pathogenic organisms. Before discharge into the Effluent Transfer System and Puget Sound, the water is dechlorinated using sodium bisulfite to protect marine life from chlorine toxicity.
F. Solids Handling (Resource Recovery)
King County is a leader in biosolids reuse. The solids handling train includes:
- Thickening: Primary sludge is thickened using gravity thickeners; WAS is thickened using Dissolved Air Flotation (DAF).
- Digestion: The blended sludge undergoes anaerobic digestion in mesophilic digesters (approx. 98°F). This process stabilizes the solids, reduces volatile content, and generates methane-rich biogas.
- Dewatering: Digested sludge is dewatered using high-speed centrifuges to produce a “cake” typically around 20-25% solids.
- Reuse: The resulting Class B biosolids are branded as “Loop®” and are widely used in forestry and agriculture applications in Eastern Washington as a nutrient-rich soil amendment.
4. Infrastructure & Facilities
A. Physical Plant
The South Plant campus spans over 90 acres in Renton. The site is notable for its architectural integration with the surrounding commercial area, featuring significant landscaping. Key structures include the Headworks, Primary Sedimentation Basins, Aeration Basins, Secondary Clarifiers, Solids Handling Complex, and the Administration/Laboratory Building. The facility also houses the WTD Water Quality Planner and process control laboratories.
B. Energy Systems & RNG
South Plant is a net energy producer regarding thermal needs and a significant contributor to renewable energy.
- Biogas Scrubbing (RNG): The plant utilizes a state-of-the-art biogas scrubbing system that purifies the methane generated during anaerobic digestion. This Renewable Natural Gas (RNG) is injected directly into the commercial natural gas pipeline managed by Puget Sound Energy (PSE), generating revenue and reducing greenhouse gas emissions.
- Thermal Energy: Recovered heat and biogas are also used for process heating (digesters) and building heat.
C. Odor Control
Given its location near office parks and retail centers, South Plant employs one of the most extensive odor control systems in the region. Technologies include multi-stage chemical scrubbers (wet scrubbers) and carbon adsorption units treating air from the headworks, primary tanks, and solids handling facilities before release.
5. Recent Upgrades & Major Projects
Biogas Purification (RNG) Project (Completed 2019-2020)
- Scope: Installation of a new gas scrubbing system to convert digester gas into pipeline-quality natural gas.
- Budget: Approx. $40 Million.
- Driver: Sustainability goals and revenue generation via EPA Renewable Fuel Standard (RFS) credits (RINs).
- Technical Highlight: The system removes carbon dioxide, hydrogen sulfide, and siloxanes to meet strict pipeline specifications.
- Results: The plant generates approximately 700,000 to 800,000 MMBtu of RNG annually (equivalent), significantly reducing the carbon footprint.
Raw Sewage Pump Station (RSPS) Improvements (Current/Ongoing)
- Scope: Replacement of aging pumping infrastructure, HVAC upgrades, and seismic retrofitting.
- Budget: Estimated >$50 Million range (Phase dependent).
- Driver: Equipment end-of-life and seismic resilience.
- Status: Phased implementation to ensure continuous operation during construction.
Nutrient Reduction Evaluation (Planning Phase)
- Scope: Engineering studies to determine the feasibility and cost of upgrading the activated sludge process to Biological Nutrient Removal (BNR) for nitrogen reduction.
- Driver: The Puget Sound Nutrient General Permit (PSNGP) issued by the WA Department of Ecology.
- Implication: This represents a potential multi-billion dollar impact on the regional system over the next 20 years, likely requiring substantial new infrastructure (larger basin volumes or intensification technologies like MABR or IFAS).
6. Regulatory Compliance & Environmental Performance
A. Permit Requirements
South Plant operates under NPDES Permit WA0029581. Key effluent limitations include:
- CBOD5: 25 mg/L (Monthly Average)
- TSS: 30 mg/L (Monthly Average)
- pH: 6.0 to 9.0 Standard Units
- Fecal Coliform: 200/100 mL (Geometric Mean)
- Chlorine Residual: Strict limits requiring dechlorination to non-detectable levels.
B. Compliance History
King County WTD maintains a high level of compliance. South Plant regularly receives the “Platinum Peak Performance Award” from NACWA (National Association of Clean Water Agencies) for consecutive years of 100% permit compliance. Occasional challenges arise related to equipment reliability during extreme peak flow events, but significant violations are rare.
7. Operational Excellence
A. Staffing & Safety
The facility is staffed 24/7 by a team of certified wastewater operators, industrial mechanics, electricians, and instrument technicians. Due to the complexity of the plant (Category IV), lead operators hold the highest state certifications (Group IV). The plant emphasizes a robust safety culture, particularly regarding chemical handling (chlorine/sulfur dioxide or hypochlorite/bisulfite conversion) and confined space entry.
B. Technology & Automation
South Plant utilizes a comprehensive SCADA system for process monitoring and control. Real-time data regarding flows, dissolved oxygen levels, and pump statuses allow for automated pacing of aeration and chemical dosing. The laboratory onsite is accredited and performs daily compliance testing and process control analysis.
8. Challenges & Future Planning
A. Nitrogen and the Puget Sound General Permit
The most significant challenge facing South Plant is the regulatory shift toward nitrogen removal. Conventional activated sludge plants were not originally designed for nitrogen removal. Retrofitting South Plant for BNR (Biological Nutrient Removal) is technically complex due to site constraints and the need to maintain treatment capacity during construction. WTD is currently developing a Nitrogen Optimization Plan to maximize removal with existing infrastructure while planning for long-term capital expansion.
B. Asset Management
With major components dating back to the 1960s and 1980s, the plant faces an “infrastructure cliff.” WTD uses a strategic asset management program to prioritize the replacement of critical mechanical and electrical systems, including the main substation and raw sewage pumps.
C. Seismic Resilience
Located in a seismically active region, ongoing projects focus on hardening critical structures (digesters, galleries) to withstand significant earthquake events and ensure rapid recovery of operations.
9. Technical Specifications Summary
| Parameter | Specification |
|---|---|
| Facility Type | Secondary Treatment (Activated Sludge) |
| Design Capacity (AWWF) | 115 MGD |
| Peak Hydraulic Capacity | 325 MGD |
| Average Daily Flow | ~90 MGD (Annual Average) |
| Primary Treatment | 12 Rectangular Sedimentation Tanks |
| Secondary Treatment | Conventional Activated Sludge (Fine Bubble Aeration) |
| Disinfection | Sodium Hypochlorite + Sodium Bisulfite (Dechlorination) |
| Biosolids Processing | Anaerobic Digestion (Mesophilic) + Centrifuge Dewatering |
| Biogas Utilization | Scrubbed to RNG for Pipeline Injection + Onsite Heating |
| Outfall | Deep marine outfall in Puget Sound (off Duwamish Head) |
| NPDES Permit | WA0029581 |
| Service Area Population | ~800,000 |
| Site Area | ~90 Acres |
10. Related Facilities
The South Treatment Plant is part of a regional system that includes:
- West Point Treatment Plant: The system’s largest facility (Seattle), utilizing high-purity oxygen activated sludge.
- Brightwater Treatment Plant: A newer MBR (Membrane Bioreactor) facility serving the northern service area.
- Carnation Treatment Plant: A small satellite facility.
- Combined Sewer Overflow (CSO) Facilities: Various wet-weather treatment stations located throughout the collection system.
11. FAQ Section