City Of Spokane Riverside Wastewater Treatment Plant

1. INTRODUCTION

The Riverside Park Water Reclamation Facility (RPWRF) serves as the cornerstone of wastewater infrastructure for the Inland Northwest. Located on the banks of the Spokane River, this facility is the largest advanced wastewater treatment plant in Eastern Washington, with a design capacity of 44 million gallons per day (MGD) and a peak hydraulic capacity exceeding 135 MGD. Commissioned in 1958 and operated by the City of Spokane, RPWRF processes wastewater for approximately 250,000 residents.

The facility recently completed a historic $126 million upgrade known as the “Next Level of Treatment” (NLT) project, commissioning one of the largest pressurized membrane filtration systems in the United States. This technological leap was driven by the critical need to reduce phosphorus discharge into the Spokane River, a sensitive waterway prone to dissolved oxygen depletion. By achieving tertiary treatment standards, RPWRF has positioned itself as a model for nutrient removal and environmental stewardship in the Pacific Northwest.

2. FACILITY OVERVIEW

A. Service Area & Coverage

RPWRF serves the City of Spokane and portions of the surrounding urban growth area. The collection system comprises approximately 900 miles of sanitary sewer mains and 30 pump stations. A significant characteristic of the service area is the presence of Combined Sewer Overflows (CSOs) in older sections of the city, which necessitates rigorous wet-weather flow management strategies at the plant.

B. Operational Capacity

• Design Average Flow: 44 MGD
• Current Average Daily Flow: ~30–34 MGD
• Peak Hydraulic Capacity: 135 MGD (to manage CSO events)
• Population Equivalent: ~250,000 residents

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The facility operates under significant seasonal flow variations driven by snowmelt and stormwater intrusion. To mitigate this, the City has invested heavily in upstream CSO tanks to dampen peak flow arrivals.

C. Discharge & Compliance

The plant discharges treated effluent directly into the Spokane River (WRIA 54). The river is a cold-water trout habitat with strict Total Maximum Daily Load (TMDL) requirements, particularly for Dissolved Oxygen (DO), requiring aggressive phosphorus removal, and Polychlorinated Biphenyls (PCBs).

3. TREATMENT PROCESS

The RPWRF utilizes a sophisticated treatment train evolving from conventional secondary treatment to advanced tertiary membrane filtration.

A. Preliminary Treatment

Raw influent enters the headworks where it passes through mechanical bar screens to remove large debris. Following screening, flow enters aerated grit chambers where heavier inorganic solids (sand, gravel, coffee grounds) settle out. The grit is washed and dewatered before landfill disposal. The headworks is equipped with extensive odor control scrubbers to mitigate nuisance odors near the adjacent Riverside State Park.

B. Primary Treatment

Wastewater flows into five rectangular primary clarifiers. These tanks reduce flow velocity, allowing settleable solids to drop to the bottom as primary sludge, while grease and oils float to the surface for skimming.
Performance: Primary treatment typically removes 30-40% of BOD and 50-60% of TSS. During extreme high-flow events, the plant can utilize Chemically Enhanced Primary Treatment (CEPT) to aid settling.

C. Secondary Treatment (Activated Sludge)

The biological treatment phase utilizes a conventional activated sludge process. The facility operates multiple aeration basins equipped with fine-bubble diffusers to introduce oxygen, encouraging microbial growth that consumes organic matter.

• Configuration: Plug-flow aeration basins.
• Clarification: Mixed liquor flows to secondary clarifiers where biological floc settles.
• Solids Management: A portion of settled solids is returned to the aeration basins (RAS), while excess growth is wasted (WAS) to the solids handling train.

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D. Tertiary Treatment (The “Next Level”)

Commissioned in 2021, the tertiary stage is the technological centerpiece of RPWRF. Effluent from secondary clarifiers is pumped to the membrane filtration facility.

• Technology: Pall Aria AP-Series Pressurized Microfiltration (MF) Membrane Modules.
• Capacity: The system is designed to treat up to 50 MGD.
• Phosphorus Removal: Chemical addition (typically Aluminum Chlorohydrate or similar coagulant) precipitates phosphorus, which is then physically filtered out by the membranes.
• Performance: The system reduces Total Phosphorus to below 50 micrograms per liter (µg/L) in the effluent, meeting strict TMDL requirements.

E. Disinfection

Post-filtration, the water undergoes disinfection using chlorine gas to eliminate pathogenic organisms. Prior to discharge into the Spokane River, the effluent is dechlorinated using sulfur dioxide (or sodium bisulfite) to ensure zero chlorine residual, protecting aquatic life.

F. Solids Handling

• Thickening: Gravity belt thickeners concentrate waste activated sludge.
• Digestion: The plant features iconic egg-shaped anaerobic digesters. These operate at mesophilic temperatures (~98°F) to break down volatile solids and produce methane gas.
• Dewatering: Digested sludge is dewatered using high-solids centrifuges.
• Disposal: The dewatered biosolids are transported to a commercial composting facility (typically Barr-Tech), where they are converted into Class A compost for soil amendment.

4. INFRASTRUCTURE & FACILITIES

A. Energy Systems & Cogeneration

RPWRF is a leader in energy recovery. The methane gas produced in the anaerobic digesters is captured and treated via a siloxane removal system. It then fuels on-site cogeneration engines.

• Generation: The system generates electricity and heat.
• Utilization: Recovered heat is used to maintain digester temperatures and heat buildings, while electricity offsets grid consumption.
• Sustainability: This process significantly reduces the facility’s carbon footprint and operational energy costs.

B. Odor Control

Given its proximity to recreational trails and residential areas, RPWRF employs a multi-stage odor control strategy. This includes chemical scrubbers at the headworks and biofilters for solids handling areas. The distinct egg-shaped digesters also aid in odor containment compared to floating cover designs.

5. RECENT UPGRADES: The “Next Level of Treatment”

6. REGULATORY COMPLIANCE

A. NPDES Permit

Permit Number: WA0024473

The facility operates under a strict National Pollutant Discharge Elimination System (NPDES) permit administered by the Washington State Department of Ecology. Key parameters include:

• Total Phosphorus: Seasonal average limits < 50 µg/L.
• CBOD5: Monthly average limits (standard secondary).
• PCBs: The Spokane River has a variance for PCBs, but the facility is required to implement Best Management Practices (BMPs) to track down and eliminate upstream PCB sources.
• Ammonia: Seasonal limits to prevent toxicity.

B. Environmental Stewardship

RPWRF plays a vital role in the regional effort to reduce PCBs (Polychlorinated Biphenyls) in the Spokane River. Through the Toxics Management Plan, the City actively traces sources of PCBs in the collection system. The membrane filtration system also provides incidental removal of PCBs attached to particulate matter.

7. CHALLENGES & FUTURE PLANNING

A. PCB Removal Standards

While the NLT project addressed phosphorus, the ultra-low limits for PCBs in the Spokane River remain a significant regulatory challenge. Conventional treatment cannot remove dissolved PCBs to the quadrillion-part levels sometimes discussed in water quality standards. The facility focuses on removing particle-bound PCBs via the new membranes.

B. Combined Sewer Overflows (CSOs)

Spokane has older infrastructure where stormwater and sewage share pipes. While the City has built millions of gallons of storage tanks to reduce overflows, managing peak hydraulic surges at the plant during rapid snowmelt or heavy storms requires precise operational control to prevent washing out the biological mass.

C. Emerging Contaminants

Like all modern facilities, RPWRF is monitoring the regulatory landscape regarding PFAS (Per- and polyfluoroalkyl substances). Future upgrades may be required if stringent PFAS limits are applied to effluent or biosolids.

8. TECHNICAL SPECIFICATIONS SUMMARY

9. FAQ SECTION

Technical Questions

1. What specific membrane technology is used at RPWRF?
The facility uses Pall Aria AP-Series pressurized microfiltration modules. These hollow-fiber membranes effectively remove total suspended solids and particulate phosphorus.

2. Does RPWRF perform biological nutrient removal (BNR)?
The plant relies primarily on chemical precipitation followed by membrane filtration for phosphorus removal, rather than a purely biological phosphorus removal (EBPR) process, to ensure consistent compliance with strict low-level limits.

3. How are biosolids handled?
Biosolids are anaerobically digested, dewatered via centrifuges, and transported off-site to a commercial composting facility (Barr-Tech) for beneficial reuse.

4. What is the peak flow management strategy?
The facility utilizes upstream CSO storage tanks to attenuate flows. At the plant, primary treatment can be chemically enhanced (CEPT) to handle surges that might otherwise overwhelm biological secondary treatment.

Public Interest Questions

5. Is the water safe to enter the Spokane River?
Yes. The effluent meets Class A reclaimed water standards for turbidity and low phosphorus, and is disinfected to remove pathogens, helping protect the river’s ecosystem and recreational safety.

6. Does the plant smell?
While wastewater treatment naturally generates odors, RPWRF uses advanced chemical scrubbers, biofilters, and enclosed egg-shaped digesters to minimize odor impact on the surrounding Riverside State Park.