Boulder Wastewater Treatment Plant

1. INTRODUCTION

The Boulder Water Resource Recovery Facility (WRRF) represents a premier example of the paradigm shift from wastewater treatment to resource recovery in the American West. Situated on a 55-acre site northeast of the city limits, the facility serves the City of Boulder and the Gunbarrel sub-community. While the plant treats an average daily flow of approximately 13.5 million gallons per day (MGD), it is hydraulically rated for 25 MGD, providing critical redundancy and storm-flow management for a service area prone to flash precipitation events.

Historically known as the 75th Street Wastewater Treatment Plant, the facility was rebranded to WRRF to reflect its operational philosophy: generating Class B biosolids for agricultural land application and converting biogas into Renewable Natural Gas (RNG). Following a landmark $16 million biogas enhancement project completed in 2020, the facility now injects pipeline-grade natural gas into the commercial grid, establishing Boulder as a leader in municipal energy neutrality and circular economy infrastructure.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The WRRF manages wastewater for the entirety of the City of Boulder, including the University of Colorado Boulder campus, federal labs (NOAA, NIST), and the Gunbarrel General Improvement District. The collection system spans approximately 400 miles of sanitary sewer mains, utilizing gravity flow for the majority of the service area, supplemented by strategic lift stations. The demographics include a unique mix of residential, high-tech industrial, and brewing/distilling industries, the latter of which contributes distinct high-strength BOD organic loading slugs that require robust process control.

B. Operational Capacity

The facility operates well within its hydraulic design parameters, ensuring consistent compliance even during diurnal peaks.

• Design Hydraulic Capacity: 25.0 MGD
• Average Daily Flow: 12.5 – 14.0 MGD
• Organic Loading Capacity: Designed for high-strength variance due to local brewing industries.
• Peak Instantaneous Flow: Approx. 58 MGD

Current capacity planning accounts for the region’s managed growth policies, focusing on process intensification and nutrient removal optimization within the existing footprint rather than hydraulic expansion.

C. Discharge & Compliance

Treated effluent is discharged into Boulder Creek, a effluent-dominated stream during low-flow months. Consequently, the Colorado Department of Public Health and Environment (CDPHE) enforces stringent limits on ammonia, nitrate, and aquatic life toxicity parameters. The facility operates under a Class A NPDES permit (CO-0024147), consistently achieving compliance with Regulation 85 nutrient limits and Regulation 31 temperature standards.

3. TREATMENT PROCESS

The Boulder WRRF utilizes a Trickling Filter/Solids Contact (TF/SC) process configuration. This hybrid system combines the energy efficiency and shock-loading resilience of attached-growth media with the high effluent quality of suspended-growth activated sludge.

A. Preliminary Treatment

Raw influent enters the headworks via two main interceptors (60-inch and 42-inch).

• Screening: Two mechanical multi-rake bar screens (6mm spacing) remove large debris. Screenings are washed, compacted, and disposed of at a sanitary landfill.
• Grit Removal: Vortex grit chambers separate inorganic solids (sand, gravel) to protect downstream pumps.
• Flow Monitoring: Magnetic flow meters and Parshall flumes provide real-time influent data to the SCADA system.

B. Primary Treatment

Flow enters three primary clarifiers (105-115 ft diameter). These circular units reduce flow velocity, allowing settleable solids to drop to the hopper for removal as primary sludge. Scum skimmers remove grease and oils.

• Efficiency: Typically achieves 60% TSS removal and 30-35% BOD removal.
• Ferric Chloride/Polymer Addition: Chemical piping exists to enhance coagulation/sedimentation during high-load events or for phosphorus precipitation.

C. Secondary Treatment (TF/SC)

The heart of the plant is the coupled Trickling Filter/Solids Contact system:

• Trickling Filters: Two rock-media trickling filters and one plastic-media biotower serve as the “roughing” biological stage. Wastewater is distributed over the media via rotary arms. The biofilm metabolizes the bulk of the soluble carbonaceous BOD (cBOD) and initiates nitrification. This stage is highly energy-efficient, utilizing natural draft ventilation.
• Solids Contact Basin (Aeration): Effluent from the trickling filters flows to the Solids Contact Basins. This is an activated sludge process operated at a low Mean Cell Residence Time (MCRT). Here, mixed liquor suspended solids (MLSS) polish the remaining BOD and flocculate fine solids.
  • Nutrient Removal: The basin configuration includes anoxic zones to facilitate denitrification (conversion of nitrate to nitrogen gas), crucial for meeting Regulation 85 Total Inorganic Nitrogen (TIN) limits.

• Secondary Clarifiers: Three circular clarifiers separate the MLSS from the treated water. Return Activated Sludge (RAS) is pumped back to the solids contact basin, while Waste Activated Sludge (WAS) is sent to thickening.

D. Tertiary Treatment & Disinfection

• Nitrification Polishing: The process is tuned to ensure complete ammonia conversion prior to disinfection to prevent toxicity in Boulder Creek.
• Disinfection: The facility utilizes a high-intensity Ultraviolet (UV) disinfection system. This replaced the legacy gaseous chlorine system, eliminating the need for dechlorination chemicals (sulfur dioxide) and improving operator safety. The UV system is flow-paced to ensure pathogen inactivation (E. coli limits).

E. Solids Handling

• Thickening: Primary sludge is typically gravity thickened or co-thickened. WAS is thickened via Dissolved Air Flotation (DAF) or Rotary Drum Thickeners to 4-5% solids.
• Anaerobic Digestion: Thickened sludge is pumped to mesophilic anaerobic digesters (98°F). The volatile solids reduction (VSR) process stabilizes the sludge and produces methane-rich biogas.
• Dewatering: Digested sludge is dewatered using high-speed centrifuges, producing a cake of approximately 20-25% solids.
• Biosolids Reuse: The final product meets Class B standards and is land-applied on local agricultural sites (wheat and corn farms) in Boulder and Weld counties, recycling nitrogen and phosphorus back into the soil.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The site encompasses approximately 55 acres within the Boulder Creek floodplain. All critical process structures are elevated or flood-proofed to resist 100-year and 500-year flood events. The architectural design of recent additions, such as the RNG building, utilizes local sandstone and earth tones to minimize visual impact on the surrounding open space.

B. Energy Systems & RNG

The Boulder WRRF is a net energy producer regarding thermal needs and offsets significant electrical demand.

• Biogas Production: The digesters produce approx. 200,000 to 300,000 cubic feet of biogas daily.
• Renewable Natural Gas (RNG): Historically, gas was used in cogeneration engines (CHP). In 2020, the plant transitioned to an RNG conditioning system. This system removes hydrogen sulfide, siloxanes, CO2, and moisture to meet strict pipeline specifications.
• Grid Injection: The purified gas is injected into the Public Service Company of Colorado (Xcel Energy) pipeline, generating D3 Renewable Identification Numbers (RINs) under the EPA Renewable Fuel Standard.

C. Odor Control

Given the proximity to the Boulder Creek trail system and residential zones, odor control is paramount. The facility utilizes dual-stage biofilters and activated carbon scrubbers at the headworks and solids handling complex to capture and treat H2S and organic odors.

5. RECENT UPGRADES & MAJOR PROJECTS

WRRF Biogas Use Enhancement Project (2018-2020)

• Project Budget: ~$16 Million
• Funding: Revenue bonds and state grants.
• Scope: Construction of a new conditioning facility to convert biogas to RNG. Installation of a membrane gas separation system and a pipeline interconnect station.
• Contractors: Tetra Tech (Design), Short Elliott Hendrickson Inc. (SEH).
• Results: The facility now recovers 100% of generated biogas for beneficial use, reducing site greenhouse gas emissions and creating a new revenue stream via RIN credits. It displaces the equivalent of 500,000 gallons of diesel fuel annually.

Nutrient Removal Optimization & Sidestream Treatment

• Scope: Upgrades to aeration control systems and the implementation of centrate treatment strategies.
• Driver: Compliance with Colorado Regulation 85 (Nutrient Management Control Regulation).
• Technical Highlight: Implementation of ammonia-based aeration control (ABAC) to optimize blower energy usage while ensuring nitrification stability.

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. Permit Requirements

The facility operates under NPDES Permit CO-0024147. Key parameters include:

• CBOD5: 30-day avg limit of 25 mg/L (typically achieves < 5 mg/L)
• TSS: 30-day avg limit of 30 mg/L (typically achieves < 10 mg/L)
• Ammonia (NH3-N): Seasonally variable limits, often < 2-3 mg/L in summer months.
• E. Coli: 126 CFU/100 mL (30-day geometric mean).

B. Compliance History

The Boulder WRRF maintains an exemplary compliance record with the CDPHE. The facility has received multiple peak performance awards from the National Association of Clean Water Agencies (NACWA) for consecutive years without permit violations.

7. OPERATIONAL EXCELLENCE

A. Staffing

The facility is staffed by approximately 30-35 full-time employees, including operations, maintenance, instrumentation/control technicians, and laboratory staff. Operations staff are required to hold Colorado Class A or B Wastewater Operator certifications.

B. Technology & Automation

The plant utilizes a Rockwell/Allen-Bradley based SCADA system with a high-speed fiber backbone. Operators utilize mobile tablets for field rounds and asset management tracking (using CMMS software). The lab is state-certified for microbiology and wet chemistry, allowing for rapid process control decisions without waiting for external lab results.

8. CHALLENGES & FUTURE PLANNING

A. Current Challenges

• Aging Infrastructure: Components of the secondary treatment system date back to the 1980s and require ongoing rehabilitation.
• Temperature Limits: Reg 31 imposes strict temperature limits on effluent to protect aquatic life. As influent temperatures rise (due to indoor plumbing trends) and stream flows decrease, thermal compliance becomes technically challenging.
• Emerging Contaminants: Monitoring for PFAS (per- and polyfluoroalkyl substances) is becoming a regulatory priority across Colorado.

B. Future Planning

The Wastewater Utility Master Plan outlines the 20-year capital vision. Key focuses include:

• Phosphorus Removal: Potential need for chemical polishing or enhanced biological phosphorus removal (EBPR) upgrades if limits tighten further.
• Resiliency: continued hardening of electrical systems against grid failures and flood protection reinforcement.

9. TECHNICAL SPECIFICATIONS SUMMARY

10. FAQ SECTION

Technical Questions

1. What is the specific nutrient removal mechanism at Boulder WRRF?
The facility utilizes a hybrid Trickling Filter/Solids Contact process. The trickling filters perform carbon oxidation and partial nitrification. The subsequent solids contact basins are operated with anoxic zones to facilitate denitrification, reducing Total Inorganic Nitrogen (TIN) before clarification.

2. How does the RNG system interconnect with the grid?
The biogas is conditioned to remove impurities (H2S, VOCs, CO2) using a membrane separation system. It is then pressurized and injected into a nearby Western Disposal pipeline which interconnects with Xcel Energy’s distribution network.

3. Is the facility equipped for phosphorus removal?
Yes, the facility can utilize chemical precipitation (ferric chloride/alum) if biological uptake is insufficient to meet permit limits, though biological strategies are prioritized.

General Interest Questions

4. Does the plant smell?
Odor control is a high priority. The plant uses biofilters (beds of organic media) and carbon scrubbers to treat foul air from the headworks and sludge processing areas. While minor odors may occur during maintenance, the system is designed to contain odors within the fence line.

5. What happens to the “solids” removed from the water?
The organic solids are digested (broken down by bacteria in a heated tank), which reduces their volume and kills pathogens. The resulting “biosolids” are used as a nutrient-rich fertilizer on local farms to grow crops like wheat and corn.