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City of Boulder Water Resource Recovery Facility

The Boulder Water Resource Recovery Facility (WRRF) is designed to treat 25 million gallons of wastewater per day using a multi-stage treatment process.

  • Currently, approximately 13 million gallons of wastewater is treated per day and a high-quality effluent (treated wastewater) is returned to Boulder Creek.
  • Water resource recovery is carried out using state-of-the-art processes, including:
    • Physical separation;
    • Microbiological nutrient removal; and
    • Ultra-violet (UV) light disinfection.
  • Treatment concludes with disinfection of harmful bacteria, viruses and protozoa.
  • Rigorous sample collection and analysis occurs daily to ensure that the final discharge is meeting or exceeding the discharge permit that has been issued by the State of Colorado.
  • The City of Boulder is regulated under the National Pollution Discharge and Elimination System (NPDES) permit number CO-0024147 as issued by the Colorado Department of Public Health and Environment (CDPHE).

The WRRF cleans sewer discharged water to permitted standards or better and afterwards returns water to Boulder Creek in as clean a state as is possible by the WRRF treatment system. Solids removed during treatment are used to produce Class B biosolids that are land applied on agricultural fields as part of a beneficial reuse program. Biogas is also generated during solids treatment and is further cleaned via a state-of-the-art process that produces a methane rich-biogas that can be injected onto the natural gas grid where it is purchased by Western Disposal Services or could be used at the WRRF to meet heating needs. 

During the past 10 years, the one-megawatt solar photovoltaic system at the Water Resource Recovery Facility has generated more than 13 million kilowatt-hours of electricity, saving utility ratepayers more than $500,000. The system began generating clean, renewable power in August 2010 and has operated efficiently and reliably ever since, producing about 14 percent of the facility’s annual power needs. In 2020, a second Solar PV system was added to the WRRF campus. The new 500-kW rated system further expands the renewable source power profile and reduces dependence on the electric grid.

The city's main Water Quality Laboratory, Industrial Pretreatment Program, and Stormwater Program are also located at the Water Resource Recovery Facility.


Construction finished in September 2020 on the city's new biogas-to-renewable-natural-gas (RNG)-conditioning system, which replaced the WRRF's cogeneration system that was at the end of its useful service life. The project is one of only a handful of similar projects in Colorado. The new system is on track to generate $600,000 in annual revenue through the sale of RNG to community partners such as Western Disposal and federal renewable fuel incentive credit revenue.   Learn more about the city's Biogas Use Enhancement Project  . 

The City of Boulder is one of many communities performing wastewater surveillance to learn more about COVID-19. In May, the city began by sampling raw wastewater entering the Water Resource Recovery Facility   and sending samples out for analysis by Biobot Analytics in an effort to detect COVID-19 indicators in our community. Biobot works with cities across the country to develop an early warning system for health departments and communities by analyzing what is in the water. Since then, the city joined a Colorado-based collaboration now sponsored by CDPHE that includes many industry partners all of whom are directing efforts toward increasing sampling and advancing front range understanding of the findings. During all efforts, the city has been coordinating with Boulder County Public Health to review the findings and to evaluate trends over time. It has not yet been determined how the information will be shared with the community. Refinements in methodologies are taking place as experts learn more about wastewater-based epidemiology and how it can be leveraged against COVID-19.

WBE is an epidemiological approach that has potential to complement current infectious disease surveillance systems via providing a possible early warning system for disease outbreaks. Through the analysis of population pooled wastewater, the emergence of new disease outbreaks at the community level can be monitored comprehensively and can serve as a leading indicator at a relatively low cost.

Wastewater surveillance is not a new area of research. It has been critically important in detecting the presence of poliovirus to support the Global Polio Eradication Initiative, and more recently, to investigate opioid use in communities. Today’s analytical techniques for wastewater surveillance are more cost efficient than testing individuals and can quickly detect disease indicators within large populations.

It is possible that WBE may complement information gathered by health agencies and epidemiologists to provide information about virus levels. Viral shedding within the stool of those infected with SARS CoV-2 begins several days before the onset of symptoms. The potential for WBE to help with the current pandemic is promising. For more information visit

The city, along with other front-range communities, have been collaborating with the  Colorado Department of Public Health and Environment  , Metropolitan State University and Colorado State University to assemble an in-state program to support the application of wastewater surveillance in Colorado to help address this pandemic since August 2020. For one year, wastewater samples will be sent to CSU for analyses two times per week and CSU will upload data to a dashboard maintained by the   CDPHE   

The Colorado Department of Public Health and Environment is leading this effort through its Clean Water Program, which includes acting as the liaison between the collaborative and state epidemiologists. 

To learn more about the Colorado WBE Program, please reach out to Colorado Department of Public Health and Environment’s Communications Manager, MaryAnn Nason at   [email protected]

The Boulder Water Resource Recovery Facility treats an average of 13 million gallons of wastewater per day. Wastewater reaching the facility goes through a multi-stage treatment process. 

Treatment Process


Physical Removal


  • Coarse debris such as twigs, trash, sand, and grit is removed using screens and aerated grit chambers.

  • The debris is hauled to a local landfill while the grease and solids are sent through an onsite biosolids treatment process.

  • The wastewater flows into primary clarifier tanks that remove approximately 70 percent of the total suspended solids and 50 percent of the biochemical oxygen demand.

  • The remaining liquid moves on to the biological treatment phase.

Biological Treatment



  • The biological nutrient removal (BNR), activated sludge treatment process uses large populations of microorganisms to break down organic materials in the wastewater and uptake available nutrients.

  • Dissolved material is converted to particulate material (solids) that is separated from the water.

  • During nitrogen treatment, bacteria convert ammonia into nitrate (nitrification), then convert the nitrate into nitrogen gas (denitrification) and release it into the atmosphere. This process occurs in large aeration basins where the proper conditions for the microorganisms to do their work are present and maintained.

  • The liquid flows into large secondary clarifiers, where the solids and microorganisms settle out, and are returned to the aeration basins to repeat the cycle.

  • A portion of the solids is removed from the liquid stream and sent to the dissolved air flotation thickener to concentrate the solids prior to sending them for further treatment in the solids treatment system.

Ultraviolet Light Disinfection


  • Since 2013, ultraviolet (UV) light technology has been used for disinfection. The UV technology eliminates both the need to use chemical treatment for disinfection and hazardous gas storage onsite.
  • The UV light is a highly effective disinfectant for pathogens in wastewater, including E. coli bacteria, viruses and protozoa.

Solids Treatment Processes



  • All of the solids that were removed from the treatment process via clarifiers or wasting station are thickened, treated, stabilized, and then dewatered.

Solids Thickening

  • Gravity thickeners settle and thicken the sludge removed from the primary clarifiers.

  • Dissolved air flotation thickeners use micro-bubbles to thicken secondary waste activated solids.

Digestion and Stabilization

  • Anaerobic digestion is a solids treatment process that reduces the solids volume and pathogen content.

  • The solids end products are: biosolids that can be beneficially used as a Class B fertilizer, and biogas produced during solids treatment.

  • Biosolids are dewatered to about 22 percent solids by weight to keep transportation costs down and to provide a more beneficial soil conditioner and fertilizer product.

  • Biogas is upgraded from ~60% methane to ~98% methane using a membrane-based process. The high quality renewable natural gas (RNG) is then injected into the natural gas pipeline and used to fuel waste collection trucks to offset diesel fuel usage. 

Pollutants of Concern in Wastewater Discharges

The City of Boulder's specific pollutant limitations are listed in Boulder Revised Code, Title 11, Chapter 3, Section 5.   

These limits are designed to control pollutant discharges including:   

  • Metals which can be found in household products
  • Flammable liquids that can create fire or explosion  hazards in the wastewater collection system
  • Corrosives
  • Pollutants that may cause the formation of toxic gases, vapors or fumes in wastewater collection lines
  • Oils and grease
  • Oxygen-demanding pollutants


The goal of Boulder's silver reduction program is to reduce the amount of silver entering the city's wastewater treatment facility.  Boulder's Water Resource Recovery Facility is not currently configured to treat elevated levels of toxic metals such as silver.

Silver Sources

The primary source of silver is from the discharge of untreated or improperly treated silver-bearing fixer from photographic material processing such as development of silver bearing film, x-ray film and photographic paper.   

Why recover silver?

Boulder Revised Code 11-3 Industrial and Prohibited Discharges requires that all facilities generating waste photographic fixer treat for silver removal prior to discharge to the sanitary sewer. Discharges from silver treatment systems may not exceed 100 mg/L. The alternative to on-site treatment is to have the fixer transported off-site for disposal. Failure to comply with regulations is a violation of law and can result in penalties up to $1,000 per day for each violation.   

Off-Site Disposal

Several local vendors offer fixer pick-up services.

On-Site Disposal

You can purchase or lease a silver recovery unit to remove silver from spent fixer.

Disposing of Pharmaceuticals

Trace   amounts of pharmaceuticals   are    f  ound in water supplies across the country, the City of Boulder Utilities Division    is    provid   ing    residents with    water treatment information   , recommended pharmaceutical disposal practices and other city programs.   

  • For prescription drug drop off information in Boulder County, call 303-413-7350
  • For general inquiries, contact Colorado Department of Public Health & Environment at 888-659-1831 ext. 3320 (toll-free)
  • Locate a prescription drug drop-off box

Environmental Impacts

Recent studies indicate that medications flushed down the toilet or drain may have an adverse impact on the environment, including streams, fish and other aquatic life. Boulder's municipal Water Resource Recovery Facility is not equipped to completely remove    all of    the chemicals and compounds found in various medications. Recent studies have shown that these chemicals can be detrimental to aquatic life and ultimately can end up in drinking water supplies.   

The Keep It Clean Partnership has developed programs to    meet    EPA    requirements regarding stormwater regulations.

Water Monitoring and Treatment

The City of Boulder Utilities Division owns and operates two drinking water treatment plants. The Federal Safe Water Drinking Act (SDWA) requires that water monitoring be performed at these plants as well as various other sample locations. The city monitors the quality of treated drinking water for    many constituents, as required under SDWA, but currently does not test for pharmaceuticals as it is not required   nor has the city been identified for having any significant risk or    vulnerability   .   

The responsibility for clean water rests with everyone. What you put back into Boulder Creek has a substantial effect on water quality. Your action will help keep water supplies clean.