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Wastewater Treatment Process

Wastewater Treatment Process

The 75th Street Wastewater Treatment Facility treats an average of 12.5 million gallons of wastewater per day. Wastewater reaching the facility goes through a 20-hour, multi-stage treatment process.

75th Street Wastewater Treatment Facility pdf

Treatment Process




  • 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.




  • The biological, nutrient removal, activated sludge treatment process uses large populations of microorganisms to break down organic materials in the wastewater.
  • 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 that provide the proper conditions for the microorganisms to do their work.
  • The liquid flows into large secondary clarifiers, where the solids and microorganisms settle out, and a portion is 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 handling facility.

Ultraviolet Light Disinfection


  • The new ultraviolet (UV) light disinfection system replaced the existing chlorine gas and sulfur dioxide gas systems, which had been in use since 1990 and posed a significant safety risk.
  • The new technology is more efficient and eliminates the need to store and use hazardous gases.
  • The UV light is a highly effective disinfectant for bacteria, viruses and protozoa.




  • All of the biosolids that were removed during the wastewater treatment process are concentrated (thickened), treated, stabilized and then dewatered.

Solid Concentration

  • Gravity thickeners settle and concentrate the sludge removed from the primary clarifiers.
  • Dissolved air flotation thickeners use micro-bubbles to concentrate secondary waste solids.

Digestion and Stabilization

  • Anaerobic digestion breaks down the biosolids, producing an end product that is stabilized and can be beneficially used.
  • The by-products of this process are methane, other gases, and heat, which fuel two generators that produce electricity. The electricity is used in the Wastewater Treatment Facility to offset the city's energy costs from the Xcel Energy grid. This is known as Cogeneration.
  • Heat is also captured and provided for facility buildings, and as auxiliary heat for the digesters.



  •  The biosolids are further concentrated using polymer and centrifuges.
  • The biosolids are dewatered to about 20 percent solids by weight to keep transportation costs down and to provide a more beneficial soil conditioner and fertilizer product.