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Wastewater Treatment Process

Wastewater Treatment Process

The Boulder Water Resource Recovery Facility treats an average of 13 million gallons of wastewater per day. Wastewater reaching the facility goes through a 20-hour, multi-stage treatment process. Key treatment stages are highlighted below.

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 (BNR), 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 treatment system.

Ultraviolet Light Disinfection


  • Since 2013, ultraviolet (UV) light technology has been used for disinfection. The UV technology is more efficient and eliminates the need to use and store hazardous gases.
  • The UV light is a highly effective disinfectant for pathogens in wastewater, including E. coli bacteria, viruses and protozoa.




  • All of the solids 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 solids, reduces volume and pathogens producing a biosolids end product that is stabilized and can be beneficially used. The biosolids are further concentrated using polymers and centrifuges.
  • The 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.