The wastewater from Boulder's houses and businesses travels through the sewer system to the Boulder Wastewater Treatment Plant, located on 75th Street. The Boulder WWTP treats an average of 17 million gallons of wastewater per day using a 10-hour, multi-stage cleaning process involving biologic and chemical treatments. Industrial businesses are required to meet City discharge limits and may need to pre-treat their wastes prior to discharge into the City's sewer system (see the City of Boulder's Industrial Pretreatment website for more information). Chemicals used in the home can also affect water quality and should not be disposed through the sewer system but through a hazardous waste collection program (see Boulder County's Household Hazardous Waste Website for more information). There are many alternatives to household cleaners that are less hazardous and do not need special handling.

When wastewater from our sewers reach the WWTP, coarse debris, such as twigs and rags, is first removed from the waste stream by bar screens. Inorganic materials, including sand and grit, are then removed in aerated grit chambers. The wastewater then flows into primary settling tanks. This process removes total suspended solids (TSS) along with organic material. The wastewater then passes through a trickling filter process that removes dissolved organics. Microorganisms growing on rock media in the trickling filters use organic matter in the waste as a food source, converting these materials into biomass and carbon dioxide. The biomass is removed from the wastewater and treated. Part of the wastewater is then pumped to the nitrifying trickling filter (NTF). Nitrifying bacteria growing on the NTF plastic media convert ammonia (which can be toxic to aquatic life) to nitrate. This stream is then blended with the rest of the wastewater and treated with chlorine to remove pathogens. Since chlorine can be very toxic to fish, the chlorine remaining after disinfection is destroyed by adding sulfur dioxide to the treated wastewater. The dechlorinated water is then discharged to Boulder Creek (just above 75th Street, east of Boulder) in accordance with regulations contained in the city's discharge permit issued by the Colorado Department of Public Health & Environment (CDPHE). (Excerpted from the City of Boulder's Public Works-Utilities-Water Website)