Treatment Processes
The Eagle River Water and Sanitation District (District) Wastewater Treatment Facilities (WWTFs) use four stages of treatment: Preliminary, Primary, Secondary and Disinfection. The final product of the wastewater treatment process is an effluent discharge that is typically cleaner than what is required by the Clean Water Act. The solids extracted during the treatment process are converted to biosolids which can be used as a soil amendment for certain land applications. Additionally, the District WWTFs have state-of-the-art odor control systems for treating odors produced during the treatment process. A brief summary of the wastewater, solids handling and odor control treatment processes follows.
Preliminary Treatment
Screening
Screening involves the removal of large objects from wastewater such as cans, bottles, sticks and plastics. These materials need to be removed because they can plug pipes, damage pumps and consume valuable treatment tank space. Screenings are hauled to the District's Biosolids Containment Facility (BCF) for drying and final disposal at the Wolcott Landfill.
Grit Removal
Grit is the heavier inorganic material in wastewater that will not decompose or break down (i.e. sand, eggshells and gravel). It is removed in aerated grit tanks which are designed to keep the organics suspended while settling out the inorganic material.
Primary Treatment
Primary Sedimentation
Primary treatment involves the use of settling tanks, also known as sedimentation tanks, to remove the organic suspended and floating solids from the raw wastewater. The primary reason for removing organic material is to reduce the biochemical oxygen demand (a measure of the amount of oxygen needed to biochemically degrade the organic matter in the wastewater). Some Fats, (Oil and Grease) are also removed from treatment during this stage.
The District wastewater facilities cover the primary sedimentation basins to help protect them from the weather elements and to control odor.
Secondary Treatment
Secondary treatment consists of three steps: Aeration, Sedimentation in the secondary clarifiers, and Nitrification. These treatment stages are designed to remove the dissolved and colloidal organic material and nutrients not removed by primary treatment. Approximately 90 percent of biochemical oxygen demand, ammonia and suspended solids are removed from wastewater during secondary treatment.
Aeration or Activated Sludge
Activated sludge is a biological slurry, or active biomass, that is composed of bacteria, protozoa, amoebae and a range of other filter feeding microbes. These microbes are responsible for removing the majority of the remaining ammonia, phosphorus and soluble and fine suspended material in the wastewater. The activated sludge process, which takes place in the aeration basin, involves mixing air with the wastewater leaving the primary clarifiers. Here, the microorganisms, in the presence of oxygen, use the organic matter and nutrients in the wastewater to sustain their life processes. The resulting effluent leaves the aeration basins with suspended activated sludge that is then removed by the secondary clarifiers.
Secondary Clarifiers
The secondary clarifiers are responsible for settling out the activated sludge from the aeration basin effluent. The clear water resulting from this process is sent to the nitrification cells for further treatment. Most of the settled sludge, called return activated sludge, is sent back to the aeration process to replenish the active biomass. A portion of the remaining settled sludge, called waste activated sludge is wasted, or removed, and sent to the solids handling process.
Nitrification
The clear effluent leaving the secondary clarifiers is sent to the nitrification tanks for processing. Nitrification, simply put, is the biological oxidation of ammonia nitrogen (a compound toxic to aquatic life) to nitrate nitrogen. The nitrification process is carried out in an oxygen rich environment by two species of bacteria referred to as nitrifiers (Nitrosomonas and Nitrobacter). These bacteria work to remove the majority of remaining ammonia from the clarifier effluent water. This treated water is then sent to disinfection.
Disinfection
Ultraviolet Light Irradiation
Disinfection is the final treatment stage of wastewater before it is discharged to the receiving water. Ultra-violet (UV) irradiation is a non-chemical disinfection method. It works by destroying the reproductive capacity of pathogenic protozoa, bacteria and viruses in wastewater through the application of ultra-violet light. As of 2007, the Avon, Edwards and Vail facilities have been retrofitted with UV disinfection systems (replacing the more dangerous and toxic chlorine disinfection systems).
Solids Handling Process
Autothermal Thermophilic Aerobic Digestion, or ATAD, is a high temperature microbiological process that combines heat, air and thermophilic bacteria (bacteria that thrive under high temperature conditions) to convert wastewater solids into environmentally safe biosolids. The process begins by pumping the solids from the primary sedimentation tanks and secondary clarifiers to waste tanks. There they are mixed with polymers and thickened with the use of a belt press (equipment that separates the liquid from the solids). The solids are then sent to the ATAD digestion process where the thermophilic bacteria work to degrade the organic solids and reduce both sludge mass and volume. The heat released during the microbial degradation maintains thermophilic temperatures. Solids treated by the ATAD process result in Class A biosolids (solids that comply with the U.S. EPA 40 CFR Part 503 Class A biosolids regulations and can be used for land applications for beneficial re-use) which are a biologically stable product. The biosolids are held for up to 10 days at temperatures between 130°F to 140°F before being sent through a centrifuge which removes as much as 70% of the water. Biosolids are available for public landscaping use and stored at the District Biosolids Containment Facility.
Odor Control
Odorous gases are produced during the wastewater and solids handling treatment processes. Sulfur compounds including mercaptans, dimethlyl disulfide, and dimethyl sulfide, generated by ATAD are among the most offensive. Reducing sulfur compounds to acceptable or undetectable levels is challenging due to the fact that the human nose can detect these odors at extremely low concentrations. Because of the close proximity of treatment facilities to our communities, the District recognizes the importance of eliminating nuisance odors produced by the wastewater treatment processes. State-of-the-art odor control systems have been installed at each WWTF and are designed to eliminate or reduce odor compounds to levels that are undetectable to the surrounding communities.