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Wastewater Treatment Plant Process

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The Louisville Wastewater Treatment Plant (WWTP) is located in Boulder County on the east side of Louisville. The facility discharges into Coal Creek, which is approximately ¼ mile east of the plant. The initial WWTP was constructed in 1952 and consisted of a settling pond followed by an aerated lagoon. Growth within the City was very slow for a number of years, and the plant was not expanded until 1965 when an Extended Aeration Activated Sludge plant (Race Track) was added along with chlorination to provide a higher degree of treatment.

To meet continuing growth, a four-cell lagoon system was added in 1973. During that period, the effluent from the Race Track process discharged into the lagoon system during high flows. Two additional lagoons were added in 1978 to meet demand. In the early 1980’s the City grew substantially and by 1982, the plant was occasionally over loaded and therefore required a more effective way of treating sewage. A 900,000-gallon per day Extended Aeration Activated Sludge treatment plant was constructed to meet growing demand.

Soon after the 1982 expansion, the WWTP had many difficulties meeting discharge permit limits because the treatment processes once again approached design capacity. In 1986, construction of a clarifier, aeration basin, and chlorine contact tank was necessary to address the increased hydraulic and organic loadings. A laboratory was also added in 1986 to assist operators in controlling the various treatment processes. An administration building was added along with a secondary clarifier, pump building, ultra-violet disinfection system, and upgrade of the aeration system. 

To meet the demands of a growing population and more stringent nutrient limits from the Colorado Department of Public Health and Environment (CDPHE), the City built and commissioned a Biological Nutrient Removal (BNR) Modified University of Cape Town process in 2017. The new treatment system has significantly reduced the wastewater effluent nutrient loading into Coal Creek.

This advanced system has the capacity to treat 2.53 MGD, 5,515 lb/day BOD, 6,340 lb/day TSS, 664 lb/day NH3, and 106 lb/day Phosphorus. Beyond increasing the organic loading capacity, many technological improvements have been installed as well. Upgrades in equipment include high speed turbo aeration compressors, variable frequency drive controllers, fine screen solids removal, grit removal, and an aluminum sulfate dosing system to further settle out pollutants in the clarifiers. The BNR design is expected to be viable until the year 2034.


The City of Louisville’s WWTP utilizes microorganisms to treat raw sewage from homes and businesses within the City. The raw sewage flows through a network of collection pipes throughout the City which discharges into the treatment plant. In the preliminary treatment phase, fine screen equipment removes rags, sticks, and other debris found in sewage. After the screening phase, particles too small to be caught by the screens are removed by a vortex grit removal system. Next, plant influent flow is measured and the raw sewage flows into a lift station that pumps the wastewater into the influent channel. The secondary treatment process begins after the influent channel where sewage comes into contact with the activated sludge. Activated sludge is mostly water—99.7% by volume—but it also contains a sludge biomass that is light brown in color with a musty odor.

In the final zone of the secondary treatment process, the aerobic zone, sludge is aerated to convert ammonia to nitrate through a process called nitrification. At the end of the aeration zone, Mixed Liquor Recycle (MLR) pumps move some sludge back to the anoxic zone for removal of nitrate through denitrification. Activated sludge not returned to the anoxic zone flows from the aeration basin into the secondary clarifiers where the solids settle to the bottom. This separation of material produces clean water that flows through the ultraviolet system where harmful bacteria such as E. Coli, giardia, and cryptosporidium are deactivated by ultraviolet light. Finally, the treated water is discharged into Coal Creek.

The following sections describe the processes in more detail.


The method of secondary treatment is a Modified University of Cape Town process, which consists of three concrete basins each with a capacity of one million gallons. Each basin is divided into five different zones. The purpose of the zones in the secondary process is to provide an optimal environment for specific microorganisms to flourish and reproduce so that they can breakdown and digest organic and inorganic materials in the raw wastewater. The mixture of microorganisms and water is called “activated sludge.” The activated sludge flows through a series of five zones: sludge, anaerobic, anoxic, swing, and aerobic. These zones are explained below.

 Sludge Zone

A portion of the solids that settle to the bottom of the clarifier are returned to the beginning of the treatment train. This portion of sludge is called Return Activated Sludge (RAS). Returning sludge to the beginning of the treatment train increases the overall contact time between the microorganisms and the sludge and allows for the Microorganisms to stay in the treatment process, increasing the overall age of the sludge. As the sludge gets older, the microorganisms in the sludge become more adapted to decompose the inorganic and organic compounds in the raw wastewater.

Anaerobic Zone

Following the Sludge Zone, the wastewater flows into the anaerobic zone, which is designed to create optimal conditions for Phosphorous Accumulating Organisms (PAOs). PAOs are a special type of organism discovered in South Africa in the 1970s. They are special because these microorganisms can consume and hold Phosphorus up to 8% of their total mass. All other known organisms, including you and me, can hold only 1% of their total mass as Phosphorus. PAOs consume fermented Volatile Fatty Acids (VFAs) from the influent flow. VFAs allow the PAOs to grow and reproduce quickly.

 Anoxic Zone

Denitrification of the wastewater occurs in this zone. The wastewater is denitrified in the anoxic zone as nitrate-rich wastewater is recycled from the tail end of the aerobic zone and mixed with anaerobic zone effluent.

The anoxic zones allow for Nitrogen Oxidizing Bacteria (NOBs) to convert Nitrite (NO2) and Nitrate (NO3) to Nitrogen gas (N2). NOBs are not obligate aerobes, meaning they can consume oxygen from other sources, such as Nitrate or Sulfate. Instead of using dissolved oxygen in the water, these microorganisms are able to complete the conversion of Nitrite to Nitrogen gas through a process called denitrification. Denitrification occurs when the NOBs pull the oxygen from the NO3 molecule. The remaining Nitrogen combines with other Nitrogen atoms to form Nitrogen gas (N2). The Nitrogen gas rises out of the anoxic zone and into the atmosphere.

 Swing Zone

The swing zone is aerated in the winter and is left anoxic in the summer. This zone allows the microorganisms to be treated dependent on the specific needs of the microorganisms in the specific time of year. The swing zone gives operators the ability to extend the area of the anoxic zone or to extend the area of the aerobic zone.

 Aerobic Zone

A majority of carbon compound oxidation and nitrification is accomplished in the aerobic zone. In this zone, phosphorus removal occurs through PAOs storing phosphorus as polyphosphate. The aerobic zones treat ammonium by providing an environment for Ammonia Oxidizing Bacteria (AOBs) to flourish. AOBs convert ammonium (NH4+) to Nitrite and Nitrate. This Nitrite and Nitrate are recycled back to the anoxic zone for denitrification. AOBs are obligate aerobes, meaning they require a high level of Dissolved Oxygen (DO) in the wastewater to breathe and complete their work of oxidizing ammonia. Oxygen is provided to these microorganisms through High Speed Turbo Compressors (HSTC). These compressors use real time data from probes in the aerobic zones to pump more or less air into the aerobic zone, depending upon the DO setting in SCADA. Using real time data allows the compressors to operate efficiently, pushing only as much air into the zones as is necessary, which reduces electricity costs.


Sludge from the aerobic zone flows into secondary clarifiers. A healthy sludge is one that flocculates, settles, and compacts.  In the clarifiers, the mixed liquor settles to the bottom of the tank and the clear water flows over the top of the clarifier weir to the disinfection system. Microorganisms that settle in the secondary clarifiers are pumped either to the beginning of the treatment basins as RAS or to the aerobic digester for pathogen removal and dewatering at the centrifuge.

The secondary clarifier units provide time for the separation of the liquids and solids. The clarified liquid that flows over the weir flows by gravity to the ultraviolet disinfection system where pathogenic (disease causing) bacteria and viruses are inactivated. To control the biological process and maintain a healthy population of microorganisms, a portion of the solids that settle to the bottom of the clarifier are removed from the system.


The treatment plant uses ultraviolet disinfection for its effluent water. Ultraviolet disinfection does not kill the harmful bacteria that could be present but instead alters the cell’s DNA and RNA so that the cell is unable to reproduce.  After passing through the ultraviolet disinfection system, the final effluent from the treatment process is used in different ways. Most of the effluent is discharged to Coal Creek, helping to maintain aquatic life and habitat in the stream. In dry months, effluent flow to Coal Creek is 50 times greater than the flow upstream of the plant’s discharge.


After disinfection, a portion of the effluent is diverted from Coal Creek and pumped to the Reuse Water Facility for further treatment through a cloth media disc filter. The facility maintains a 3 million gallon holding tank for Reuse Water. The Reuse Water is pumped to the Louisville Sports Complex, Miner’s Field, Coal Creek Golf Course, and Community Park. From there the Reuse Water is used for irrigation. Reuse Water contains valuable nutrients and soluble metals that improve plant growth and soil structure.


Sludge removed from the secondary clarifiers which is not returned to the aeration basin as RAS is pumped to a Rotary Drum Thickener (RDT). The RDT removes excess water from the sludge, thickening the sludge from 1% Total Solids up to 4% Total Solids. The thickened sludge is then pumped to two aerobic digesters for further treatment. The microorganisms in the digesters are not fed any new wastewater, and therefore begin to deteriorate in the digesters. The microorganisms decrease in size through endogenous respiration and some break apart, releasing water, inorganic and organic compounds. 


Solids that are removed from the activated sludge process are stabilized in the Aerobic Digester for approximately 20 days. Stabilized sludge is then removed from the digester and pumped to a Centrifuge for further dewatering.  Dewatered sludge is known as Biosolids. Biosolids are hauled by a third party contractor to farms in Weld County who use the Biosolids’ high nutrient content as a soil amendment.


The administration building contains the office of the Plant Superintendent and the Environmental Compliance Specialist. A combined break room/meeting area and locker room/shower facility are also located within the administration building.

The laboratory building houses the operator’s office area and computer room, which controls, monitors, and records daily functions and activities throughout the WWTP. Upgrades to the Supervisory Control and Data Acquisition (SCADA) system and operator workspace were completed in 2017. An integrated alarm system monitors operations 24 hours per day for equipment failure or process variances.

The lab area is used to perform analysis of sludge, influent, and effluent samples for process control. Certified operators perform tests in order to ensure compliance with the Environmental Protection Agency and CDPHE stringent discharge permit regulations for Coal Creek. Proper analysis is imperative to effectively and efficiently operate the plant as well allowing staff to sustain public health by removing water’s impurities.  


The City’s Industrial Pretreatment Program (IPP) was adopted by the Environmental Protection Agency in 1983. The IPP has gone through numerous changes to comply with new regulatory requirements over the years. The primary duties consist of regulating and monitoring all industrial and commercial businesses located within city limits of Louisville. Permitted industries are sampled on a regular basis to verify compliance with local and federal pretreatment regulations. Currently, there are three IPP permitted Significant Industrial Users (SIUs) in the City. All new commercial and industrial businesses in the City are inspected before they can discharge from their facilities. The Environmental Compliance Specialist performs annual inspections of regulated businesses and also inspects restaurant grease interceptors on a quarterly basis to make sure the restaurants are maintaining their interceptors.

Stringent discharge limits imposed on industrial and commercial businesses are required to protect the general public and City employees as well as the sewer collection system and wastewater treatment processes. Other benefits of a Pretreatment Program include: protecting the receiving stream (Coal Creek) from industrial pollutants that may pass through the WWTP, reducing the chance of illegal dumping of hazardous or toxic wastes into the sanitary sewer system, and protection of the City’s Biosolids from contamination by industrial waste.

For more information about the plant, you may contact Justin Elkins at (303.335.4784).