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Wastewater treatment in Calgary

Calgary has three wastewater treatment facilities:

  • Bonnybrook Wastewater Treatment Plant
  • Fish Creek Wastewater Treatment Plant
  • Pine Creek Wastewater Treatment Plant

Together, these treatment plants meet the wastewater and sewage needs of over one million Calgarians each day. For an in-depth look at the wastewater treatment process, we invite you to take the wastewater treatment tour.

Wastewater treatment

The treatment of wastewater is a complex process consisting of three stages. Once the wastewater reaches one of these treatment plants, it goes through a series of processes before it is released as clean water into the Bow River.

First, The City collects all the wastewater that comes out of homes, schools, businesses and car washes in a sanitary sewer system. The wastewater flows through the pipe system by gravity or is pumped to one of our wastewater treatment plants.


When wastewater reaches a treatment plant, it passes through screens that remove larger materials such as plastic bags, toilet paper, toys, sticks, and tennis balls. The wastewater then travels into grit tanks where the heavier material settles to the bottom and is taken to the dump. This water, known as Primary Influent, flows by gravity to the Primary Clarifiers.

Primary Clarifiers

These are open-air tanks used for settling and skimming. The water stays in these tanks for about three hours. After that, more material settles at the bottom. This sludge and top skimming waste are pumped to the digesters for anaerobic (without air) decomposition. The overflow from these tanks (Primary Effluent) goes to the secondary process for further treatment.

The treatment process is then separated into two parts:

  • Liquid treatment processing
  • Sludge-handling processing 

Liquid-treatment Processing


These large open-air tanks mix Primary Effluent with a high concentration of naturally occurring microbes. These micro-organisms eat the dissolved nutrients (phosphorous and ammonia) and organic material to help them reproduce.

Compressed air is injected near the bottoms of the aeration tanks and bioreactors. Oxygen in the air is used by the micro-organisms in their life processes. The injected air also mixes the micro-organisms and brings them into contact with the organic materials. This biological treatment process is called the air-activated sludge process. The mixture of activated sludge and primary effluent is known as mixed liquor.  After a seven-hour average retention time,  the mixed liquor flows to the secondary clarifiers.

Secondary Clarifiers

In the secondary clarifiers, which are large, open-air tanks, the mixed liquor is retained for about seven hours. During this period, the activated sludge settles by gravity and the clear secondary effluent overflows the clarifiers' weirs and flows to the ultraviolet-light disinfection facility. Most of the settled activated sludge is returned to the aeration tanks and bioreactors to repopulate it with micro-organisms. Excess activated sludge is pumped to five dissolved-air flotation tanks for thickening. As in the primary clarifiers, scum and grease are skimmed off the surface of the secondary clarifiers and pumped to the anaerobic digesters.

Disinfection (U.V.)

Effluent from the secondary clarifiers flows to the ultraviolet-light disinfection facility, where it is exposed to ultraviolet light. This light disrupts the micro-organisms' genetic materials and renders them incapable of reproduction. They can no longer cause disease. Ultraviolet-light disinfection, unlike chlorine, adds no harmful chemicals to the plant's final effluent.

Final Effluent

Final effluent flows from the ultraviolet-light disinfection facility and is discharged into the river. The final effluent consistently meets the stringent effluent standards set by Alberta Environmental Protection. The effluent is clear, colourless, high in dissolved oxygen, and very low in organic solids, suspended solids, phosphorus, ammonia nitrogen and pathogenic (disease-causing) micro-organisms.

Sludge-handling Processing


In the covered anaerobic digesters, the combined primary and activated sludges are digested over a 25-day period by anaerobic bacteria (bacteria that grow only in the absence of oxygen). The naturally-occurring bacteria break down complex organic materials into simple and stable substances such as water, methane, and carbon dioxide. The digested sludge becomes less odorous and many disease causing organisms are destroyed. This biological sludge treatment process, or anaerobic digestion, is enhanced by constant mixing, using compressed digester gas, and heating to maintain digester temperature at 35 degrees Centigrade, the ideal temperature for anaerobic bacteria growth. Digested sludge is pumped to the Shepard Sludge Lagoons for ultimate reuse on farmland under the Calgro program. Anaerobic digestion produces digester gas, which consists of approximately 65 per-cent methane and 35 per-cent carbon dioxide.

Shepard Sludge Lagoons

Digested sludge, often called biosolids, is pumped from the anaerobic digesters of the Bonnybrook and Fish Creek wastewater Treatment plants to the Shepard Sludge Lagoons, located in southeast Calgary. The sludge lagoons consist of six summer cells, two winter cells, and two separated-liquid (supernatant) cells. They encompass a 20-hectare area and can store 743,000 cubic metres. The water at the top of the lagoons is pumped back to the Headworks at the treatment plant to once again move through the treatment process.


This program involves taking biosolids, the final product from the lagoons to the farmers' fields. After about six weeks in the lagoons, the biosolids can be used as organic fertilizer and soil conditioner. Learn more about Calgro.