The City of Markham's sanitary sewer system collects and moves domestic/industrial wastewater to the Duffin Creek Wastewater Pollution Control Plant located in Pickering, Ontario. The Region treats wastewater and releases the cleaned water into Lake Ontario. The quality of the water being released after treatment must meet high standards as set by the Ministry of the Environment, Conservation and Parks.

Once Markham's wastewater reaches the Duffin Creek Wastewater Pollution Control Plant, it goes through an extensive cleaning process. This process includes four general steps.

Step 1: Screening

As wastewater (sewage) enters the Plant, screening removes large, inorganic objects. These objects could include things such as rags, diapers, personal hygiene products, and any other inorganic objects which are removed and sent to landfill.

Step 2: Primary Treatment (Physical)

Organic materials (ie. human waste and food) in the wastewater are removed using settling tanks. These settlement tanks work by using gravity and time to allow the solids to settle to the bottom of the tank where it is collected. A skimmer is then used to remove fats, oils, and grease that float to the top and a chemical is added to remove phosphourous. The remaining liquid portion of the wastewater moves on to secondary treatment.

Step 3: Secondary Treatment (Biological)

Secondary treatment removes small pieces of organic materials from the wastewater using aeration lanes which pump air into the tanks giving helpful bacteria the oxygen they need to break down the organic material and absorb nutrients. Settling tanks are used again to allow the material and bacteria to settle to the bottom where they are collected and removed. The clean water (effluent) from this stage then flows into its final stage, disinfection.

Step 4: Disinfection

Chlorine (sodium hypochlorite) is added to kill and deactivate any remaining harmful bacteria (i.e.. E.coli.). Afterwards, another chemical, sodium bisulphite, is used to remove the chlorine before the clear treated water is released into Lake Ontario.