Wastewater is what residents and industry flush down toilets and empty down sinks and drains. This material then travels through the sanitary sewer system to one of four wastewater treatment plants where it is treated to the Ministry of the Environment, Conservation and Parks (MECP) standards and released back into the lake.

The safe and effective treatment of wastewater is important to the continued health and well-being of Toronto’s residents.

Toronto’s wastewater treatment process operates under strict regulations and meets or exceeds standards set by the province and federal government to protect public health and the environment.


Wastewater is collected and treated 24 hours a day, 7 days at week via:

  • 3,730 km of sanitary, 1,411 km of combined and 401 km of trunk sewers
  • 507,548 sewer service connections
  • 87 wastewater pumping stations
  • four wastewater treatment plants

How treatment works

Preliminary treatment

  • Removes large debris, including rocks, sticks, sand, silt and gravel that is washed into the sewer system.
  • Removes hygiene products, including tampons, dental floss and wipes, which are not meant to be flushed.

Primary treatment

  • Separates finer organic solid matter (human waste) from wastewater.
  • Solids are removed and treated during solids processing (more below).

Secondary treatment

  • Plant technicians cultivate and create the optimal environment for microscopic organisms that reduce and or convert pollutants in the wastewater.
  • Microorganisms multiply and grow in numbers in order to help produce a clear, clean effluent.
  • A combination of physical and biological processes are used and optimized in order to achieve clear, clean effluent that meets all Ministry standards.
  • Removal and reduction of nutrients, including nitrogen and phosphorus, are also achieved.


  • Water is disinfected to ensure that any remaining harmful pathogens are destroyed.
  • The effluent wastewater is then safe and meets all Ministry standards to be returned to the natural environment.
  • In most plants, some of the water is recycled within the facility for internal process use.

Solids processing

  • Solids removed during primary and secondary treatment are pumped into digesters.
  • A biological process using microorganisms, as well as heat, mixing and a long (10-20 day) holding time, breaks down the complex organic matter into simpler form and also creates methane as a biproduct.
    • The solids that leave processing are called biosolids, which are rich in organic material and nutrients.
    • The methane can be used to produce energy (heat and electricity) for the plant, reducing treatment costs and resources.

The City of Toronto has four wastewater treatment plants. Review the most recent reports submitted to the Ministry of the Environment, Conservation and Parks (MECP) regarding operations, capital projects, bypasses, maintenance costs, staffing, and health and safety initiatives.

The goal of odour management is to ensure odours are captured and treated on-site and do escape into the surrounding communities. Each plant has comprehensive systems under construction to manage odours produced during the wastewater treatment process. The main components include:

  • Enclosures and ventilation systems to contain and capture odourous air from larger sources in order to prevent it from leaving the plant property.
  • Biofilters, which use naturally occurring microorganisms to remove odorous compounds from the captured air.
  • Activated carbon scrubbers, which use carbon to clean the odourous air from numerous smaller sources.
  • Controlled ventilation systems, to collect and disperse treated air.
  • Best practice plant operations and housekeeping.

Biosolids are the nutrient-rich, organic materials resulting from the treatment of sewage. Learn how the City manages biosolids.