Biosolids are the nutrient-rich, organic materials resulting from the treatment of sewage in a wastewater treatment facility (i.e. treated sewage sludge). They are a beneficial resource, containing essential plant nutrients, organic matter and are recycled as a fertilizer and soil amendment.

Biosolids are the solids that have come through the final stage of wastewater treatment, Solids Processing. The Ashbridges Bay and Highland Creek wastewater treatment plants process approximately 195,000 tonnes of biosolids each year.

The City’s biosoilds management can be categorized as follows:

  • Environmental:
    • Land application for agricultural and other uses
    • Pelletization to create fertilizer
    • Alkaline stabilization to create fertilizer product, landfill cover, or to increase the pH of acidic soil
  • Landfill
  • Incineration

Read more in the City’s Biosolids and Residuals Master Plan (BRMP).

Methods Used to Manage Biosolids

Land application (also called “recycling”) returns nutrients to soil that are removed when we harvest and consume food. Biolsolids:

  • Like animal manure, contain a number of nutrients and organic matter that are beneficial for crop growth, as well as enhancing soil and improving its ability to retain water.
  • Reduce the need for, and environmental impacts of producing commercial fertilizers.
  • Can be applied to forest land and tree nurseries, and have been applied to agricultural land in Canada and internationally for over 40 years.
  • Can be applied to strip mines and can also help reclaim and vegetate the land.

In 2016, 26% of the City’s biosolids were used in land application. Numerous studies have shown that biosolids land application is safe and has significant benefits.

A pelletizer facility operates at the Ashbridges Bay Treatment Plant.

  • Biosolids are heated to evaporate water content, producing a finished material that is 90% solids or greater.
  • The high temperatures reduce pathogens (harmful microorganisms) to below detection level, making the pellets a registered fertilizer product.
  • The finished material is dust-free, low-odour pellets 2 to 4 mm in size that can be stored for long periods of time.
  • The reduction in mass and volume cuts down on transportation and storage costs.

In 2016, 49% of the City’s biosolids were made into pellets.

Chemical stabilization does the opposite of composting because it decreases rather than accelerates bacterial activity.

  • The most common form of chemical treatment consists of mixing biosolids with lime.
  • This process, called Alkaline Stabilization, controls pathogen levels by increasing temperature and pH.
  • It converts biosolids into an alkaline soil conditioner suitable for use as a fertilizer product, landfill cover, or to increase the pH of acidic soil.

In 2016, 25% of the City’s biosolids were treated using chemical stabilization.

Biosolids can be disposed of in landfills.

  • In some cases, the biosolids are used to cover landfill, which:
    • reduces the need for soil cover
    • limits waste seeping into the ground
    • discourages rodents and other disease-carriers from accessing the waste

In 2016, none of the City’s biosolids were brought to landfills.

Highland Creek Treatment Plant incinerates all of its biosolids.

  • The emissions are tested annually and are well below the regulatory limits.
  • None of the other City of Toronto treatment plants use incineration.