"Biosolids" is the term we use for digested sewage solids. They are a nutrient-rich, organic material. The users of the Toronto wastewater system generate about 195,000 tonnes of biosolids every year (about 72% of this is water).
The wastewater you send to us from your toilets, drains and showers travels through the City's 9,000 kilometres of underground pipes to one of our four wastewater treatment plants. At the plants, wastewater is cleaned to remove solids, chemicals and other undesirable material before it is released to Lake Ontario.
In Toronto, the first stages of wastewater treatment (PDF) allow the solids in the sewage to settle. The settled solids are sent to enclosed tanks called "digesters". There, microorganisms consume the solids as food. Their digestion process takes the organic solids and produces carbon dioxide, ammonia and methane. The methane is collected and some is used as boiler fuel to heat the sewage plant.
Because most of the microbial "food" is consumed by microorganisms in the digesters, the solids are then less likely to support further microbial activity. This is why we refer to them as "stabilized".
Toronto's biosolids are all treated at the Ashbridges Bay Treatment Plant (78%) and the Highland Creek Treatment Plant (the other 22%). Throughout the world, biosolids are returned to the environment in a variety of ways. Some methods require little or no processing, while others use sophisticated technology. Although the City continually pursues beneficial reuse options, we currently use a combination of methods to manage biosolids. The table below shows where our biosolids went in 2010:
Biosolids Reuse/Disposal Methods
|wet tonnes||% total|
|Incineration at Highland Creek||38243||22%|
A pelletizer facility began commissioning in the summer of 2007 at Ashbridges Bay, the City's largest treatment plant. The facility is planned to process about half of the biosolids treated at Ashbridges Bay.
The City's Biosolids and Residuals Master Plan (BRMP) will provide direction on the future management of biosolids and water residuals produced by the City's water and wastewater treatment plants to the year 2025. The plan will ensure that the City's management of its biosolids and water residuals is cost efficient, environmentally sound and sustainable.