From wastewater to fertilizer: Waste recovery

Last updated May 3, 2024
Reading time: 1 min

The city produces an average of 47,000 tonnes of ash each year through wastewater treatment. In 2022, 26% of this ash was used as agricultural fertilizer. Our goal is to increase this number to 80% by 2030.

How is sewage sludge ash (SSA) produced?  

Montréal’s wastewater is piped through the sewer system to the Jean-R.-Marcotte wastewater treatment plant, located in the far east of the island. Wastewater is decanted in huge ponds to remove organic matter, called sludge. The sludge is removed and pressed as much as possible to extract the water. At the end of the process, the compacted sludge is placed in an incinerator, becoming what is known as sewage sludge ash, or SSA.  

Combustion of sludge at 840°C removes all organic matter, microorganisms and residues of hormones and drugs. Preliminary data from the most recent studies also suggest the absence of perfluorinated compounds (PFAS) in SSA.

Perfluorinated compounds, or perfluoroalkylated and polyfluoroalkylated substances, are highly stable chemical compounds that persist in the environment for a long time. Unfortunately, they are found in our wastewater because they are present in many everyday consumer products.

What do we do with SSA?

Sewage sludge ash is branded under the name “Fertili Cendres” (Ash Fertilizer). Fertili Cendres, which is certified by the Canadian Food Inspection Agency, is considered safe for growing plants for animal consumption (such as maize for livestock), especially since its quality is monitored. Montréal hires firms that specialize in waste fertilizer management to distribute sewage sludge ash from its wastewater treatment plant. SSA is mostly spread on farmland as a replacement for chemical fertilizers. 

SSA that is not used as fertilizer is sent to a landfill.

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