Some municipalities and private companies are looking at additional options for managing biosolids, including using them as a fuel to produce electricity. This option has been encouraged by Ontario’s commitment to developing new, renewable sources of electricity generation through Ontario’s Green Energy Act.
Sewage biosolids and pulp and paper biosolids are defined as “biomass” under the Electricity Act and are permitted for use in anaerobic digestion and thermal treatment facilities under the Environmental Protection Act.
Anaerobic digestion
Anaerobic digestion is a biological process carried out in an enclosed vessel in the absence of oxygen. Anaerobic digestion is commonly used to stabilize sewage sludge.
A wide array of micro-organisms are involved in the stabilization process which results in the conversion of volatile solids to volatile organic acids and the production of biogas (consisting primarily of methane and carbon dioxide). Depending on the system design, biogas can be:
- collected and combusted to run a generator to produce electricity and/or heat
- cleaned and used as a natural gas replacement
Anaerobically digested material has a reduced pathogen content and odour and can be beneficially used on land.
Diagram showing anaerobic digestion inputs, generation unit and end uses, such as energy, heat, compost product or fertilizer.
Thermal treatment
Thermal treatment is an alternative method for managing biosolids and producing energy. In addition to traditional combustion technologies, such as fluidized bed combustion, another thermal treatment technology that may have application for biosolids is pyrolysis. Any thermal treatment technology is acceptable provided it can meet the Ministry of the Environment’s approval and air quality standards.
Thermal treatment involves the addition of heat through the thermal conversion of biomass to produce an energy-rich gas called syngas as well as solid and liquid residues. The syngas is then burned to produce energy.
