Municipalities in Ontario currently operate more than 300 mechanical sewage treatment plants. These plants separate the solid residues (sludge) from the liquid portion of the wastewater. The liquid portion is typically clarified and is then disinfected before being discharged into a nearby watercourse. The sewage sludge undergoes further biological and physical-chemical treatment and is stabilized through anaerobic or aerobic digestion processes to reduce volume, pathogens and odour. After treatment, nutrient-rich sewage biosolids remain.
Sewage is delivered to the wastewater treatment plant via sewers. The sewage undergoes primary treatment, where liquids are passed into the secondary treatment chamber prior to undergoing clarification (i.e. liquid and solids separation) followed by disinfection (e.g., chlorination, ultraviolet disinfection) before being released into the receiving water body. In the primary treatment chamber, solids settle to the bottom and typically enter a digestion unit for further treatment prior to transportation or storage.
Aerobically digested sewage sludge is digested in the presence of oxygen and anaerobically digested sewage sludge is digested without the presence of oxygen. Both processes involve digestion by micro-organisms to convert the organic matter into volatile organic compounds, carbon dioxide and water (aerobic) or methane (anaerobic). The processes greatly reduce pathogens and odour while further breaking down the solid residues into liquid and gas. The digestion process treats the sewage sludge so that it is able to meet the standards required for biosolids.