Toronto moves forward with anaerobic digestion of residential SSO

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Courtesy of BioCycle Magazine

In June 2007, Toronto's City Council approved the “Target 70” plan to achieve 70 percent diversion by 2010. Target 70 includes construction of two, 55,000 metric tons/year anaerobic digestion plants to process residential source separated organics (SSO). City Council says that the digester facilities, along with adoption of single stream recycling and a pay-as-you-throw funding system, will enable the city to meet its goal of 70 percent solid waste diversion - and extend the life of its Green Lane Landfill until about 2034.

City Council's commitment makes Toronto the first major city in North America to select anaerobic digestion (AD) as the first phase of treatment for organics in the municipal solid waste stream. The City has been operating a digester at its Dufferin transfer station since 2002, and since 2004 has been processing in excess of its design capacity of 25,000 metric tons/year. The remaining SSO collected has been composted at public and private facilities in Ontario and Quebec. In a report to City Council in advance of the June 2007 decision, Toronto's solid waste staff noted that “experience to date with contracted processing capacity teaches that disruptions and failures are common, and that the facilities are often unable to respond with adequate contingency measures.” This has led the City to sign a number of short-term SSO management contracts, which staff feels “is not sustainable in the long-term.”

The Dufferin plant utilizes the BTA anaerobic digester technology, which involves a wet preprocessing stage followed by digestion. Dewatered solids have been sent off-site for composting. Toronto allows residents to place its sorted organics - including all food waste, soiled paper, diapers and other items - in plastic bags, which are then set out at the curb in Norseman Plastics' green bins for collection. Those bags are opened in the BTA hydropulper unit, which uses a “rake” to skim plastic off of the surface on the tank. Heavier contaminants drop to the bottom. Additional removal of contaminants takes place in a hydrocyclone, prior to loading material into the digester.

The decision to site the new organics processing facilities within the City's borders supports selection of AD, due to its significantly smaller footprint than composting plants of similar scale. Notes the 2007 report to City Council, “the in-City sites can accommodate receiving, preprocessing and anaerobic digestion operation at capacities less than 110,000 metric tons/year. No in-city site can accommodate aerobic composting or compost curing or storage operations. … The remaining processing operations - aerobic composting, curing and storage - would be completed at a secondary facility located at an external site, or elsewhere.”

PROJECT TIMEFRAME
One of the new AD plants will be located at the existing Dufferin Organics Processing Facility. The other will be constructed at the Disco Transfer Station. The city is committed to having the Disco plant operational by January 2011. The Dufferin plant would come on line about 12 months later.

The consulting firm of Conestoga-Rovers & Associates was awarded the contract to prepare and issue the Requests for Proposals (RFP) for both facilities. The RFP for the Disco site is expected to be issued this fall. The RFP for Dufferin is due out in the spring of 2009. The RFP will reflect the city's desire to utilize wet separation for preprocessing, followed by anaerobic digestion.

Estimated capital costs for the two new primary processing facilities for SSO is about $69 million (Cdn), equivalent to $54/metric ton. Estimated operating costs for the two plants is about $10 million/year (Cdn), or about $91/metric ton, which includes estimated revenue from electricity sales of $23/metric ton.

Brian Van Opstal, Acting Manager of Operational Planning for the City of Toronto's Solid Waste Management Services, will be speaking at the 8th Annual BioCycle Conference on Renewable Energy From Organics Recycling on Monday, October 6th. He will discuss the new anaerobic digestion program in Toronto, and address how experiences at the existing Dufferin AD plant will be reflected in design and operation of the new facilities. - N.G.

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