Anaerobic treatment processing technology offers value and efficacy for cities that seek resilience in managing wastes and that wish to lead in resource stewardship.
Toronto’s new Disco Road green bin processing facility is North America’s first full-scale municipal source-separated organic (SSO) waste processing operation to use anaerobic digestion (AD). By processing organics with this technology, the city is now able to divert more waste from disposal, generate and collect biogas as a recoverable resource, and supply digested nutrient-rich material for commercial compost.
The plant is one of Toronto’s most significant new infrastructure assets.
The green organic waste bin has become as ubiquitous as its blue recycling counterpart in Toronto since its launch city-wide (for single-family homes collection) in 2002. The program has been a remarkable success; the city collected more than 100,000 tonnes of organics in 2012. Nearly 90 per cent of residents reported they regularly use their green bins, according to research from the Municipal Waste Association (MWA). This is one of the highest participation rates in the country.
The Disco Road organics facility expands and improves Toronto’s current SSO processing capacity.
Toronto’s work with anaerobic digestion technology began in 2002 when it opened the Dufferin green bin processing facility. Designed to serve as a technology testing site, the Dufferin plant was initially designed for a throughput capacity of 25,000 tonnes of organics per year. With tests demonstrating the viability of processing and digesting the city’s collected green bin material, Toronto announced plans in 2009 to build a new and larger anaerobic digestion plant at its existing Disco Road waste management property, located near the city’s north-west border.
The $74 million design-build-operate project involved AECOM as the prime contractor and designer, its joint-venture partner E.S. Fox as the constructor, and its sub-contractor Veolia Water as the operator. The facility’s anaerobic digestion system was supplied by CCI BioEnergy (the Canadian licensee of BTA International, a Germany-based firm holding the technology patent).
Commissioning is to commence in June 2013 and will continue until January 2014.
Anaerobic digestion’s established track record in the treatment of municipal wastewater made it an ideal technology to apply to Toronto’s organic waste. Drawing on lessons from the Dufferin plant and the prime contractor’s team (who worked on similar projects in Europe), the Disco Road plant was designed to adapt and optimise anaerobic digestion for the city’s needs.
Trucks deliver curbside-collected green bin material to the facility by trucks, depositing their loads onto a tip floor. From there, the organics are transferred into two hoppers via a front loader. The organics are then conveyed into one of three hydropulpers, and process water is added for batch processing.
The hydropulpers remove any green bin contaminants by separating them into a light fraction (e.g., plastic, wood and textiles) and a heavy fraction (e.g., bone, stone, glass, batteries, cutlery and other metals) and transforms the organics into a homogenized waste suspension. (The ability to separate these incidental materials supports wider public participation as residents can use plastic bags to collect and store their household organics.)
A grit removal system removes sand, glass splinters and other fine materials not removed in the hydropulpers. This step serves two purposes: contaminant removal safeguards the processing equipment downstream (i.e., digesters and pumps). It also ensures the final compost product meets the most stringent Category AA level (established under Ontario’s new guidelines for commercial compost).
The waste suspension is then transferred to a suspension buffer tank, which feeds two anaerobic digesters continuously (24/7). The digestate is mixed by re-injection of a portion of the produced digester gas back into the digesters via internal gas lances; external tube heat exchangers maintain the digesters at mesophilic temperatures (32 to 35 degree C). The digesters achieve 21 days hydraulic retention time when the facility is operating at 55,000 tonnes per year throughput capacity (and 14 days retention time for 75,000 tpy of throughput capacity).
The digester gas (or biogas) generated from the digesters is used in dual-fuel boilers within the facility (biogas and natural gas). Remaining biogas is flared off using a waste-gas burner.
Plans currently being reviewed by the city for consideration include upgrades to allow the conversion of the biogas into biomethane, which could be fed into the local gas distribution system.
The digested solids are then dewatered by two centrifuges. (The liquids collected from the centrifuges are captured for re-use in the facility’s operations.) The digested dewatered solids are loaded into waiting truck trailers for eventual composting.
Smart urban footprint
Besides its AD processing system, the Disco Road facility’s second innovation is its compact design and stringent odour and waste-water management systems; these allow organics to be safely and efficiently processed inside Toronto’s boundaries.
The facility is located approximately 30-ki-l-ome-tres northwest of the city’s downtown core, just east of Toronto Pearson International Airport. This location’s proximity to major arterial roads and inner city highways helps achieve lower transport times and fleet fuel usage. However, it also makes it necessary to eliminate potential environmental impacts on neighbours.
A team of AECOM geotechnical, wastewater, air management and environmental engineers (and other specialists) managed an integrated design approach to prepare the facility’s site and ensure its operations in accordance with the city’s needs.
The design incorporates the waste receiving and pre-treatment system, AD system, a wastewater treatment system, administration offices and mess facilities, as well as a surface-water storage pond and access road — all within a 10,000-square metre site. The location sits atop a former landfill site, which required incorporating a piled foundation and a passive landfill gas collection system to the site plan.
Controlling odours was critical for the city. The facility operates under a negative-pressure environment while fans draw odorous air from within to a high capacity bio-filter system consisting of six cells of inorganic media in a concrete vessel. Treated air is then dispersed via a 40 metre high stack.
As AD is water-intensive, a water re-use strategy minimizes the use of potable water. Wastewater is treated on-site and then re-enters the system (to a large extent to be used again for processing).
A range of quality grades was established to determine the water quality required for different processing steps. For example, the water used for the hydropulper batch process only requires the lowest quality grades; liquids are sourced from the facility’s tip floor sumps, press waters and floor drains as well as from the centrifuges after solid-liquid-separation.
To satisfy the demand for higher grade waters within the facility, other components and source sites within the plant are used. These provide a total of six separate grades of water, including a passive rainwater collection system.
Potable water is thereby limited to a small number o
f specialized functions, such as the facility’s odour control unit, the cleaning of sensitive monitoring instruments, and a back-up source for topping up buffer tanks. The system’s remaining wastewater is treated on-site before it’s sent to the municipal sewer system (meeting city bylaw discharge standards).
Building on a success
The Disco Road plant helps Toronto increase its processing capacity by up to 75,000 tonnes per year. But the city needs to further grow its capacity as it seeks to eventually expand its green bin program to condos and multi-unit apartments. With this goal in mind, Toronto is currently enlarging its Dufferin facility. When completed in 2016, the expanded Dufferin plant will complement Disco Road’s processing capacity by an additional 55,000 tonnes.
For additional details, visit www.toronto.ca/involved/projects/disco_greenbin/
Ian Dickenson is Director, Alternative Delivery, Water with AECOM in
Markham, Ontario. Contact Ian at