BioCycle August 2005, Vol. 46, No. 8, p. 42
Two facilities in Ontario are processing residential organics from municipal three-stream sort programs using anaerobic digestion technologies.
BY THE END of 2005, all single family households – about 500,000 – in the city of Toronto, Ontario will be receiving curbside collection of source separated organic (SSO) materials. This represents about 105,000 metric tons/year of food scraps, soiled paper, disposable diapers, pet waste and other biodegradable residuals. Another 15,000 metric tons are being diverted from small commercial establishments serviced by the city’s collection program.
Organics diversion is part of a three-stream solid waste management strategy designed to help Toronto achieve Ontario’s goal of 60 percent waste diversion by 2008. As neighborhoods are added to the collection program, households receive a 12-gallon Norseman green wheeled cart (the “Green Bin”) that is set out weekly. Commingled recyclables and trash are cocollected with the Green Bins on alternating weeks. Split body trucks are used to collect materials.
Two single-stream materials recovery facilities (MRFs) service Toronto’s curbside recycling collection. Trash is long-hauled to Michigan for landfill disposal. The city solid waste department does not impose any penalties on residents who don’t use the green cart, however with trash collection every other week, “if they don’t participate, they won’t like keeping their trash around that long,” says Anne Wheatley, Supervisor of Organics Processing Operations for the city of Toronto.
Residents are allowed to put SSO in plastic bags before placing materials into the carts. In a focus group prior to starting the SSO collection roll-out, Toronto residents indicated they would be more apt to participate if they could use plastic bags. Collected organics are processed at a variety of locations, including the city’s Dufferin Organics Processing Facility; Halton Recycling’s plant in Newmarket, Ontario; three composting sites in Quebec (Conporec, Fertival and GSI, all about an 8-hour trip one-way); and the city of Guelph, Ontario’s cocomposting plant. Both the Dufferin facility and the Halton Recycling plant utilize anaerobic digestion technology supplied by BTA in Germany. Despite the similarity in equipment, the two plants are operated in different manners: Dufferin digests separated solids whereas Halton Recycling captures methane from the liquid fraction. To our knowledge, these are the only two facilities in North America processing (at full-scale) segregated municipal solid waste using anaerobic digestion technology.
DUFFERIN AD PLANT
About three years ago, a demonstration-scale anaerobic digestion (AD) plant began operating at Toronto’s Dufferin solid waste transfer station. The Dufferin Organics Processing Facility, with 25,000 metric tons/year of throughput capacity, uses the BTA wet mesophilic anaerobic digestion technology. It is operated under contract by Canada Composting, Inc. A trommel screen and MRF-type sorting system were installed adjacent to the tipping floor to open plastic bags and sort contaminants. In addition to opening bags, the trommel separated materials into three sizes – 2.5 inches, 10-inches and overs. The 10-inch and overs fractions were conveyed to a sorting line and passed through magnetic and eddy current separators. The 2.5 inch minus fraction and the sorted materials were loaded into the hydropulper, the first stage of the AD system.
While the plant originally operated in this fashion, the city no longer uses the trommel or the sorting station. “The preprocessing system was designed to positively sort recyclable materials and organics from a mixed waste stream, however in 2001, we decided in favor of source separation of organics versus mixed waste processing,” explains Brian Van Opstal, Senior Engineer in the Solid Waste Management Services office of the city of Toronto. “The quantity of recyclable materials in the SSO stream is too small to be economically recovered. The preprocessing system does enable efficient removal of film plastics or other contaminants as they would have to be manually removed. Therefore preprocessing adds cost but no benefit and is therefore not operated beyond the times when the operator runs the trommel and conveyors to move material to the hydropulper.”
The facility processes an average of 100 metric tons/day of SSO – about 120 metric tons/day in the summer and 90 metric tons/day in the winter. In the current operating mode, about 10 tons/batch of SSO are loaded into the hydropulper. It is filled with water recycled from the previous batch. An agitator mounted at the top of the tank spins very quickly, breaking open the plastic bags and releasing and pulping the contents. Once the pulping is complete, a rake-like unit passes through the liquid to remove plastics and other floating light fraction materials. A trap in the bottom of the pulper captures heavy objects such as glass, coins and silverware. “We operate the pulper with the goal of opening the plastic bags but not shredding them,” Wheatley explains. “There is some shredding, however.”
The pulp moves from the hydropulper into a fiberglass surge tank, which acts as a holding tank between the batch operation of the pulper and the continuous feed mode of the digester. A second contaminant removal system, known as the hydrocyclone, works in tandem with the surge tank. It removes very small pieces of heavy fraction material, such as pieces of glass that are smaller than a grain of rice, collectively referred to as grit. Pulp from the surge tank is cycled through the hydrocyclone and then returned to the surge tank in a continuously operating closed loop. The tank has two compartments – one for clean slurry and the other for gritty slurry. “Out of the 100 metric tons/day going in, we get about one metric ton/day of grit,” says Wheatley. “We use the hydrocyclone to keep the material going into the digester as ‘bug-friendly’ as possible.”
Overall processing time – from when SSO is put into the pulper until the batch enters the surge tank and passes through the hydrocyclone – is 70 minutes on average. “Having materials in plastic bags really adds to the processing time, and energy,” says Wheatley, who estimates that it costs $130 (Cdn)/metric ton to operate the plant.
DEWATERING AND DIGESTION
Operating hours of the Dufferin facility are Monday through Friday, from 6 a.m. to 10 p.m. Pulped organics with a solids content of six percent are fed into the digester from 7 a.m. to 8 p.m. Optimum temperature of the infeed material is 37°C (98.6 F). During colder months, a heat exchanger is used to maintain the optimum temperature.
Retention time in the digester, a glass-lined steel tank, is two weeks. The tank has a working volume of 3,000 m3. Material is continuously withdrawn and a screw press is used to separate the liquids and solids. Liquids are recycled back to the hydropulper. The dewatered digestate, at about 25 to 30 percent solids, is transported to All-Treat Farms in Arthur, Ontario for composting. The hauling and tip fee are included in the $130/metric Overall, the estimated mass balance of the Dufferin organics processing facility, based on 100 metric tons/day, is as follows: 50 percent methane and effluent (which goes to a wastewater treatment plant); 25 percent digestate; and 25 percent residue (which is Because the digester is not fed on a 24 hour, seven day a week schedule, methane gas production varies. This makes energy recovery difficult, as a cogeneration plant – which Toronto has considered installing – needs a steady feed of biogas. The economics of such a plant have yet to be determined, notes Wheatley. “Another question we had when starting out three years ago was what the quantity and quality of the biogas would The digester generates about 110 m3 of biogas per metric ton of SSO processed, standardized to 0°C and 1 atm (1 atmosphere, a standard engineering unit for gas pressure), notes Van Opstal. On average, the biogas is 56 percent methane by volume. “These results compare favorably with published results of laboratory studies and operations at similar full-scale facilities,” he says. “The quantity and quality of biogas that result from the digestion of Toronto’s SSO streams seems to be such that, for a larger-scale operation, a biogas energy recovery system should be an economically viable option.”
He adds that one minor technical challenge to overcome is reducing the daily and weekly variation in the biogas methane content, which ranges from a high of 73 percent to a low of 45 percent. The variation seems to be directly related to the weekly pattern of intermittent feeding during plant operating hours. “Continuous feeding should stabilize the biogas methane content at near the reported average value,” says Van Opstal.
The Dufferin site, which is in a commercial office park setting, has a site neighbor not far from the boundary line and in the direction of the prevailing wind. A biofilter was installed to treat air from the processing building. In addition, the city installed fast-acting doors at the tipping floor entrance to minimize release of odorous air. “The doors do a pretty good job,” says Wheatley, noting that the facility receives about a half-dozen serious odor complaints annually.
With the single-family household collection program almost completely rolled out, the City of Toronto is moving to the other half-million households living in multifamily buildings. It is running pilots in about 30 different buildings, including high-rises and low-rises. A variety of collection options are being tested, including carts, Molok units (see “Toronto Takes Three Streams To Different Levels,” May 2005) and bulk lift containers like the ones used in apartments for garbage, but modified to hold organics. It is estimated that another 30,000 to 40,000 metric tons/year could be diverted from multifamily households.
A previous request for proposals, which resulted in contracts with private processors including Halton Recycling and others, took care of 70,000 metric tons/year of SSO currently diverted. “The city of Toronto is developing a plan for an expanded public source separated organics processing system for the total quantity of SSO material in excess of the 70,000 metric tons/year,” says Van Opstal. “The plan will address the future of the Dufferin Organics Processing Facility.”
Options being considered are limited to: Expansion to 50,000 metric tons/year using the same AD technology with a biogas energy system and improvements to the materials receiving area and the odor control system; Continued operation at 25,000 metric tons/year with a biogas energy system and improvements in the materials receiving area and odor control; or Decommissioning processing operations with possible use of the building for SSO transfer. “Options for the Dufferin facility are limited to those three because of site area constraints – insufficient area for compost curing or storage – and out of a desire to limit the risk of off-site odors,” he explains.
After three years of operation, Wheatley is fairly positive about the use of anaerobic digestion technology to process source separated organics from the MSW stream. “Plant operations are expensive but this facility has worked since it was commissioned. It recovers most of the good material from the incoming SSO and generates a decent product that can be composted. In short, this AD system does what it is supposed to do in the way it is supposed to be running.”
ENERGY RECOVERY, COMPOSTING IN NEWMARKET
The organics processing facility in Newmarket, Ontario, just north of Toronto, was built in 2000 by Canada Composting Inc., which also has the license to market the BTA anaerobic digestion technology in North America. The plant is designed for an annual throughput of 120,000 metric tons. Canada Composting opened the facility to service municipal collection contracts and institutional, commercial and industrial generators.
In 2004, the facility was acquired by Halton Recycling Ltd. The company retrofitted equipment and installed a composting plant on-site. (Canada Composting shipped dewatered pulp to an off-site processor.) The facility is processing about 100 to 150 metric tons/day of source separated organics from Green Bin programs in Toronto and the York Region, which also has been rolling out three-stream collection programs similar to Toronto’s (see “Residential Organics Diversion Moves Forward In Ontario,” September 2004). It has the capacity to process 400 metric tons/day. (The city of Toronto signed a contract with Halton Recycling in 2004 to receive up to 77,000 tons/year of Toronto’s SSO stream.)
Unlike the Dufferin installation, Canada Composting did not install a sorting line after the trommel screen. Otherwise, the two plants operate in a similar manner for the pulping and hydrocyclone stages.
Trucks unload on the tipping floor and are loaded onto a conveyor that feeds a trommel screen. Recently, a second trommel was installed that has 4-inch and 6-inch holes. The original trommel has only 6-inch holes. The new trommel has “knives” on the inside to open the bags, which facilitates screening out contaminants. “Basically, we use the trommels to remove large contaminants, although we are able to remove some plastic as well,” says Tom Darlow, Process Technologist with Halton Recycling. A magnet is located at the end of each trommel to remove metal.
The Newmarket plant has three hydro-pulpers, each with capacity of 32m3. The pulpers are filled with water recycled from dewatering at the end of the process, and the material is agitated to liquefy the organics. Similar to the Dufferin facility, a rake is used to remove plastic bags and other lightweight items from the surface of the pulper; heavy items are captured in the drain. The pulpers feed into a surge tank, which in turn feeds the hydrocyclone to remove grit. Darlow estimates that the entire upfront processing phase takes about 1.5 hours/batch.
METHANIZATION AND COMPOSTING
It is at this point in the process that operations at Newmarket and Dufferin diverge. After the surge tank and hydrocyclone, the liquefied organics are run through a screw press. The solids (35 percent) are fed into a Seko blender, where they are mixed with wood chips greater than a half-inch in size. Halton Recycling uses ground green waste, Christmas trees and other woody materials dropped off at its facility. The mixture is then conveyed to vertical silo composting cells supplied by VCU Technology, Ltd. There is a total of 20 cells, each 25 m3, or a total of 500 m3 of composting capacity on-site.
The cells are passively aerated. Each time material is fed from the blender into the top of the cell, an equivalent amount of material is unloaded via a drag chain conveyor from the bottom. “It is a 7-day cycle from when the material enters the system to when it is harvested,” says Darlow. “Right now, we are loading and harvesting two times a day, once per shift.” The Halton facility operates on two shifts, six days a week. The composted solids are transported to a soil blending facility where the material is cured and then sold in bulk and bags.
The liquid fraction (at about six percent solids on average) is fed into the bottom of the digester tank, which has 2,600m3 capacity. There are eight pipes that feed the bottom of the tank, which forces the liquid at the top to pour into a weir about two-thirds of the way up the tank. As the liquid falls, it releases the methane, which is captured and cleaned prior to being fed into the cogeneration equipment. “The optimal temperature of the liquid going into the tank is 37.5°C,” notes Darlow. “Extra heat from the generator is used to heat the liquid to keep it at the optimum temperature.”
Halton Recycling refers to the tank as a “methanizer,” because it is not digesting solids, explains Darlow. The methanizer operates 24 hours/day, seven days a week to maintain a steady state. Gas captured in the tank is about 65 percent methane, 34 percent CO2 and about one percent inert gases and moisture. When organics-laden liquid is not being fed in, process water from a storage tank inside the plant is piped to the methanizer. About 1,640 kW/hour of electricity can be generated by the Caterpillar cogeneration equipment and used to operate the plant. Any excess is sold to the local utility.
A 40,000 cfm fan draws air from the organics processing line and methanizer, which is treated in a biofilter. The biofilter is enclosed in a Coverall building. In addition to biofiltration, Halton Recycling is working in other areas of the plant to minimize the generation of odors.
While satisfied with overall operations, Darlow echoes Anne Wheatley’s view about the challenge of having plastics enter the organics processing system. “I think it takes twice as long in the system to process with plastic,” he says. “Ideally, we should not put these organic materials in plastic bags at all.”