In-Vessel Composting of Residential Organics

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Courtesy of BioCycle Magazine

In 1994, the Regional Municipality of Peel began work on a system for residential organic residuals collection and composting. Although Peel had been promoting backyard composting, greater diversion rates were sought. The public was surveyed to determine what level of cooperation could be expected for source separation and setting out of food, soiled paper products, yard trimmings, etc. “We knew that some people would not cooperate because they resent the fact that the government is telling them what to do,” recalls Paul Della Bianca, Peel’s supervisor of landfill operations. “But the vast majority of people said they would participate, especially if they were given their collection container for free.”

Peel’s Regional Council and staff toured Europe looking for a composting system that could deliver simple operation, reduced odors and protection from weather conditions. Peel chose to purchase and install two in-vessel Herhof Biocell Composters at its Caledon Sanitary Landfill Site near Brampton, Ontario. (See “Modular Management Of Residential Organics,” February, 1996.) Herhof Biocell Composters are used extensively in Germany. Peel is the first site in North America to use this technology. The program serves the Town of Caledon in the Region of Peel, which includes 13 communities and hamlets, diverting 99 percent of the yard trimmings and 25 to 30 percent of its residential organics.


The equipment, facility construction and public outreach cost $1,117,000, with the Ontario Ministry of Environment contributing $239,210. Purchases included a shredder, front-end loader, screen, baler and conveyors. The process included a public hearing; however, an environmental assessment was not necessary as the location was adjacent to the existing landfill.

With such a significant investment, an extensive public education campaign was launched to ensure that the separation and collection system would work effectively. “It took a team of people to knock on doors and explain the upcoming system and its objectives,” says Della Bianca. “Literature with the collection schedule and composting tips soon followed. We had to return to the same households three times: once to explain the program, a second time to deliver the cart, and a third time to check out any problems and offer encouragement. All in all, we were received with a positive response.”

Since organics collection began in December, 1994, two additional biocells have been added to increase composting capacity. To receive a Certificate of Approval for expansion, the facility underwent an extensive study by the University of Guelph that monitored the feedstocks, compost quality and odor emissions. The conclusions were positive, with heavy metal concentrations of the incoming and outgoing material well within Ministry of Environment guidelines. Odor problems were of little concern, and approval was granted in November, 1995. The expansion was completed in 1996 at a cost of $1,895,000.

About 7,500 of the approximately 10,000 households have received a $100 wheeled, plastic Schaffer cart for free. “The lightweight ventilated carts were chosen for their convenience, both to transport materials as well as to clean after use. They have a life expectancy of ten years, so we’re investing about $10/year, which isn’t so bad,” explains Della Bianca. Yard trimmings that do not fit in the cart can be bundled up and set beside it for pick up. The residents excluded from service are in the more remote rural areas. While some participants already place organics in backyard composting bins — which Peel encourages them to continue doing — they use the carts for excess volume and items they prefer not to compost themselves, such as meat and dairy products.

“Presently, using over 56 percent of our capacity of 6,000 (metric) tons per year, the municipality is collecting 3,360 (metric) tons of organic kitchen and yard waste from the community,” says Della Bianca. From 25 to 30 percent of the food is generated in the home. “With Canada’s climate changes, the feedstock from residents changes dramatically from spring to winter. In April and May, we receive a lot of grass clippings, which we are moving towards banning from landfill in 2001. In the fall, it is mostly dry leaves and brush. With the exception of Christmas trees, the winter months bring us little or no yard waste, so we rely on a supply of collected yard waste which we keep on hand (for amendment).”


Picked up biweekly, the organics are delivered to the composting site, where the trucks are weighed and then directed to the processing building. The materials are unloaded directly onto the floor space for visual inspection of potential contamination. They are then combined with additional yard trimmings loads and conveyed under magnets to a Banner Jenz AZ50 slow-speed shredder, which achieves particle sizes of less than 75mm. Materials are continuously loaded into the biocell composters with a front-end loader.

Located inside a 33-by-20-meter building, the biocell reactors are insulated, reinforced concrete boxes measuring eight by four by three meters. In Peel’s experience, they each can process about 1,500 metric tons/year. They each have a 60-cubic meter capacity and are equipped with a stainless steel hermetically sealed door and floor plates. The seal and insulation prevent interference from outside temperature, humidity, etc. The units also are completely computer controlled with a 15-minute interval record and continuous readout.

 In a typical batch process, materials stay in the composting units for an average of seven days, including an initial warming stage, several days at 45° to 55°C, and a few more days at 60°C for pathogen control. Air is forced into and circulated through the box from a series of holes within the floor. It is recycled continually until the carbon dioxide concentration in the reactor reaches four percent by mass, which triggers release of the reactor air and introduction of fresh air.


Exhaust air is vented through a biofiltration system that consists of three stacked units laid out at one by 1.2 meters in cross section, with each level .66 meters high. The three sections of the filter contain .3 meters each of cured compost, wood chips and bark. Exhaust from the biofilter exits through a stack.

When airflow falls below 2,200 m3/hour of air, the system indicates that the biofilter media must be changed, which generally happens every six months. “When there’s too much moisture, the particles swell up and prevent air from coming in,” says Della Bianca. “The easiest way we found to detect when to change the biofilter is to monitor the air that flows through.”

In eight experiments done over about one year in 1996, Lambert Otten and Lindsay Dennison of the University of Guelph concluded that there was very little risk of odor problems from this facility. In all but one instance, the concentrations of dimethyl disulfide (DMDS) were found to be below the threshold of human odor detection before being released from the exhaust stack. The detection threshold of dimethyl sulfide (DMS) is much lower than that for DMDS; however, on a few occasions the measured concentrations of DMS were above the threshold value. It was believed that after release from the stack, atmospheric dilution would reduce the risk of nuisance odors.

Leachate is drained from the bottom of the biocells and recirculated by being sprayed onto the top of the compost. Excess moisture is evaporated by the heat of the compost and exits with the exhaust air. The ideal moisture level within the composting units is 45 to 60 percent.

A three-day cooling and drying stage follows composting, during which the reactors are continually flushed with ambient air. Following that step, the compost is removed and placed in windrows for 35 days of curing. The piles rest on about two meters of clay soil at the peak of closed cells at the landfill so that leachate filters through the garbage instead of the ground, says Larry Conrad, manager of landfill operations for Peel. Weather permitting, the windrows are turned weekly.

The compost then passes through a Powerscreen 615 trommel with a half-inch screen to separate the remaining larger woody material and any other contaminants that escaped earlier detection. Overs are composted again and inerts are used for daily landfill cover. After about one week of additional curing, the mature compost is tested to ensure that it meets Ministry of Environment guidelines.

While a small amount of compost is distributed free of charge to residents during special events, the vast majority is sold for $35/metric ton at the Caledon and Britannia Sanitary landfill sites. Demand from nurseries, soil blenders, residents and other markets exceeds supply, notes Conrad.

Because most of Peel is farmland, and crop field variability is a major concern for Ontario farmers, a project was initiated to investigate potential uses of the compost. Typically, farmers treat yield problems by increasing the application of synthetic fertilizers. The four-year study is testing the agronomic and environmental effects of the compost on agricultural fields using variable rate technology, and measuring the effects on plant development and crop yields over three growing seasons. The conclusion from the first full growing season of the collaborative research agreement (among Peel, Agriculture and Agi-Food Canada, and HUWS Corp., the North American manufacturer of the Herhof system) was considered a success and the project continues.


In 1999, Peel spent $3.8 million to purchase four more biocells, support equipment and a facility to produce “Stabilate,” or refuse derived fuel (RDF), from municipal solid waste, as part of a pilot project. Garbage collected from Peel residents is shredded down to two to four inches, then placed in the biocells. After about six days of processing, moisture decreases to 15 percent. Next, the material is run through a screen to remove metals. The RDF is then baled, shrink wrapped and stored in preparation for shipping.

The RDF plant is in the start-up phase; full-scale operation is expected next year. “The impetus for this was to further reduce what is going to the landfill,” says Conrad. “There are about ten years left in the landfill without the RDF program. We hope to have somewhere around 70 to 80 percent reduction in the tonnage going to landfill.” No markets for the material have been established yet. The RDF can be used by facilities that burn soft coal, such as concrete kilns or waste-to-energy plants. “We can store our material for a year or two,” adds Conrad. “Part of our problem is that the average concrete plant in North America probably burns in one morning what we produce in a year. Down the road, I think we’ll find a paying customer who recognizes its value.”

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