An in-vessel technology
In-vessel composting systems are fully contained units that aren’t exposed to ambient air; they typically consist of a box-like retention unit and have sophisticated air handling and moisture control systems. There’s a high degree of control over the retention period, typically 10-28 days (followed by compost curing).
Wright Environmental Management Inc. (WEMI) of Richmond Hill, Ontario manufactures in-vessel composting tunnels. The term “composting tunnel,” borrowed from the mushroom composting industry, hearkens back to the development of this Canadian technology by James Wright, an air handling specialist, and Bert Baillie, a mushroom composter. In 1989, the first unit for testing was installed in Powell River, British Columbia. In 1993, after further development, their flagship system was installed at the Ontario Science Centre in Toronto.
Two features make this technology unique: it’s one of the few composting systems with capable of small or large (up to 30 tonnes per day per tunnel) onsite models and it’s the only in-vessel technology that incorporates agitation within the composting process.
Organic waste — including leaf and yard waste, biosolids and food wastes — are mixed with bulking amendments such as wood chips (if necessary). A conveyor system takes this mixture into a series of transporter trays that form the floor of the tunnel, supported above the air plenums on channels along each wall and along the centre of the tunnel. As one tray is pushed into position by a hydraulic ram, the trays move through the tunnel as a unit and the last tray in the tunnel is pushed out the exit.
The tunnel is divided into two zones. Spinners, located midway, create a void space between the zones to allow spray nozzles to add water during the mixing process. (See Figure 1.) Typically, composting material is retained in each zone for seven days. Material passing through the spinners is mixed to regain porosity and is re-moistened. At discharge the moisture content is around 50 per cent.
Exhaust fans continually maintain negative pressure so that odorous off-gases cannot escape untreated to the atmosphere. Each zone in the tunnel has a separate air supply fan that works on a control system.
In the first zone temperature set points are typically around 58C; pathogen reduction activity takes place in this zone. In the second, the set point is held at around 53C. According to regulations, the material must be at 55C or greater for three days or more. The separation of the two zones ensures that there is no migration of pathogens from the hotter area to the cooler area, thus avoiding recontamination.
Temperature ports along the side of each tunnel are equipped with controllers that send information back to the control panel. The panel contains a programmable logic controller (PLC) that monitors temperatures and turns on supply fans as required.
Maintaining the aforementioned temperature set-points also maintains oxygen levels within the tunnel at 18 per cent and ensures aerobic conditions for the microorganisms. These measurements are made with temperature probe ports situated at the midpoint of the material depth. The ports are about 1.25 metres above the level of the trays and oxygen probes are inserted about one metre into the mass.
After the main composting phase, curing takes place outside the tunnel and typically lasts 21 to 28 days.
Almost 50 composting tunnels are installed across North America and the UK at onsite and central composting facilities. There are approximately 40 onsite composting tunnels — ranging in capacity from 135 kilograms to 10 tonnes per day (tpd) — at a number of different locations: federal, provincial, state and municipal government facilities; institutions such as hospitals and universities; military bases; and, private sector organizations such as waste haulage firms.
In Canada there are 30 onsite tunnels. Two installations are located at the Canadian Air Force Base at Trenton, Ontario. These are a 2,000 pound per day tunnel and a 1,500 pound per day tunnel, when loaded at 28 day retention (capacity can double at 14 day retention). Food waste is separated at three mess halls. About one tonne of organic waste is loaded into the tunnel each day during winter months. During this season, the compost material has a residence time of 28 days. When the population increases over the summer months the quantity of organic waste doubles to two tonnes per day and the tunnel operates at 14-day retention. After windrow curing, the compost is screened and used for landscaping and as a greenhouse growing medium.
In Meaford, Ontario a 0.5 tpd tunnel has been in operation since 1998. Source separated organic waste –about 140 tonnes per year — is collected in plastic bags from 2,400 homes. Meaford has won “Communities in Bloom” awards in the years since the installation, receiving special kudos for the use of their high quality compost in gardens and parks.
There are also a number of larger centralized composting facilities. A facility that uses two 25 tpd tunnels is operated in conjunction with an energy-from-waste facility by Capital Compost in Albany, New York. Commercial customers dispose high proportions of food, yard and paper waste at the composting facility. Finished compost is then marketed to local growers.
It’s apparent that the composting tunnels can be used in a variety of venues. A testament to this technology’s flexibility is work on a unit that may function on extraterrestrial site in the future. Wright Environmental is currently working with NASA’s life support team to develop and test composting equipment for use on the space station. This of course begs the question — does compost float?
Paul van der Werf is principal of 2cg Waste Management Consulting Services in Canada and Senior Consultant for Environment & Resource Management Ltd. in Ireland.