Vegetable Waste Management - an Energy Opportunity
Anywhere from 5% to 75% of the fruit and vegetables that are processed in Ontario contribute to an estimated 200,000 tonnes of an under-utilized energy resource. Some of this ends up as animal feed and some is returned to the land as a nutrient. Yet locked in each tonne of vegetable matter is a potential energy source - methane. According to the Ontario Centre for Environmental Technology Advancement, up to 50% of vegetable matter could be potentially converted to this fuel. Realistically, we should expect a lower energy yield, but in an era where we have seen electricity and energy prices increase two- to fourfold since 1998, the competitive advantage of generating methane from by-product may benefit both growers and processors.
It is expected that the cost of energy and waste management associated with the processing of a tonne of fruit or vegetables will rise from around $5 per tonne to over $15 per tonne between 1998 and 2007. The anaerobic digestion of vegetable by-products has the potential to produce both energy (methane) and heat. Moreover, the use of anaerobically digested matter makes agronomic sense. Pound for pound, digestate has a higher nutrient value, is 99% free of bacteria and has a very low odour potential compar to raw vegetables.
Anaerobic digestion (AD) is completed by heating materials to a temperature between 35 to 55 degrees C in an oxygen free environment. Typical average retention time is 20 days (however may be higher if energy crops such as corn silage are added to the inputs to increase methane production). Most agricultural AD systems will use manure as a primary component and add materials such as vegetable waste. The manure is a consistent stabilizing input allowing a changing vegetable input over the harvest season.
In Denmark, food wastes are a much sought-after additive for manure-based anaerobic digestion facilities. In fact, Danish digestion projects actively seek this resource. And based on European anaerobic digestion projects, Ontario's fruit and vegetable byproducts may hold more than 8 million cubic meters of methane that could produce 35 million kilowatt hours of electricity and 28 million kilowatt hours of heat. That's about a $6 million a year resource that industry may soon spend over $4 million a year to manage as waste. Added together, there may be a $10 million potential benefit for companies and growers that is consistent with resource stewardship, the need for disease control and reducing the odours we currently associated with today's farm and processing practices.
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