Frequently Asked Questions

If you have any questions about this topic please email Ryan Graunke at reg1214@ufl.edu

Why divert food waste?

There are many important reasons why food should not be wasted. Growing food to sustain our society requires a huge input from the environment. Water, fertilizer, land, and labor inputs for agriculture (especially industrialized agriculture) can be extremely taxing on the environment. All of these inputs are embodied in the food we eat and throw away. By not wasting this food and reusing this food for beneficial end products, we are able to recover some of these inputs that would otherwise be sent to landfills. Once food waste is in a landfill, it does not disappear. The same process that occurs in an anaerobic digester occurs in a landfill and methane is produced from organic material. However, most of this methane is not captured for energy, and about half of the methane generated in the landfill escapes to the atmosphere and is 25 times stronger a greenhouse gas than carbon dioxide (Source: US EPA). A major source of this organic material in landfills is food waste, and by diverting food waste, methane emissions can be significantly reduced. In addition to methane, food waste in landfills can cause nutrient problems in leachate and odor and vermin problems, along with taking up unnecessary space in the landfill. Hauling and landfilling food waste represents a direct cost to businesses and many external costs to society with no economic benefits. Through diverting food waste from landfills to anaerobic digestion, two beneficial products are produced. Biogas can be used on-site to remediate energy costs or sold to the grid. Biofertilizer can be sold to farmers and gardens or used on-site for gardens and landscaping. Food waste will no longer be thought of as a waste, but a resource to generate additional revenue. Food is one of the most essential elements for life, and therefore we should have an ethical and moral obligation not to waste it. With all the issues of unsustainable agriculture and global hunger, we should not be needlessly throwing good food into the landfill. Therefore we must simultaneously reduce the food waste that is produced while putting to good use that food that is not consumed. In addition to these reasons, Florida has also passed a 75% recycling by 2020 goal. In order to meet this goal, we must find creative and sustainable alternative for waste disposal. Food waste and organics represents a significantly proportion of our organic waste stream, and by diverting this material, we help to make this goal a reality.

How much food waste is there?

The Florida DEP reports the 1.7 million tons of food waste were generated in 2008, with only a 1% reported recycling rate. This represents 8% of the total unrecycled municipal solid waste in Florida.

What is anaerobic digestion?

Anaerobic digestion is the microbial degradation of organic matter under anaerobic (without oxygen) conditions. It is a process that is ubiquitous throughout nature from lake bottoms to ruminant animals to hydric soils. The process can be captured in a controlled, man-made environment for treatment of organic wastes, such as manure, sewage sludge, crop residues, and food waste.

What are the products?

There are two primary products of anaerobic digestion: biogas and biofertilizer. Biogas is composed of methane and carbon dioxide. The gas is a natural metabolic byproduct of the microbial metabolism. Biofertilizer is the effluent from the digester. Because only the carbon is removed in the biogas, all of the nutrients found in the feedstock are conserved in the effluent. The process mineralizes these nutrients into plant available forms, so they can be a substitute for fossil-fuel derived, synthetic fertilizer.

Are there any waste products?

Because the effluent can be used as a biofertilizer, there are no inherent waste products. Just as in nature, everything is reused during anaerobic digestion.

What are the uses of biogas?

Biogas is approximately 60-80% methane with the remainder carbon dioxide. It can be readily combusted for cooking or heating. By cleaning the biogas and removing the carbon dioxide, it can be used as a direct replacement for natural gas (e.g. for electricity, pipeline injection, and natural gas powered vehicles).

What are the uses of biofertilizer?

Since the effluent is a liquid, it can be used in existing fertigation systems on-farms. Additionally it could be bottled and sold on the consumer market. This will help to bring additional revenue for anaerobic digestion. The biofertilizer could also be integrated into compost systems with high carbon material. Compost needs a carbon-to-nitrogen(C:N) ratio of around 30:1. The high nitrogen biofertilizer could be mixed with high carbon materials (i.e. paper and woody wastes) to expedite the conversion of these materials into compost.

What are the inputs for anaerobic digestion? Is any energy required?

Other than the infrastructure and the feedstock, no other inputs are required for anaerobic digestion. The process is entirely microbially driven and utilizes the microbes’ energy to convert waste into usable products. Energy external to the process itself (i.e. hauling, mixing, pumping) may be necessary, but is dependent upon the configuration of the system.

How much biogas can food waste produce?

Food waste is a widely diverse feedstock. The amount of biogas that can be generated from food waste is dependent on moisture content and composition. Food waste with a low moisture content will generate more biogas than food waste with a high moisture content. Fats and proteins also generate more biogas than carbohydrates.

Does food waste need to be pretreated for digestion?

While anaerobic digestion is possible on whole food waste, physical pretreatment helps to reduce the particle size of the material. This allows the microbes to have better access to the food and results in a faster overall process. An ideal pretreatment method would be to run the food waste through an in-sink food disposal, which is already on-site at many restaurants, dining halls, and grocery stores.

How long does the process take?

The time required for digestion is dependent on both the composition of the food waste and the configuration and operating conditions of the anaerobic digester.

Can meat, oils, and bones be fed to a digester?

Yes, unlike backyard composting, meats, oils and bones are welcome additions to anaerobic digesters. In fact meats and oils have a much higher methane production potential that vegetables and carbohydrates. Bones, if properly ground, can provide pH buffering to the digester and are a source of calcium (a plant micronutrient) in the effluent.