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Biogas – A FAQ

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Why divert food waste from a landfill to a biogas digester?

There are many important reasons why food should not be wasted. Growing food to sustain a society requires a huge input from the environment. Water, fertilizer (organic or chemical), land, mechanical and labor inputs for agriculture. Industrialized agriculture uses a higher energy (fossil fuels) input than organic agriculture. Industrialized agriculture can be extremely taxing on the environment. All of these inputs are embodied in the food we eat and throw away everyday. By not wasting food and reusing it in a beneficial manner, we are able to recover the inputs that would otherwise be sent to a landfill. Once food waste is in a landfill, it doesn’t just disappear rather methane is produced.

Most of this methane produced in a landfill is not captured for energy, but rather half of it escapes into the atmosphere and methane is 25 times stronger greenhouse gas than carbon dioxide. (Source: US EPA) A major source of methane is from organic material in landfills — food waste. By diverting food waste instead to a biogas digester, also known as an anaerobic digester, methane can be captured and used significantly reducing emissions into the atmosphere. In addition to methane, food waste in landfills can cause nutrient problems in the leachate, odor and problems with vermin. Hauling food waste to a landfill represents a significant cost to businesses and many external costs to society with zero economic benefits. In fact landfills are a drain on the economy, not to mention people’s pocketbooks.

Diverting food waste from landfills to an anaerobic digester two very beneficial products are produced. Biogas and a fertilizer which be sold to farmers or gardens. By keeping it out of a landfill food waste is no longer thought of as a waste, but rather as a resource that can generate a significant revenue. Food is one of the most essential elements of life, and therefore we should have an ethical and moral obligation not to waste it or mistreat it. It should be elevated and though of as life giving, sustaining and renewable.

With all the issues of unsustainable industrial agriculture, global hunger and pollution, we should not be needlessly throwing food into a landfill. We must both reduce food waste while putting food that is not consumed to good use instead of unceremoniously dumping it in a landfill. Food waste and organic material represents a significant proportion of our waste stream which could actually become a source of revenue while doing something positive.

How much food waste is there?

20 lb. per month per person X 311.6 people in the U.S. X 12 months = 75 thousand lb. / year (2011)
This includes the waste which starts at the farm, transporter, warehouser, wholesaler, grocery store and finally in the home. I am sure there is food waste that goes unaccounted but 75 thousand pounds a year is close enough. It could easily be twice that amount. That amount of food could easily feed the poor and hungry in the U.S. without financial strain on anyone in fact if the food is sold at cost, at wholesale then the farmer and others would make a substantial amount.

What is anaerobic digestion?

Anaerobic digestion is the microbial break down of organic matter without the presence of oxygen. It is a process that is ubiquitous throughout nature from lake bottoms, swamps to ruminant animals. The process can be captured in a controlled, man-made environment for treatment of organic wastes, such as manure (human and animal), slaughter-house waste, crop residues, and food waste. Biogas digesters have most numerously been built-in China, India, Germany, and other countries where energy is in short supply and there is no waste treatment. The only thing an anaerobic digester can not accept are man-made chemicals — fossil fuel based all other organic matter is acceptable.

What is the result of anaerobic digestion?

There are two primary products of anaerobic digestion: biogas — a mix of methane and carbon dioxide, and a high quality fertilizer. The gas is a natural metabolic byproduct of the microbial metabolism that goes on naturally. The fertilizer is the resulting effluent from the digester when methane production is complete. Nutrients found in the feedstock are conserved in the effluent which is why the effluent make such a good fertilizer. The process makes minerals more availabel, the nutrients in the feedstock, for plant available forms, which makes them a better option to fossil-fuel derived, synthetic fertilizers.

Are there any waste by-products?

Because the effluent can be used as a fertilizer, there are waste by-products. Just as in nature, everything can be reused from an anaerobic digester without harm.

What are the uses of biogas (methane)?

Methane can be used for cooking, heating and lighting primarily. Biogas is approximately 60-80% methane with the remainder being carbon dioxide, some moisture and minor other items. It can be burned easily for cooking or heating without any sort of refinement. Cleaning the biogas to remove everything but the methane it can be used as a direct replacement for natural gas inject directly into the pipeline for use.

What are the uses of the fertilizer?

Since the effluent is a semi-liquid, it can be used in its existing state directly on gardens and farms. Additionally it could be bottled or dehydrated and sold to consumers. The fertilizer could also be integrated into compost systems with a high carbon content. Compost needs a carbon-to-nitrogen (C:N) ratio in order to properly break down. The high nitrogen fertilizer from the digester could be mixed with a carbon source (i.e., leaves, shredded paper and other woody waste like sawdust) to convert these materials into compost which can be sold as a soil amendment.

What other inputs are there for anaerobic digestion to take place?

Other than the infrastructure and the feedstock, no other inputs are required for anaerobic digestion. The process is entirely microbial utilizing the energy of microbes to convert waste into biogas and a high quality fertilizer. Any external energy to the process itself (i.e., hauling, mixing, pumping) may be necessary to facilitate the process but the digestion does not require them.

How much biogas from food waste can be expected?

Food waste is a diverse feedstock made up of plant and animal byproducts. The amount of biogas that can be generated from food waste is dependent on moisture content and the composition of the initial material. Food waste with a low moisture content will generate more biogas than food waste with a high moisture content. Fats and proteins can also generate more biogas than plant base material.

Does food waste need to be pre-treated?

While anaerobic digestion is possible on whole food waste, physical pretreatment helps to reduce the particle size of the material which helps produce a more even supply of biogas. This allows the microbes better access to the feedstock and results in a smoother overall process. An ideal pretreatment method would be to run the food waste through a food disposal system — blender.

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. In most cases you want the waste to stay in the digester from 30 up to 70 days for heavier material. Material needs to be retained long enough for maximum methane production and to prevent any disease from being able to survive after it leaves the digester.

Can meat, oils, fats, and bones be added to a digester?

Yes, unlike backyard composting, meats, oils & fats are welcome additions to anaerobic digesters. Bones are not suggested as they can accumulate in the bottom of a digester that have to be removed separately. Meats, oils and fats have a much higher methane production potential than vegetable based matter. That’s not to say you want a digester handling only animal by-products a mix makes for a much healthier environment within the digester.

Are there any biogas digesters currently in use?

China and India are two countries where digester have been used widely to break down everything from pig waste to household kitchen waste. In China digesters were built underground to keep them warm in colder winter months accepting animal, human and kitchen waste. In India people tend to be vegetarian living in the city so all the kitchen waste was used to generate enough gas for cooking and heating water for cleanliness. In more recent years Germany, Sweden and some other European countries have built biogas digesters not to handle waste but to create a renewable energy source — methane. There are currently several millions of biogas digesters all around the globe.

Could biogas digesters ever become viable way to obtain enough energy?

We currently waste more than 55% of the energies original source mainly due to inefficiencies. With improvements and better use, not using the gas to create a fertilizer or hydrogen, we could produce enough gas for cooking, heating and as a back up for electric production when other renewables are not available. The answer is yes but we have to change the way we do things for that to happen.

 


Filed under: biogas, climate change, energy, food, methane, sustainability Tagged: anaerobic digester, biogas, biogas digester, climate, economic benefit, electricity, elements of life, energy, energy fossil fuels, environment, fertilizer, gobar gas, greenhouse gas, methane, natural gas, renewable energy, science

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