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In the history of human ingenuity, many inventions and technologies are inspired by naturally occurring phenomena. The fastest, most efficient trains in the world — the Shinkansen trains of Japan — move at over 186 miles per hour. To eliminate the sonic booms such velocity yields, designers streamlined the trains to resemble the Kingfisher's elongated beak, since that bird flies and swims with very little splash. Similarly, those designing 21st-century wind turbines use whale fins for models: the ungainly mammals are nevertheless the most graceful swimmers, jumpers and divers in the seas because of the serrated nature of the fins. The decomposition of organic material happens in nature all the time. Whenever dead flora, fauna, fallen leaves, abandoned grass clippings, livestock manure, food waste and sewage get sealed off from oxygen — as with a landfill or compost heap — these substances are subject to anaerobic digestion. Broken down by bacterial formations, this disintegrated matter releases methane gas (CH4) and carbon dioxide (CO2). Recognizing the economic and environmental value of harnessing methane for energy, scientists developed machines to replicate, and accelerate, anaerobic digestion. These are sometimes called bio digesters.
Anaerobic digestion passes through four stages to get to the methane. HYDROLYSIS — the organic matter consists of polymers, the chemical bonds of which are broken down by the bacteria to form simple sugars, amino acids and fatty acids. ACIDOGENESIS — the by-products of hydrolysis are further deconstructed by fermentative bacteria. The end products of acidogenesis are volatile fatty acids, ammonia, CO2 and hydrogen sulfide. ACETOGENESIS — the compounds are broken down further to release acetic acid, as well as CO2 and hydrogen. METHANOGENESIS — the remainders of the preceding reactions are converted to methane, carbon dioxide, and water. Together, they compose what is known as biogas. Biogas is often processed after production to remove contaminants and other chemicals that would impede its performance.
A bio-digester, or eco digester, is a tank that allows no oxygen (O2) in. It does what nature does with manure, compost etc except that those materials — or substrate — undergo immediate O2 deprivation, inciting biodigester chemical reactions. First, the substrate is received and collected after which it may undergo pre-treatment to make the process smoother. Once secured in the digester itself, the organic matter then passes through the four stages noted above. When the micro-organisms have done their work, the biogas is separated from the digestate, i.e. the solid matter that remains.
Because biogas is used to generate electricity, on the one hand, and also used for vehicular fuel and heating, there are different schemes for its anaerobic production. Covered anaerobic lagoon digesters — the organic matter settles in a large, enclosed pool covered with a flexible canopy. Plug flow digesters — work best when the substrate, e.g. cow manure, is scraped from floor slats, as in a dairy facility. Complete mix digesters — often an above-ground tank, enclosed and heated with a hydraulic system for gas mixing, set upon a concrete foundation. Induced blanket reactors — operate on the principle of sludge digestion. A blanket of sediment forms that maintains the bacteria necessary for anaerobic digestion.
Anaerobic digestion separates biogas from substrate leaving something called digestate. This can be more on the fibrous side or more akin to slurry — like much farm digestate. Most often, this is applied like fertilizer, as a soil conditioner, with good results. Digestate is also a component of certain plastic composites as well as bedding for cattle. They need not be large — small-scale bio-digester systems serve households, restaurants and small farms, demonstrating that the benefits of anaerobic digestion are accessible at every scale.
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