Thursday, August 9, 2012

Anaerobic Digestion and Stonyvale Farm

Dariy Cow

Dairy farms have a lot of cows. Cows are great as they produce milk, which is essential when eating cookies (as any Fogler relative will know), among many other things. However, in addition to milk, cows produce a lot of manure, which is essential for... Well not much really. It can be used as smelly fertilizer, but one wishes to find a better use, especially if you work on a diary farm and have to deal with tons of it every day

In fact, one can use the manure waste to produce heat and energy using a very similar process to that which the cow uses to generate the manure using a facility called an anaerobic digester. This is exactly what many farms in Europe are doing, and now some in the USA. In fact my uncle and cousins at Stonyvale Farm in Exeter, Maine have just installed the first such facility in Maine and formed two companies Exeter Agri-energy and Biogas Energy Partners to manage their new facility.  

How can we take cow manure and produce energy? A cow has a special relationship with some microorganisms contained in its stomach which help it break down plant matter into amino acidsfatty acids, and simple carbohydrates which it can use. Similarly, we can develop a special relationship with another unique bacteria called a methanogen that breaks down the waste further into biogas - carbon dioxide and methane, which we can then burn to create heat an electricity.  As you may know, methane is also known as natural gas - a very common fuel for producing heat and electricity. Let's investigate this process starting with the plants the cows eat, continue with how the cow digests the plants with the help of special microorganisms, and end with how we can use a similar process to cow digestion to further digest the cow's waste to produce renewable energy.

Have you ever eaten a huge salad with lots of lettuce and leafy vegetables and a few hours later felt really really hungry even though you ate such a large amount of food? This is because most of the energy content in the salad was not available to you because you can't fully digest it. Why not? Digestion is a fairly complicated chemical process, but basically when you eat something, your digestive system secretes various proteins called enzymes that break food down into simpler chemicals such as fatty acids, simple carbohydrates, and amino acids which are then absorbed by the body through the small and large intestines. Plant cells are surrounded by cell walls composed of a polymer called cellulose. It is really tough stuff and it is what gives plants their rigid structure. Our digestive system doesn't have enzymes that break down cellulose, therefore even though we can extract some energy and nutrients from plants, most just passes through trapped in the cellulose.

Cows on the other hand, are huge animals that exclusively eat vegetation like grass and corn. They can survive on this diet because they have developed a unique digestive system that includes a symbiotic relationship with various microorganisms that are able to break down the cellulose of plant cell walls into carbohydrates, short chain fatty acics, and amino acids. Cows themselves don't produce the enzymes to break down cellulose, but contained in their huge 4 compartment stomach are a wide variety of bacteria, protozoa, fungi that do produce the enzymes capable of breaking down cellulose into smaller molecules that can then be converted to carbohydrates, fatty acids, and amino acids that are absorbed in the small intestine. Some of the most important types of bacteria found in the cow stomach include  Butyrivibrio fibrisolvens and Bacteroides succinogenes which produce the enzymes to digest cellulose, and methanogens, which are a special type of bacteria that consume hydrogen and carbon dioxide produced during cellulose digestion and convert it to methane. You can learn more about the organisms in the cow stomach here. Although cows are pretty efficient at digesting plant matter, there is still a fair bit of energy left in their manure.

The environment inside the cow stomach is optimized to produce simple carbohydrates, fatty acids, and amino acids and use the methane producing methanogens simply for regulation. However, what if we could design a system similar to the cow stomach, but optimized to produce methane with the methanogens. An anaerobic digester is a facility that does exactly that. An anaerobic digester takes waste from the cows and combines it with food waste from restaurants and farms in a huge vat containing many of the same microorganisms as are found in the cow stomach. The biggest difference is that the equilibrium is shifted such that it is optimized to produce methane rather than carbohydrates, fats, and amino acids. After being mixed and heated for several days, the bacteria and microorganisms further break down the plant matter and convert as much as possible to methane. What's left over is some fluid enriched with nutrients useful for fertilizer and some solid waste that can be used as bedding for the cows. The collected methane can then be burned to produce heat and electricity.

Photo of the anaerobic digestion facility in Exeter, Maine. Photo courtesy of

By implementing the digester into the cycle of milk production on a dairy farm, we end up with an almost completely closed system for generating milk and renewable energy. We start with converting carbon dioxide in the atmosphere to cellulose by growing plants. The cows then eat the plants and convert some of it to milk and some of it ends up as manure. We then take the manure and convert it to methane and carbon dioxide, plus some fertilizer. Finally, we burn the methane to generate heat and electricity and producing carbon dioxide. The carbon dioxide is then released to the atmosphere where it can again be converted to plant matter with the help of the fertilizer completing the cycle. Without the digester, the manure can be used as fertilizer, however, much of it ends up in landfills where it decomposes producing methane, which when released to the atmosphere acts as a greenhouse gas with more climate changing potential than carbon dioxide.

Here is a video about Stonyvale Farm and the new project.

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