Household Anaerobic Digestion Technology and Bio-inspired Design

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This blog is about household digesters, my experience in designing and building them, and lessons I’ve learned after discovering that nature had already made perfect digesters long before I gained an interest in designing them. I apologize in advance if verbiage within this writing seems intended for an audience of anaerobic digestion enthusiasts. In an effort to bridge this gap, I will provide a brief list of just a couple of terms that might be useful:

Feedstock: The content that is fed to an anaerobic digester (food waste or manure)

Digestate or Effluent: The liquid tea fertilizer that is produced by a digester

Acidogenic Bacteria: Bacteria that convert manure or food waste into acids

Methanogenic Bacteria: Bacteria that convert the acids into methane or “biogas”

Of all the renewable energy and waste treatment technologies that are employed in the human pursuit of environmental protection and freedom from fossil fuels, anaerobic digestion (AD) may have the most potential in finding technological advancement through bio-inspired design. Anaerobic digestion also has the unique ability of employing an environmental synergy not had by the other technologies. It is a technology that holds three environmental benefits: the treatment of waste, the generation of energy, and the production of high-quality fertilizer. Anaerobic digestion processes have been employed by humans for centuries, where waste is placed within an air-tight vessel and it is consumed by methanogenic bacteria for the production of burnable biogas and fertilizer. The technology is currently employed at the large-scale for manure and food waste treatment in developed countries, while developing countries benefit from AD in the form of household systems that convert manure and food waste into biogas for cooking and other purposes. Technological advancements are needed in both realms. My research interest is digestion that treats household food waste at its point-of-source, which is the backyard of residences where the food waste is generated.

Point-of-source household digesters have sustainability advantages over large-scale AD food waste treatment infrastructures. In many parts of Europe, it is common practice for households to sort their food waste and put it in a bin next to their trash can. This separate bin is taken to the curb with the trash bin for pickup from the local municipality. From there, the waste is taken to a large-scale digestion facility where it is treated and converted into biogas and fertilizer. The biogas is consumed for onsite operations in many cases, while the digestate fertilizer is delivered to local farms. Large-scale food waste pickup and the delivery of the digestate have negative consequences on the economics and environmental factors concerning this process. Fuel is needed for trucks to pick up food waste and deliver the digestate to local farms. There is nothing more local and sustainable than processing food waste in your own backyard and self-consuming the end byproducts. There is a place for large-scale systems in the worldly pursuit of combatting growing issues with environmentally sound food waste treatment methods, and I applaud their use, but point-of-source treatment has inherent advantages in respect to carbon-footprint.

In parts of Europe, fines are incurred when residents do not separate their food waste and dispose of it in the proper bin. A rare few states and municipalities in the U.S. are following suit through developing food waste pickup programs. As food waste disposal becomes more and more of a growing concern, the future may eventually have all Americans being required to sort and dispose of their household food waste in a responsible manner. Although food waste sorting practices and wide-spread non-compliance fines for Americans may be a ways out, now is the time to explore all options in relation to food waste treatment in the United States.

Sorting and storing food waste for responsible disposal is messy business that most Americans may find inconvenient. I believe that if it becomes a requirement in the U.S., that there are many who may want to reap the benefits of point-of-source treatment, rather than sorting and storing the food waste prior to municipal pickup. Those in the suburbs with appropriately sized backyards that hold vegetable and flower gardens are prime candidates for household digesters. As are those in rural areas that may be outside the scope of a local municipality-driven food waste pickup program.

There are existing issues with household digester operation, but there is opportunity to employ bio-inspiration in the pursuit of design approaches that remedy current problems faced by the end-user. Difficulty in routinizing the operation of household digesters often arise from technical issues in the form of temperature, clogging, bacteria ecosystem instability, and ease of use. These technical issues sometimes lead to digester failure and frustration for the end user, which in turn, leads to innovation rejection. I believe observing what I will refer to as “walking digesters”, may allow scientists to draw design knowledge that can overcome existing technology adoption issues. For some time, biologists have applied biomimetic connections between animal digestive ecosystems and digester bacteria management, but this has been geared towards digester feedstock load management and an effort to better control the bacterial ecosystem that the digester relies upon for successful waste-to-fuel conversion. I believe there needs to be a large shift towards examining nature’s design of perfected digestion systems, from an anatomic perspective, and apply this knowledge directly into the digester design process.

Humans and other animals are walking digesters. Many of the same methanogenic bacteria employed to digest the waste in man-made digesters, also live within the guts of humans and other animals. Bodies of humans and animals have evolved to host methanogenic bacteria in an environment that is hospitable to their growth and performance. Among the evolved attributes that allow for “good digestion” is the ability of digestive tracts to prevent solid accumulation and clogging, as well as full processing of feedstock. Sometimes digesters “sour” and become inoperable due to accumulation of acidogenic bacteria that overtake a required healthy population of methanogens. In many cases, the digester is continually fed food waste, while the bacterial ecosystem turns sour, unbeknown to the digester operator. In turn, digestate produced during these feedings may contain higher levels of pathogens. When this digestate is fed to vegetable gardens, there are health risks.

The first picture in this blog reveals my original cold weather digester design, which is a one stage treatment process (more info here: Feedstock enters one pipe and the fertilizer exits via an outlet pipe through displacement. Even with a healthy digester bacterial ecosystem, this design inherently allows some of the newly introduced food waste to exit the system and mingle with the fertilizer that exits during the feeding. Occasionally, unacceptable numbers of pathogens from the new food waste may find their way out of the outlet pipe and into the digestate that is intended for the vegetable garden. Clogging is another issue. Undigested solids, in the form of lignin, float to the top and eventually clog the outlet pipe. The unfortunate answer to the clogging problem is having to drain the digester periodically to get the solids out and then start over. It’s a messy and time consuming business.

The two-stage digester treatment approach, which has been used in large-scale systems long before my improved household design was implemented, reduces the risks of inadequate feedstock processing. Many animal guts isolate acidogenic and methanogenic bacteria into different digestive chambers for successful digestion. The picture of the experimental two-stage digester I operate at my home illustrates how breaking down the digestion process into two stages can lead to a higher destruction of volatile solids (see picture 2). Large-scale digesters have their bacterial ecosystems closely monitored to ensure successful operation and resulting pathogenic destruction. Home digester systems lack this monitoring for most users, so the two-stage approach provides a logical measure at pathogen reduction in the end fertilizer product. The two stage approach, which mimics the foregut and hindgut of an animal, allows for pretreatment in the first tank, where there may be a higher level of acidogenic bacteria. Then when the content overflows into the second treatment tank during feeding, the abundant methanogenic bacteria in the second tank are less challenged in their duty of destroying pathogens. I implemented the two-stage design a few years ago. I incorporated it into my design because I had seen this approach successfully employed for large-scale digesters, but it wasn’t until recently that I made the connection that the biological world was the inventor of the two-stage treatment method.

Now, about clogging. Evolution has engineered our bodies to not clog, given an unresolved clog could be deadly. For years, I built cylindrical digesters that clogged. Unprocessed solids (lignin) would float to the top of the digester and eventually, the outlet pipe would clog. I’ve taken lids off of digesters where the top third of the digester was a floating mass of unprocessed wood, thus reducing the operable volume of the digester by 30%. Lignin is the stuff in plants and trees that make them stand up straight. The methanogenic bacteria don’t eat lignin. Except for particular methanogens in the guts of termites that easily process wood, but that is another BioBuild blog altogether- and an interesting one. Let me just add that it goes without saying, that nature may provide more than one means to an end in resolving issues within household digester operation.

Long, tube-shaped digesters seem to do better than unmixed cylindrical digesters when it comes to pathogen destruction and clogging. They are shaped more like digestive tract pathways and what goes in one end is highly processed, as it is slowly pushed to the outlet pipe during each succession of feeding (see picture 3). I only made this distinction in design and performance, between tube and cylindrical shaped digesters, after being exposed to the BioBuild program here at VA Tech and learning what design knowledge can be drawn from the biological world. Was the first tube digester created through bioinspired thought? I don’t know, but I’ve come to realize that sometimes iterative design processes that neglect bio-inspired thought sometimes still steer towards a solution that nature could have revealed to the designer on day one (with few or zero reinventions). The tool of bio-inspiration could have saved me many years of reinvention in my digester designs.

In this writing, I have provided two examples of how anaerobic digestion technologies have mimicked nature: A two-stage digester tank design that mimics compartmentalized bacterial chambers in animals for full processing of feedstock, and tube digesters that reduce clogging found in cylindrical household digesters. For over a decade, I was aware of these designs, yet it was only recently that I understood why these designs are successful. Anaerobic digester design may be the poster child for bio-inspired design, but there are infinite possibilities to apply knowledge from the natural world in all fields of design and invention.