Get our free book (in Spanish or English) on rainwater now - To Catch the Rain.
E305 backyard biodigester
Understanding the Market
Inspired by the Park Spark Project in Cambridge, MA which was created by an MIT student, the idea of recycling dog doo at the local dog park is an efficient way to produce clean-burning energy from waste.
- 55-gallon-drum, food-grade, metal barrel
- Boat propeller
- Hand crank
- Various gas valves
- Inner tube or balloon for gauging methane collection
- Various metal tubes
- Access to welding equipment
- Drain valve
- PVC pipe
The object of our project is to take organic waste including dog, cow, and horse excrement, as well as kitchen waste and recycle it. Methane is a harmful greenhouse gas that can potentially be harnessed as a clean-burning renewable fuel.
- Wed February 29th- Completed welding attachments onto barrel.
- Sat March 3rd- Organic matter in barrel building methane.
- Sat April 7th- Within two months, have clean burning gas and gas storage facilities in place.
- Sun May 6th- Continued monitoring inflatable gas detection device.
These criteria were chosen as a means of evaluating the project based on maintenance considerations, safety, replicability, effectiveness, educational capacity, and aesthetics.
|Criteria||Constraints||Our weight (0-10)|
|Maintainability||Must be able to maintain functionality on less than 0.5 human labor hrs a week||7|
|Safety||Must be able to function without possible injury to users or observers||10|
|Replicability||Must be made of materials that can be easily sourced||10|
|Effectiveness||Must produce enough methane gas through anaerobic activity to keep a constant flame||10|
|Storage ability||Must be able to be stored and maintain function in a moist environment such as Humboldt County, California||6|
|Ease of use||Must be suitable from an age range of 12 years and above without adult supervision||7|
|Transportability||Must be able to be transported for educational outreach||2|
|Social justice||50 % of materials must come from a recycled source||6|
|Educational value||Must be able to provide an understanding of the functionality and use of the device, within a two minute viewing span||9|
|Aesthetics||Must be pleasing to the eye and look professional||8|
|Implementation||Time constraint of 8 weeks for construction so the bacteria has ample time to degrade the waste and produce gas||10|
|Simplicity||The necessity to have a construction plan with easily available materials is a desired criteria.||8|
We were going for a relatively simple and affordable design which could be easily replicated with local resources. We acquired the supplies from local hardware stores, resale lumber yards, neighborhood farmers and in junk yards. The final cost was under $200. The main structure consists of a 55 gallon food grade oil drum with four important attachments.
- The first is a down spout constructed of 4 inch pvc pipe for waste to be inserted into the top biodigester.
- Second is a stir rod to stir the slurry mixture. It's built from a welded-together boat prop, a 1/4 piece of rebar and a hand wheel.
- Thirdly, a pipe needs to be welded to the top of the drum for methane to escape then we attached a two way valve for methane collection or gas burn off as desired.
- Lastly, there is a drainage valve attached to the bottom of the drum for the release of fertilizer.
Organic Material Collection
After assembling the mini-digester, the next step is to collect organic material. We chose to use cow pies as they have the ideal moisture content and balance of methane to carbon.
|Quantity||Material||Source||Cost ($)||Total ($)|
|1||55 gallon food grade drum||Re-sale Lumber||40.00||40.00|
|1||1 inch ball valve||Re-sale Lumber||20.00||20.00|
|1||Poo Stirring Stick||Arcata Scrap Yard||20.00||20.00|
|1||4 inch x 4 feet pvc pipe with resealable top||Ace Hardware||30.00||30.00|
|1||Sealed Bearing||Ace Hardware||10.00||10.00|
|1||2 way valve and gas line||Ace Hardware||20.00||20.00|
|1||Grill burner and inflatable gas container||Ace Hardware||20.00||20.00|
In the beginning of the semester, we were bouncing ideas off each other trying to figure out what kind of design we would go with. Once we had figured out a blue print for the digester, we began searching for material needed to construct the digester. We visited local hardware stores, getting expert help on gas valves and gas lines. We then headed to Re-Sale Lumber to get PVC piping and our 55 gallon barrel. Once all the materials were gathered, assembly was the next step. We were able to sway a welder friend to help weld 4 holes into the barrel for us in exchange for a 12 pack of Pabst Blue Ribbon. After the holes were in the barrel we started collecting the other components and brainstormed on ways to attach them in a air tight manner. We came to the conclusion that epoxy would be the most economical way to attach the individual parts because we had a surplus on hand. Once we had the biodigester completely constructed the next step was to fill it up. To our surprise going door to door trying to locate cow owners willing to share poop took a bit longer than expected. Once the cow dung was collected, we filled the biodigester that day and mixed in water with some pre-made slurry from the Arcata treatment marsh's biodigester. Thirty days ago we started our mix and we are still waiting on collection observing minor fluxes in the gas holding container.
We plan on continuing our household waste to the digester. We also plan on adding a better gas-holding tank, we were just short of funds mid-semester and hope to put more money into it when summer funds are saved. We are still hoping to have a BBQ for our friends with the meal cooked entirely from homemade gas.
Although we were able to source our materials locally and keep within our budget, our time estimates for gas collection were under-evaluated. Ample time, as well as an airtight facility, is needed for proficient methane collection. Humboldt County does not have ideal weather conditions to promote anaerobic digestion through sunlight. As temperature of at least 110 degrees is needed, a heat source from the methane gas once enough has collected will be the most efficient way to heat the digester. Our plans include continuing monitoring the project beyond the semester's term to close the methane cycle in this way.
Literature Review - Jess B.
Biomimicry, Innovation Inspired by Nature, by Janine M. Benyus
This book describes the benefits of mimicking nature in natural processes such as how to break down waste, how to produce strong adhesives and countless other innovations, in order to achieve cutting edge innovations to improve our modern world. There are countless examples of how different farms and organizations utilize this technology. One in particular is of interest to this project, a section describing grass farmers raising cattle in the Midwest. Instead of managing the system in a traditional model where grain feed is brought to the cows, these farmers let the herd out into the fields to graze off grasses. The symbiosis of these cows and their cow pies on the farmer’s land allows for a new type of management, where farmers “consider themselves solar harvesters now--- turning sunlight into grass and then into meat and milk.” (Benyus 45). Some additional, beneficial information that I extracted from this section of the book was that in healthy microbial conditions a cow pie should break down within three weeks in a warm environment.
Cradle to Cradle, by William McDonough & Michael Braungart
Describing ways for industries to close their production loop cycle is relevant to all business ventures. “Today, with our growing knowledge of the living earth, design can reflect a new spirit. In fact, the authors write, when designers employ the intelligence of natural systems—the effectiveness of nutrient cycling, the abundance of the sun's energy—they can create products, industrial systems, buildings, even regional plans that allow nature and commerce to fruitfully co-exist” (MBDC). Having a closed loop cycle by building a bio-digester is the ultimate way to balance the intake of biological waste and provide a clean burning fuel at the same time.
The Humanure Handbook, A Guide to Composting Human Manure by Joseph Jenkins
A great read on how the nature of human’s relationship to their own waste should change and multiple reasons to see why it should become more of a closed loop cycle. This book has a plethora of information regarding the finer details of this project. Undoubtedly, this text will be referred to on countless occasions. The section I found most helpful and interesting had to do with sewage sludge. In addition to our project’s goal, we will include the ability to create fire from methane gas, an excellent by-product of rich fertilizer will be available after a few months of digestion. It cites how much better of a system this is in places like Duisberg, Germany where a 100 year old sewage plant composts over 100 tons of domestic refuse a day(Jenkins 231).There are numerous other cities that provide “biobins” for collecting household compost refuse. The book implies that we have to look forward to advanced collection of humanure by municipalities and gives the following stats on America’s sewage sludge innovation. “In 1988, in the U.S. alone, there were only 49 operating municipal sludge composting facilities. By 1997, there were over 200. The U.S. composting industry grew from less than 1000 facilities in 1988 to nearly 3800 in 2000 and that number will only increase” (Jenkins 231)
Methane Gas Process and Apparatus by Oliver W. Boblitz
This interesting peer reviewed article is the application for a U.S. patent for a bio-digester with the intention of producing methane gas. The schematics are a bit larger than the project size that we are undertaking, however, there was some pertinent information on how the breakdown process works for methane. Specifically cited, the slurry is to remain between the pH of 6 and 8; kept between a temperature of 100 and 140 degrees F and heating the slurry in this way for up to 10 days to reach the desired affects. There is implementation of solar collection for heating that would be something to consider.
Literature Review - Josh Bancroft
The methane digester built on the Foster Brothers farm in Middlebury, Vermont was designed with the goal of turning something negative into a positive opportunity. The brothers designed a system eight feet deep with an underground cement tank. The anaerobic process breaks down the poo of 350+ cows and releases methane. A pipe transfers the methane to another room to scrub out the hydrogen sulfide present in the gas, this is what makes the poo smell. The cleaner methane is then funneled into an internal combustion engine that is hooked up to generator producing 360,000 kWh of power to run the entire farm.
The brothers have some problems with the project. When implementing the idea of a digester they received plenty of proposals. They had to pick the best one for their farm. They turned down outlandish proposals such as a system that required seven acres of tanks. An original idea of the brothers was to sell the electricity produced to the surrounding community, but in time learned that the economic loss was to great and eventually decided to use the energy to power the farm and the houses located on the property. 
Methane is the main component of natural gas, which is derived from ancient plants and algae. Methane is produced naturally in landfills and can also be produced at a sewage treatment center where natural decomposition of human waste occurs. The use of methane digesters is an excellent idea to harvest this natural gas. It is awesome to see the use of digesters in under developed countries giving them a very cheap alternative fuel source to light and warm their homes.
It is unfortunate that methane digesters aren’t as widely used in developed countries, there is so much potential energy being wasted. The natural gas could power your homes electricity, it could heat your water, and can warm your house for fraction of the cost to use gas provided by a company such as PG&E.
Bacteria grow with the absence of light and oxygen. This bacteria helps digest organic material and with this a by-product of natural gas is produced. When the gas is mixed with oxygen it can be used for fuel. I found it very cool that the heat produced form the burning gas “can be used with a heat engine, which converts thermal energy to mechanical or electrical energy.” (Mulvaney pg 36)  Biogas should be more present in everyday life. In some locations such as Arcata, solar power is not the best option for alternative energy source. But no matter the weather conditions, alternative energy can still be produced with a digester.
Methane is a natural gas created by the anaerobic decomposition of organic materials. The anaerobic process happens naturally in boggy areas. It works here because the body of water is airtight with a swampy sludge keeping oxygen at bay. The anaerobic process occurs in three stages. First a group of microorganisms change the organic material into another group of organisms that will form organic acids. Bacteria will then form and complete the process of turning the material into a sludge that will release methane
Literature Review - Chris DeFoney
ANAEROBIC DIGESTION: This summarized the process of digestion starting with fermentative bacteria’s creating acid forming bacteria and acetate from organic material. Higher energy foods or poo have sugar and breakdown faster can actually create to much acid which inhibits the bacteria due to a high pH which can halt the anaerobic digestion all together. But as long as you start with a large quantity of fermentative bacteria found in low energy fuel like cow dung the Methane forming bacteria will consume all the acetate and acids formed in the first step of digestion, giving off methane Co2 and other gases. This brief chapter also gave the temperature range for the occurrence of methane bacteria and a good idea of the time it takes to produce quantities of methane at different temperatures. 
Cow power: This article was about a large methane digester constructed on a cow farm in Birdport Vermont which measured 72’ by 100’ @ 14’ deep and pumped out 1.75 million kwh a year. This farm was lucky enough receive the USDA Rural development renewable energy grant which kicked off this powerful and profitable cow power program. This system has been connected into a local grid in central Vermont and customers can choose what percentiles of energy bill they would want to be powered by cow Power. This is one of the most successful program of its kind in the country and I see its popularity continuing grow in the future. 
The next book I read gave very specific information for the construction and implementation of backyard biodigester and the recommended ratios of water to manure for the slurry. The recommended mixture is 50% water at 50% manure but the total weight will be about 90% water and the slurry should be chunk less. If the mixture is right, a ph between 7-8 the methane production should start within 8 to 10 days. Cow, Chicken, horse and pig manure are all recommended because they have a good carbon to nitrogen ratio and have fermentative bacteria. 
Lastly I looked into the economic values of biogas and the public health benefits. In the environment burning methane helps in reducing greenhouse gases, organic waste, odor, and pathogens. Anaerobic digestion is a carbon-neutral technology to produce biogas that can be used for heating, generating electricity, mechanical energy, or for supplementing natural gas. Biodigesters are being used allover the world and are expanding rapidly in growing impoverished countries which is a great sign for or generation and this great technology. 
- Benyus, Janine M. Biomimicry: Innovation Inspired by Nature. New York: Harper Collins, 1997.
- McDonough, William, and Michael Braungart. Cradle to Cradle: Remaking the Way We Make Things. New York: North Point, 2002. Print.
- Jenkins, Joseph. The Humanure Handbook. Grove City, PA: Chelsea Green Publishing, 2005.
- Boblitz, Oliver W. "Methane Gas Process and Apparatus." Google. Bio-Gas Corporation, 3 Sept. 1976. Web. <http://www.google.com/patents?hl=en>..
- Pahl, Greg. "Biomass." In The citizen-powered energy handbook: community solutions to a global crisis. White River Junction, Vt.: Chelsea Green Pub. Co., 2007. 177-178.
- Chiras, Daniel D.. "Whats on the Horizon?." In The homeowner's guide to renewable energy: achieving energy independence through solar, wind, biomass and hydropower. Gabriola, BC: New Society, 2006. 283-284.
- Mulvaney, Dustin. "Biogas." In Green energy an A-to-Z guide. Thousand Oaks, Calif.: Sage Publications, 2010. 36-38.
- . Energy Savers: How Anaerobic Digestion (Methane Recovery) Works." EERE: Energy Savers Home Page. http://www.energysavers.gov/your_workplace/farms_ranches/index.cfm/mytopic=30003 (accessed February 10, 2012).
- Lusk, P.. Methane recovery from animal manures the current opportunities casebook. Golden, Colo.: National Renewable Energy Laboratory, 1998. Print.
- Pahl, Greg. The citizen-powered energy handbook: community solutions to a global crisis. White River Junction, Vt.: Chelsea Green Pub. Co., 2007. Print.
- Doerr, Beth, and Nate Lehmkuhl. METHANE DIGESTERS. North Fort Myers: Echo Technical Note, 2001. Print.
- Anaerobic Digesters | Center for Climate and Energy Solutions. Center for Climate and Energy Solutions | Working Together for the Environment and the Economy. Center for Climate and Energy Solutions, 9 Feb. 1211. Web. 13 Feb. 2012. <http://www.c2es.org/technology/factsheet/anaerobic-digesters>.