Final Report: Floating Dome Biodigester
Abstract
A biodigester is a device that anaerobically converts animal waste to methane and fertilizer; they are lauded for their environmentally sustainable approach to energy production and the efficient reuse of waste materials. Grupo Fenix is a community partner in Nicaragua that requested our assistance in improving their biodigester design. To approach this project we first consulted with Grupo Fenix as to their desired design specifications; after receiving their feedback, we committed to keeping this design cheap, simple, and durable, and able to be transported. A final specification that Grupo Fenix requested was efficient production of methane gas and fertilizer, but because we knew we would not be able to truly test this design we did not ultimately commit to meeting this exact specification. We explored the various designs for biodigesters––the plastic bag model vs. the floating dome model––and ultimately determined the floating dome would best meet the design specifications of Grupo Fenix.
Background
Grupo Fenix
Grupo Fenix is working in developing communities to create a sustainable source of clean energy. Currently, the community members use wood-burning stoves to cook 3 times a day. While wood may be an effective source of energy, it can have a negative effect on people’s health and the environment. Furthermore, users must maintain a constant supply of wood either by buying it, or through physical labor. Grupo Fenix is looking to supply the community with an alternative energy source, and their solution may come in the form of biodigesters.
What is a biodigester?
Biodigesters are products that use manure and biomass to create methane gas and compost. When mixed with water, raw, organic materials undergo anaerobic digestion and produce methane gas, carbon dioxide, and nitrogen gas. These products are completely harmless to humans, and can be used just like any other cooking fuel. The “neutralized” solid waste that is leftover can be used as a fertilizer for crops.
There are clear advantages to using biogas rather than other fuels. Biogas is inexpensive; once the product is built or bought, it uses waste products at no extra cost to those surrounded by livestock. Also, it is better for the environment in that it doesn’t require a constant supply of natural resources. Just 1 cubic meter of biogas can either power a 60 Watt lightbulb for 6 hours, provide fuel to cook 3 meals, or run a 1 horsepower engine for 2 hours. One of the major disadvantages, and subsequent challenges for us and Grupo Fenix, is to combat the “taboo” of using gas produced from manure and biomass for cooking.
Design Options and Decision Process
There are basically 3 models of biodigesters currently in production. The most expensive one is the fixed dome digester model. The fixed dome model involves an underground chamber with inlet and outlet pipes. This kind of digester is sturdy and long-lasting, but too expensive to implement on a large scale.
In contrast, the most inexpensive model of biodigesters is the plastic bag model. These digesters involve a plastic bag inflating while filling up with biogas, and are low cost along with easy to install. However, there is a much greater chance of these products malfunctioning, breaking, or leaking. We have also decided against implementing this in the community because among people who are already hesitant to use the biodigesting method, we want to introduce a more durable and reliable product.
Therefore, we chose to develop the third type of biodigester, known as the floating dome model. The floating dome digester uses 2 chambers, with a lower one to hold materials and an upper one to collect gas. Floating dome digesters are more durable than the plastic bag model, and can even be made from leftover materials, including buckets, oil drums, trash cans, or other already existing products.
Our Design: The Floating Dome
Materials
The final list of materials came down to:
-2 5-gallon (19-litre) buckets with lids
-1 gas faucet
-1 water faucet
-2 brass fittings
-2 8-32 screws and accompanying nuts
-1 PVC pipe with cap --> measurement should be 3.5" across outer diameter; 19" (~50cm)
-1 Weather-Resistant EPDM Foam, Tube, 7/16" OD, 1/4" ID, (Recommended: 3' or 1 metre long)
Tools:
-Permanent marker
-Measuring tape or ruler
-Drill
-Saw (Jigsaw)
-Caulking glue (or glue of equivalent strength; if tube of 10.5 oz, less than 1/4 tube will be used)
We chose the cheapest materials we could find to try and keep the cost down. We used plastic buckets as the 2 chambers because they were durable, inexpensive, and easy to work with. They are also a good size for a private, single family digester. The buckets came with good quality locking lids as well, so we didn’t have to worry about leaking issues. We also needed durable faucets, so we opted for brass pieces that we could easily attach to the plastic. This product had to be airtight, so we used foam and caulking glue to ensure that there were no leaks.
Design
For the actual construction of our product, we need to cut the buckets to a specific size, hence the necessity of the plastic material. We only needed cutting tools for trimming the 2 buckets and for cutting a hole in the lid. Aside from that, we need to drill a total of 4 holes, Everything else came down to gluing and assembling by hand. We didn’t want to create a product that was impossible to replicate outside of our engineering lab. The actual construction instructions can be found in the appendix.
Essentially the floating dome digester is comprised of 1 5-gallon bucket sitting upright, but with its bottom cut off, inside the 2nd 5-gallon bucket, which sits upside down and with its bottom cut off. A PVC pipe sits several inches above the top bucket and runs through two the middle of the bottom bucket, and this pipe is where any material can be inserted. The PVC pipe has a cap to ensure everything remains airtight, and the buckets are arranged tightly enough that the whole digester is airtight. This system operates on a 1:1 ratio of water:material, so water should be poured in to fill up about half of the bottom bucket. As material is inserted down the PVC pipe the water will help the material be digested, and as methane gas is released it will be sent into the gas chamber (the top bucket). The location of the PVC pipe ensures that the gas chamber remains airtight.
Prototype Processes
Our major objectives in building our prototype included making the design able to be used for individual homes, keeping the cost low, and most importantly developing a product that wouldn’t fail. We really wanted to make something that would make people excited about biogas, not disgusted by using waste.
Our first prototype involved two upside-down buckets, with the top bucket meant for collecting gas fitting over the lower one. We also had a tray attached to the bottom bucket meant to collect the solid waste. We were hoping the lid on the bottom bucket would easily come off so we could remove the tray and clean out the digester. However, this design had some flaws. First of all, there were some leakage issues where we didn’t have enough caulking glue. Also, we realized that the top bucket should fit inside the bottom bucket, in order to prevent gas from escaping. Finally, the the other major issue the bottom. The lid was very difficult to remove, and we realized that we could just separate the buckets to clean out the digester instead of bothering with the tray.
For our second prototype, we major some slight, but significant adjustments.
1) We flipped the top bucket so that it rests inside the lower one, to better ensure the gas is sealed inside the biodigester and won't escape.
2) We designed the two buckets to be taken apart easily, but they fit together securely enough to not separated unintentionally. This allows the top bucket to rise when fill with methane gas, giving a visual indicator when methane is ready to be released. Also, it allows us to easily empty out and clean the bottom after most of the water has been drained.
3) We also changed the faucet we were using for gas to one which will fit with a propane connection. both faucets are reinforced with extra caulking, as the design is dependent on there being no water or gas leaks.
Instructions for Use
On the outside of each bucket is a faucet; one is to release water and the other is to release gas. When the bucket becomes full, a user would attach their gas tank to the faucet to release any methane that has been produced. Then, the user would get ride of any excess water by releasing the water faucet. At this point the user would be able to separate the top and bottom buckets (by simply pulling the top bucket off); the bottom bucket holds the remaining material and can be used to transport the compost to a field––or dumped into a better bucket for carrying, if one is available. The ability to separate the buckets also allows the user to clean each bucket.
We do not yet have an idea as for how often the buckets would need to be cleaned, or how quickly this design will process material and produce gas, but we imagine every few days to one week would be the expected time. The production efficiency will, of course, depend on how much material is inserted, climate, etc. We have yet to perform a test run on our 2nd prototype (we're waiting for the parts for the gas faucet to arrive), but made some design improvements after a test run on our 1st prototype.
Conclusion
In building our design we most kept in mind the design specifications of affordability, durability and simplicity of design; we feel that our final prototype maintains these specifications. If being built in the U.S., the production cost of this particular design range is between $30-$40, but we imagine the cost of products in Nicaragua will be less. We are told that 5-gallon buckets (or barrel drums, if this idea is scaled up) are generally available, which would cut out this cost, and we imagine that other parts are available for purchase in markets or appliance/hardware stores. We hope that the overall cost is cheaper than the cost of production in the U.S.; either way, we are satisfied that this version is cheaper than other models that we have examined.
Because we used 5-gallon buckets we are confident that this model is more durable than the plastic bag biodigester models that have been used in the past in the Grupo Fenix community. The plastic of the buckets is thick and will not be eroded due to weather; additionally we are not concerned that any animal will be able to bite its way into the material (as was the concern with the plastic bag models). We suggest that users ensure that the biodigester be placed on a flat, hard surface to ensure that it is not knocked over, but otherwise we feel we have met the design specification of durability.
Finally, we believe this design is fairly simple and that we have met this specification. Classmates seemed to understand how to use the biodigester, and Lyndsay Chapman’s reaction was positive when we presented to her. We are unsure of the amount of time a user will have to wait for gas to be produced, or material to be fully decomposed, and so do feel obliged to note this aspect of simplicity that may not be met. However, we imagine the user will get a better sense once it is put to the test.
Design Shortfalls
Due to difficulty of obtaining animal feces, as well as general concern for violating health code within the classroom setting, we were unfortunately unable to truly test our prototype with the actual material with which this device will be used. We are confident in this design’s ability to avoid leaks; to effectively hold waste; to drain water; and to transport a certain amount of material. Nonetheless, we would be remiss to mention our inability to truly test it, and we would be foolish to be fully confident that this prototype will effectively and efficiently produce methane gas and compost material into fertilizer. We are hopeful that the trip to Nicaragua will prove fruitful and that this device will run effectively, but we are prepared to receive feedback that certain parts need to be reworked or improved.
Community Reception
We have not yet received the full reaction of Grupo Fenix, but Lyndsay Chapman’s reaction during our class presentation was generally positive and appreciative. She had several questions about the production efficiency––which, as aforementioned, we could not confidently answer––but our sense was that she understood the way to operate the system. We really wanted to create a prototype that was user friendly and simple, so we were satisfied that this was her reaction to our final prototype. We look forward to the reaction by Grupo Fenix in January; additionally, because this design was meant primarily for the restaurant of the organization, which is run by foreign volunteers, we will be interested to know if the reaction of the initial foreign recipients is more or less positive than the reaction of Nicaraguan families. Grupo Fenix mentioned that they hope, if it works for volunteers in their restaurant, that Nicaraguan families will choose to use this model on a greater scale. We will be interested to check back in several months or years to see if the floating dome model is scaled up for more extensive use, if a different prototype is developed, or if the idea is abandoned altogether.
Appendix: Instruction Manual for Construction
Instructions:
- Go from bottom of first bucket and draw line 2" (~5 cm) up from the end around the entire width of the bucket
- Repeat above step, but use second bucket and draw line 3 3/4" (~9.5 cm) around width of bucket
- Using pliers (or other tool), remove handles from bucket
- Pick a point on the line of each bucket and drill a hole big enough for blade of jigsaw to enter
- Use jigsaw to cut along line of each bucket; dispose of end pieces.
6) Flip first bucket upside down (keeping lid attached) and let it sit on table. With second bucket, take lid off and keep bucket upright. Place second bucket inside of first bucket; this should be a tight arrangement, but will prevent any gas or water leakages.
7) On second lid, find center point and draw circle with diameter 3 3/4" (~9.5 cm)
8) Use drill to make a hole on one point of circle on lid; use jigsaw to cut out circle
9) Stick PVC pipe through this circle; should be wide enough to push PVC pipe through and hold it in place.
10) Take a 13 1/2" (~34 cm) measurement of foam tube and use glue to affix foam tube to PVC pipe at approximately 4" from the top of PVC pipe. When PVC pipe is sitting inside the circle of the top bucket lid, this foam should touch the bucket lid, leaving 4" of PVC pipe above lid and 15" of PVC pipe below lid.
11) Glue the foam to the lid and make the seal airtight with caulking glue. Use a generous amount and leave overnight to dry.
12) To attach the water valve (blue), first drill a 1" diameter hole in the second (bottom) bucket, 5.5" up from the bottom. Then drill 2 small .2" diameter holes on either side of the larger. The smaller holes should line up with the gaps on the sides of the valve itself
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Stick the threaded part of the valve into the hole, and screw on the brass fitting from the inside. Thread two 8/32" screws through the smaller holes, and secure them with two 8/32 nuts on the inside. Seal the valve in place using a generous amount of caulking, both on the inside of the bucket and on the outside around the water valve. This needs to be airtight and leakproof.
13) To attach the gas valve (red), drill a .65" diameter hole in the top bucket 5.25" down from the lid. Insert the longer end of the valve into the hole, and attach the thin fitting from the inside of the bucket. Secure with caulking, sealing any gaps on the outside and the inside.
