1. Design of Scalable Biogas Digester for the Developing World
By: Tiffany Cheng, Thomas Davis
Dawn Schmidt, Kyle Schroeder, Andrew Wu
BME 272
1/21/10
Advisors:
Dr. Dave Owens – Owen Graduate School of Management
Dr. Paul King – Vanderbilt University School of Engineering
Scalable – reduce the cost per unit by mass producting

2. Meet the Rezas 6 members 1 cow Make $60 per month
Two parents, four children
1 cow
Make $60 per month
Spend $10 on petroleum fuel per month
Spend 2 weeks per year collecting additional fuel
Interested in neighbor’s biogas digester
To improve their standard of living
For our project, we will be focuing on Bangladesh. The standard of living in Banglaldesh is one of the bottom 25% of the world. The Rezas are a typically family from Bangulash. The family has 6 family members, two parents, 4 children and one cow. The parents make about 60 dollars per month in US dollar, and spend 10 dollars on Petroleum fuel. And they would also spend 2 weeks per year collecting straw and burn them as fuel source. Few months ago, the Rezas noticed that since their neighboring family had installed a biogas digester with the help of NGO. From then on, the Rezas observed that the neighboring family had additional money to purchase new clothing. And they are really interested in having a biogas digester as well to improve their standard of living. So The Rezas talked to the neighbors to learn more about biogas.

3. What is a BioGas Digester?
The neighboring family explained that the biogas digester converted cow manure into biogas which is used as cooking fuel. Water is mixed with manure to create a slurry, which is then fed into a large sealed tank where bacteria anaerobically digest the organic matter to produce biogas. Biogas is composed of about 65% methane, 25% carbon dioxide and 5% other gasses. Usually 1kg of manure produces 360 liters of biogas which is almost enough to cook for one person for a day.(1)
With the help of Non-Governmental Organizations, the neighboring family installed a biogas digester. The Rezas noticed that a few months later the neighboring family had additional money to purchase new clothing. The Rezas talked to the neighbors to learn more about biogas. The neighboring family explained that the biogas digester converted cow manure into biogas which is used as cooking fuel. Water is mixed with manure to create a slurry, which is then fed into a large sealed tank where bacteria anaerobically digest the organic matter to produce biogas. Biogas is composed of about 65% methane, 25% carbon dioxide and 5% other gasses. Usually 1kg of manure produces 360 liters of biogas which is almost enough to cook for one person for a day.(1)
However the Biogas digester right now are too expensive
1)Unicef Article
Saubolle, B., & Bachmann, A. (1983). Fuel Gas from Cowdung. Kathmandu: Sahayogi Press.
Previous slide:
Converts cow manure into biogas
65% Methane
25% Carbon dioxide
10% Other gases
Methane can be used for cooking
1kg manure  360L of biogas almost enough to cook for one person for one day (Saubolle & Bachmann, 1983)
1kg manure  360L of biogas
(~enough to cook for 1 person for 1 day)
(Saubolle & Bachmann, 1983)

4. Neighbor’s Digester Cost $200 Expensive materials Hard to install
Requires specialist
Pros:
Requires little maintenance.
During the hot months, the neighbor’s two cows produce enough manure to cook for 14 people.
Cons:
NGO helped pay for a $200 digester
It uses expensive materials (concrete)
Takes about a week to install
Requires specialist for installation.

5. Design Criteria: Costs $80 or less Produces 2800 liters of biogas/day
15,000 Kcal
Cooks for 6 people
Lasts at least 5 years
Easy to install
Goal: To Design an Affordable BioGas Digester for the Developing World
To do:
Cite Unicef

6. Design Method Scale: The more you produce the cheaper it becomes.
Goal
Scalable
Appropriate Materials
Efficiency Techniques
Heating
Mixing
Slurry additives
Scale: The more you produce the cheaper it becomes.
Materials: Cheaper/ more easily obtainable materials = reduced cost
Efficiency:
Biogas production vs residence time (residence time is proportional to the size of the tank)
size is calculated by taking the amount of material added per day and multiplying it by the number of days it must remain in the digester.
Slurry additive: improves nitrogen to carbon ratio… helps feed bacteria
“Apply principles of scale, appropriate materials and other strategies to achieve our goal.
Other strategies: Heating, mixing, and slurry additives. “
To do:
Input graph of biogas production vs residence time

7. Design Method Scale: The more you produce the cheaper it becomes.
Goal
Scalable
Appropriate Materials
Efficiency Techniques
Heating
Mixing
Slurry additives
Scale: The more you produce the cheaper it becomes.
Materials: Cheaper/ more easily obtainable materials = reduced cost
Efficiency:
Biogas production vs residence time (residence time is proportional to the size of the tank)
size is calculated by taking the amount of material added per day and multiplying it by the number of days it must remain in the digester.
Slurry additive: improves nitrogen to carbon ratio… helps feed bacteria
“Apply principles of scale, appropriate materials and other strategies to achieve our goal.
Other strategies: Heating, mixing, and slurry additives. “
To do:
Input graph of biogas production vs residence time

8. Biogas Digesters Worldwide
Floating Drum
Egg Shaped
Fixed Dome
Plastic Bag
The first step is to develop an understanding of current approaches
Floating Drum
More expensive
High level of maintenance
Short expected lifetime
Fixed Dome
Challenging construction
Frequent gas leaks
Fluctuating gas pressure
Gas production not immediately visible 
Egg shaped (industrial)
Difficult construction
Plastic Bag
Short lifespan
Plastic Cylinder
To Do: Import pictures and add some cons on why we don’t like some of them.

9. Our Full-Scale Designs
Brick Design
Masonry and cement
Readily available materials
Requires sealant
Plastic Design
Polyethylene
Easier to install
Mass producible
Mention for the brick design, we would have compost around the structure to warm it.
Also put dome above ground so compost can be next to the digesting portion of the digester
(maybe redraw)
Say:
Both Cylindrical, underground, with compost around outside.
Mention that we are waiting on feedback from contacts in Bangladesh on pricing for Brick vs Cement.
We are also looking into the cost to develop a mold for the plastic model
To do:
Make drawing(s)

10. Selection Matrix:
Material for Digester

11. Testing Prototypes Testing the effects of: Stirring Heating by compost
Changing material type
Brick
Plastic
Plastic no stirring, no compost
Plastic no stirring, compost (control)
Plastic stirring, compost
Brick no stirring, compost
Cement no stirring, compost
To Do:
Draw pictures (add on right side)

12. Brick Prototype 3.5u Dome 10.1u (add 0.1u) Wall Floor 10.2u (add 0.1u)
V = u3
10.1u (add 0.1u)
*Thickness = u, (assuming full scale device has 1in thickness with 10.2u = 7.2ft and 10.1u = 7.1 ft)
Method of Implementation
Compare brick dimensions (thickness = 0.12u) and digester dimensions to determine number of bricks for dome, walls, and floor.
Construction of dome
Position bricks appropriately for dome shape formation and brick ring
Leave hole that fits pipe for gas
Option: Leave hole that fits thermometer
Insert pipe into hole with pressure safety valve installed on other end
Drill hole into pipe to allow insertion of thermometer at an angle and insertion into slurry
Seal with sealant
Construction of wall
Position bricks to create open cylinder on one end and floor on the other
Construction of floor
Position bricks to create curvature for floor
Wall and floor should be sealed with sealant
Dig a hole
Width should allow space for compost
Depth should be enough so hole is level with ground surface
Place digester (wall and floor) into hole
Prepare compost
Pack compost beside walls
Prepare manure
Mix manure with water and other additives
Place 825u^3 of resultant slurry in digester
Mix manure in digester
Place dome on wall and sealed with sealant
Pack a layer of compost onto dome, then dirt, leaving hole and tube exposed
V = 825u3
Wall
Floor
10.2u (add 0.1u)

13. Plastic Prototype (Bucket)
V = u3
1.5u
V = 825u3
Method of Implementation
Buy bucket and lid made of appropriate plastic material
Create a hole in the center of the lid for pipe insertion
Drill another hole for compost thermometer insertion
Seal the pipe in place
Other end of pipe has safety pressure valve installed
Dig a hole
Width should allow space for compost
Depth should be enough so hole is level with ground surface
Place digester into hole
Prepare compost
Pack compost beside walls
Prepare manure
Mix manure with water and other additives
Place 825u^3 of resultant slurry in digester
Mix manure in digester
Pack a layer of compost onto digester, then dirt, leaving tube exposed
10.1u (add 0.1u)
10.2u (add 0.1u)

14. Future Work Timeline
Talk to technicians and receive feedback from Bangladesh (Pricing)
Jan 21-Jan 28
Determine thickness of plastic
Jan 29-Feb 4
Obtain materials and site location
Jan 29- Feb 4
Detail design of prototypes
Feb 5-Feb 11
Thickness or strength? We need to estimate how strong the plastic AND the brick need to be to withstand the maximum pressure.

15. Cont. Build prototypes for testing Begin digestion
Feb 12-Feb 18
Begin digestion
Feb 19- March 4
Detail the design of full scale digester
Feb 19-March 4
Obtain experimental data

16. Additional Citations
Saubolle, B., & Bachmann, A. (1983). Fuel Gas from Cowdung. Kathmandu: Sahayogi Press.