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
Two parents, four children
1 cow (60-65% of families own at least 1 cow in rural Bangladesh)
Make $45 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 focusing 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 45 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.
“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 family is looking for new opportunities to improve their standard of living. 2

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.
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
To do:
Cite source
Put in picture of digester for steps (water +manure  slurry) Tiffany
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) 3

4. Biological Processes

5. Biogas Production Technology: An Indian Perspective
(Nagamani, B. and K. Ramasamy, 1999)
On average, a cow in India produces 3.6m3 or 3600 L of biogas per day. This yields approximately 76,280 BTU/animal/day.

6. 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.

7. Overall Design Specifications
Costs $89 or less (present value)
-Calculated using Grameen Bank’s simple interest 8% housing loans with $8 monthly payments
Produces 2800 liters of biogas/day
15,000 Kcal (59,500 BTU)
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 7

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. Portable Small Digester
Brainstorming
Improve C:N ratio
Mixing in Digester
Heating by Compost
Portable Small Digester
Improve C:N ratio  Cow manure has good enough C:N ratio already. If we were to add more additive, it will effect the pH.(optimum pH= )The anaerobic bacteria is really sensitive to the pH. Thus it will require a lot of technical knowledge to maintain the digester in good working condition
Optimum 30:1
Mixing Since we are building a small digester, mixing is not as important. However, inserting a stick through the exit or inlet from time to time would be good to break up the slum.
Heating by compost  Increase heating wont effect much ? (Graph) Temp= around 35
Portable small digester  prone to error, Transporting. Impossible to mass produce for everyone.
Greatest opportunity to improve the cost efficieny… is material choice.
Materials

10. Specific Design Criteria
Cost
Material Availability
Efficiency of gas product
Longevity
Ingenuity
Maintenance
Ease of Use
Assembly
Production

11. Pairwise Comparison Matrix
Cost
Material
Efficiency of gas
Longevity
Ingenuity
Maintenance
Ease of use
Assembly
Production
Total - 1 8
Material Availability 5
Efficiency of gas product 4 3
Ease of Use 7
Pairwise Comparison Matrix
1) determine qualitatively which criteria are more important - i.e. establish a ranking of the criteria, and
2) assign each criterion a quantitative weight so that the qualitative ranking is satisfied.

12. 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)

13. Hybrid Digester Design
Top View
Side View
Plastic Cover
Brick Digester Tank

14. Decision for Choosing Design Criteria
Brick
Plastic
Hybrid
Quality
Weight
Value
total
Cost 8 3 24 4 32
Ease of Use 7 21 28
Material Availability 5 25 15 20
Maintenance 2 10
Efficiency of Gas Product 12
Assembly 1 9
Longevity 6
Production
Ingenuity
Total
114
121
135

15. Hybrid Digester Design: Bottom
PVC Hooks
0.15m
2.4m
2.4m
1.39m
Brick and Mortar

16. Black Plastic Cover
PVC Elbow
PVC Pipe
2.3m
Glue
Plastic Cover
2.3m

17. PV = nRT (Assume T is a constant)
Slurry Exit
PV = nRT (Assume T is a constant)
Plastic Cover
Gas Produced
Digester
Some biogas is produced
Plastic Cover is not yet taught
Volume Increased
Pressure remain about constant.
No biogas produce
No pressure difference

18. Calculation for Required Pressure to Supply Biogas Stove
Plastic Cover
Gas Produced
Hagen-Poiseuille Equation
Qv =  ΔP/R = πD4ΔP/(128μL)
u = 1.71X10-5 Pa s at 30 °C
L ~ 20m
D = 1/2" PVC pipe for flow L/hr
Qv = L/hr
ΔP = Pa – Pa
The above pressure range is needed to achieve flow rates of 200 L/hr and 450 L/hr, respectively.
ρgh
Digester
Plastic Cover is taught
Volume Increased
Pressure Increased
Height Difference in Slurry Levels.

19. Next steps for Plastic Covering
Identify ideal plastic covering material
Determine how to deliver relatively constant pressure
Protect digester from rain
Use this added layer to provide solar heating
Prevent damage at digester-plastic interface to improve lifespan of cover

20. Future Work Building Testing Scale down the hybrid design
Order parts for the digester
Testing
Measure amount of gas produced
Dr. Speece’s Wet Tip Gas Meter
Measure composition of produced gas
Dr. Debelak’s Gas Chromatography