Design of Scalable Biogas Digester for the Developing World

Vanderbilt University, School of EngineeringBME 273, Group 19

Submitted on April 27, 2010

Group Members:Tiffany Cheng (ME)

Thomas Davis (BME)Dawn Schmidt (ChemE)

Kyle Schroeder (CE)Andrew Wu (BME)

Sponsors:Project Pyramid

Vanderbilt University, School of Engineering

Advisor:Dr. Paul King

Dr. Dave Owens

Table of Contents

I. Introduction1.1 Project Pyramid, Bangladesh, and Biogas Digester Review 11.2 Goals 21.3 Previous Biogas Digester Designs 4

II. Methodology2.1 Identification and Evaluation of Opportunities for Cost Reduction 52.2 Evaluation of the Candidate Materials 72.3 Evaluation of Plastic Materials 72.4 Theoretical Framework 82.5 Prototype Experimental Setup 82.6 Gas Chromatography 92.7 Cost Analysis 10

III. Results3.1 Mass Balance 103.2 Gas Chromatography 113.3 Safety Considerations 113.4 Economic Analysis 12

IV. Conclusions 14

V. Recommendations 145.1 Future Work 145.2 Ethical Concerns 155.3 Societal Impact and Related Concerns 165.4 Final Thoughts 16

References 17

Appendix 18

Abstract

Biogas digestion is the process of allowing organic material to undergo anaerobic

conversion to biogas facilitated by the presence of bacteria. Project Pyramid, a student

organization at Vanderbilt University’s Owen Business School that addresses issues of poverty,

realized in March of 2009 that the country of Bangladesh has a need for the cheap and available

energy that biogas provides. Specifically, biogas was identified as an existing technology with

considerable opportunity for growth in this country; however, the current situation regarding

biogas technology is economically and pragmatically unsatisfactory. Biogas digesters, the

device that facilitates the production of biogas, cost over $200, which is more than 4 months of

an average family’s income. Furthermore, the technology was difficult to understand, maintain,

install, and therefore, spread across the rural parts of Bangladesh. As part of Vanderbilt’s School

of Engineering, we sought to design a new biogas digester that would meet the energy needs of

an average family household while 1) costing no more than $89, 2) producing 2800L of

biogas/day, 3) adequately function for ≥ 5 yrs, and 4) be easily installed. We determined that

through changing the biogas digester material to polyethylene terephthalate (PET), biogas

digesters could be cheaply made en mass and still provide quality biogas and usability. Two 5

gallon PET bottles were set up as prototypes to test the feasibility of biogas production using this

new material and determine rates of production. Through financial analyses and data collected

from the prototype experiments, we concluded that it is possible to use PET to construct biogas

digesters for Bangladesh for $97. Although this is higher than our target retail price, it is cheaper

than what rural Bangladeshi’s currently spend on biogas energy. Future work will continue to

improve and further this project until the ultimate goal of having Bangladesh self-sufficiently,

safely, and efficiently meet its energy needs is realized.

I. Introduction

1.1 Project Pyramid, Bangladesh, and Biogas Digester Review

The Vanderbilt University Engineering School’s Senior Design class was approached by

Project Pyramid, a business venture focusing on alleviating poverty by means of business

incentives sponsored by Owen School of Management and other Vanderbilt University graduate

schools, to help design a scalable biogas digester for the developing world.

            A biogas digester is a device that uses anaerobic digestion to convert cow manure and

other organic material into biogas, a feasible energy source. The cow manure and other organic

bio-waste are collected and mixed with water to produce slurry. The slurry is then placed into the

air-tight digester where the bacteria that naturally reside in bio-waste begin the anaerobic

digestion process.  Organic matter, which includes carbohydrates, proteins, and fats, is initially

digested by acid-forming bacteria into simple organic acids (Information and Advisory Service

on Appropriate Technology [ISAT], n.d, vol. 1 p.9).  These simple organic acids (acetic acid,

propionic acid, butyric acid, and formic acid) are further broken down by other methane-forming

bacteria to produce a gaseous mixture composed of 40-70% methane, 30-60% carbon dioxide

and the final 1-5% is an assortment of other inert gases (ISAT, n.d, vol. 1 pp.4 and 9). After the

digestion process is complete, the residual slurry is forced out of the digester by the addition of

fresh slurry.  This digested slurry exits the digester at ground level, where users can obtain and

use it as a fertilizer.

           Project Pyramid’s interest in biogas began after a third visit to Bangladesh during which

the organization found a great need for both sustainable energy and improved sanitary

conditions. The country is wrought with malnutrition, short life spans, high child mortality rates,

and water-borne diseases (CIA, n.d.).  These issues are largely due to the unsanitary practices of

open defecation and overflowing latrines.  The consequences of these practices can be alleviated

through the use of biogas digesters which offer an economic incentive to properly contain

sewage.  The country is primarily agrarian and utilizes livestock for economic purposes and

basic subsistence.  Any additional feed required for the digester could be obtained from these

livestock.  As for the need for sustainable energy, Bangladesh is currently going through an

energy crisis with a supply shortage of natural gas, the country’s principle source of energy

(Khan, 2010). Biogas can be used as a replacement for traditional natural gas, which would

reduce the country’s dependence on natural gas supplies. Bangladesh is also an opportune

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location to market digesters because it has over 116 million rural residents, which allows for a

large potential customer base for this product (CIA, n.d.). The concept of biogas is also not a

new idea to Bangladeshi’s.  Non-governmental organizations (NGOs) have set up biogas

digesters in villages and individual rural homes.  However, these digesters cost over $200 and

most Bangladeshi families cannot afford them on their own (S. Songy, personal communication,

March 5, 2009).  Considering all of these factors, we found that Bangladesh is an appropriate

environment for designing a biogas digester for developing countries.

1.2 Goals

            Before fully developing our design goal, it was necessary to specify the focus and

expound on two aspects of this project: the design of the biogas digester and the target

population of this product, specifically the rural population of Bangladesh.  For the design

portion, we considered the features of our ideal digester such that it would adequately meet the

energy needs of the target population.  For example, we considered the life span, durability, gas

permeability, affordability, and availability of the biogas digester material.  Research aided us in

this process of determining how to specifically approach the design of a cost-efficient digester.

In order to gather information on Bangladeshi population demographics, their typical rural

lifestyles, price points on materials and services in Bangladesh, and other important facts, we

established and maintained contact with Project Pyramid correspondents both at Vanderbilt and

in Bangladesh.  Over the course of three months, the necessary facts and literature about digester

designs and Bangladeshi's rural demographic were accumulated and organized to explicitly

finalize our project's goals.

            We determined that our goal is to design a biogas digester with the ability to meet the

cooking related energy needs of an average-sized Bangladeshi family by utilizing the bio-waste

of one cow. The typical Bangladeshi family has six members (i.e. two parents and four children),

and about 65% of families in Bangladesh own at least one cow (K.S. Salehin, personal

communication, January 22, 2010). The average family makes about forty-five dollars a month,

and spends approximately ten dollars per month on cooking fuel for the family (K.S. Salehin,

personal communication, January 22, 2010). They also spend approximately two weeks per year

collecting wood and straw as additional fuel (K.S. Salehin, personal communication, January 22,

2010).  We wish to design a biogas digester that is cheap enough so that a typical Bangladeshi

family can realistically finance the purchase of a digester.

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            Affordability for the people of rural Bangladesh is our main design criterion for the

digester. In order to be affordable, it must have a retail cost of $89 or less.  In this way, the

biogas digester can be financed over a 12 month period with payments of eight dollars per month

at an eight percent simple interest rate. If the digester is able to produce enough gas to generate

59,500 BTU of energy per day, it will completely replace their current fuel costs of ten dollars

per month (Saubolle, 1983, p. 10). This would save them two dollars per month for the first year

and ten dollars per month for every year after. In order for the product to produce large economic

effects for the family, the product must have a lifespan of at least five years. This would save the

family $500 over the product’s lifetime, which is equivalent to almost one year’s salary.

            It was also important to know how much waste an average Bangladeshi cow can

produces. This amount was estimated using a study done by Nagamani, B. and K. Ramasamy, on

the manure productivity of Indian cows (1999). This study was selected because India shares

many environmental, economic, and agricultural similarities with Bangladesh. The study

determined that the average Indian cow produces ten kilograms of manure per day, which yields

approximately 3.6 cubic meters of biogas per day. This yields approximately 76,280 BTUs per

cow per day, which is more than enough for the required 59,500 BTU needed to cook for a

family of six (Saubolle, 1983). Given this data, an 80% efficiency rating on the biogas digester is

enough to meet our design specifications.

            The two digester design criteria of cost effectiveness (maximum $89 retail price) and

minimum required energy production (59,500 BTU or more) combined to provide a foundation

for further investigation into the overall design of a scalable biogas digester that would be the

most beneficial for the energy needs of the people of Bangladesh.

1.3 Previous Biogas Digester Designs

We began our approach to this project by gathering information on various biogas

digester designs found around the world, each with their own advantages and disadvantages.  We

found that there are four major digester designs: the floating drum digester, the egg-shaped

digester, the plastic bag digester, and the fixed dome digester.

    The floating drum digester design consists of an underground digester tank and a gas tank.

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The gas tank floats on the slurry (or

sometimes a water jacket) contained in the

underground digester tank (ISAT, n.d, vol. 1

p. 17).  As gas is produced and collected in

the gas tank, the tank floats up (or down if

gas is consumed) and is guided by a frame

to prevent tilting (ISAT, n.d, vol. 1 p. 17).

The volume of gas available is clearly

visible, the construction is relatively easy,

and it is not highly sensitive to construction

mistakes (ISAT, n.d, vol. 1 p. 17).

However, the costs are quite high, especially for the floating drum, which is usually made of

steel, which is also susceptible to corrosion (ISAT, n.d, vol. 1 p. 17).  Therefore, floating drum

designs usually have short lifespans and higher maintenance costs (ISAT, n.d, vol. 1 p. 17).

    The egg-shaped digester is the best solution as far as overall efficiency of the gas digestion

and the structural strength (ISAT, n.d, vol. 2 p. 20). The reason for this efficiency is the surface

to volume ratio is ideal for anaerobic digestion (ISAT, n.d, vol. 2 p. 20). By having a very large

and curved bottom that tapers off towards

the top in a curve manner allows for the gas

to naturally rise and allows the pressure to

dissipate more evenly throughout the

system. The shape though comes with a

cost. It is very hard to construct and thus

much more expensive than the other

digester because it requires very strong

material like reinforced concrete and

metallic cladding as an outer shell (Gilbert,

2010). This is why the egg-shaped digester is mostly utilized by large scale and industrial sized

settings.

4

Figure 1: Floating drum digester. http://www.snisd.org.cn/images/05126b.JPG

Figure 2: Egg shaped digester. http://www.water-technology.net/projects/reading_sewage/images/Island-Road-2.jpg

    The plastic bag designs, also called balloon digesters, consist of a digester bag that holds the

manure and collects the gas in the upper portion

(ISAT, n.d, vol. 1 p. 16).  The inlet and outlet tubes

are attached directly to the plastic skin, and gas

pressure is achieved through the bag's natural

elasticity and by adding weights to the balloon

(ISAT, n.d, vol. 1 p. 16).  This design is relatively

cheap, easy to transport, understand, maintain, and

construct (ISAT, n.d, vol. 1 p. 17).  On the other

hand, it has a very short life span and high

sensitivity to damage (ISAT, n.d, vol. 1 p. 17).

    The fixed dome design consists of a digester tank and a fixed gas holder that sits on top of the

digester tank (usually sealed to the tank for gas

barrier purposes), which stores that gas and creates

the necessary gas pressure (ISAT, n.d, vol. 1 p. 17).

Fixed dome designs are relatively cheap to construct,

have no moving or rusting parts, have a long life

span, save space because they are planted

underground, and thus are also protected from

significant temperature changes (ISAT, n.d, vol. 1 p.

17).  The main disadvantage of this design is the lack

of gas barriers in the dome portion (i.e. cracks from

construction) or in the interface between the dome

and tank (ISAT, n.d, vol. 1 p. 17).  Therefore, this design usually requires an expert technician on

site (ISAT, n.d, vol. 1 p. 17).

II. Methods

2.1. Identification and Evaluation of Opportunities for Cost Reduction

After establishing a strong, fundamental understanding of the existing state of biogas

technology, an emphasis was placed on understanding the cost drivers of a biogas digester, since

in the end this is the underlying goal of the project. It is readily evident that the cost of materials

and the corresponding construction or manufacturing due to the size of the digester is a

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Figure 4: Fixed dome digester. http://www.inforse.dk/asia/images/M_III_biogas3.jpg

Figure 3: Plastic bag digester. http://www.saintsfarm.org/photos/biogas_digester_2_large.jpg

significant determinant of the cost of a digester. Nevertheless, other opportunities for reducing

the cost of the digester were explored. Namely, by improving the efficiency of biogas

production, the retention time of manure in the digester could be reduced. Since the size of the

digester is proportional to the retention time and the volume input of slurry, decreasing the

retention time could significantly reduce the cost of the digester. Several strategies for improving

the efficiency of the digester were evaluated: improving the carbon:nitrogen ratio, heating, and

mixing.

In a meeting with Dr. Alan Bowers, it was confirmed that the inherent carbon:nitrogen

ratio of cow manure is approximately 20:1, or close to an already ideal value (A. Bowers,

personal communication, January 26, 2010). Efforts to marginally improve the carbon:nitrogen

ratio would introduce organic material that would put the digester at risk of maintenance

complications, specifically clogging that would induce additional work and cost. Though heating

and mixing both appeared to have potential to improve digestion efficiency, no literature

suggested any significant improvement in efficiency—at least none that would in itself be

capable of meeting the targeted price point. Meanwhile, the selection of appropriate materials for

the construction of a digester presented considerable opportunity for decreasing the cost of the

digester since the material itself is a significant cost driver. Consequently, the scope of the

project was focused on identifying the most appropriate material for the digester.

While identifying candidate materials, the group reached out to Bangladeshi contact Kazi

(Shaheer) Salehin to learn more about the current cost of materials in Bangladesh (K. Salehin,

personal communication, January 22, 2010). From this, it was determined that the current cost of

concrete digesters is 10% more expensive than the cost of a brick digester. Concurrently, a list of

materials was developed without respect to any preconceptions regarding the utility of that

material. The list included materials as diverse as tar, glass, wood, and natural rubber. Example

brainstorming included digging a hole and lining it with tar. After this brainstorming phase, the

ideas were evaluated for their feasibility. For example, tar was found to be toxic to cells and

consequently an unviable solution. Following this process of ideation, the top three methods

were identified—brick, plastic, and a hybrid of the two materials. Masonry was chosen because

of its availability in Bangladesh, whereas plastic was chosen because of its wide range of

properties and usage.

2.2. Evaluation of the Candidate Materials

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In order to evaluate the proposed materials, a list of design criteria was made, which

includes parameters such as cost, longevity, and material availability. From this, the criteria were

evaluated relative to each other to produce a Pairwise Comparison Matrix (See Appendix A.1).

The matrix showed that cost was the top priority for material choice, followed closely by ease of

use and then material availability and maintenance. From this, each criteron was assigned a

weight that was inputted into the Selection Matrix. A value for each material choice was then

assigned to each respective design criterion based on initial sketch prototypes used for each

proposed material (See Appendix A.2). The product of the value and the weight was then

computed. The sum of products for each material is then computed and compared to identify the

most appropriate material—plastic. Appendix A.3 contains a copy of the results.

2.3. Evaluation of Plastic Materials

Designing with plastic is a particularly exciting area since there is considerable

opportunity for creativity and unexplored opportunity for cost reduction. Brick, on the other

hand, is a rather standard material with less opportunity for innovation. However, before

considering a variety of plastic shapes to minimize cost, it was essential to identify the most

appropriate plastic material. In doing so, several plastic materials were considered with the

leading candidates being

polypropylene (PP), polyethylene

teraphthalate (PET), polyethylene

(PE), and polyvinyl chloride

(PVC). A matrix (See Appendix

A.4) was created to evaluate

durability, mechanical properties,

permeability, and cost. Though no

data were readily available for

permeability of plastics to

methane, a method of estimating permeability relative to other permeability coefficients was

used (T. Harris, personal communication, March 12, 2010). Based on the compiled data specific

to the four analyzed plastics, PET was identified to have the most appropriate combination of

characteristics.

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Figure 5: Price and permeability are plotted for four plastics.

Though PET does posses a strong set of properties that include high yield strength, low

permeability, and low cost, there was initially some concern regarding the degradability and

specifically the biodegradability of the plastic. However, literature shows that degradation of

PET below its glass temperature due to hydrolytic and thermal degradation is rather insignificant

(Pirzadeh et al., 2007, p. 1544-1549). Additionally, PET is shown not to biodegrade in the

presence of high concentrations of microbes and lipases, suggesting low biodegradability (Zhang

et al., 2004, p. 1089 – 1096).

2.4. Theoretical Framework

Following the identification of the most appropriate plastic material, a theoretical mass

balance equation was written to describe the availability of biogas within the digester for

continuous flow digestion:

From this equation, individual terms can be experimentally identified. By focusing on the first

term on the right hand side, the size of the digester can be optimized so that the digester is

neither too big and consequently too expensive nor too small and unable to meet the fuel

demands.

2.5. Prototype Experimental Setup

The above theoretical mass balance equation models the continuous flow digestion as a

summation of batch processes and assumes no mixing. Therefore, the tested prototype is

designed to evaluate the biogas potential for a specific input of cow manure (See Appendix A.5

for detailed diagrams).

Unfortunately, when starting a new digester, the bacteria generally require considerable

time to reach equilibrium, or a pH level at which digestion can occur. In order to avoid this lag in

digestion onset, the prototype was filled with 100% seed from the Biosolids Facility located in

downtown Nashville, as suggested by Dr. Dick Speece (See Appendix A.6). This experimental

design requirement necessitated the construction of two prototypes—one filled with pure seed

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and another filled with the same volume of seed plus a measured amount of fresh cow manure.

The difference in biogas production is assumed to be the gas produced by the fresh cow manure.

Two 5 gallon PET bottles were purchased

and each filled with 17 L of 100% seed. The

variable digester was filled with an additional 1.5 L

of freshly collected cow manure from Triple L

Ranch near Franklin, Tennessee. Both PET bottles

were capped with carboy caps purchased from

Nashwood Winery (Dallas, TX) and tightened using

hose clamps. Plastic tubing with inner diameter ½"

was connected to a 3/8" brass T-tube using plastic

adapters. Radiating from each end of the bifurcation

were two plastic tubes each connected to a 3/8" brass ball valve, which also required plastic

adapters. From one ball valve, plastic tubing ran into a 5 gallon bucket filled approximately ¾

full with water. A 2 L graduated cylinder was inverted for collection of biogas from the tube.

Half inch tubing was used since the literature indicates that this is the preferred tubing diameter

for biogas delivery in home settings (ISAT, n.d, vol. 2 p. 49). All experiments were conducted in

a hot room set at 35°C to model typical temperatures in Bangladesh.

Due to the high rate of biogas production and the inability to invert a graduated cylinder

of appropriate size, the rate of biogas production was calculated by measuring the volume of

biogas produced in one hour intervals approximately every twelve hours. This rate of production

during each one hour interval was assumed to be the rate of biogas production across the

respective twelve hour period.

Unfortunately, due to a series of unexpected events including gas leaks in the tubing and

foaming (See Appendix A.5), the data collection process had to be restarted multiple times.

Consequently, only seven days of continuous data were successfully collected. In order to

identify the gas leaks, a soapy water mixture was sprayed on the prototypes and air was blown

through the tubing. Bubbles indicated gas leaks, which were addressed by adding hose clamps or

parafilm to the plastic adapters. Foaming, however, remains an unresolved problem within the

biogas field. According to Dr. Speece, the cause of this phenomenon is unknown.

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Figure 6: The output from each digester is collected in an inverted graduated cylinder.

2.6. Gas Chromatography

In order to confirm successful production of methane, gas chromatography was

performed in Dr. Kenneth Debelak's laboratory. Due to machine malfunction, Dr. Debelak's

laboratory was unable to analyze all the biogas constituents. Instead, an HP 5890a GC system

was used, which was only able to identify organic constituents such as methane due to the

method of analysis used in the system. The area under the curve of the methane peak from each

biogas sample is related to the area under the curve of the pure methane sample, the percent

composition of methane in the biogas samples is determined.

2.7. Cost Analysis

The method of estimating cost of manufacturing a plastic biogas digester was determined

through conversations with engineer Michael Sykes at Plastic-Mart.com. Plastic-Mart.com uses

rotational molding to manufacture its product. The company estimates the cost by first

calculating the cost of materials and power and utilities. Labor is then estimated to be 20% of the

sum of the aforementioned cost. The machine payment is then calculated similarly as a

proportion of the cost of materials and power and utilities. A gross profit margin of

approximately 32% is then assumed. Finally, two molds will be needed, each with a cost of

$65,000. The cost of tooling is a fixed cost, and the allocated cost per unit thereby decreases as

the production level increases. Finally, the cost of accessories is estimated based on online prices

for items such as PVC piping and PVC cement.

III. Results

3.1. Mass Balance

Resulting from complications

due to foaming and initial gas leaks

in the connections, the

aforementioned experimental

procedure was repeated.

Unfortunately, only seven days of

data were collected for analysis.

From this, we observed a differential

total of gas production between the

variable and control digester of 21 L.

10

0

5

10

15

20

25

0 2 4 6 8Time (Days)

Bio

gas

Prod

uced

(L)

Figure 7: The difference in rate of biogas production between the variable and control digesters was calculated to determine the volume of biogas produced due to the 1.5L additional input of fresh cow manure.

Therefore, 1.5 L of input fresh manure yielded 21 L of biogas within one week. Though

published literature reports an expected 60 L of biogas production, the rate of biogas production

remains relatively constant at 1.5 mL/min (2.2 L/day) for the last 3 days of data collection in

Figure 7 (Saubolle, 1983, p.13). This suggests that significant biogas generation potential

remains within the prototype digester. Raw data and analyzed data are available in Appendix B.1

and Appendix B.2, respectively.

3.2. Gas Chromatography

Figure 8 indicates that the collected biogas produced in the prototype has an average

methane content of 66.5%. The areas under the curve (AUC) of the first two major peaks

(162.442 and 154.7695) in the gas chromatography correspond to biogas samples 1 and 2,

respectively. These

areas are compared to a

pure methane sample

with an area under the

curve of 238.424. The

ratio of AUC for each

respective biogas

sample to that of the

pure methane sample

concludes that sample 1

and sample two have a

methane composition of 68.1% and 64.9%, respectively. These results are consistence with

literature review, which expects biogas to contain 40%-70% of methane.

3.3. Safety Considerations

Three major safety considerations of the biogas digester are the gas composition,

foaming, and flooding in the Bangladesh countryside. When biogas mixes with air at a ratio

between 1:6 and 1:20, a highly explosive gas mixture forms (Bell, 1973, p.22). Consequently,

two areas of focus must be appropriately addressed to minimize this risk. First, when the digester

is initially being filled with manure substrate, it is essential that the user of the biogas fuel not

use the biogas that is produced at the beginning of the process. Therefore, the user should allow a

volume of biogas to be produced that is equal to the headspace in the digester so that the

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Figure 8: By computing relative areas under the curve for biogas samples 1 and 2 using gas chromatography, the produced biogas contains 68.1% methane and 64.9% methane, respectively.

explosive ratio of gases is removed. Second, gas leaks in the pipes contained in enclosed areas

can pose a safety hazard. Therefore, it is essential to ensure proper ventilation to further dilute

the leaking biogas. This can be achieved by ensuring that nearly all piping is outside the home

with only the most distal end emerging in the home for cooking.

Foaming is a phenomenon during which the digester unexpectedly burps its contents

through the piping. Generally, this disrupts the digestion process and provides an inconvenience

to the user. However, foaming has been reported to produce more violent eruptions, causing the

top of a digester to come off at the Biosolids Facility in downtown Nashville. Unfortunately,

according to Dr. Dick Speece, the cause of foaming remains unknown and, therefore, preventing

this phenomenon is currently unrealizable.

Finally, the environmental conditions of Bangladesh pose unique concern with respect to

the presence of biogas digesters. Namely, Bangladesh frequently experiences flooding which can

expose contained waste to flood waters. Subsequent design iterations should be taken to

determine methods of fully sealing the digester, a process which can also be designed to prevent

flood water from uplifting the digester. Other safety considerations are addressed in a complete

DesignSafe report given in the Appendix B.3.

3.4. Economic Analysis

Though Bangladesh is a country with a small landmass, its population is staggering. The

country is estimated to be home to over 260 million, making it the most densely populated

country in the world—excluding city-states. According to the Central Intelligence Agency public

database, 73% of the Bangladeshi population is rural. Among the country's rural inhabitants, 60-

65% of families own at least one cow. The average family size is 5-6 people. Therefore, we

estimate that over 21.5 million families live in rural Bangladesh who own at least one cow (CIA,

n.d.).

Bolstering this market potential is Bangladesh's high population density, which improves

the efficiency of the supply chain. Additionally, Bangladesh is fortunate to neighbor both China

and India and is commonly referred to as "Chindia." Each neighboring country is home to over 1

billion people—each of which remain highly rural countries. Clearly, the market potential is

significant.

The threshold value used to evaluate the success of the product through a cost-benefit

analysis is $89. This value significantly weighs a rural and poor family's risk aversion to

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investing in a new technology as a source of cooking fuel. This target price point was

conservatively calculated so that a retail price of the digester higher than the target price can still

have a high probability of being a viable product. A more detailed explanation of the method

used to calculate the target price point can be

found in Appendix B.4, Target Price Point

Determination.

The currently projected cost of an 8m3

digester is $97. However, there are some

assumptions that must be explicitly discussed.

First, the estimate is based on the volume and

thickness of the digester. The current price is

calculated for an 8m3, though it is possible that

the size optimization will be near 6m3.

Additionally, future tests specific to determining

the minimum acceptable plastic thickness for

various digester shapes must be performed in

order to determine the total weight of raw

material needed. More complete evaluation of

both of these intrinsic design characteristics

could affect the cost estimate significantly in

either direction.

Second, the method of estimating the

cost of production at a 100,000 unit level of

production is based on an ongoing conversation

with engineer Michael Sykes at Plastic-

Mart.com. The same method and ratios used by

Plastic-Mart.com for estimating the cost of

manufacturing were applied to estimating the

cost in Table 1 (M. Sykes, personal communication, March 31, 2010, and April 23, 2010).

Finally, the estimated costs of installation, maintenance, and transportation were not

included. Installation and maintenance are expected to have a small affect on the retail cost, due

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Table 1: The per unit cost of manufacturing the proposed biogas digester is calculated using the procedure outlined through conversations with engineer Michael Sykes at Plastic-Mart.com.

0.7m

1.4m

1.35m0.1m

Slurry Intake

Gas Release

SlurryExit

0.5mm

PVC

Gas Holder

Digester

0.7m

1.4m

1.35m0.1m

Slurry Intake

Gas Release

SlurryExit

0.5mm

PVC

Gas Holder

Digester

Figure 9: The dimensions of the PET digester are used to estimate the cost of materials.

to the longevity of the digester and the low cost of labor. Transportation, however, is highly

dependent on the efficiency of the supply chain.

IV Conclusions

   While the design criteria have not been fully met, the current status of the design process is one

that has reached many important progress markers that provide considerable direction to

catalyzing future work. Namely, a simple methodology for optimizing the size of the digester

was developed, which can be easily applied in the field to test Bangladeshi cows. To evaluate the

underlying theoretical framework, we designed a prototype model that was able to determine the

rate of production of biogas from a given amount of manure. It should be noted that this

prototype experiment was unable to reach expected completion because progress was hindered

and/or delayed by gas leaks, faulty gas plugs, and foaming. However, despite those setbacks we

were able to collect 21 liters of biogas from 1.5 liters of manure, and we found that the biogas

production rate continuous to decrease but had not yet reached a negligible rate of production for

completing the size optimization.  

    Preliminary economic analyses and experiments indicate that it is economically and

pragmatically feasible to manufacture a biogas digester of eight cubic meters for $97 for rural

Bangladesh, which is eight dollars higher than our pre-determined price point of $89.  However,

it is still an acceptable cost given the average family income per month, for our data suggests that

instead of paying $8 per month for a year under a loan with 8% interest, the family can pay $9

per month, which is still cheaper than the $10 per month the family currently spends on fuel.

Furthermore, over time (i.e. after a year) the reduction in cost will be quite substantial.  Also,

these estimates are based on preliminary approximations of the optimal thickness and size of the

digester, and will change depending on results from future experiments.  The $97 estimate

suggests that an affordable biogas digester is possible and that future experiments should be

performed concerning this topic. Ultimately, these price estimates along with gas production data

strongly suggest that Project Pyramid’s endeavor to meet rural Bangladesh's energy needs can be

both profitable and beneficial to the Bangladeshi people.

14

V. Recommendations

5.1. Future Work

Before realizing the ultimate end goal of implementing a full-scale, plastic digester in

Bangladesh, the current studies and work will need to be verified and built upon with more

testing and research.  Specifically, the short-term (i.e. within months) goals are:

To repeate the prototype experiments to verify our data

To calculate the required residence time of the slurry in the digester once biogas

production rate approaches zero

To calculate the required input and output rates of slurry

To calculate the appropriate dimensions of the full-scale digester (i.e. tank, input and

output stations, pipes) by using the calculated residence time of the slurry

To contact plastic manufacturers in Bangladesh to find source of mass production of

plastic digesters

Our long-term (i.e. within years) goals are:

To determine the minimum digester production number that yields a digester retail

price of no more than $89

To obtain and test sealants for plastic cap and tank interface

To obtain and test PVC pipes for durability, leaking, etc.

To locally construct and test a full-scale plastic digester

To implement a full-scale plastic digester in Bangladesh

5.2. Ethical Concerns 

          Considering that the first biogas digester implemented in Bangladesh was in 1972, and that

digesters have propagated in use throughout the country since then, there are few new ethical

concerns that our project faces (Jamil, 2007, p. 9).  The novelty of our design involves the cheap

mass production and efficient design material of our biogas digester, characteristics that should

not impede any ethical concerns in Bangladesh.

          However, existing ethical barriers to the implementation of biogas digesters should be

noted, most of which arise from cross-cultural differences.  Some Bangladeshi’s recognize

religious beliefs that have strict regulations regarding cleanliness, especially within the context

of human and animal waste (ISAT, n.d., vol. 1 p. 39).  If these religious beliefs were to be in any

way suppressed, then the installation and use of these digesters might be viewed negatively and

15

thereby, potentially barred or otherwise unused and inefficient.  Therefore, the approach to

implement this product in Bangladesh must be done so with consideration of the current

domestic social institutions. 

5.3. Societal Impact and Related Concerns 

          In order to handle any socio-cultural taboos that may arise such as the issue with

cleanliness, highly respected members of a community or educational institutions can participate

and overcome these taboos by way of example.  Of course, these persons and groups would need

to be thoroughly verified for cultural acceptance before allowing these parties to become a

mediator for this project. 

          It has been found that another socio-cultural taboo outside of the religious realm involves

the use of digested sludge for fertilizer, as it is viewed as culturally “unclean” (ISAT, n.d., vol. 1

pp. 39-40).  This concern is particularly prominent in those areas that are new to biogas digester

technology.  One way this can be resolved is to conduct demonstrations with credible parties to

prove the hygienic and safety characteristics of the digester.  Unfamiliarity with the digester

technology may also cause users to irregularly maintain and use the digester, which can cause

inefficient biogas production because of fertilizer build-up or low amounts of slurry in the tank. 

To correct for this, the user would need to be trained on how to use the digester in regards to

when and how to load and unload the slurry and fertilizer, respectively.

5.4. Final Thoughts

The research and work described in this paper provides a solid foundation on which the

aforementioned future work can not only build upon, but be justified by.  The vision of this

project will be realized when the rural areas of Bangladesh can self-sufficiently meet their energy

needs without NGO intervention or constant expert advice.

16

References

Bell, C., Boulter, S., Dunlop, D., & Keiller, P. (1973). Methane: Fuel of the Future. Dorset, England: Prism Press.

Central Intelligence Agency. (n.d.). The World Factbook: Bangladesh. Retrieved April 25, 2010, from https://www.cia.gov/library/publications/the-world-factbook/geos/bg.html

Gilbert, J. (n.d.). Reading Sewage Treatment Works. Retrieved April 25, 2010, from Thames Water: http://www.thameswater.co.uk/cps/rde/xchg/corp/hs.xsl/5671.htm

Information and Advisory Service on Appropriate Technology. (n.d.). Biogas Digest (Vols. 1-4). Retrieved April 25, 2010, from GTZ: http://www.gtz.de/de/dokumente/en-biogas-volume1.pdf

Khan, S. (2010, April 26). The Power-Rush. Retrieved April 26, 2010, from The Daily Star: http://www.thedailystar.net/newDesign/news-details.php?nid=135927

Nagamani, B., & Ramasamy, K. (1999). Biogas Production Technology: An Indian Perspective. Current Science , 77 (1), 44-55.

Pirzadeh, E., Zadhoush, A., & Haghighat, M. (2007). Hydrolytic and Thermal Degradation of PET Fibers and PET Granule: The Effects of Crystallization, Temperature, and Humidity. Journal of Applied Polymer Science , 106 (3), 1544-1549.

Polak, P. (2009). Out of Poverty (1st Edition ed.). San Francisco: Berrett-Koehler Pub.

Saubolle, B., & Bachman, A. (1983). Fuel Gas from Cowdung. Kathmandu, Nepal: Sahayogi Press.

Zhang, J., Wang, X., Gong, J., & Gu, Z. (2004). A Study on the Biodegradability of Polyethylene Terephthalate Fiber and Diethylene Glycol Terephthalate. Journal of Applie Polymer Science , 93 (3), 1089-1096.

17

Appendix- Table of Content

I. MethodologyA.1 Pairwise Comparison Matrix for Evaluating Specific Design Criterion 19A.2 Initial Sketch Prototypes Used for Each Proposed Material 20A.3 Evaluation Chart for the Three Proposed Designs 21A.4 Material Properties of Plastic 22A.5 Prototype Experimental Setup 23A.6 Emails and Meeting Notes with Dr. Speece 25A.7 Ideation Process IWB Conflict Map 32

II. ResultsB.1 Raw Data of the Gas Production of the Two Prototypes 40B.2 Data Analysis of the Prototypes on Gas Production 41B.3 Target Price Point Determination 42B.4 DesignSafe Report 61

18

Appendix A.1: Pairwise Comparison Matrix for Evaluating Specific Design Criterion

19

Appendix A.2: Initial Sketch Prototypes Used for Each Proposed Material

Figure 10: Schematic Diagram of Brick Designhttp://www.journeytoforever.org/biofuel_library/methane_nepal.html

Figure 11: Schematic Diagram of Plastic Design

Figure 12: Schematic Diagram of Hybrid Design

20

http://www.ruralcostarica.com/biodigester.html

Appendix A.3: Evaluation Chart for the Three Proposed Designs

Criteria Brick Plastic HybridQuality Weight Value Total Value Total Value TotalCost 8 3 24 4 32 3 24Ease of Use 7 3 21 3 21 3 21Material Availability 5 5 25 3 15 4 20Maintenance 5 2 10 4 20 5 25Efficiency of Gas Product 4 3 12 3 12 3 12Modular 3 1 3 5 15 3 9Longevity 3 5 15 4 12 2 6Production 1 4 4 2 2 3 3Ingenuity 0 1 0 3 0 3 0

Total 114 129 120

21

Appendix A.4: Material Properties of Plastic

  High Density Polyethylene (High Density, High Molecular Weight)

Polyvinyl Chloride (Rigid Molding and Extrusion)

Polypropylene UV Stabilized- PP (Homopolymer, low flow)

Polyethylene Terephthalate (Unfilled Amorphous)

  HDPE PVC PP-UV PETDurability        Fresh water Very Good Very

GoodVery Good Very Good

Weak alkalis Very Good Very Good

Very Good Good

UV radiation Good Very Good

Good Good

Thermal properties        Max. service temperature (°C) 110 50 56.2 55Mechanical Properties        Yield Strength (MPa) 19.3 41.4 23.6 50Permeability (cm^3*mm/(m^2*day*atm))

       

CO2 234 31.7 357 7.75CH4* 286.65 38.83 437.33 9.49Price (USD/kg) 1.85 1.52 2.14 1.61* Estimating permeability of CH4 relative to CO2 with the following equation:

, where D is the free diffusion coefficients and P is the permeability.

Email with Dr. Thomas Harris regarding estimating the permeability of CH4

From: Harris, Thomas R <thomas.r.harris@vanderbilt.edu>Date: Sun, Mar 14, 2010 at 5:54 PMSubject: RE: Permeability of different plasticsTo: "Davis, Thomas Edward" <thomas.e.davis@vanderbilt.edu>

Thomas:The best thing is to seek for direct data on the internet. If that doesn'twork, you can approximate it from the ratio of the free diffusioncoefficients. I assume the gases you have the permeability data for are ina mixture of oxygen and air or CO2 and air. You can estimate perm of methaneby multiplying the permeability for any of those substances times the ratioof the diffusivity of methane in air to that for CO2 in air. Try it for allthe materials for which you have data and take the mean.TRH

22

Appendix A.5: Prototype Experimental Setup

Figure 13: 100% biosoild seed was collected from the Metro Water Services Biosolids Facility located in downtown Nashville.

Figure 14: Freshly collected cow manure was collected from Triple L Ranch near Franklin, Tennessee

Figure 15: Prototype Experimental Setup

23

5-gallon PET bottle

Inverted 2L graduated cylinder

Water Seal

Fermentation carboy cap

Plastic tubing with inner diameter ½"

5-gallon Bucket

3/8" brass T-tube

Plastic adapters

Hose Clamps

3/8" brass ball valve

Figure 16: To fix the problem with gas leak, hose clamps or parafilm is added to the plastic adapters

Figure 17: Foaming is a phenomenon during which the digester unexpectedly burps its contents through the piping

Figure 18: Biogas produced by the manure is collected inside the inverted graduated cylinder.

24

Biogas Produced

Appendix A.4: Emails and Meeting Notes with Dr. Speece

Dr. Speece is an Emeritus Professor from Vanderbilt University’s Civil and Environmental Engineering Department. He specializes in anaerobic digestion. Below are some relevant email conversations and meeting notes regarding the senior design project in their respective order:

From: tiffany.t.cheng@vanderbilt.edu Tiffany Timyu Cheng To: dick.speece@vanderbilt.edu Date: Tue, 26 Jan 2010 13:13:06 -0500 Subject: Re: Engineering Senior Design Regarding Biogas Digester Dear Dr. Speece,

Hi, I dont know if you remember me, my name is Tiffany Cheng, a seniorMechanical Engineering Student at Vanderbilt University. I emailed youbefore Thanksgiving break regarding my senior design project on biogasdigester. I was wondering would you be available this week for a meeting.Maybe Thursday at 11am? Or any time that would be best for you. My groupwould really appreciated any advices.

Thank you for your time.

Best,Tiffany

From: dick.speece@vanderbilt.edu To: tiffany.t.cheng@vanderbilt.edu Tiffany Timyu Cheng Date: Tue, 26 Jan 2010 23:29:42 -0500 Subject: Re: Engineering Senior Design Regarding Biogas Digester TiffanyNice to hear from you again.I can meet with you at 11 AM on Thursday. However it would be moreconvenient to meet you between 2 and 4 PM on Thursday if that isconvenient for the group.Let me know soon.r.e.speece

25

When: January 28, 2010, 11:00 am Purpose: Discuss with Dr. Speece regarding our senior design project Location: Jacob Hall’s Conference Room Parties Present: Dr. Speece, Tiffany Cheng, Thomas Davis

Summary: Briefly talked about the basic information on biogas digester. Informed Dr. Speece about our concerns about pH, C:N:P:S ratio, temperature change,

materials. Dr. Speece agreed that focusing on material would be a good approach. Dr. Speece told us that about his invention, Wet Tip Gas Meter, and how it works. He

imformed us that we can borrowed one from him for our project.

From: tiffany.t.cheng@vanderbilt.edu Tiffany Timyu Cheng To: dick.speece@vanderbilt.edu Date: Tue, 23 Mar 2010 21:53:00 -0500 Subject: Re: Engineering Senior Design Regarding Biogas Digester Dear Professor Speece,

I hope you are enjoying the warm weather in Nashville! I was wondering ifyou have time this week for a meeting with our biogas senior design group.We have come up with the prototype that we will be testing and we would liketo hear some of your inputs regarding it. We would really appreciate youradvices. Also, we were wondering if you have your Wet Tip GasMeter available for us to use in our prototype to measure how much gas isbeing produced.

Thank you so much for your time.

Regards,Tiffany

From: dick.speece@vanderbilt.edu To: tiffany.t.cheng@vanderbilt.edu Tiffany Timyu Cheng Date: Tue, 23 Mar 2010 22:17:50 -0500 Subject: Re: Engineering Senior Design Regarding Biogas Digester TiffanyYes I can make time Thursday or Friday. I will bring a gas meter for youto use. Let me know when are possibilities for meeting.r.e.speece

26

When: March 26, 2010, 2:00 pm Purpose: Discuss with Dr. Speece regarding our senior design project Location: Jacob Hall’s Conference Room Parties Present: Dr. Speece, Tiffany Cheng, Thomas Davis

Summary: We informed Dr. Speece that we are planning on using five-gallon PET bottle as our

digester prototype. Dr. Speece bought his Wet Tip Gas Meter. We talked through how to set up the experiment and how to analysis our result. Since our project is in a time constraint as it will take a while for the anaerobic

bacteria to develop, Dr. Speece suggested that we obtain some 100% biosoild seed from Nashville to start our digestion. The two different types of bacteria will digest the cellulose and produce methane. He proposed the following setup:

o Use two 5-gallon PET bottle 1st bottle is the control with 16L of seed, where bacteria are already

growing already 2nd bottle is the variable with 16L of seed with 2 additional liters of

slurry. The slurry contains ½ water and ½ cow manure. Batch system will be use as oppose to continuous feeding system.

Our goal is to be able to tell the farmer how much manure then have to input in order to produce a certain amount of biogas and produce an optimal size for the biogas digester.

Below are the graphs that we hope to produced in our experiment.

We also ta Other than collecting the amount of gas produced, we could also measure the amount

of volatile solid in the manure. The method of measuring the volatile acid is as followed:

27

Time/day Time/day

Am

ount

of V

olat

ile

acid

in th

e so

lid

Am

ount

of m

etha

ne

gas p

rodu

ced

o Measured out a 15ml sample of manure in a crucible. o Place the sample inside a bake oven to about 100°C to evaporate water

content.o Once the sample is dried, measure the dry mass. o Place the sample inside a muffle furnace at a temperature of 550°C to burn out

all the organic content. o Measured the sample again to measure the mass of the non-volatile content. o The difference between the two measured mass would be the volatile solid.

Since we are planning on setting up two digesters, we will require two Wet-Tip Gas Meter to monitor the gas produced. However, Dr. Speece can only lend us one. Therefore, we, along with Dr. Speece, decided to measure the gas produced by inverting a graduated cylinder over a water seal.

From: tiffany.t.cheng@vanderbilt.edu Tiffany Timyu Cheng To: dick.speece@vanderbilt.edu Date: Wed, 31 Mar 2010 15:15:58 -0500 Subject: Re: Engineering Senior Design Regarding Biogas Digester Dear Dr. Speece,

I hope you got the chance to take a walk and enjoy the beautiful weather inNashville today! I have talked to Rossane and we have a muffle furnace wecould use, and graduated cylinders for the gas collection. Our next step isto get the seed and we can start experimenting! During our last meeting, Iremember that you mentioned that that there is a place in Nashville that wecan collect some seed for the digester. I was wondering if you know the fullname of the company that have the seed. My group has tried searching onlinefor the location, but we couldn't find it. If you know the name of thecompany, that would be really helpful.

Thank you for your time!

Regards,Tiffany

From: dick.speece@vanderbilt.edu To: tiffany.t.cheng@vanderbilt.edu Tiffany Timyu Cheng Date: Wed, 31 Mar 2010 18:45:34 -0500 Subject: Re: Engineering Senior Design Regarding Biogas Digester TiffanyThe City of Nashville has a Central WWTP on Second ave. that has 4 verylarge digesters. The manager is Ron Taylor and he can tell you how to geta sample. Roseanne can probably give you his number.

dick

28

From: tiffany.t.cheng@vanderbilt.edu Tiffany Timyu Cheng To: dick.speece@vanderbilt.edu, thomas.e.davis@vanderbilt.edu Thomas Edward Davis Date: Thu, 8 Apr 2010 15:12:27 -0500 Subject: Re: Engineering Senior Design Regarding Biogas Digester Hi Dr. Speece,

I hope you are doing well! Thanks for the information. Last Friday, we wentdowntown to the Nashville Biosolids Facility and got some 100% seed. Wetried to contact Ron Taylor, but he was out of town. We were fortunate thatRon Denney ( a Vanderbilt Alum) was there to help us to collect some seed tobegin our experiment.

We set up the experiment this past weekend, which we have set up as we lastdiscussed. However we are running into some problems that we would like tohear your opinion. Below are the description of our experiment, and theattached Microsoft Word Document has pictures of the experiment.

*Experiment *

Namely, we have two 5 gallon PET bottle each filled with 17 liters of 100%seed which we obtained at the Biosolids Facility. To one bottle, we added1.5 liters of manure mixed with water at a 1:1 ratio. We then capped thebottles with carboy caps used for wine production and connected plastictubes to the cap that then run into the inverted graduated cylinder.

*Progress*

Unfortunately, we have still not collected any biogas though we have beenoperating for almost a week now. We have been trying to troubleshoot andhave identified the following potential causes. We would like to hear youropinion regarding our thoughts and to see if you have any other suggestionsfor us.

1. Gas leak 2. Overcome water pressure in order for gas to enter inverted graduated cylinder 3. Equilibrium of bacteria has not been met yet 4. Gas solubility in the slurry and, to some small degree, water that is in the tubing leading into the inverted graduated cylinder

*Gas Leak*

The most likely point of gas leak is at the carboy cap. Though it is designfor 5 gallon bottles, there may still be leaking. We have been unable tofind any sealant that interfaces well with both PET and PE.

*Water Pressure*

29

It is possible that enough pressure has not built up inside the PET bottlesto overcome the water pressure. However, as we placed the inverted graduatedcylinders in our buckets, there is only a couple inches of water pressurethat would need to be overcome.

*Equilibrium*Potentially, the bacteria are no longer in equilibrium due to their beingadded to the PET bottles.

*Gas Solubility*Through conversations with our TA, we believe that the gas may be dissolvinginto the slurry solution. Once the gas reaches saturation within the bottle,we will be able to collect effluent.

*Questions*What do you believe is the most likely cause of our lack of gas production?Is there anything we should try to remedy this problem?

Though we have not produced any gas to this point, we have considered takingmultiple mass balance measurements to determine the loss of volatile solidsover time, which would indicate that digestion is indeed occurring.

Finally, our TA expressed some concern in taking multiple mass balancemeasurements while trying to collect gas from the same container as this maydisrupt our ability to remain at saturation and thereby reduce ourcollection of biogas effluent. For the last several days, we have notdisrupted the digesters in any way but have still not produced any gas.

We would greatly appreciate any feedback. Thanks for all the time you havealready spent guiding us in this project!

Warmest regards,

Tiffany and Thomas----

From: dick.speece@vanderbilt.edu To: tiffany.t.cheng@vanderbilt.edu Tiffany Timyu Cheng Date: Fri, 9 Apr 2010 10:23:54 -0500 Subject: Re: Engineering Senior Design Regarding Biogas Digester Tiffany

Is the system set up in the 35 oC "hot room"?

Of course leaks must be prevented. If you inject 50 mL of gas from asyringe with a small hypodermic needle inserted through the wall of thetubing, do you note 50 mL of gas in the inverted cylinder? If you spray asoap solution on the connection with the 5 gal bottles and blow into thetubing coming from the bottle, do you see bubbles?

30

I doubt that the problem is gas solubility. Likewise I doubt that theproblem is to overcome the slight pressure differential. There should beno problem with "equilibrium of bacteria".

My past experience is that there is little problem with taking a sludgesample back to the lab and getting gas production the next day.

Give me a call at XXX-XXXX to try to talk this through.

r.e.speece

From: tiffany.t.cheng@vanderbilt.edu Tiffany Timyu Cheng To: dick.speece@vanderbilt.edu Date: Wed, 14 Apr 2010 14:52:16 -0500 Subject: Biogas Production Dear Dr. Speece,

Yesterday we cleaned the pipes and the water buckets and continued the thirdtrial of the experiment. We find that the the 2000L graduated cylinders arefilling up every hour and half! We concluded that it would be veryinefficient to repeatedly setup the inverted cylinder every 1.5 hours.

So, we were wondering if it would be a reasonable procedure for calculatingthe total amount of gas produced by the following method.

1) Measure how much gas is produced in 30 minutes every 12 hour period.2) Calculate the rate of the production in that 30 minutes3) Use the rate to calculate the total biogas production in that 12 hoursperiod.

We understand that there could be a lot of error in this estimation. We werewondering if you had any suggestions for improving our measurement of thetotal amount of biogas produced.

Thank you for your time.

Regards,Tiffany

From: dick.speece@vanderbilt.edu To: tiffany.t.cheng@vanderbilt.edu Tiffany Timyu Cheng Date: Wed, 14 Apr 2010 14:58:10 -0500 Subject: Re: Biogas Production TiffanyCould you get 2 more 5 gallon carboys and fill them with water and invertthem in a pan filled with water as a water seal and put your gas tubes inthe mouth so that the gas is collected in a 20 liter rather than 2 litercontainer.Give me a call if this needs clarification.XXX-XXX-XXXX cell phone.r.e.speece

31

32

Appendix A.7: Ideation Process IWB Conflict Map

Vanderbilt University- Engineering Senior Design Project 2009- Group 19

Design a Scalable Biogas Digester for Developing WorldIdeation Process

Innovation Situation Questionnaire 1. Brief description of the problem

Biogas digesters in the developing world are a proven technology; however, the current designs are too expensive for the rural population to purchase themselves. Additionally, open defecation and overflowing latrines are a widespread problem in Bangladesh.

2. Information about the system 02.1 System name

Biogas digester2.2 System structure

The digester consists of three main components. (1) Mixing tank with inlet pipe (2) Digester for anaerobic digestion and gas holding (3) Slurry outlet

2.3 Functioning of the system The primary function of the digester is to produce methane from anaerobically digested biomass--both animal and human waste. The methane can then be used as fuel for cooking or electricity.

2.4 System environment Biogas digestion is dependent on many environmental factors: temperature, quality of the substrate, and pH.

3. Information about the problem situation 03.1 Problem that should be resolved

More appropriate materials should be selected in order to optimize the relationship between cost and efficiency within the financial constraints of the target market.

3.2 Mechanism causing the problem Currently, there is no value associated with human waste, which provides no incentive for the removal of waste from latrine pits.

3.3 Undesired consequences of unresolved problem Without coupling biogas technology to human and animal waste, Bangladeshis will continue to suffer from a lack of access to energy and sanitation.

3.4 History of the problem Currently, biogas digesters in Bangladesh cost approximately $200, which is too expensive for the general population. Adding to the population's risk aversion for purchasing this product, biogas digesters tend to fail primarily due to malfunctioning gas pipes in 60% of the cases. However, many of the problems fundamentally lie with problems associated during the installation process.

3.5 Other systems in which a similar problem exists With respect to gas pipe leakages, this is a common problem in many homes. The fixed dome digester has inconsistent gas pressure that is not visualized.

3.6 Other problems to be solved-Shape of the digester is hard to construct-Stirring the digester can improve and reduce efficiency of biogas production depending on how much the slurry is mixed. -Temperature and temperature fluctuations can profoundly impact efficiency of production.

33

4. Ideal vision of solution Open defecation and overflowing latrins that cause a harmful effect will be removed from the water system.

Appropriate choice of materials will increase local availability and decrease cost.

A new digester design will increase efficiency of methane production.5. Available resources

Materials: bricks, polyethylene water tanksInexpensive manual laborSubstrate: Cow dung and human wasteSources of heat: sun, ground, compostTime: Continuos vs. batch digestion

6. Allowable changes to the system Completely changing the system is allowed since the biogas digester is a relatively new product in Bangladesh. However, the use of human waste must be analyzed to determine if such a change in input is culturally acceptable.

7. Criteria for selecting solution conceptsLow costProduct life expectancy greater than 5 yearsPathogen-free output Easy to installEasy to maintain

8. Company business environment Not applicable

9. Project data Name: Design of a Scalable Biogas DigesterObjective: To design a digester that is affordable for the developing world that can increase access to energy and sanitation.Timeline:January 14th--Design and sketch digester protype digesterFebruary 15th--Select design and begin construction of functional prototypeMarch 15th--Test and refine funtional prototypeApril 1st--Finalize prototype and collect dataTeam: Group 19Contact information: www.bme.vanderbilt.edu/srdesign/2009/group19/index.htm

34

Problem Formulation 1. Build the Diagram

2. Directions for Innovation 12/7/2009 9:34:26 PM Diagram1

1. Find a way to eliminate, reduce, or prevent [the] (Monetary Stress/Poverty) in order to avoid [the] (People do not buy biogas digesters.), under the conditions of [the] (Expensive biogas digester design).

2. Find a way to eliminate, reduce, or prevent [the] (Energy Depletion) under the conditions of [the] (Expensive biogas digester design).

3. Find a way to eliminate, reduce, or prevent [the] (Expensive biogas digester design) in order to avoid [the] (Energy Depletion), (Monetary Stress/Poverty) and (Little incentive to clean open feces), under the conditions of [the] (Expensive materials) and (Difficult Installation/Labor Issues).

4. Find a way to eliminate, reduce, or prevent [the] (Little incentive to clean open feces) in order to avoid [the] (Unhealthy/unsanitary environment), under the conditions of [the] (Expensive biogas digester design).

5. Find an alternative way to obtain [the] (Scalable biogas digester design) that offers the following: provides or enhances [the] (Incentive to clean open feces), (Relieves economic burden) and (Renewable, cheap energy), eliminates, reduces, or prevents [the] (Expensive biogas digester design), does not require [the] (Cheap, available biogas digester materials).

35

6. Find an alternative way to obtain [the] (Incentive to clean open feces) that does not require [the] (Scalable biogas digester design).

7. Consider transitioning to the next generation of the system that will provide [the] (Incentive to clean open feces) in a more effective way and/or will be free of existing problems.

8. Find an alternative way to obtain [the] (Relieves economic burden) that does not require [the] (Scalable biogas digester design).

9. Consider transitioning to the next generation of the system that will provide [the] (Relieves economic burden) in a more effective way and/or will be free of existing problems.

10. Find an alternative way to obtain [the] (Renewable, cheap energy) that does not require [the] (Scalable biogas digester design).

11. Consider transitioning to the next generation of the system that will provide [the] (Renewable, cheap energy) in a more effective way and/or will be free of existing problems.

12. Find an alternative way to obtain [the] (Cheap, available biogas digester materials) that offers the following: provides or enhances [the] (Scalable biogas digester design), does not require [the] (Obtain bricks, polyethylene water tanks).

13. Find an alternative way to obtain [the] (Obtain bricks, polyethylene water tanks) that provides or enhances [the] (Cheap, available biogas digester materials).

14. Find a way to eliminate, reduce, or prevent [the] (People do not buy biogas digesters.) under the conditions of [the] (Monetary Stress/Poverty).

15. Find a way to eliminate, reduce, or prevent [the] (Unhealthy/unsanitary environment) under the conditions of [the] (Little incentive to clean open feces).

16. Find a way to eliminate, reduce, or prevent [the] (Expensive materials) in order to avoid [the] (Expensive biogas digester design), under the conditions of [the] (Gas tight seal requirement), (Temperature/Pressur Resistant) and (Longevity).

17. Find a way to eliminate, reduce, or prevent [the] (Difficult Installation/Labor Issues) in order to avoid [the] (Expensive biogas digester design), under the conditions of [the] (Time), (Number of people) and (Gas tight seal requirement).

18. Find an alternative way to obtain [the] (Gas tight seal requirement) that does not cause [the] (Expensive materials) and (Difficult Installation/Labor Issues).

19. Try to resolve the following contradiction: The useful factor [the] (Gas tight seal requirement) should be in place in order to fulfill useful purpose and should not exist in order to avoid [the] (Expensive materials) and (Difficult Installation/Labor Issues).

20. Find an alternative way to obtain [the] (Temperature/Pressur Resistant) that does not cause [the] (Expensive materials).

21. Try to resolve the following contradiction: The useful factor [the] (Temperature/Pressur Resistant) should be in place in order to fulfill useful purpose and should not exist in order to avoid [the] (Expensive materials).

22. Find an alternative way to obtain [the] (Longevity) that does not cause [the] (Expensive materials).

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23. Try to resolve the following contradiction: The useful factor [the] (Longevity) should be in place in order to fulfill useful purpose and should not exist in order to avoid [the] (Expensive materials).

24. Find a way to eliminate, reduce, or prevent [the] (Time) in order to avoid [the] (Difficult Installation/Labor Issues).

25. Find a way to eliminate, reduce, or prevent [the] (Number of people) in order to avoid [the] (Difficult Installation/Labor Issues).

Prioritize Directions1. Directions selected for further consideration

First Priority 1.Find a way to eliminate, reduce, or prevent [the] (Monetary Stress/Poverty) in order to avoid [the] (People do not buy biogas digesters.), under the conditions of [the] (Expensive biogas digester design).

3.Find a way to eliminate, reduce, or prevent [the] (Expensive biogas digester design) in order to avoid [the] (Energy Depletion), (Monetary Stress/Poverty) and (Little incentive to clean open feces), under the conditions of [the] (Expensive materials) and (Difficult Installation/Labor Issues).

4. Find a way to eliminate, reduce, or prevent [the] (Little incentive to clean open feces) in order to avoid [the] (Unhealthy/unsanitary environment), under the conditions of [the] (Expensive biogas digester design).

12. Find an alternative way to obtain [the] (Cheap, available biogas digester materials) that offers the following: provides or enhances [the] (Scalable biogas digester design), does not require [the] (Obtain bricks, polyethylene water tanks).

13. Find an alternative way to obtain [the] (Obtain bricks, polyethylene water tanks) that provides or enhances [the] (Cheap, available biogas digester materials).

14. Find a way to eliminate, reduce, or prevent [the] (People do not buy biogas digesters.) under the conditions of [the] (Monetary Stress/Poverty).

15. Find a way to eliminate, reduce, or prevent [the] (Unhealthy/unsanitary environment) under the conditions of [the] (Little incentive to clean open feces).

16. Find a way to eliminate, reduce, or prevent [the] (Expensive materials) in order to avoid [the] (Expensive biogas digester design), under the conditions of [the] (Gas tight seal requirement), (Temperature/Pressur Resistant) and (Longevity).

17. Find a way to eliminate, reduce, or prevent [the] (Difficult Installation/Labor Issues) in order to avoid [the] (Expensive biogas digester design), under the conditions of [the] (Time), (Number of people) and (Gas tight seal requirement).

18. Find an alternative way to obtain [the] (Gas tight seal requirement) that does not cause [the] (Expensive materials) and (Difficult Installation/Labor Issues).

19. Try to resolve the following contradiction: The useful factor [the] (Gas tight seal requirement) should be in place in order to fulfill useful purpose and should not exist in order to avoid [the] (Expensive materials) and (Difficult Installation/Labor Issues).

20. Find an alternative way to obtain [the] (Temperature/Pressur Resistant) that does not cause [the] (Expensive materials).

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22. Find an alternative way to obtain [the] (Longevity) that does not cause [the] (Expensive materials).

Long Term 2. Find a way to eliminate, reduce, or prevent [the] (Energy Depletion) under the conditions of [the] (Expensive biogas digester design).

7. Consider transitioning to the next generation of the system that will provide [the] (Incentive to clean open feces) in a more effective way and/or will be free of existing problems.

11. Consider transitioning to the next generation of the system that will provide [the] (Renewable, cheap energy) in a more effective way and/or will be free of existing problems.

21. Try to resolve the following contradiction: The useful factor [the] (Temperature/Pressur Resistant) should be in place in order to fulfill useful purpose and should not exist in order to avoid [the] (Expensive materials).

Out-of-Scope 5. Find an alternative way to obtain [the] (Scalable biogas digester design) that offers the following: provides or enhances [the] (Incentive to clean open feces), (Relieves economic burden) and (Renewable, cheap energy), eliminates, reduces, or prevents [the] (Expensive biogas digester design), does not require [the] (Cheap, available biogas digester materials).

6. Find an alternative way to obtain [the] (Incentive to clean open feces) that does not require [the] (Scalable biogas digester design).

8. Find an alternative way to obtain [the] (Relieves economic burden) that does not require [the] (Scalable biogas digester design).

9. Consider transitioning to the next generation of the system that will provide [the] (Relieves economic burden) in a more effective way and/or will be free of existing problems.

10. Find an alternative way to obtain [the] (Renewable, cheap energy) that does not require [the] (Scalable biogas digester design).

23. Try to resolve the following contradiction: The useful factor [the] (Longevity) should be in place in order to fulfill useful purpose and should not exist in order to avoid [the] (Expensive materials).

24. Find a way to eliminate, reduce, or prevent [the] (Time) in order to avoid [the] (Difficult Installation/Labor Issues).

25. Find a way to eliminate, reduce, or prevent [the] (Number of people) in order to avoid [the] (Difficult Installation/Labor Issues).

2. List and categorize all preliminary ideas

A) Construction Techniques: 1) Don’t use a dome design in order to simplify masonry construction.2) Use solar energy or compost to increase temperature of the digester.

B) Material Choices:1) Use local brick2) Use tar to seal ground

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3) Use water tanks, which are commonly seen in developing world 4) Invest in higher quality gas pipes and seals in order to ensure longevity.

C) Substrate Quality: 1) Optimize ratio of animal:human waste 2) Add other forms of biomass to improve C/N ratio

Develop Concepts01. Combine ideas into Concepts

A) Construction Techniques: 1) Place the Biogas Digester in a location underground where it can receive the most sun

light on its surface. 2) No moving Parts

B) Material Choices:1) Gas-tight2) Cheap3) Available in adequate quantities in Bangladesh

C) Substrate Quality1)Need an easy method of measuring pH or some other test to determine quality of

effluent2)Help digester operators to mix biomass in appropriate proportions

2. Apply Lines of Evolution to further improve Concepts

Line of Evolution: 1. Methane production noticed in marshes in Western world2. Concurrent/previous product evolution in China3. Chinese biogas digester model (fixed-dome) is then adapted to Indian floating-drum

model4. Other less expensive models have been made but use plastic sheeting which is easily

damaged

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Evaluate Results 1. Meet criteria for evaluating Concepts

Evaluation of material selection: In order to use a certain material, we must consider factors such as gas permeability, how that material reacts with digesting bacteria, and how easily that material can be sourced. Though tar appears to be readily available in Bangladesh, the material appears to kill bacteria.

Evaluation of construction technique: Using a design that does not require a dome would simplify construction. However, what role does the dome have in the collection of biogas?

Substrate quality: What is carbon/nitrogen ratio of human waste and how does that affect the efficiency of biogas production. What can be done to improve biogas production efficiency?

2. Reveal and prevent potential failures

Potential failure: gas pipes leakIdeal Scenario: Rugged gas pipes are available at little costFailure scenario: A tree falls that breaks the pipeFailure prevention: Place gas pipe underground where it will be more protected from damage

Potential failure: Temperature will fluctuate too much causing methane production to be significantly inhibitedFailure prevention: Insulate digester with the ground or compost

Potential failure: Mixing the contents of the digester too much will disrupt the stratification of the bacterial stages of digestion.Failure prevention: Design a digester that can only mix the desired levels of sludge to a certain amount.

3. Plan the implementation

Below are several questions that might be answered by experts in different professional areas of expertise.

Question: What is the ideal C/N ratio?Question: How long should the sludge be digested? Batch or continuous?Question: What is the ideal temperature for digestion?Question: What materials are available in Bangladesh?Question: How much does it cost to create a simple mold of polyethylene?

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Appendix B.1: Raw Data of the Gas Production of the Two Prototypes

      Control Variable

Name Date Start TimeVolume (ml)

Δ Time (min)

Rate (ml/min)

Volume (ml)

Δ Time (min)

Rate (ml/min)

Thomas & Tiffany 4/15/2010 22:00:00 140 30 4.67 150 30 5.00      270 60 4.50 370 60 6.17      390 90 4.33 535 90 5.94Dawn & Andrew 4/16/2010 9:39:00 0 30 0.00 70 30 2.33      75 60 1.25 230 60 3.83      140 90 1.56 400 90 4.44Tiffany 4/16/2010 17:42:00 110 30 3.67 150 30 5.00      110.2 60 1.84 355 60 5.92      225 90 2.50 420 90 4.67      340 120 2.83 570 120 4.75Andrew 4/17/2010 4:35:00 0 30 0.00 40 30 1.33      55 60 0.92 140 60 2.33      110 90 1.22 270 90 3.00Thomas 4/17/2010 17:30:00 0 30 0.00 165 30 5.50      55 60 0.92 245 60 4.08Thomas 4/18/2010 11:00:00 0 30 0.00 220 30 7.33      55 60 0.92 285 60 4.75Thomas 4/18/2010 21:30:00 0 30 0.00 160 30 5.33      70 60 1.17 245 60 4.08Dawn 4/19/2010 21:38:00 0 30 0.00 0 30 0.00      13 60 0.22 70 60 1.17      21 90 0.23 123 90 1.37Tiffany and Andrew 4/20/2010 11:00:00 9 30 0.30 10 30 0.33      20 60 0.33 17 60 0.28      84 90 0.93 21 90 0.23Tiffany 4/21/2010 0:00:00 6 30 0.20 55 30 1.83      10 60 0.17 150 60 2.50      70 90 0.78 220 90 2.44Tiffany 4/21/2010 14:40:00 4 30 0.13 90 30 3.00      10 60 0.17 180 60 3.00      140 90 1.56 180 90 2.00      140 97 1.44 270 97 2.78Dawn 4/21/2010 21:51:00 5 30 0.17 15 30 0.50      8 60 0.13 57 60 0.95      12 90 0.13 140 90 1.56Andrew 4/22/2010 12:53:00 5 30 0.17 13 30 0.43      9 60 0.15 30 60 0.50      13 90 0.14 119.5 90 1.33

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Appendix B.2: Data Analysis of the Prototypes on Gas Production

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Appendix B.3: DesignSafe Report

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Appendix B.4: Target Price Point Determination

Determination of Target Price Point

A significant goal at the onset of the design process was to determine a target retail price point for the average rural Bangladeshi denizen. Though Project Pyramid had initially identified the potential of the biogas digester from general observation, there were little data suggesting what price an individual in Bangladesh would be willing to pay that would overcome the transitional barrier to biogas.

Initial Information

Resulting from Project Pyramid's most recent survey of villagers in Baroipotol during a trip to Bangladesh in March 2009, villagers spent on average $10 per month for liquefied petroleum gas (LPG) for cooking use. Additionally, villagers spent two full weeks collecting straw as an complementary fuel source.

Follow-Up Questions for Bangladeshi Contact

In order to more fully identify the market potential and to develop a more complete understanding of rural Bangladesh, Thomas Davis contacted a personal acquaintance from Project Pyramid's previous trip for more information and confirmation of previous knowledge. Kazi (Shaheer) Salehein spoke with villagers, engineers, and masons to determine the following information:

1. Average number of people living in a home in rural Bangladesh: 5-62. 60-65% of families in rural Bangladesh own at least 1 cow.3. Average monthly income in rural Bangladesh of families that own 1 or 2 cows: $37 – $44.50 (2,500 BDT – 3,000 BDT)4. $76.89 (5,200 BDT) for 1,000 bricks5. Size of brick: 4.5 inches x 9.5 inches x 2.75 inches6. 100 cubic feet = 850 bricks, 100 cubic feet concrete = $72.45 (4,900 BDT)7. Amount spent per month on LPG for cooking: $9.91 – 10.35 (670 – 700 BDT)

The poor require a high return on investment (ROI)

In Paul Polak's book Out of Poverty, the author claims that the poor generally require a 300% ROI in order to make an investment. This is because when an individual is living at the margin, his or her investment must yield large returns considering the severe implications of an investment's failure.

Polak's work therefore means that we cannot determine a price point based on finding the present value of a family's fuel expenses throughout the useful life of a digester because the risk remains too high for a family to transition to a new technology.

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Therefore, our design criteria included a minimum 5 year life expectancy of the digester and would be priced based on the present value of a family's cooking fuel expenses for the first year. Therefore, after making monthly payments for 1 year, the family would no longer pay for cooking fuel. Expected maintenance must also be minimized in order to avoid unexpected negative cash flows.

Grameen Bank offers 8% simple interest housing loans

Founded by Nobel laureate and Vanderbilt alumnus Muhammad Yunus, Grameen Bank is a rather ubiquitous bank in Bangladesh that focuses on providing access to credit among the country's poor. Considering that this loan would be realistically attainable for many rural Bangladeshis, this interest rate was used to compute the present value.

Computation of retail price point

As a group, we decided to give ourselves a challenging yet well substantiated target price point. Therefore, we assumed that a consumer would make 12 equal monthly payments of $8. The $2 differential between currently monthly expenses for cooking fuel and the monthly payment is considered a contingency fund.

Total payments in a year are $96. Using Grameen Bank's method of simplifying the present value calculations, the present value is $96/1.08 or $89.

The present value of the contingency fund is $22.

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