Background:

• Bangladesh suffers from many poverty-related issues, such as affordable energy.

• Project Pyramid from the Owen Graduate School of Management has identified biogas digesters as a potential energy solution.

• Bacteria digest organic material in anaerobic conditions to produce biogas, which is generally 65% methane, 25% carbon dioxide, and 10% other gases

• However, biogas digesters are currently inaccessible and expensive (≥ $200) in Bangladesh.

• We propose designing a cost-effective biogas digester to improve the standard of living in Bangladesh.

Average Bangladeshi Family (6 members):

• Bangladeshi family makes $45 per month• $10 per month spent on cooking fuel• 60-65% of families own ≥ 1 cow

Figure 5. Gas chromatography of biogas concluded that produced biogas contained approximately 66.5% methane.

Experimental Goal: To develop methods for quantifying the first term of the right hand side of the above equation:

1. Gas Volume Production• Control: One 5 gallon, polyethylene terephthalate (PET)

bottle is filled with 17 L of 100% digester seed.• Variable: A second 5 gallon, PET bottle is filled with 17L

of 100% digester seed and 1.5 L of cow manure.• (Gas Production)variable- (Gas Production)control = (Gas Production)input manure

2. Gas Chromatography is used to calculate volumetric production of methane gas.

Design of Scalable Biogas Digester for the Developing World Tiffany Cheng1, Thomas Davis2, Dawn Schmidt3, Kyle Schroeder4, Andrew Wu2

Advisors: Paul King, PhD; Dave Owens, PhDVanderbilt University, Mechanical Engineering1, Biomedical Engineering2, Chemical and Biomolecular Engineering3 & Civil Engineering4

We would like to thank the following for their support in making our senior design project possible: Dr. Dick Speece, PhD Dr. Kenneth Debelak, PhD The Lee family of Triple L Ranch

• Continue collecting biogas until rate of biogas production is insignificant in order to calculate rn

• Determine minimum acceptable digester thickness for a sealed and filled container

• Apply experimental procedure to Bangladeshi cow manure in order to determine the optimal size for full-scale biogas digester

• Work with companies to develop commercialization plan

INTRODUCTION

DESIGN APPROACH

RESULTS

FUTURE DIRECTION

ACKNOWLEDGEMENTS

PROTOTYPE EXPERIMENTAL SETUP

Figure 2. Determined path of design approach (gold) to minimize digester cost.

Figure 4. 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.

Figure 3. Filled PET bottle connected to inverted graduated cylinder.

THEORETICAL FRAMEWORK The theoretical mass balance equation describes the net biogas available within the

digester over time. By performing experiments to determine values of the corresponding terms within the equation, the optimal retention time and consequently, the optimal size of the digester can be determined.

N

n

Vbinbb

ILSSEPLCLtfsquareQTVrdt

Vd

1

),(*),,()(

rn = rate of biogas generationVi = volume of slurry input/loadτ = residence timeQv = biogas volumetric flow rate

CL = connection leakage ratePL = permeability leakage rateSSE = slurry solution exit rateIL = inlet loss rate (diffusion)

N = retention days Χ = solids fractionρb = biogas densityVb = biogas volume

OBJECTIVES• Biogas digester will meet the energy needs of an

average family household.• The ideal biogas digester will:

• Improve waste management• Create quality fertilizer• Reduce energy costs• Improve human health

DESIGN CRITERIA• The digester must:

• Sell at a retail price of ≤ $89• Produce 2800L of biogas/day• Adequately function for ≥ 5 yrs• Be easily installable

OutflowOutflow

Biogas CollectionBiogas Collection

Removable CoverRemovable CoverSealSealInletInlet

BiogasBiogas

SlurrySlurry

Figure 1. Schematic of biogas digester.http://www.journeytoforever.org/biofuel_library/methane_nepal.html

68% Methane

65% Methane

100% Methane

Biogas Sample 2

Pure Methane

Biogas Sample 1

Cost Drivers

C/N Ratio Materials Heating Mixing

Brick Hybrid Plastic

PP PET PE PVC

Volume of Digester

Experimental Design

f = freq. of biogas uset = time

MANUFACTURING COSTRaw PET Plastic MaterialsPower and Utilities5

$26.32$12.52

Labor of Manufacturing5

$7.77

Machine Payment5 $6.94 Subtotal $53.55 Profit Margin (32%) 5 $17.14

Subtotal $70.69

Tooling at 100,000 unit production level5

$1.30

Subtotal $71.99

ADDITIONAL COSTInstallation TransportationAccessories

Varies*Varies*$25.00

Estimated Total $96.99

PER UNIT COST BREAKDOWN

Rosanne DelappRoy Denney at Biosolids FacilityMichael Sykes at Plastic-Mart.com5

* Dependent on the efficiency of the supply chain

Figure 6. Current full-size design of PET plastic biogas digester with an estimated manufacturing cost of

$71.99.

N

n

in TVr1

),,(

0.7m

1.4m

1.35m

0.1m

Slurry Intake

Gas Release

Slurry Exit

0.5mm

PVC

Gas Holder

Digester