This document presents a proposal for an anaerobic digestion system to process food waste from Clemson University's dining halls. It estimates that 262.5 tons of food waste is produced annually that could be used to produce biogas through anaerobic digestion. The goals of the project are to destroy 60% of volatile solids and produce 70% of the theoretical methane yield from the food waste. The document discusses governing equations, preliminary data collection, system design considerations, energy output estimates, and sustainability measures for the proposed anaerobic digestion system.

1. ANAEROBIC DIGESTION OF
CLEMSON’S CAFETERIA
FOOD WASTE
Biting Li and Jessica Ketchum
Senior Design Project
Biosystems Engineering
Clemson University

2. INTRODUCTION

3. WASTING OUR WASTE!!
• In the United States, food waste is the 2nd
largest component of municipal waste.
• The EPA estimates 14.5% of the 251 Million
Tons of MSW in 2012 was food.(36.4 M.tons)
• More than 97% was dumped in landfills
when it could have been composted or
anaerobically digested.
• Anaerobic digestion can produce
biological methane, allowing the US to rely
less on non-renewable energy, and also
nutrient rich digestate that can be utilized
as plant fertilizer or soil amendment.
http://www.epa.gov/epawaste/nonhaz/municipal/

4. CLEMSON’S DINING HALL FOOD WASTE
• 262.5 tons annually produced by the main two
dining halls Harcombe and Schilletter can be used
for AD
• Summer: 500 lb/day of feedstock input into the
anaerobic digester
• Fall & Spring: 2000 lb/day of feedstock input into the
anaerobic digester

5. PROJECT GOALS FOR THE DESIGN
• Processing of Clemon’s Dining Halls’(Harcombe & Schilletter) Food Waste
• 60 % of volatile solids destruction among 262.5 tons FW/year
• 70% of CH4 yield from total biogas produced from Clemson’s 262.5 tons
of food waste annually (Harcombe and Schilletter)
• 84,814 m3/year
• ~0.3231 m3 CH4/ Kg of food waste (M2)
• 137,813 m3/year
• ~0.5251 m3 CH4/ Kg of food waste (M2)
• (0.06L-H2/g-COD of conversion efficiency)

6. POSSIBLE CONSTRAINTS OF DESIGN
• Budgetary……
• $700 budget total
• E.g. sampling tests, fabrication cost
• Skills…….
• Limited lab experience
• Time……
• Amount and sources of food waste depends on time
• High variability b/w spring, summer, fall, Football food waste both in types and amount
• “Potential Errors in the Quantitative Evaluation of Biogas Production in Anaerobic
Digestion Processes” (Walker et. al.)
Note: Up to 10% difference in Volume corrections at STP (IUPAC vs. NBS)
in many Anaerobic Digestion recent publications

7. CONSIDERATIONS DURING DESIGN
• Safety & Health issue
• potential pathogens
• Ecological & Ethical concern
• emission of product
• storage for CH4
• global warming problem
• Life Cycle
• supplement ions for bacteria (Fe, Mg)
• Ultimate Use
• compost
http://hajahubacademy.tumblr.com/post/27818028851/2012-07-23-workshop-permaculture-
with-uni

8. QUESTIONS OF USER, CLIENT AND
DESIGNER
• User’s questions
• How big the digester will be?
• Can I have one in my backyard without smelling bad odor?
• How do I check or evaluate functions of a digester?
• Client’s questions
• How long will it take to produce good amount of methane (and hydrogen)?
• How much will it cost to build a digester?
• Is it easy to move and assemble?
• Designer’s questions
• How to minimize the emission of methane into atmosphere?
• How big the reactor should be according to the food waste input?
• How much it costs?

9. LITERATURE REVIEW
Governing Equations, Literature Data, etc.

10. ANAEROBIC DIGESTION PROCESS
http://www.intechopen.com/books/biomass-now-sustainable-growth-and-use/
microbial-biomass-in-batch-and-continuous-system

11. GOVERNING EQUATIONS
1. Volumetric Organic Loading Rate V’=(Ci)*(Q/V)
2. Hydraulic Retention Time, HRT Ĩ=V/Q
3. Bushwell and Mueller (1952) (Formula/VS to obtain CH4 yield)
(Curry & Pillay, 2012)
4. Alkalinity and pH (Bicarbonate/Carbonate/NH4
+↔NH3)
pH= - log[H+]
5. C6H12O6→3CO2 + 3CH4
6. Henry’s Law
p=kHc

12. GOVERNING EQUATIONS…
• The mass of the substrate can be converted to biogas in multiple ways.
• Buswell and Mueller (1952)-Takes mass of waste, using VS and chemical
composition, to covert to volume of biogas produced.
• Using a conversion factor for kg COD/kg VS for specific substrate, biogas volume
can be obtained with a VS value.
• Other Factors:
• Microbes: pH, Alkalinity, Temperature and Toxicity
• Mixed Culture to perform different steps of AD
Theoretical Yield of Biogas will be greater than actual due to Assumptions:
1. All VS=Organic Matter
2. Inhibition Factors not considered
3. Retention Time long enough for full AD

13. HARD DATA FROM LITERATURE
• Analysis of Food Waste
(Curry & Pillay, 2012)
• Approach 1: Elemental basis
C H O N S
% 48 6.4 37.6 2.6 0.4
Kg/ mol 5.45 0.46 7.26 6.35 14.55
• Approach 2: Molecular formula basis
Carbohydrate Protein Lipid
Formula C6H10O5 C5H7NO2 C57H104O6
% 59 33 8

14. DATA & EQUATIONS FROM
LITERATURE…
• Organic Loading Rate (OLR) for food waste
• Optimal: 5 – 10 kg VS/ m3
• Remaining stable & producing biogas
• High OLR  mechanical failure
• High viscosity of the slurry
• Pumping ability
• Dryness of Input (b = dryness)
• D = 1 for b </= 15%
• D = 1- exp(-0.3/(b-0.1))for b > 15%
• Digester Sizing Consideration
• Volume [m3]= Flow Rate [m3/day] * VS Concentration [kg/m3] / OLR
[kg/(m3*day)]
• Cvs = (VS/TS) [%] * Density of dry substances [kg/m3]
(Curry & Pillay, 2012)

15. PRELIMINARY DATA
• Summary Table of Measurements
PH Density (ρ)
[g/cm3]
Water Content
(Ɵ) [%]
TS/Wet [%] VS/Wet [%] VS/TS[%]
6.313 0.9812 76.14 23.86 22.92 95.935
• Experimental Procedures for Determining TS and VS
Weight [g] Empty
bowl
Filled bowl
with wet
food
Wet food
waste
(t=0)
FW after
24hrs @
105°C
FW after
48hrs @
105°C
FW after
7.5hrs @
550°C
No. 1 46.6302 63.3120 16.6818 3.9681 3.9558 0.1638
No. 2 44.7587 59.5740 14.8153 3.5433 3.5293 NA
No. 3 44.9524 61.7362 16.7838 4.0516 4.0381 0.1609

16. DESIGN METHODOLOGY
& MATERIALS

17. BOUNDARY SYSTEM
Feedstock
(262.5 tons/ year)
Pretreat Anaerobic
Digester
Biogas
(CH4)
Solid
digestate
Liquid
Compost
CV
Energy Input Energy Output
(Temp. maintenance,
mixing…)
(Energy of CH4 generated
from a gas turbine)
Heat loss
(Radiation…)

18. THEORETICAL YIELD – METHOD 1
• Elemental basis: C, H, O, N & S
• Assume 150 tonnes of total food waste & 100% of FW is broken down
• Ratio between C:H:O:N = 22:34:13:1
• C22H34O13N
• Using Buswell’s equation: a=22, b=34, c=13, d=1
• C22H34O13N +7.75 H20  11.625 CH4 + 10.375 CO2 + NH3
• V (biogas) = 1.0186 m3/kg VS
• Correction factor: the practical percentage of organic matter broken down
in the digester ranges from 40% ~ 65%
• Biogas yield = 0.4074 m3/kg ~ 0.6621m3/ kg
• Our goal is to yield 70% of CH4 out of biogas
• Methane yield = 0.2852 m3/kg ~ 0.4635 m3/kg
(Curry & Pillay, 2012)

19. THEORETICAL YIELD – METHOD 2
• Molecular formula basis: Carbohydrate, protein & lipid
• Assume 150 tonnes of total food waste & 100% of FW is broken down
• Ratio between C:H:O:N = 9.75:16.53:4.07:0.33
• C9.75H16.53O4.09N0.33
• Using Buswell’s equation: a=9.75, b=16.53, c=4.09, d=0.33
• C9.75H16.53O4.09N0.33 +3.82 H20  5.795 CH4 + 3.955 CO2 + 0.33NH3
• V (biogas) = 1.154 m3/kg VS
• Correction factor: the practical percentage of organic matter broken down
in the digester ranges from 40% ~ 65%
• Biogas yield = 0.4616 m3/kg ~ 0.7501m3/ kg
• Our goal is to yield 70% of CH4 out of biogas
• Methane yield = 0.3231 m3/kg ~ 0.5251 m3/kg
(Curry & Pillay, 2012)

20. SYSTEM DESIGN STEPS
• Digester type: CSTR - continuous flow &
mixing
• Dryness of input
• Density = 1- exp(-0.3/(0.2286-0.1)) = 0.903 dry
tons/m3 = 903 dry kg/m3
• Step 1: Mass flow rate = 226.796 kg/day ~
907.185 kg/day
• Step 2: Our dryness is 22.86 %
• Step 3: No; >15 %
• Step 4: Add water to the food waste 
make the dryness smaller than 15 % 
Density = 1kg/m3
(Curry & Pillay, 2012)

21. SYSTEM DESIGN STEPS…
• Step 5/6: Q = mass flow rate / density 
Q = 226.796 m3/day ~ 907.185 m3/day
• Step 7: Choose 4 different HRT – V=HRT*Q
• Step 8: OLR = Q*Cvs/V
• Cvs = 95.935 % *903 kg/m3 = 866.3 kg/m3
• Step 9: No; OLR is too big.
(Curry & Pillay, 2012)
V [m3] 15 d 20 d 25 d 30 d
Q min 3401.94 4535.92 5669.9 6803.88
Q max 13607.78 18143.7 22679.63 27215.55
OLR 15 d 20 d 25 d 30 d
Q min 57.75 43.315 34.65 28.88
Q max 57.75 43.315 34.64 28.88

22. HRT, V AND OLR OF REACTOR
• Volume=Q*Cvs/OLR
• Same Flow Rates
• Varying V w/HRT
Step 10

23. HYDRAULIC RETENTION TIME VS.
ORGANIC LOADING RATE
• HRT=Cvs/OLR=(Constant for all Q)
• Literature suggests an optimal OLR
range of 5-10 kg VS/m3*day
• Anaerobic Digestion HRT can vary
• Optimal range HRT for food
waste is 25-35 days

24. MIXING AND FLOW
Contents of unmixed digester become stratified into following layers:
Gas
Scum
Supernatant
Active Digester Sludge
Digested Sludge
Grit
• CSTR- Homogeneous 2-Layer remains after mixing
• Mixing options:
-Impeller
-CO2 Injection
• Energy Required?
http://en.wikipedia.org/wiki/Chemical_reactor

25. ENERGY OUTPUT & YIELD
• Energy value of methane
• 1m3 CH4  36MJ = 10 kWh
• Theoretical Energy Output from Methane
Energy
[kWh/day]
M1 (L) M1 (H) M2 (L) M2 (H)
500lb/day 646.822 1051.20 732.78 1190.91
2000lb/day 2587.29 4204.80 2931.11 4763.63
• Theoretical Energy Generated from the system(η = 35%)
Energy
[kWh/day]
M1 (L) M1 (H) M2 (L) M2 (H)
500lb/day 226.39 367.92 256.473 416.82
2000lb/day 905.55 1471.68 1025.89 1667.27

26. SAVING BILLS
• The least electricity bills we could save
per day is in summer:
• 226.39 kWh/day * 11 cents/kWh =
$24.9/day
• The most electricity bills we could save
per day during fall or spring semester:
• 1667.27 kWh/day *11 cents/kWh =
$183.4/day
http://www.npr.org/blogs/money/2011/10/27/141766341/the-price-of-
electricity-in-your-state

27. ALTERNATE DESIGN
• Currently focusing on single CSTR
• Interested in 2-stage CSTR
• 1st Stage containing acid forming bacteria
• May increase stability since methanogens have a high pH sensitivity (Bonomo,
2011)
Acetogenesis &
Methanogenesis
Acidogenesis (2)
(1)
HRT 1 < HRT 2

28. SUSTAINABILITY MEASURES

29. SUSTAINABILITY MEASURES
• Contributions
• Economic: produce energy & save bills
• Ecological: reduce environmental issues
• Social: bring alternative energy
• Ethical: green & concern
• Efficiency
• Societal issues
• Less FW, less rodent/insect issues
• Odor emission of H2S
• Active Carbon or Iron Oxide Coated wood chips
• C & H2O footprint
• Lower Carbon Footprint; but be aware
• Burning H2 small amount H20
http://www.ptj.com.pk/Web-2011/04-2011/Dyeing-Benninger.htm

30. LCA-LIFE CYCLE ASSESSMENT
• LCA Cradle to Grave
• Consider Impacts on Human Health, Ecosystem, Climate Change, Resources
• Important Consideration when comparing AD to Landfill life cycle—TIMELINE
• (1 yr? 5 yrs? 10 yrs?
Michael Carbajales-Dale, Asst. Professor, Clemson University, Intro to LCA, 2014.
Inputs:
-Water
-Energy
-Raw
Materials
Outputs:
CO2
Methane
H2S
Digestate
Michael Carbajales-Dale, Asst. Professor, Clemson University, Intro to LCA, 2014.

31. BUDGET

32. ANAEROBIC DIGESTION:
3 SOURCES OF VALUE
1. Electricity Generation: Converting biogas through electric generator with FIT contact
-Sold to Grid at price range (0.132$/kWh) to (0.269$/kWh)
($30-$60/day in Summer) ($220-$450/day in Spring and Fall)
-2009--CU purchased 133,410,000 kWh for $7.16 million
-2011--Decrease in use/rising energy cost (122,127,434 kWh at $10.2 million)
Total Savings $$ $60-125,000/year
2. Heat Generation: Burning the biogas or capturing heat given off when run through electrical
generator
3. Tipping Fees- Fee paid for AD of organic waste
(Waste from restaurants, farms and meat processing plants)
http://www.investopedia.(Banks, 2006) com/terms/f/feed-in-tariff.asp

33. CAPITAL COSTS:
CSTR
The first method calculates the base capital cost
by multiplying the base generator size by the
estimated average capital cost per kilowatt (kW).
• Minimum capital cost set to $300,000
The second method is the one that is currently
being used by the workbook. This method has a
minimum capital cost of $250,000 with an addition
$5,000 added per kW of capacity
(Anderson, 2012)

34. TIME LINE

35. REFERENCE
1. Banks, C.J. et. al. (2011). Anaerobic digestion of source-segregated domestic food
waste: Performance assessment by mass and energy balance. BioResource
Technology, 102(2), 612-620.
2. Dr. Sandra Esteves and Desmond Devlin-Technical report food waste chemical
analysis, PDF of Final Report produced March 2010, Company: Wales Center of
Excellence for Anaerobic Digestion.
3. Curry N. & Pillay P. (2012). Biogas prediction and design of a food waste to energy
system for the urban environment. Renewable Energy, 41 (2012) 200-209.
4. http://www.ptj.com.pk/Web-2011/04-2011/Dyeing-Benninger.htm
5. http://hajahubacademy.tumblr.com/post/27818028851/2012-07-23-workshop-permaculture-
with-uni
6. http://www.alternative-energy-action-now.com/hydrogen-power.html

36. APPENDICES
• Theoretical yield – Method 1
• Assume 1 mol of N; Percentage of C, H, O, N, S and their kg/mol values are given
• N= (150 tonnes) * (1000kg/tonnes) * (2.6%) /(6.35kg/mol) = 614.173 mol
• C = (150 tonnes) * (1000kg/tonnes) * (48%) /(5.45kg/mol) = 13211.009 mol
• H = (150 tonnes) * (1000kg/tonnes) * (6.4%) /(0.46kg/mol) = 20869.565 mol
• O = (150 tonnes) * (1000kg/tonnes) * (37.6%) /(7.26kg/mol) = 7768.595 mol
• C:H:O:N = 13211.009 : 20869.565 : 7768.595 : 614.173~~ 22 : 34 : 13 : 1
• Buswell’s equation: a=22, b=34, c=13, d=1
• (4a-b-2c+3d)/4 = 7.75; (4a+b-2c-3d)/8 = 11.625; (4a-b+2c+3d)/8 = 10.375
• C22H34O13N +7.75 H20  11.625 CH4 + 10.375 CO2 + NH3
• 1 mol C22H34O13N  11.625 mol CH4
• (150 tonnes) * (1 mol C22H34O13N/ 520 g) * (1/1 mol C22H34O13N) * 11.625 mol CH4 *
(16g/1mol CH4) = 53.654 tonnes CH4
• Density (CH4) = 0.66kg/m3  V (CH4) = 81294 m3
• 1 mol C22H34O13N 10.375 mol CO2  density (CO2)=1.842kg/m3  71489 m3
• Total biogas generated for 150 tonnes of food waste = 152783 m3

37. APPENDICES…
• Theoretical yield – Method 2
• Using weighted average method
• C: 6*59 % +5*33 % + 57*8 % = 9.75
• H: 10*59 % +7*33 % + 104*8 % = 16.53
• O: 5*59 % +2*33 % + 6*8 % = 4.09
• N: 0*59 % +1*33 % + 0*8 % = 0.33
• C9.75H16.53O4.09N0.33
• Buswell’s equation: a=9.75, b=16.53, c=4.09, d=0.33
• (4a-b-2c+3d)/4 = 3.82; (4a+b-2c-3d)/8 = 5.795; (4a-b+2c+3d)/8 = 3.955
• C9.75H16.53O4.09N0.33 +3.82 H20  5.795 CH4 + 3.955 CO2 + 0.33NH3
• 1 mol C9.75H16.53O4.09N0.33  5.795 mol CH4
• (150 tonnes) * (1 mol C9.75H16.53O4.09N0.33/ 203.59 g) * (1/1 mol C9.75H16.53O4.09N0.33) *
5.795 mol CH4 * (16g/1mol CH4) = 68.314 tonnes CH4
• Density (CH4) = 0.66kg/m3  V (CH4) = 103506.061 m3
• 1 mol C9.75H16.53O4.09N0.33  3.955 mol CO2  density (CO2)=1.842kg/m3 
69605.86 m3
• Total biogas generated for 150 tonnes of food waste = 173111.921 m3