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?
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
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 =
$183.4/day
http://www.npr.org/blogs/money/2011/10/27/141766341/the-price-of-
electricity-in-your-state
27. ALTERNATE DESIGN
• Currently focusing on single CSTR
• Interested in 2-stage CSTR
• 1st Stage containing acid forming bacteria
• May increase stability since methanogens have a high pH sensitivity (Bonomo,
2011)
Acetogenesis &
Methanogenesis
Acidogenesis (2)
(1)
HRT 1 < HRT 2
29. SUSTAINABILITY MEASURES
• Contributions
• Economic: produce energy & save bills
• Ecological: reduce environmental issues
• Social: bring alternative energy
• Ethical: green & concern
• Efficiency
• Societal issues
• Less FW, less rodent/insect issues
• Odor emission of H2S
• Active Carbon or Iron Oxide Coated wood chips
• C & H2O footprint
• Lower Carbon Footprint; but be aware
• Burning H2 small amount H20
http://www.ptj.com.pk/Web-2011/04-2011/Dyeing-Benninger.htm
30. LCA-LIFE CYCLE ASSESSMENT
• LCA Cradle to Grave
• Consider Impacts on Human Health, Ecosystem, Climate Change, Resources
• Important Consideration when comparing AD to Landfill life cycle—TIMELINE
• (1 yr? 5 yrs? 10 yrs?
Michael Carbajales-Dale, Asst. Professor, Clemson University, Intro to LCA, 2014.
Inputs:
-Water
-Energy
-Raw
Materials
Outputs:
CO2
Methane
H2S
Digestate
Michael Carbajales-Dale, Asst. Professor, Clemson University, Intro to LCA, 2014.
32. ANAEROBIC DIGESTION:
3 SOURCES OF VALUE
1. Electricity Generation: Converting biogas through electric generator with FIT contact
-Sold to Grid at price range (0.132$/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)
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
