ANAEROBIC DIGESTION OF CLEMSON’S CAFETERIA FOOD

WASTEBiting Li and Jessica Ketchum

Senior Design Project

Biosystems Engineering

Clemson University

INTRODUCTION

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/

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

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)

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

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

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?

LITERATURE REVIEW Governing Equations, Literature Data, etc.

ANAEROBIC DIGESTION PROCESS

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

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

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 Matter2. Inhibition Factors not considered3. Retention Time long enough for full AD

HARD DATA FROM LITERATURE

• Analysis of Food Waste• Approach 1: Elemental basis

• Approach 2: Molecular formula 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

Carbohydrate Protein Lipid

Formula C6H10O5 C5H7NO2 C57H104O6

% 59 33 8

(Curry & Pillay, 2012)

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)

PRELIMINARY DATA

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

• Summary Table of Measurements

• 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

DESIGN METHODOLOGY & MATERIALS

BOUNDARY SYSTEM

Feedstock(262.5 tons/ year)

Pretreat AnaerobicDigester

Biogas(CH4)

Solid digestate

Liquid

Compost

CVEnergy Input Energy Output

(Temp. maintenance, mixing…)

(Energy of CH4 generatedfrom a gas turbine)

Heat loss(Radiation…)

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)

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)

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)

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

HRT, V AND OLR OF REACTOR

• Volume=Q*Cvs/OLR

• Same Flow Rates• Varying V w/HRT

Step 10

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

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

ENERGY OUTPUT & YIELD• Energy value of methane

• 1m3 CH4 36MJ = 10 kWh

• Theoretical Energy Output from Methane

• Theoretical Energy Generated from the system(η = 35%)

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

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

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

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(2)Acidogenes

is (1)

HRT 1 < HRT 2

SUSTAINABILITY MEASURES

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

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-RawMaterials

Outputs:CO2MethaneH2SDigestate

Michael Carbajales-Dale, Asst. Professor, Clemson University, Intro to LCA, 2014.

BUDGET

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)

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.com/terms/f/feed-in-tariff.asp(Banks, 2006)

Total Savings $$

$60-125,000/year

CAPITAL COSTS:

CSTRThe 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)

TIME LINE

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

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

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