ANAEROBIC DIGESTION OF CLEMSON’S CAFETERIA

FOOD WASTEBiting Li and Jessica Ketchum

Senior Design Project

Biosystems Engineering

Clemson University

INTRODUCTION

CLEMSON’S DINING HALL FOOD WASTE

• 313 tons annually produced by the main two dining halls Harcombe and Schilletter can be used for AD• Summer: 360 lb/day of food waste

• Fall & Spring: 2,600 lb/day of waste food

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/

WHERE DO WASTES GO?

• Problems associated with landfill• Running out of space

• Causing health & environmental problems

• Example: Cairo in Egypt• Manshiyat Naser, a ward of Cairo

• “Garbage City”, slum settlement

• Zabbaleen: the garbage collectors

• Increasing & extending landfill is not the best option…

(Curry, 2010)

Figure: “Garbage City” located outside Cairo, Egypt (L), Zabbaleen family (R)

WHAT ARE OTHER WAYS TO TREAT WASTES?

• Incineration (burn)• Advantages

• Reduce volume

• Produce heat and steam generate power

• Disadvantages• Toxicity of the gases and ash

• Containing heavy metals

(Curry, 2010)

• Pyrolysis / Gasification• Temp. > 430°C• Chemical decomposition

• 2 main byproducts

• Syngas: CO + H2

• Used as fuel, b/c

~1/2 energy content

of natural gases

• Biochar ash

• Rich in C; used as

fertilizers

• Carbon negative

BIOLOGICAL PROCESSING WAYS

• Composting

• Aerobic reaction

• Organic constituents CO2 + heat + stable fertilizer

• “Carbon-neutral”

• No energy produced

• Anaerobic Digestion

• Anaerobic reaction

• Organic materials biogas + neutral digestate sludge

• CH4 can be used as fuel

• Digestate can be used as soil conditioners • Provide moisture content

• Supply nutrients

• Protect from soil erosion

(Curry, 2010)

BRIEF COMPARISON

Table: Efficiency Comparison of Renewable Technologies

PROJECT GOALS FOR THE DESIGN

• Processing of Clemon’s Dining Halls’(Harcombe & Schilletter) Food Waste

• 60% of the biogas produced from Clemson’s 313 tons of food waste annually are CH4 (Harcombe and Schilletter)

• 38074.74 m3 of biogas (fall & spring) • 22844.844 m3 CH4 (60%)

• 3305.24 m3 of biogas (summer & winter)• 1983.146 m3 CH4 (60%)

• Biogas total production: 41380 m3/year

• CH4 total production: 24828 m3/ year

POSSIBLE CONSTRAINTS OF DESIGN

• 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

• Safety……

• “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• Energy & Soil conditioner

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. Buswell 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 Matter

2. Inhibition Factors not considered

3. Retention Time long enough for full AD

INCREASING C/N RATIO

• Microbes desired C/N ratio ranges from 20:1 to 30:1

• Problem of the mono-substrate anaerobic digester

• Low C/N ratio of 13.8 – 18.2

• Protein

• Co-digestion method

• Why?

• Balance C & N sources

• Improve digestion rate & biomethane yield

• Our Design?

• Adding paper towels & shredded paper

HARD DATA & EQUATIONS FROM LITERATURE

• Analysis of Food Waste

• Ultimate Analysis: Elemental basis

• 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]

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

(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

ADM1-ANAEROBIC DIGESTION MODELING 1 SOFTWARE

• The IWA Anaerobic Digestion Modeling Task Group

• Established in 1997 by Congress

• Goal- To develop a generalized AD model

• Includes disintegration & 4 Major steps of AD in all their complexity

• Implemented as a differential and algebraic equation (DAE) set

• 32 dynamic concentration state variables using MATLAB AND SIMULINK software

http://spectrum.library.concordia.ca/7485/1/Curry_MASc_S2011.pdf

WHY IS THIS A GOOD APPROACH?

• Numerous Papers state Buswell Percent Error 275-

300% (Both Lab and Industrial Scale)

• ADM1 <35 % error

• ADM1 with Transformer <10% error

DESIGN METHODOLOGY & MATERIALS

BOUNDARY SYSTEM

Anaerobic

Digester

Biogas

CVEnergy Input Energy Output

(Temp. maintenance, mixing…)

(Energy of CH4 generatedfrom a gas turbine)

Heat loss(Radiation…)

Digestate

Paper towel

Food waste

Water

THEORETICAL YIELD

• Elemental basis: C, H, O, N & S

• C22H34O13N (C:H:O:N = 22:34:13:1)

• 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

• Experimental biogas yield (300% overestimated)

• Corrected biogas yield = 0.3395 m3/kg VS

• Our goal is to yield 60% of CH4 out of biogas

• CH4 yield = 0.2037 m3/kg VS

(Curry & Pillay, 2012)

SYSTEM DESIGN STEPS

(Curry & Pillay, 2012)

Calculation of density needs 15%

• 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

• Mass flow rate =

• Our dryness is 22.86 %

• Add water to the food waste

SYSTEM DESIGN STEPS…

FLOW RATE, HRT AND VOLUME OF REACTOR

# lbs. Daily * Weeks /Density of FW

PT mass flow rate was calculated by C:N

Divide by 0.75 to

get a Working

Volume with a

Headspace of

25%

Final Reactor Volume=348 m3Substrate Fall/Spring Winter/Summer

Food Waste 1.50 0.21

Paper Towels 0.86 0.12

Added Water 8.09 1.12

Total Q (QFW+QPT+QH2o) 10.45 1.45

Flow Rate (Q) [m3/day]

Ƭ (HRT) [days] Fall/Spring Winter/Summer

15 156.8 21.8

20 209.0 29.0

25 261.3 36.3

30 313.5 43.5

35 365.8 50.8

Volume V=Ƭ*Q [m3]Ƭ (HRT) [days] Fall/Spring Winter/Summer

15 209.0 29.0

20 278.7 38.7

25 348.3 48.4

30 418.0 58.1

35 487.7 67.7

Working Volume WV=V/0.75

REACTOR TANK

Height [ft] Height [m] Radius [ft] Radius [m]

40 12.2 10.4 3.2

25 7.6 13.15 4

15 4.6 17 5.2

Volume of a Cylinder: V=r2h∏

h=40’

D=20.8’

h=25’

D=26.3’

h=15’

D=34’

Best Optionhttp://www.gosuma.com/Ruehrwerke_Biogas_E.php?we_objectID=106

• Pre-fabricated Concrete Panels

• Required Volume is 348 m3

• Cladded, rigid insulation on exterior

http://www.fairtex.com.ng/insulation.php

REACTOR COVER• Double Membrane Gas Storage –

Tank Mounted | DMGS TM

• Company: SATTLER

• ¼ to ¾ Spherical, bolted to concrete tank

• Hermetically sealed

• Structural Analysis for Snow Load and Wind Pressure

http://www.sattler-global.com/environmental-engineering/gas-

storage-dmgs-tm-1083.jsphttp://www.sattler-global.com/environmental-engineering/gas-storage-dmgs-tm-1083.jsp

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

REACTOR MIXING

http://www.gosuma.com/Ruehrwerke_Biogas_E.php?we_objectID=106

• Specifically designed for Biogas Tanks

• Up to 39.4 ft. Depth

• Adjustable Height with Cable Winch

(Cleaning Made Easy/No Disruption of AD)

• Positioned 47” horizontally into the tank

• Height Indication Controls

• 135º +/- Angle Rotation

• Running 50% time

• Product Name: MGD

(SUMA America, Inc.)

http://www.gosuma.com/Ruehrwerke_Biogas_E.php?we_objectID=106

REACTOR HEATING• Constant Mesophillic Range Needed (35-40º C)

• Specifically designed for thermal processing of manure, sludge and liquid biomass

• Cold substrate can be circulated against heated substrate for heat recovery purposes

• Economical heat recovery for digester

• Extremely low energy loss

• Heat exchange surface: 12.3 m²

DOUBLE HELIX HEAT EXCHANGERS

STAINLESS STEEL PIPING

http://www.farmatic.com/en/biogas-components/heat-exchangers.html

REACTOR DESIGN SAFETY MEASURES

• Pressure Relief Valve

• Flame Arrester

• Vacuum Breaker

• Burst/Rupture Disk

http://www.turbosquid.com/3d-models/water-pressure-relief-valve-3d-model/470491http://www.atechsis.com/en_EN/arrete_flammes/arrete_flammes_en_li

gne.html

http://www.lowes.com/pd_21507-34146-MVB+3/4_0__?productId=3353894

http://singapore.corrom.com/Rupture_Disc.htm

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%)

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)Acidogenesis(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

-Raw

Materials

Outputs:

CO2

Methane

H2S

Digestate

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. BioResourceTechnology, 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

• 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