Mass Balance Computational

Procedure in Landfill Assessment

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Jae K. (Jim) Park, Professor

Dept. of Civil and Environmental Engineering

University of Wisconsin-Madison

Mass Balance in Landfills

Estimate the remaining site life for disposal of refuse

Step 1. Develop a general indication of the solid waste specific weight

Step 2. Estimate the volume of soil required to cover each day’s waste from the appropriate solid waste-to-cover ratio (e.g., 4 to 1)

Step 3. Estimate the tonnage of refuse that may still be landfilled, given site volume remaining.

=

2

+ + +Refuseplaced

Mass leavingin leachate

Mass leavingin gas

Waste remaining

Mass transformed to other products

Quantities of Gas Produced

Methanogenic decomposition

CaHbOcNd + 1/4(4a-b-2c+3d) H2O 1/8(4a+b-2c-3d)CH4

+1/8(4a-b+2c+3d)CO2 + dNH3

Estimate an upper bound on the gas production relative to the quantity of substrate utilized.

Because acid-phase anaerobic and aerobic decomposition gives rise to CO2 and not to CH4, there is a higher CO2

content in the gas generated than predicted from eq. above.

a = 60, b = 94.3, c = 32.8, d = 1, noncombustibles = 53.4, and H2O = 44.4 53% CH4 520 L/kg

Theoretical: 300 to 500 L of landfill gas produced from 1 kg of municipal refuse (5 to 8 ft3/lb)

Full-size landfill projected: 50 ~ 400 L/kg (0.8 ~ 6.4 ft3/lb)

Optimum moisture content: 75 ~ 100% of the refuse dry wt.3

Theoretical CH4 & CO2 Production (1)

In 100 lb of solid waste, 56 lb decomposable

C60H94.3O37.8N+18.28H2O → 31.96CH4+28.04CO2+NH3

1433.1 g 329.0 g 511.4 g 1233.5 g 17 g

Specific wt: CH4 – 0.0448 lb/ft3; CO2 – 0.1235 lb/ft3

Solution

CH4 = 511.4/1433.1ⅹ56 lb = 20 lb

CO2 = 1233.5/1443.1ⅹ56 lb = 48.2 lb

Volume

CH4 = 20 lb0.0448 lb/ft3 = 446.4 ft3

CO2 = 48.2 lb0.1235 lb/ft3 = 390.3 ft3

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Theoretical CH4 & CO2 Production (2)

Fraction of CH4 and CO2

CH4(%) = 446.4(446.4+390.3)ⅹ100=53.3%

CO2(%) = 390.3(446.4+390.3)ⅹ100=46.5%

Gas production based on dry wt. of org. material

(446.4+390.3) ft356 lb=14.9 ft3/lb=0.93 m3/kg

Gas production based on total wt. of org. material

(446.4+390.3) ft3100 lb=8.4 ft3/lb=0.52 m3/kg

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Empirical Formula of Solid Waste

Element Wt, gAtomic wt.,

g/molMole

Mole ratio (N=1)

C

H

O

N

S

Ash

27.39

3.62

22.97

0.54

0.10

3.48

12.01

1.01

16.00

14.01

32.07

-

2.28

3.584

1.436

0.038

0.003

-

60

94.3

37.8

1.0

0.1

-

C60H94.3O37.8N → C60H94O38N

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Factors Affecting Gas Production

Refuse composition, age of refuse, moisture content, pH, microbial population present, temperature, and quantity and quality of nutrients

Rate of methane generation

Cumulative gas produced

C = Co (1 - e-kt)

Assume that the factor limiting the rate of methane generation at a landfill is the quantity of material remaining in the landfill.

tk

1 2

0 69/

.C C eo

kt dC

dtkC

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Estimation of CH4 Production

The Scholl Canyon model may be used to estimate emissions using the following first-order decay equation (IPCC, 1997):

Gi = Mi× k × L0× exp-(k × ti)

where:

Gi = emission rate from the ith section (kg CH4/year);

Mi = mass of refuse in the ith section (ton);

k = CH4 generation rate (1/year);

L0 = CH4 generation potential (kg CH4/ton of refuse);

ti = age of the ith section (years).

IPCC (1997), Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories,

Vols. 1 and 3, Intergovernmental Panel on Climate Change, Bracknell, U.K. 8

CH4 Generation Potential

L0 = (Mc× Fb× S)/2

where:

Mc = kg of carbon per kg of waste landfilled;

Fb = biodegradable fraction; and

S = stoichiometric factor = 16/12.

L0: 4.4 to 194 kg CH4/ton of waste (Pelt et al., 1998)

EPA: L0 = 165 kg CH4/ton of waste

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Organic portion of municipal wastes

Readily decomposable: food wastes (t1/2 = 0.5~1.5 yrs)

Moderately decomposable: paper (t1/2 = 5 ~ 25 yrs), wood grass, brush, greens, leaves, oils, and paint

Non-decomposable: plastics, leather, rubber, and rags

Landfill methane generation

Lag phase

Active methane generation phase

Life of methane gas generation for economic recovery: 5 to 20 years

Gas Production Rate

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Decomposition Time in Landfill

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Waste product Decomposition time

Banana skin 3~4 wksPaper bag 1 monthCardboard 2 monthsWool sock 1 yrOrange peel Up to 2 yrsCigarette butts Up to 12 yrsPlastic bags* Up to 20 yrsPolyfilm wrapping (clingwrap)* 25 yrsLeather shoe Up to 45 yrsTin cans 50 yrsPlastic bottle* 450 yrsPlastic 6-pack holder* 500 yrsDisposable nappies 550 yrsPolystyrene cups > 500 yrsAluminum cans > 1 million yrs or forever?Glass 1~2 million yrs

* Even though these products break down in the times indicated they are still petrochemical products and will always remain in the environment.

Example - Solution

Example: Calculate CO2, CH4, and water consumed in the formation of landfill gas per kg of MSW. MSW empirical formula: C68H111O50N

C68H111O50N + 16H2O 33CO2 + 35CH4 + NH3

(1741) (288) (1452) (560) (17)

Water consumed = 288/1741 = 0.165 kg H2O/kg MSW

= 0.165 kg H2O/0.435 m3 gas/kg MSW

= 0.38 kg H2O/m3 gas

CO2 and CH4: 95 to 99% of landfill gas

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Typical Landfill Gas Composition

Typical conc.

Component Source (% by vol.) Concern

Methane Biodegradation 50~70 Explosive

Carbon dioxide Biodegradation 30~50 Acidic in GW

Hydrogen Biodegradation < 5 Explosive

Mercaptans (CHS) Biodegradation 0.1~1 Odor

Hydrogen sulfide Biodegradation < 2 Odor

Toluene Contaminant 0.1~1 Hazardous

Benzene Contaminant 0.1~1 Hazardous

Disulfates Contaminant 0.1~2 Hazardous

Others Biodegradation Traces Hazardous

or contamination

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Leachate Production Rates

Water quantity

Water present in the waste (small)

Water produced during decomposition (negligible)

Water added to the landfill - percolation through the landfill surface, horizontal flow through the sides, and upward flow through the bottom (major)

Hydrologic water balance Formation of surface water runoff, evaporation directly

to the atmosphere, transpiration to the atmosphere through vegetation surfaces, or infiltration into the cover soils and refuse at the surface of the landfill Infiltrates may be held in surficial soil and percolate through the refuse (leachate).

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Leachate Quality

Cellulose is a major carbohydrate in domestic refuse.

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CelluloseGlucose &

cellobiose

CO2, H2, ethanol, &

acetic, propionic,

butyric, valeric and

caproic acids

CH4 & CO2

pH increase

to 7~8

Cellulose:hemicellulose:lignin = 70:15:15

Major Compositions of Papers Landfilled

Cellulose: A long chain of glucose molecules, linked to one another primarily with glycosidic bonds. Only a small number of enzymes are required to degrade this material.

Hemicelluloses: Branched polymers of xylose, arabinose, galactose, mannose, and glucose. Hemicelluloses enhance the stability of the cell wall. They also cross-link with lignin, creating a complex web of bonds which provide structural strength, but also challenge microbial degradation.

Lignin: A complex polymer of phenylpropane units, which are cross-linked to each other with a variety of different chemical bonds. This complexity has thus far proven as resistant to detailed biochemical characterization as it is to microbial degradation. Lignin degradation is primarily an aerobic process, and in an anaerobic environment lignin can persist for very long periods.

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Biological Decomposition in Landfill

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Aerobic decomposition

High CO2, temp. , pH , high COD, BOD, and

specific conductance, high concentrations of most

inorganic constituents

Anaerobic acid phase

Highest COD, BOD, and specific conductance

Methanogenic phase

pH ~7, COD, BOD, and specific conductance

LandGEM

EPA Landfill Methane Outreach Program, Program Development Handbook: http://www.epa.gov/lmop/publications-tools/handbook.html

LandGEM program: http://www.epa.gov/ttn/catc/dir1/landgem-v302.xls

Useful reference: http://www.stormh2o.com/MSW/Editorial/LandGEM_the_EPAs_Landfill_Gas_Emissions_Model_15957.aspx

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