Biological Transformations of Refuse

Transcription

1 Biological Transformations of Refuse Aerobic decomposition Organic matter + O 2 CO 2 + H 2 O + NH 3 + Heat NH 3 + O 2 NO 3 This is composting - air is supplied to refuse Anaerobic decomposition Organic matter ---> CO 2 + CH 4 + NH 3 + H 2 S This occurs in landfills Methane is only produced in the absence of oxygen Copyright Morton A. Barlaz, NC State University 1

2 Municipal Solid Waste Composition Discarded Waste (2006) Yard trimmings, 12.9 Food scraps, 12.4 Other, 3.4 Paper, 33.9 Wood, 5.5 Rubber,leather & textiles, 7.3 Plastics, 11.7 Metals, 7.6 Glass, 5.3 Copyright Morton A. Barlaz, NC State University 2

3 Refuse Decomposition Cellulose: (C 6 H 10 O 5 ) n + nh 2 O 3n CO 2 + 3n CH 4 Hemicellulose: (C 5 H 8 O 4 ) n + nh 2 O 2.5n CO n CH 4 Copyright Morton A. Barlaz, NC State University 3

4 Organic Composition of Residential Refuse (% Dry Wt.) Reference Barlaz et al, 1989 a Eleazer et al Rhew and Barlaz, 1995 Ress et al., 1998 Barlaz, Unpublished Price et al., 2003 Price et al., 2003 Barlaz, Unpublished Canadian Landfill Cellulose Hemicellulose Lignin (Cellulose + hemicellulose)/lignin Volatile Solids nm 71.4 nm nm nm 86.0 nm Copyright Morton A. Barlaz, NC State University 4

5 Organic Composition of Residential Refuse (% Dry Wt.) News print Office OCC Coated Paper Branches Grass Leaves Food Waste Cellulose Hemicellulose Lignin Volatile solids

6 Residential Refuse Composition Other 21.7% Lignin 15.2% Cellulose 51.2% Hemicellulose 11.9% Copyright Morton A. Barlaz, NC State University 6

7 CO 2 H 2 0 heat O 2 Biological Polymers Soluble Monomers Alcohols Butyric Acid Propionic Acid Cellulose Hemicellulose Sugars Amino Acids Fermentative Hydrolytics Anaerobic Biodegradation H 2,CO 2 Acetic Acid CH 4 CO 2 Methanogens Copyright Morton A. Barlaz, NC State University 7

8 Where are we? Paper, yard waste and food waste are comprised of cellulose and hemicellulose These compounds are converted to CH 4 and CO 2 by bacteria under anaerobic conditions Several groups of bacteria are involved Copyright Morton A. Barlaz, NC State University 8

9 Refuse Decomposition Refuse decomposition is affected by: Climate, surface hydrology, ph, temperature, operations Exerts an influence on: Gas composition and volume Leachate composition Copyright Morton A. Barlaz, NC State University 9

10 Refuse Decomposition Description of refuse decomposition in phases Useful for understanding the status of a landfill and how decomposition occurs Data measured in lab-scale landfills The picture presented here applies to a small quantity of refuse undergoing uniform decomposition Copyright Morton A. Barlaz, NC State University 10

11 1. Aerobic Phase - Oxygen present when refuse is buried supports aerobic decomposition - End products: H 2 O, CO 2 - "CO 2 bloom" - Waste heat of aerobic decomposition causes a temperature increase - Carbon source is soluble sugars based on chemical composition Copyright Morton A. Barlaz, NC State University 11

12 Aerobic Phase: Leachate Production And Quality Only useful if leachate represents a known area of refuse Limited volume in young landfills Strength may be high as water released from compacting refuse Refuse below field capacity leachate due to preferential flow paths Copyright Morton A. Barlaz, NC State University 12

13 2. Anaerobic Acid Phase (Commonly Referred to as Acid Phase) All oxygen consumed, no significant mechanism for replenishment Carboxylic acids accumulate due to an imbalance in microbial activity Butyrate CH 3 CH 2 CH 2 COOH Propionate CH 3 CH 2 COOH Acetate CH 3 COOH High CO 2 concentrations, CH 4 just detected, may be some H 2 Copyright Morton A. Barlaz, NC State University 13

14 2. Anaerobic Acid Phase Leachate Quality High COD, BOD (70% - 90% is acids) Low ph High metal dissolution Some solids (cellulose, hemicellulose) hydrolysis The acid phase explains the lag between burial and methane production in sanitary landfills May not be observed in an older landfill producing methane Copyright Morton A. Barlaz, NC State University 14

15 3. "Accelerated" Methane Production Phase Rapid increase in methane production rate Typical concentration 50% - 70% CH 4 Carboxylic acid concentrations decrease, ph increases to 7-8 Activity of the three groups of bacteria is balanced The sharp increase and decrease presented in the figure are dampened in full-scale landfills Many combine phases 3 and 4 Copyright Morton A. Barlaz, NC State University 15

16 Phase 3: Leachate Quality Similar to acid phase depending on landfill May not be observed if this leachate flows through well decomposed waste below prior to collection Copyright Morton A. Barlaz, NC State University 16

17 Leachate Quality Interpretation Leachate Fresh Refuse Older Refuse Leachate Collection Geomembrane Compacted clay Copyright Morton A. Barlaz, NC State University 17

18 4. "Decelerated" Methane Production Phase Carboxylic acids (soluble substrate) are depleted Rate of CH 4 production is dependent upon solids hydrolysis Activity of the three groups of bacteria is balanced Gas composition constant Leachate quality COD is significantly lower Acids are a much lower fraction of COD Humic materials representative of very mature refuse are present Copyright Morton A. Barlaz, NC State University 18

19 5. Complete Stabilization (Theoretical) degradable solids completely consumed O 2 infiltrates the landfill and is not consumed may only occur over geologic time Copyright Morton A. Barlaz, NC State University 19

20 Trends in Methane, COD, and ph CH 4 Production Rate ph COD Copyright Morton A. Barlaz, NC State University 20

21 Methane Production From Landfills Landfill gas = CH 4 + CO 2 Methane production rate = CH 4 Must specify temperature and pressure Copyright Morton A. Barlaz, NC State University 21

22 Methane Production From Landfills Composition under steady methane production CH % CO N O indicates over pumping H CO Trace* * petroleum hydrocarbons, chlorinated aliphatics, alkanes, ketones, aldehydes, alcohols, terpenes, siloxanes, H 2 S Pumping scenario will influence oxygen and nitrogen content significantly Copyright Morton A. Barlaz, NC State University 22

23 Copyright Morton A. Barlaz, NC State University 23

24 Landfill Gas Modeling Q n k L 0 n i j 0.0 M i 10 e k t i, j Q n is annual methane generation for a specific year t (m 3 CH 4 /yr); k is first order decay rate constant (1/yr) L 0 is total methane potential (m 3 CH 4 /ton of waste); M i is the annual burial rate (wet tons) t is time after initial waste placement (yr); j is the deci-year time increment Landfill Gas Emissions Model (LandGem)

25 Landfill Gas Modeling Understand difference between production and collection Must assume a collection efficiency to apply over entire landfill life Decay rate will vary dependent upon climate and operating conditions Copyright Morton A. Barlaz, NC State University 25

26 Methane Production Rate Curve for One Year of Waste 3.00E E+06 Methane Rate (m3/yr) 2.00E E E E E Time (Yr) Copyright Based Morton on 286,000 A. Barlaz, NC short State University tons of refuse at time zero 26 and Lo = 1.5 ft 3 /wet lb (93.5 m 3 /wet Mg)

27 Methane Production Rate Curve for Five Years Waste Methane Rate (m3/yr) 1.50E E E E E+06 Year 1 Year 2 Year 3 Year 4 Year 5 total 0.00E Copyright Morton A. Barlaz, NC State University 27 Time (Yr)

28 Effect of L 0 on Methane Production Methane Production (m3 per year) 1.80E E E E E E E E E E Year Copyright Morton A. Barlaz, NC State University 28

29 Effect of Decay Rate (k) on Methane Production Methane Production (m3 per year) 1.20E E E E E E E+00 Based on 286,000 short tons of refuse annually for 20 years and Lo = 1.5 ft 3 /wet lb (93.5 m 3 /wet Mg) Copyright Morton A. Barlaz, NC State University Year 29

30 Effect of Decay Rate (k) on Methane Production % of Cumulative Methane Based on 286,000 short tons of refuse annually Copyright Morton for 20 A. Barlaz, years NC and State University Lo = 1.5 ft /wet lb (93.5 m /wet Mg) 5 30 Year

31 Landfill Gas Modeling Must be careful to use appropriate waste composition and quantity data Mass of construction debris differs from a mass of food waste Use multiple waste fractions Model results Data should be presented as a range given uncertainty Decreasing waste quantities will affect model predictions Copyright Morton A. Barlaz, NC State University 31

32 U.S. EPA Defaults L 0 = 100 m 3 CH4/wet Mg (1 Mg = 1 metric ton = 1000 kg) k = 0.04 yr -1 in regions that receive >62.5 cm annual precipitation 0.02 in regions that receive <62.5 cm annual precipitation Copyright Morton A. Barlaz, NC State University 32

33 Effect of Decay Rate on Gas Collection Methane Rate (m3/yr) 8.00E E E E+06 k = 0.04 Collection Efficiency = 69.6% Methane Production Methane collection Methane Rate (m3/yr) 8.00E E E E+06 k = 0.12 Collection Efficiency = 60.6% Methane Production Methane collection 0.00E E Time (Yr) Time (Yr) Copyright Morton A. Barlaz, NC State University 33

34 Stoichiometric Methods 1. General stoichiometric formula which includes all organics some organics do not degrade 2. Estimate from methane potential of refuse C n H a O b N c + (n- a/4 - b/2 + 3c/4)H 2 O ----> (n/2 - a/8 + b/4 + 3c/8) CO 2 + (n/2 + a/8 - b/4-3c/8) CH 4 Copyright Morton A. Barlaz, NC State University 34

35 Limitations to Stoichiometric Methods Enables prediction of total remaining methane based on 100% conversion Some refuse will not degrade Chemically unavailable (lignin) physically unavailable (plastic) Lowest remaining cellulose unknown: Hard to sample, diluted with soil Values as low as 3-6% have been measured Limitation: Landfills are difficult to sample Copyright Morton A. Barlaz, NC State University 35

36 Methane potential of remaining refuse: Biochemical Methane Potential (BMP) Test Measure maximum attainable methane production Bioavailable cellulose/hemicellulose Sample is ground to a powder Incubated for 60 days in culture medium with an inoculum of microbes acclimated to refuse Methane production is measured Representative sampling still an issue Copyright Morton A. Barlaz, NC State University 36

37 Measured Yields Lab-scale data (ultimate versus actual) Assumptions to fit field data mass of waste and time of burial collection efficiency CH4 (m 3 /min) Expansion Landfill k=0.04, L0=100 m^3/mg k=0.1, L0=100m^3/Mg k= 0.07, L0=100 m^3/mg 1 m 3 /min = 35.3 cfm Year Copyright Morton A. Barlaz, NC State University 37

38 Effect of Decay Rate on Gas Collection Methane Rate (m3/yr) 8.00E E E E+06 k = 0.04 Collection Efficiency = 69.6% Methane Production Methane collection Methane Rate (m3/yr) 8.00E E E E+06 k = 0.12 Collection Efficiency = 60.6% Methane Production Methane collection 0.00E E Time (Yr) Time (Yr) Copyright Morton A. Barlaz, NC State University 38