Jae K. (Jim) Park Dept. of Civil and Environmental Engineering University of Wisconsin-Madison 1

 Physical: breakdown or movement of the refuse components by physical degradation and by the rinsing and flushing action of water movement  Chemical: hydrolysis, dissolution/precipitation, sorption/desorption, and ion exchange of refuse components; generally results in altered characteristics and greater mobility of refuse components  more chemically uniform.  Biological: most important process; occurs with naturally present bacteria  Byproducts of MSW decomposition: Decomposed solid wastes, new biomass, generated gases, contaminants into solution (leachate), and heat 2

 Requires oxygen; thus occurs on initial placement of the refuse; rapid compared with anaerobic decomposition Degradable waste + oxygen  CO 2 + H 2 O + biomass + heat + partially degraded materials + NH 3 CH z O b N c + 1/4(4a-2b-3c)O 2  CO 2 +1/2(a-3c)H 2 O + cNH 3  Produces CO 2 as high as 90% and raises temperature as high as 70°C  High CO 2  Acidic pH (CO 2 + H 2 O  H 2 CO 3 )  Leachate is not usually produced because the refuse has not reached field capacity in this early stage. If produced, it is due to channelization through highly permeable pathways or voids in the refuse. Typically particulate matter, soluble salts, soluble organic matter.field capacity 3

 Involves facultative microorganisms that become dominant as the oxygen is depleted.  Called ‘acid or acetogenic’ phase  High concentrations of organic acids, ammonia, hydrogen, and CO 2 are produced. Degradable wastes  CO 2 + H 2 O + Organism growth + Partially degraded organics  Due to high production of CO 2 and organic acids, pH of the leachate ranges from 5.5 to 6.5, which in turn causes the dissolution of other organics and inorganics.  A chemically aggressive leachate with high specific conductance 4

 Produces CO 2, CH 4, and water along with some heat  Slow decomposition rates 4H 2 + CO 2  CH 4 + 2H 2 O CH 3 COOH  CH 4 + CO 2  Consumption of organic acids  pH increase to the range of 7 to 8  Volatile fatty acids: at high conc. inhibitory and toxic  Near-neutral pH, low volatile fatty acids, and low TDS  Optimal conditions for methanogenic bacteria  pH: 6.7 to 7.5 (5 to 9)  Temp: 30 to 35°C (mesophilic); ~45°C (thermophilic)  C:N =16:1 5

Complex organics: e.g., food, grass, paper (polysaccharides, proteins, fats, and oils) Higher organic acids, residual organics, H 2 O, NH 3 -N, Org.-N, H 2 S Acetic acid Acetate decarboxylation Reductive methane formation CH Acetogenic bacteria 35 CH 4 CO 2 Methane bacteria 70 Hydrolysis and fermentation by many bacteria 6 30 Methane bacteria H 2, CO CH 4 CO 2

7 H 2 O + CH 3 CH 2 OH  CH 3 COO - + 5H + + 4e - 2e - + 2H +  H 2  H 2 O + CH 3 CH 2 OH  CH 3 COO - + H + + 2H 2  G = 1.42 kcal/mol 2H 2  4H + + 4e - 4e - + 4H + + ½CO 2  ½ CH 4 + H 2 O  H 2 O + ½CO 2  ½CH 4 + H 2 O  G = kcal/mol H 2 O + ½CH 3 COO -  CO H + + 4e - 4e H + +½CH 3 COO -  CH 4 + H 2 O  H + + CH 3 COO -  CH 4 + CO 2  G = -6.8 kcal/mol  +  +   CH 3 CH 2 OH  1.5CH CO 2  G = kcal/mol Theoretical 75%25% Measured ~65%~35% The reaction will not occur unless H 2 concentration is low, i.e., the second reaction occurs. The overall reaction will occur since  G value is negative.

 Aerobic decomposition Carbohydrates: C 6 H 12 O 6 + 6O 2  6CO 2 + 6H 2 O Stearic acid : C 18 H 36 O O 2  18CO H 2 O Short duration, rapid decomposition, organic acids  Anaerobic decomposition Carbohydrates: C 6 H 12 O 6  6CO 2 + 3CH 4 Stearic acid: C 18 H 36 O 2 + 8H 2 O  5CO CH 4 CH 4 gas: 40 to 70% by vol.; CO 2 gas: 30 to 60% by vol. 8

Soluble chemicals Readily biodegradable chemicals Poorly soluble/biodegradable chemicals Percolate through landfill surface cover Leachate contaminants -Dissolves -Complexes Stimulates biomass -Organics -Gases Leachate Temporal variations Time (yr) Leachate chemical concentration (mg/L) Moisture flux pathways 9

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Gas  pH Moisture content Infiltration Air temperature * Factors over which some control may be exerted during landfill design and operation. 11 Temperature Aeration Toxic compounds Oxidation/reduction potential AlkalinityAlkalinity (Buffer) Nutrients * (N & P) Refuse composition * Atmospheric pressure Placement and cover * Precipitation Topography * Hydrogeology CO 2 produced Biodegradation pH dropBuffer capacity Biological inhibition  Aerobic, anoxic, or anaerobic conditions Air temperature

Young Mature Aging Old Raw, undegraded Partially degraded Partially stabilized Well stabilized Landfill age BOD 5 /COD Leachate type 12 Treatment Biological Chemical No treatment

 Most important parameter in refuse decomposition and gas production  Provides the aqueous environment necessary for gas production and also serves as a medium for transporting nutrients and bacteria throughout the landfill  Methanogenic bacteria can grow in the driest of landfills.  Gas production: moderate increase up to field capacity but significant increase when over field capacity  Moisture content w = weight of the sample including water, kg; and d = weight of the sample after drying at 105  C, kg. 13

 Refuse moisture content as received at a landfill: a low of 15 to 20% to a high of 30 to 40% on a wet wt. basis - typically 25%  Vary significantly in different zones of the landfill Biochemical processes Percolation Surface runoff Groundwater infiltration Evaporation transpiration Precipitation Leachate Moisture flux at a landfill 14

Estimate the pH of leachate in contact with landfill gas. Assume that the composition of the landfill gas in contact with the leachate is 40% CO 2 and 60% CH 4 and that the landfill gas is saturated with water vapor at 50°C and atmospheric pressure. The alkalinity of the leachate is 500 mg/L as CaCO 3. Solution [Alk]  [HCO 3 - ] 15

H 2 CO 3 * ≈ CO 2(aq) ↔ H + + HCO 3 - k at 50°C = 5.07 × ; CO 2 saturation conc. = 379 mg/L (given) pH = -log [H + ] = -log (4.37×10 -7 ) =

whereP g = partial pressure of gas (atm); H = Henry’s constant (atm/mol fraction); and x g = equilibrium mole fraction of dissolved gas. Temperature, ° C H (×10 4 atm/mol fraction) CO 2 CH

CO 2 1 L H 2 O = 1000 g  18 g/mol = 55.6 mol n w >> n g ; n g = x g ×n w = 1.77×10 -4 ×55.6 mol/L = 9.82×10 -3 mol/L C s = 9.82×10 -3 mol/L×44 g/mol×10 3 mg/g = mg/L CH 4 n w >> n g ; n g = x g ×n w = 8.76×10 -6 ×55.6 mol/L = 4.82×10 -3 mol/L C s = 9.82×10 -3 mol/L×16 g/mol×10 -3 mg/g = 21.2 mg/L 18

 Affects the type of bacteria and gas production rate  Landfill gas temp.: 30 to 60°C; as high as 70°C  Aerobic decomposition: 24 to 46°C  Affects chemical solubility  CH 4 production rate k 10°C = 0.17 k 25°C T = temperature (K); E a = 20 kcal/mol; k T = CH 4 production rate (m 3 /day); R = ideal gas constant = kcal/K/mol. 19 At shallower depth, temperature was affected more by atmospheric temp. Landfill

 Redox is controlled by the microbial activity in the refuse and the introduction of oxygen through rainfall and diffusion.  Production of methane requires the ORP of < 330 mV. Particle Size on Gas Production  Affects gas production, including moisture, nutrients, and bacteria Specific Weight on Gas Production  In the typical range of 300 to 450 kg/m 3 (500 to 760 lb/yd 3 ), there is little relationship between refuse density and gas production. 20 Oxidation/Reduction Potential (ORP)

21 FactorChangeEffect on CH 4 rate Cover- Reduces percolation- - Insulates+ - Reduces O 2 (ORP)+ Overall - Depth- Insulates+ increase- Reduces O 2 (ORP)+ - Increases contact of biomass and organics+ Overall + Moisture- Increases contact of biomass increase by and organics+ recycling- Reduces O 2 (ORP)+ Overall + Additives- Lime or other buffer+ - Phosphate+ - Digesters - MSW, sludge+ Overall +

 VS content is not a good measure of the biodegradability of the organic fraction of MSW.  Biodegradable fraction, BF, under anaerobic conditions BF = LC where LC = lignin content of the VS expressed as a percent of dry weight. ComponentVS, % of TSLC, % of VSBF Food waste95~ Paper Newsprint96~ Office paper90~ Cardboard90~ Yard waste85~ Chandler, J.A., W.J. Jewell, J.M. Gossett, P.J. Van Soest, and J.B. Robertson Predicting methane fermentation biodegradability. Biotechnology and Bioengineering Symposium No. 10, pp