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EVALUATING AEROBIC EVALUATING AEROBIC LANDFILL BIOREACTIONSLANDFILL BIOREACTIONS

USING A NUMERICAL USING A NUMERICAL MODELMODEL

ByBy

G.R. Walter, S.J. Smith, L. G.R. Walter, S.J. Smith, L. Major, and J. TangMajor, and J. Tang

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BACKGROUNDBACKGROUND• SPACE FOR MUNICIPAL SOLID WASTE (MSW) SPACE FOR MUNICIPAL SOLID WASTE (MSW)

LANDFILLS IS AT A PREMIUMLANDFILLS IS AT A PREMIUM

• DEGRADATION OF ORGANIC WASTE IN LANDFILLS DEGRADATION OF ORGANIC WASTE IN LANDFILLS IS SLOW DUE TO ANAEROBIC CONDITIONSIS SLOW DUE TO ANAEROBIC CONDITIONS

• CLOSED MSW LANDFILLS CAN GENERATE CLOSED MSW LANDFILLS CAN GENERATE METHANE AND UNDERGO SUBSIDENCE FOR METHANE AND UNDERGO SUBSIDENCE FOR DECADESDECADES

• METHANE RECOVERY FOR ENERGY FROM SMALL METHANE RECOVERY FOR ENERGY FROM SMALL LANDFILLS IS UNECONOMICALLANDFILLS IS UNECONOMICAL

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CONCEPT OF A SUSTAINABLE CONCEPT OF A SUSTAINABLE LANDFILL (SWITZERLAND, 1986)LANDFILL (SWITZERLAND, 1986)• EACH GENERATION SHOULD MANAGE ITS EACH GENERATION SHOULD MANAGE ITS

WASTE TO FINAL STORAGE QUALITY (30 YEARS)WASTE TO FINAL STORAGE QUALITY (30 YEARS)• FINAL STORAGE QUALITY: Any emissions to the FINAL STORAGE QUALITY: Any emissions to the

environment are acceptable without further treatment environment are acceptable without further treatment or controlor control

• UK FINAL STORAGE QUALITY CRITERIAUK FINAL STORAGE QUALITY CRITERIA– Reduce landfill gas generation by 99.9 % (200 cubic Reduce landfill gas generation by 99.9 % (200 cubic

meters/metric ton to 0.1 cubic meters/metric ton)meters/metric ton to 0.1 cubic meters/metric ton)– Leachate quality determined by local environmental Leachate quality determined by local environmental

conditionsconditions

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CONCEPT OF THE CONCEPT OF THE AEROBIC BIOCELLAEROBIC BIOCELL

• PURPOSE:PURPOSE:– Speed the degradation of organic waste by creating aerobic Speed the degradation of organic waste by creating aerobic

conditionsconditions

• ADVANTAGES:ADVANTAGES:– Preserve disposal space in active landfills and extend their Preserve disposal space in active landfills and extend their

lifelife– Limit or eliminate methane production in closed landfillsLimit or eliminate methane production in closed landfills– Promote consolidation of refuse to improve future land usePromote consolidation of refuse to improve future land use

• OPERATING PRINCIPLES:OPERATING PRINCIPLES:– Derived from composting science and experienceDerived from composting science and experience

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CONCEPTUAL MODEL OF CONCEPTUAL MODEL OF AEROBIC BIOCELLAEROBIC BIOCELL

+ O2 + ORGANIC WASTE

+ CO2 + H2O + HEAT

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OPERATING FACTORSOPERATING FACTORS• WATER APPLICATIONWATER APPLICATION

– Maintain Desired Moisture Content to Support Microbial Maintain Desired Moisture Content to Support Microbial PopulationPopulation

– Replace Water Removed by EvaporationReplace Water Removed by Evaporation

• TEMPERATURE CONTROLTEMPERATURE CONTROL– Aerobic Biodegradation Releases Approximately 3600 Aerobic Biodegradation Releases Approximately 3600

cal/gm substrate oxidizedcal/gm substrate oxidized– Optimal Biodegradation: 40 to 60 Optimal Biodegradation: 40 to 60 ooC (100 to 140 C (100 to 140 ooF) F) – Prevent Hot Spots that May Result in Spontaneous Prevent Hot Spots that May Result in Spontaneous

CombustionCombustion• > 80 > 80 ooC (180 C (180 ooF) F)

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Composted Mixed Refuse

Newsprint

Fresh Mixed Refuse

Reference: Practical Handbook of Compost Engineering, pg.360

BIODEGRADATION RATE AS A FUNCTION OF TEMPERATUREBIODEGRADATION RATE AS A FUNCTION OF TEMPERATURE

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PRIMARY HEAT REMOVAL PRIMARY HEAT REMOVAL PROCESSESPROCESSES

• VAPORIZATION OF WATERVAPORIZATION OF WATER• ADVECTIVE TRANSPORT OF HEAT IN AIR FLOWADVECTIVE TRANSPORT OF HEAT IN AIR FLOW

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VAPORIZATION OF WATERVAPORIZATION OF WATER

HEAT OF VAPORIZATION

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Heat of Vaporization

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THERMALTHERMAL PROPERTIES OF WET AIR PROPERTIES OF WET AIR

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TYPICAL ENERGY BALANCE TYPICAL ENERGY BALANCE (% of total energy)(% of total energy)

Heat in Heat in Applied Water Applied Water

(2%)(2%)

Heat In Heat In Injected Air Injected Air

(7%)(7%)

Heat in Heat in Extracted Air Extracted Air

(24%)(24%)

Heat Associated with Heat Associated with Water Vaporization Water Vaporization

(75%)(75%)

Heat Released during Heat Released during Aerobic Degradation Aerobic Degradation

(90%)(90%)

Initial Heat Initial Heat of Refuse of Refuse

(1%)(1%)

Final Heat Final Heat of Refuse of Refuse

(1%)(1%)

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PROCESSESPROCESSES SIMULATED SIMULATED• AIR FLOWAIR FLOW

– DARCY’S LAW APPLIED TO COMPRESSIBLE FLUIDDARCY’S LAW APPLIED TO COMPRESSIBLE FLUID– LINEARIZED LINEARIZED

• HEAT TRANSPORTHEAT TRANSPORT– ADVECTION IN AIR FLOWADVECTION IN AIR FLOW– CONDUCTION THROUGH SOILCONDUCTION THROUGH SOIL– HEAT CAPACITY OF AIR FUNCTION OF TEMPERATURE AND WATER HEAT CAPACITY OF AIR FUNCTION OF TEMPERATURE AND WATER

CONTENT (ABSOLUTE HUMIDITY)CONTENT (ABSOLUTE HUMIDITY)

• OXYGEN TRANSPORTOXYGEN TRANSPORT– ADVECTION AND DISPERSION IN AIR FLOWADVECTION AND DISPERSION IN AIR FLOW

• BIOGENIC HEAT PRODUCTIONBIOGENIC HEAT PRODUCTION– IMMOBILE SUBSTRATE (REFUSE)IMMOBILE SUBSTRATE (REFUSE)– HEAT GENERATION FUNCTION OF:HEAT GENERATION FUNCTION OF:

• TemperatureTemperature• Oxygen (Monod Rate Equation)Oxygen (Monod Rate Equation)

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COUPLED, NON-LINEAR PROCESSES

• HEAT FLOW, HEAT GENERATION, AND OXYGEN TRANSPORT ARE COUPLED

• SIMPLIFICATIONS IN MODEL– AIR FLOW PROPERTIES NOT DEPENDENT ON

TEMPERATURE– TRANSPORT EQUATIONS DECOUPLED (LINEARIZED)– ACCURACY CONTROLLED THROUGH TIME STEP– AIR ASSUMED TO APPROACH 100% HUMIDITY

RAPIDLY IN REFUSE

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COMPLEXITIES ASSOCIATED COMPLEXITIES ASSOCIATED WITH USING WELLSWITH USING WELLS

COMPLEXITIES ASSOCIATED COMPLEXITIES ASSOCIATED WITH USING WELLSWITH USING WELLS

• AIR FLOW RATE AIR FLOW RATE DECREASES DECREASES RADIALLY FROM RADIALLY FROM WELLWELL

• ASSUMING ASSUMING ADEQUATE OXYGEN ADEQUATE OXYGEN AND EQUAL AND EQUAL TEMPERATURE, TEMPERATURE, BIODEGRADATION BIODEGRADATION RATE IS CONSTANTRATE IS CONSTANT

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RATE

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RELATIVE REACTION RATE VS RELATIVE REACTION RATE VS GAS OXYGEN CONTENTGAS OXYGEN CONTENT

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1 day

OXYGEN

TEMPERATURESCALE

Gas

Oxy

gen

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5 days 10 days 15 days 20 days

25 days 30 days 35 days 40 days

45 days 50days 55days 60 days

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TEMPERATURE AND OXYGEN TEMPERATURE AND OXYGEN VS DISTANCEVS DISTANCE

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SUMMARYSUMMARY• AEROBIC BIODEGRADATION REACTIONS ARE AEROBIC BIODEGRADATION REACTIONS ARE

GOVERNED BY COMPLEX INTERACTION GOVERNED BY COMPLEX INTERACTION BETWEEN AIR CIRCULATION RATES AND BETWEEN AIR CIRCULATION RATES AND TEMPERATURETEMPERATURE

• THESE INTERACTIONS CAN LEAD TO THESE INTERACTIONS CAN LEAD TO INTERESTING AND SOMETIMES SURPRISING INTERESTING AND SOMETIMES SURPRISING BEHAVIOR (AT LEAST IN NUMERICAL MODELS)BEHAVIOR (AT LEAST IN NUMERICAL MODELS)

• THE MODELS DISPLAY SELF-LIMITING THE MODELS DISPLAY SELF-LIMITING BEHAVIOR IN TERMS OF TEMPERATURE BEHAVIOR IN TERMS OF TEMPERATURE REGIMEREGIME

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T, O2, and BIORATEBIORATE AFTER 20 DAYS

100 cfm O2

300 cfm O2