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Numerical Modeling of Biodegradation

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Title: Numerical Modeling of Biodegradation


1
Numerical Modeling of Biodegradation
  • Analytical and Numerical Methods
  • By
  • Philip B. Bedient

2
Modeling Biodegradation
  • Three main methods for modeling biodegradation
  • Monod kinetics
  • First-order decay
  • Instantaneous reaction
  • Methods can be used where appropriate for
    aerobic, anaerobic, hydrocarbon, or chlorinated

3
Microbial Growth
  • Region 1 Lag phase
  • microbes are adjusting to the new substrate (food
    source)
  • Region 2 Exponential growth phase,
  • microbes have acclimated to the conditions
  • Region 3 Stationary phase,
  • limiting substrate or electron acceptor limits
    the growth rate
  • Region 4 Decay phase,
  • substrate supply has been exhausted

4
Monod Kinetics
  • The rate of biodegradation or biotransformation
    is generally the focus of environmental studies
  • Microbial growth and substrate consumption rates
    have often been described using Monod kinetics
  • C is the substrate concentration mg/L
  • Mt is the biomass concentration mg/ L
  • µmax is the maximum substrate utilization rate
    sec-1
  • KC is the half-saturation coefficient mg/L

5
Monod Kinetics
  • First-order region, C ltlt KC the equation can be
    approximated by exponential decay (C C0ekt)
  • Center region, Monod kinetics must be used
  • Zero-order region, C gtgt KC, the equation can be
    approximated by linear decay (C C0 kt)

Zero-order
region

dC
dt
First-
order
region
C
6
Modeling Monod Kinetics
  • Reduction of concentration expressed as
  • Mt total microbial concentration
  • µmax maximum contaminant utilization rate per
    mass of microorganisms
  • KC contaminant half-saturation constant
  • ?t model time step size
  • C concentration of contaminant

7
Bioplume II Equation - Monod
  • Including the previous equation for reaction
    results in this advection-dispersion-reaction
    equation

8
Multi-Species Monod Kinetics
  • For multiple species, one must track the species
    together, and the rate is dependent on the
    concentrations of both species

9
Multi-Species
  • Adding these equations to the advection-dispersion
    equation results in one equation for each
    component (including microbes)
  • BIOPLUME III doesnt model microbes

10
Modeling First-Order Decay
  • Cn1 Cn ek?t
  • Generally assumes nothing about limiting
    substrates or electron acceptors
  • Degradation rate is proportional to the
    concentration
  • Generally used as a fitting parameter,
    encompassing a number of uncertain parameters
  • BIOPLUME III can limit first-order decay to the
    available electron acceptors (this option has
    bugs)

11
ModelingInstantaneous Biodegradation
  • Excess Hydrocarbon Hn gt On/F
  • On1 0 Hn1 Hn - On/F
  • Excess Oxygen Hn lt On/F
  • On1 On - HnF Hn1 0
  • All available substrate is biodegraded, limited
    only by the availability of terminal electron
    acceptors
  • First used in BIOPLUME II - 1987

12
Sequential Electron Acceptor Models
  • Newer models, such as BIOPLUME III, RT3D, and
    SEAM3D allow a sequential process - 1998
  • After O2 is depleted, begin using NO3
  • Continue down the list in this order
  • O2 gt NO3 gt Fe3 gt SO42 gt CO2

13
Superposition of Components
  • Electron donor and acceptor are each modeled
    separately (advection/dispersion/sorption)
  • The reaction step is performed on the resulting
    plumes
  • Each cell is treated independently
  • Technique is called Operator Splitting

14
Principle of Superposition
15
Oxygen Utilization of Substrates
  • Benzene C6H6 7.5O2 gt 6CO2 3H2O
  • Stoichiometric ratio (F) of oxygen to benzene
  • Each mg/L of benzene consumes 3.07 mg/L of O2

16
Biodegradation in BIOPLUME II
17
Initial Contaminant Plume
18
Model Parameters
19
Biodegrading Plume
Original Plume Concentration Plume after two
years Extraction Only - No Added O2
20
Plume Concentrations
Plume after two years Plume after two years O2
Injected at 20 mg/L O2 Injected at 40 mg/L
21
Biodegradation Models
  • Bioscreen -GSI
  • Biochlor - GSI
  • BIOPLUME II and III - Bedient Rifai
  • RT3D - Clement
  • MT3D MS
  • SEAM 3D

22
Biodegradation Models
23
Dehalogenation of PCE
  • PCE (perchloroethylene or tetrachloroethylene)
  • TCE (trichloroethylene)
  • DCE (cis-, trans-, and 1,1-dichloroethylene
  • VC (vinyl chloride)

24
Dehalogenation
  • Dehalogenation refers to the process of stripping
    halogens (generally Chlorine) from an organic
    molecule
  • Dehalogenation is generally an anaerobic process,
    and is often referred to as reductive
    dechlorination
  • RCl 2e H gt RH Cl
  • Can occur via dehalorespiration or cometabolism
  • Some rare cases show cometabolic dechlorination
    in an aerobic environment

25
Chlorinated Hydrocarbons
  • Multiple pathways
  • Electron donor similar to hydrocarbons
  • Electron acceptor depends on human-added
    electron donor
  • Cometabolic
  • Mechanisms hard to define
  • First-order decay often used due to uncertainties
    in mechanism

26
Modeling Dechlorination
  • Few models specifically designed to simulate
    dechlorination
  • Some general models can accommodate
    dechlorination
  • Dechlorination is generally modeled as a
    first-order biodegradation process
  • Often, the first dechlorination step results in a
    second compound that must also be dechlorinated

27
Sequential Dechlorination
  • Models the series of dechlorination steps between
    a parent compound and a non-hazardous product
  • Each compound will have a unique decay constant
  • For example, the reductive dechlorination of PCE
    requires at least four constants
  • PCE k1gt TCE
  • TCE k2gt DCE
  • DCE k3gt VC
  • VC k4gt Ethene
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