Bioremediation. Biodegradation

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1 Bioremediation A technology that encourages growth and reproduction of indigenous microorganisms (bacteria and fungi) to enhance biodegradation of organic constituents in the saturated zone Can effectively degrade organic constituents dissolved in groundwater and adsorbed onto the aquifer matrix Generally requires a mechanism for stimulating and maintaining the activity of the microorganisms, e.g., addition of an electron acceptor (oxygen, nitrate); nutrients (nitrogen, phosphorus); and an energy source (carbon) Biodegradation Biodegradation microbially catalyzed reduction in complexity of chemicals Mineralization - conversion of an organic substrate to inorganic end products Growth-linked metabolism - biodegradation provides carbon and energy to support growth Maintenance metabolism - biodegradation not linked to multiplication, but to obtaining carbon for respiration to maintain cell viability; take place only when organic carbon concentrations very low

2 Biodegradation Triangle Microbial Metabolism Need nitrogen, phosphorus, sulfur, and a variety of trace nutrients other than carbon Carbon is often the limiting factor for microbial growth in most natural systems Acclimatization period - a period during which no degradation of chemical is evident; also known as adaptation or lag period Length of acclimatization period varies from less than 1 h to many months Acclimatization of a microbial population to one substrate frequently results in the simultaneous acclimatization to some structurally related molecules

3 Metabolism Modes Aerobic: transformations occur in the presence of molecular oxygen (as electron acceptor), known as aerobic respiration Anaerobic: reactions occur only in the absence of molecular oxygen, subdivided into: Anaerobic respiration Fermentation Methane fermentation Metabolism Modes Anaerobic respiration Nitrate as an electron acceptor - denitrifying and nitratereducing organisms Sulfate and thiosulfate as electron acceptors - by sulfatereducing organisms CO 2 as an electron acceptor, by methanogenic organisms Chlorinated organic compounds as electron acceptors Fermentation - organic compounds serve as both electron donors and electron acceptors Methane fermentation - consecutive biochemical breakdown of organic compounds to CH 4 and CO 2

4 Metabolism Modes Cometabolism - transformation of an organic compound by a microorganism that is unable to use the substrate as a source of energy Metabolites or transformation products from cometabolism by one organism can typically be used as an energy source by another Preferential degradation: higher energy yielding compounds degraded first, e.g., in a petroleum spill under aerobic conditions, benzene naphthalene chrysene Summary of Metabolism Modes

5 Microbial Reactions and Pathways Dechlorination - a chlorine atom is replaced with a hydrogen atom Hydrolysis - a cleavage of an organic molecule with the addition of water Cleavage - an organic compound is split or a terminal carbon is cleaved off an organic chain Oxidation - breakdown of organic compounds using nucleophilic form of oxygen (H 2 O, OH -, etc); releases electrons Reduction - breakdown of organic compounds using electrophilic form of hydrogen (H + ); takes electrons Microbial Catalyzed Reactions

6 Degradation of Aliphatic Hydrocarbons Generally an aerobic process As high as 20% of all soil microbes (bacteria, fungi and yeast) capable of degrading aliphatic hydrocarbons Most common pathway of alkane degradation is oxidation at the terminal methyl group; alkane alcohol fatty acid ketone CO 2 and H 2 O; short chain hydrocarbons (except methane) more difficult to degrade Unsaturated straight-chain hydrocarbons generally less readily degraded than saturated ones Hydrocarbons w/ branch chains and cyclic aliphatic hydrocarbons less susceptible to biodegradation Degradation of Aromatic Hydrocarbons Microorganisms capable of aerobically metabolizing single-ring aromatic hydrocarbons ubiquitous in the subsurface PAHs with two or three rings such as naphthalene, anthracene, and phenanthrene are degraded at reasonable rates when O 2 is present PAHs with four rings such as chrysene, pyrene, and pentacyclic compounds are highly persistent and are considered recalcitrant

7 Single-Ring Aromatics PAHs with 2-3 Rings

8 PAHs with 4-5 Rings Benzene Degradation Show reactions

9 Degradation of Chlorinated Aliphatic Hydrocarbons (CAHs) Can occur both chemically (abiotic) and biologically (biotic) Generally transformed only partially by microbial processes Only the less chlorinated one- and two-carbon compounds might be used as primary substrates for energy and growth, and organisms capable of doing this not widespread in the environment Microbial transformation of most CAHs depends upon cometabolism Anaerobic Degradation CAHs

10 Anaerobic Degradation CAHs Environmental Factors Microbial Factors Nutrients Temperature ph Moisture Content Oxidation-Reduction (Redox) Potential

11 Temperature Effect In Situ Bioremediation Biostimulation - adding nutrients (N, P, etc.) and electron acceptors (e.g., O 2 ) to microbial environment to stimulate the activity of microorganisms Bioaugmentation - adding exogenous microbes to the subsurface where organisms able to degrade a specific contaminant are deficient

12 Screening Criteria Biodegradability of contaminants Mineralization potential of the compounds Primary substrate or cometabolic reactions? Specific microbial, substrate, and other conditions Availability of carbon and energy Electron acceptor availability and redox condition Sufficient number of degraders Availability of nutrients - N, P Site s hydrogeologic characteristics Extent and distribution of contaminants Biogeochemical parameters DO, Eh, NH 4+, NO 3, SO 4 2, S 2, and Fe 2+ Biodegradability of Contaminants

13 Primary Substrate or Cometabolism? In Situ Bioremediation Systems

14 In Situ Bioremediation Systems Raymond process Injection Well Configurations

15 In Situ Bioremediation Systems Denitrification-based in situ bioremediation Nitrate much more water-soluble than O 2 (9200 mg/l as NaNO 3 vs. 8 to 10 mg/l as O 2 ) Regulatory and microbial toxicity considerations Pure oxygen injection Microbubbles made from low-surface-tension water containing surfactant 15 to 20% of the oxygen injected dissolved into flowing groundwater; 10% committed to biodegrading surfactant; minimum of 5 to 10% net utilization In Situ Bioremediation Systems Methanotrophic biodegradation Methane provides necessary substrate for indigenous microorganisms to produce methane monooxygenase (enzyme) which also degrades TCE 1 to 4% in the methane air mixture Nutrients also in gaseous form: NH 3 gas or nitrous oxide and triethyl phosphate Subsurface recirculation system for methane and O 2 injection

16 Subsurface Recirculation System In Situ Bioremediation Systems Enhanced anaerobic biodegradation Addition of easily biodegradable organic substrates to enhance reductive dechlorination Organic substrates: acetate, butyric acid, lactic acid, methanol, ethanol, vitamin B12, etc. Anaerobic-aerobic sequential transformation Oxygen release compounds Magnesium peroxide (MgO 2 )

17 Anaerobic-Aerobic Sequential Biodegradation Natural Intrinsic Bioremediation Significantly reduce cleanup costs Not a no action alternative Zero line - a vertical plane where rate of natural degradation of contaminants exceeds mass flux rate Assimilative capacity (show calculations!) Evaluation of natural intrinsic bioremediation collecting and analyzing site-wide groundwater quality data: loss of contaminant mass, biogeochemical indicator trends, laboratory confirmation of microbial activity

18 Concept of Zero Line Summary of Bioremediation - Advantages Is generally recognized as being less costly than other remedial options (e.g., pump and treat, excavation) Can be combined with other technologies (e.g., bioventing, soil vapor extraction) to enhance site remediation In many cases, this technique does not produce waste products that must be disposed of

19 Summary of Bioremediation - Disadvantages Injection wells and/or infiltration galleries may become plugged by microbial growth or mineral precipitation High concentrations (TPH > 50,000 ppm) of low solubility constituents may be toxic and/or not bioavailable Difficult to implement in low-permeability aquifers (<10-4 cm/sec) Re-injection wells or infiltration galleries may require permits or may be prohibited Some states require permit for air injection May require continuous monitoring and maintenance Remediation may only occur in more permeable layer or channels within the aquifer