What is Bioremediation?

Using subsurface microorganisms to transform hazardous contaminants into relatively harmless byproducts– Biodegrade– Mineralize– Biotransform

Techniques or types of bioremediation:– A component of Natural Attenuation– Enhanced Bioremediation– Bioaugmentation

Bioremediation Background

Natural Attenuation is Not fast enough, Not complete enough, Not frequently occurring enough to be broadly used for some compounds, especially chlorinated solventsThe current trend is to stimulate/enhance a site’s indigenous subsurface microorganisms by the addition of nutrients and electron donorIn some cases, bioaugmentation is necessary when metabolic capabilities are not naturally present.

Historical Perspective

~1900 Advent of biological processes to treat organics derived from human or animal wastes (and the sludges produced)~1950 Approaches to extend wastewater treatment to industrial wastes~1960 Investigations into the bioremediation of synthetic chemicals in wastewaters~1970 Application in hydrocarbon contamination such as oil spills and petroleum in groundwater~1980 Investigations of bioremediation applications for substituted organics~1990 Natural Attenuation of ’70 and ’90, and the development of barrier approaches~2000 High-rate in situ bioremediation; bioaugmentation

Most recent

National Institute of Environmental Health Sciences established the Environmental Genome Project

– Study impact of environmental chemicals on human disease

• Identify genes and their products that are sensitive to toxic chemicals in the environment

• Identify genes that encode for products that detoxify the chemicals

What are environmental contaminants?

Pollutants– naturally-occurring

compounds in the environment that are present in unnaturally high concentrations.

– Examples:• crude oil• refined oil• phosphates• heavy metals

Xenobiotics– chemically synthesized

compounds that have never occurred in nature.

– Examples:• pesticides• herbicides• plastics

Soil and Subsurface Contaminants

Benzene and related fuel components (BTEX)

Pyrene and other polynuclear aromatics

Chlorinated aromatics and solvents

Herbicides and pesticides

Nitroaromatic explosives and plasticizers

What types of treatment technologies are in use to remove contaminants from the environment?

Soil vapor extractionair spargingbioremediationthermal desorptionsoil washingchemical dehalogenationin situ soil flushing

What Makes Bioremediation a Promising Approach?

permanence– contaminant is degraded

potentially low cost– 60-90% less than other technologies

Contaminants Potentially Amenable to Bioremediation________________________________________________________________________________________

Readilydegradable_____________

Somewhatdegradable_____________

Difficult todegrade_____________

Generallyrecalcitrant_____________

fuel oils, gasoline creosote, coaltars

chlorinatedsolvents (TCE)

dioxins

ketones andalcohols

pentachloro-phenol (PCP)

some pesticidesand herbicides

polychlorinatedbiphenyls (PCB)

monocyclicaromatics

bicyclic aromatics(naphthalene)

What challenges exist for bioremediation of pollutants and xenobiotics?

Pollutants– may exist at high, toxic

concentrations– degradation may depend

on another nutrient that is in limiting supply

Xenobiotics– microbes may not yet have

evolved biochemical pathways to degrade compounds

– may require a consortium of microbial populations

Current Water Issues Associated with Gasoline Use

Widespread contaminationMajor threat to drinking water resourcesComponents of fuels are known carcinogensCurrent fuel oxygenate, MTBE, very mobile and not very degradableEthanol is due to replace MTBE, but its behavior in the subsurface is not yet understood

Chlorinated Background

Groundwater plumes of chlorinated solvents are widespread due to their extensive use at industrial and dry cleaner sites.Chlorinated compounds commonly exist as dense nonaqueous-phase liquids (DNAPLs) that act as long-term, continuing sources that slowly solubilize into groundwater.Known carcinogenic and toxic effectsNot a primary substrate for any known bacteria

DNAPL Our Most Difficult Challenge

DNAPL

Storage AreasProcess Area

Lint Trap

Sewer Lines

Flow

DRY CLEANERS

Clay Lens

Clay LensDissolved

Plume

BedrockDNAPL

Residual Tank

DNAPL sourceResidual phaseTrapped on lensesPools in low areasCreates soluble plumes for yearsExtremely hard to remediate

Why use Bioremediation?

No additional disposal costsLow maintenanceCapable of impacting source zones and thus, decreasing site clean-up time

Fundamentals of Biodegradation

All organics are biodegradable, BUT biodegradation requires specific conditionsThere is no Superbug Contaminants must be bioavailableBiodegradation rate and extent is controlled by a “limiting factor”

Biotic Transformations

Result of metabolic activity of microbesAerobic and anaerobic biodegradationReduces aqueous concentrations of contaminantReduction of contaminant massMost significant process resulting in reduction of contaminant mass in a system

Bioremediation Processes

Conversion of contaminants to mineralized (e.g. CO2, H2O, and salts) end-products via biological mechanisms

Biotransformation refers to a biological process where the end-products are not minerals (e.g., transforming TCE to DCE)Biodegradation involves the process of extracting energy from organic chemicals via oxidation of the organic chemicals

In Situ Bioremediation

Biostimulation - adding nutrients (N, P etc.) and electron acceptors (e.g., O2) to microbial environment to stimulate the activity of microorganismsBioaugmentation - adding exogenous microbes to the subsurface where organisms able to degrade a specific contaminant are deficient

Ex situ Bioremediation

Ex situ bioremediation involves removal of waste materials and their collection at a place to facilitate microbial degradation.

disadvantages and limitations. – costs associated with solid handling process e.g. excavation,

screening and fractionation, mixing, homogenizing and final disposal.

• solid-phase system (including land treatment and soil piles) i.e. composting.

• slurry-phase systems (involving treatment of solid-liquid suspensions in bioreactors).

How Microbes Use the Contaminant

Contaminants may serve as:– Primary substrate

• enough available to be the sole energy source– Secondary substrate

• provides energy, not available in high enough concentration

– Cometabolic substrate• fortuitous transformation of a compound by a microbe

relying on some other primary substrate

Fundamentals of cleanup reactions

Aerobic metabolism– Microbes use O2 in their metabolism to degrade

contaminants

Anaerobic metabolism– Microbes substitute another chemical for O2 to

degrade contaminants• Nitrate, iron, sulfate, carbon dioxide, uranium, perchlorate

Requirements for Microbial Growth

ToxicantsCarbon/ EnergySource

Electron Acceptor

(O2, NO3–, SO42-, etc.)

Nutrients (N, P)EnvironmentalConditions (Temp, pH, Eh) Trace Elements

Aerobic v. Anaerobic

If oxygen is the terminal electron acceptor, the process is called aerobic biodegradationAll other biological degradation processes are classified as anaerobic biodegradationIn most cases, bacteria can only use one terminal electron acceptorFacultative aerobes use oxygen, but can switch to nitrate in the absence of oxygen

Aerobic

Oxidation

Cometabolism

Anaerobic

Denitrification

Manganese reduction

Iron reduction

Sulfate reduction

Methanogenesis

Bacterial Metabolism

Electron Acceptor ZonesAfter O2 is depleted, begin using NO3

–

Continue down the list in this order– O2 ––> NO3

– ––> Fe3+ ––> SO42– ––> CO2

GroundWaterFlow

Plume of Dissolved Fuel Hydrocarbons

Residual NAPLMobile LNAPL Pool

Methanogenesis

SulfateReduction

Iron (III) ReductionDentrification

Aerobic Respiration

Bioremediation Practice

Understand physical and chemical characteristics of the contaminants of interestUnderstand the possible catabolic pathways of metabolism and the organisms that possess that capabilityUnderstand the environmental conditions required to:– Promote growth of desirable organisms– Provide for the expression of needed organisms

Engineer the environmental conditions needed to establish favorable conditions and contact organisms and contaminants

Oxygen is of Primary Importance

Most of the time oxygen is the primary factor limiting in situ biodegradation

In most cases if adequate oxygen can be supplied then biodegradation rates are adequate for remediation

Other limiting factors exist, but are usually secondary to oxygen

Degradation for Benzene: C6H6 + 7.5O2 ––> 6CO2 + 3H2O

Two ways to introduce oxygen in situ Dissolved in water : – Actively pumped: H2O2 , aerated water– Passively: ORC ®

In gaseous form, usually air – Bioventing above the water table– Air sparging below the water table

Oxygen Supply is the Key to Aerobic In Situ Bioremediation

Dehalogenation

Stripping halogens (generally Chlorine) from an organic moleculeGenerally an anaerobic process, and is often referred to as reductive dechlorination

R–Cl + 2e– + H+ ––> R–H + Cl–

Can occur via – Dehalorespiration (anaerobic)– Cometabolism (aerobic)

Cometabolism

Bacterium uses some other carbon and energy source to partially degrade contaminant (organic aromatic ring compound)

bacterium

cornstarch CO2 + H2O

contaminant

degradationproducts

Cometabolism

Fortuitous transformation of a compound by a microbe relying on some other primary substrateGenerally a slow process - Chlorinated solvents don’t provide much energy to the microbeMost oxidation is of primary substrate, with only a few percent of the electron donor consumption going toward dechlorination of the contaminantNot all chlorinated solvents susceptible to cometabolism (e.g., PCE and carbon tetrachloride)

Dehalorespiration

Certain chlorinated organics can serve as a terminal electron acceptor, rather than as a donorConfirmed only for chlorinated ethenesRapid, compared to cometabolismHigh percentage of electron donor goes toward dechlorinationDehalorespiring bacteria depend on hydrogen-producing bacteria to produce H2, which is the preferred primary substrate

Reductive Dechlorination

An electron donor, such as hydrogen, and an electron acceptor is needed to transfer from one product to the next

Added Danger

Dechlorination of PCE and TCE should be encouraged, but monitored closelyThe dechlorination products of PCE are more hazardous than the parent compoundDCE is 50 times more hazardous than TCEVinyl Chloride is a known carcinogen