Bioremediation

Bioremediation refers to the productive use of biological catalysis to remove or detoxify pollutants that have found their way into the environment and threaten public health, usually as contaminants of soil, water, gas stream or sediments. Microorganisms destroy organic contaminants in the course of using the chemicals for their own growth and reproduction.  Organic chemicals provide:carbon, source of cell building material, electrons, source of energy.

1.0Types of Bioremediation

• Biostimulation, which involves the addition of nutrients, electron acceptors or electron donors, and sometimes auxiliary substrate to stimulate the growth and activity of specific indigenous microbial populations. (eg. Bioventing, biobarriers). 
• Bioaugmentation - addition of exogenous, specialized microorganisms with enhanced capabilities to degrade the target pollutant.

2.0In situ Biological Treatment

• The most common approaches:  
• Bioventing  
• Water circulation systems  
• Air sparging  
• Biobarriers

Bioventing

• In situ bioremediation that uses indigenous microbes to biodegrade organic constituents adsorbed to soil in the unsaturated (vadose) zone. 
• Activity of indigenous bacteria is enhanced by inducing slow air flow into the unsaturated zone commonly by using vacuum pumps that pull air through the contaminated soil. 
• Nutrients are added if necessary.

Water Recirculation Systems (WRS)

• Applicable to the saturated zone.Also known as Raymond process (Richard Raymond-pioneer 1974).
• Contaminated groundwater is extracted and treated aboveground.
• The treated groundwater is amended with nutrients and reinjected into the aquifer to biostimulate the contaminated zone. 
• Most versatile: attempt to handle contamination above and below the water table 
• Can be used and modified to inject microbe, electron acceptor delivery systems to enhance bioremediation. 
• However, recovery of all recirculating water requires aboveground treatment 
• Time required: 6 months to 5 years. 

Air Sparging

• Involved injection of compressed air into the contaminated subsurface to deliver oxygen and strip the contaminant (volatile organic compounds) into a vapor-capture system. 
• Remediate contamination in the saturated zone of a groundwater aquifer, as well as to remediate portions of the unsaturated zone.  
• This system is also known as in situ air stripping or in situ volatilization.  
• Effectiveness depends on 2 factors. 
• Partitioning characteristics of the contaminants (vapor, dissolved, sorbed phases).  
• Permeability of soil (rate air can be injected)  This determines the mass transfer rate of the contaminants from dissolved to vapor phase.

Biobarriers

• Also known as biowalls, trench biosparge, microbial fences, bubble curtains, air curtains, bioscreens and sparge curtains. 
• Alternative to traditional impermeable barriers (containment tools designed to prevent or control groundwater from flowing into, through or from a certain location). 
• Biobarriers are permeable barriers that allow contaminants to flow through it where contaminants will be degraded.

Biobarrier's aim is to establish and maintain a biological treatment system with a high density of competent microorganisms under a controlled process that protects specific degraders and prevents their loss under environmental stress conditions.

Simple example of biobarriers

• Dig a trench and fill it with crushed stone, which serves as a medium for biofilm growth.  
• Oxygen introduced by air sparging to form an air curtain.  
• Nitrate is also introduced as another electron acceptor  
• Specialized microorganisms that can degrade the contaminants could also be injected.

3.0Microorganisms Used for Bioaugmentation

• Enrichment cultures - Enrichment of indigenous microbes isolated from contaminated areas. 
• Pure cultures specific for contaminants - Culture isolated and studied earlier on specific contaminants.  Eg: Dehalococcoides ethenogenes to remove TCE

4.0In situ bioremediation - Can be aerobic or anaerobic 

• Aerobic  supplying oxygen by:  Addition of oxygen using air spargers.  Hydrogen peroxide dissolved in nutrients.  Oxygen releasing compound buried in contaminated zone.  
• Anaerobic:  Organic substrates or hydrogen gas can be injected as electron donors (lactate, acetate, methanol or hydrogen gas).

5.0Ex-situ bioremediation - Generally selected for the treatment of highly contaminated material.  

• Applicable when :  contaminants have been extracted from source zone  
• hydrophobic pollutants (PAHs and PCBs) are not effectively removed in situ due to low availability.  
• Rapid removal of contaminants is required

Advantages of in situ bioremediation to ex-situ

• No remediation wastes are produced because treatment occurs below ground. Eliminates transportation costs and disposal charges.  
• Minimum land and environmental disturbance.  
• Treating large volumes of contaminated soil and groundwater  
• Attack hard-to- withdraw hydrophobic pollutants that may be located at depths that prohibit excavation.

Disadvantages of in situ

• In situ treatment is slower than ex-situ treatment due to inefficiencies in distributing stimulus substrates throughout the contaminated zone.  
• Difficulties to overcome mass transfer through mixing or surfactant addition, therefore slow degradation of hydrophobic pollutants.  
• Difficult to implement highly stratified soils that hinder the vertical distribution of injected air or gases through the contaminated zone.

6.0Advantages of Bioremediation

• Bioremediation is a natural process and is therefore perceived by the public as an acceptable waste treatment process.  
• Relies on natural biodegradation processes that can be faster and cheaper  
• Instead of transferring contaminants from one environmental medium to another, for example, from land to water or air, the complete destruction of target pollutants is possible. 
• Minimum land and environmental disturbance 
• Bioremediation is less expensive
• Used in conjunction with (or as a follow up to) other treatment technologies.

7.0Disadvantages of Bioremediation

• Bioremediation is limited to those compounds that are biodegradable.  
• There are some concerns that the products of biodegradation may be more persistent or toxic than the parent compound.  
• It is difficult to extrapolate from bench and pilot-scale studies to full-scale field operations.  
• Bioremediation often takes longer than other treatment options.  Biological processes are often highly specific.  
• Important site factors required for success include the presence of metabolically capable microbial populations.  suitable environmental growth conditions, and appropriate levels of nutrients and contaminants.  
• May require extensive monitoring.  
• Certain wastes such as heavy metals, are not eliminated by biological processes (they can be bioreduced or biooxidized to less toxic and less mobile forms.  
• High concentration of contaminants will inhibit microorganisms.