"Bio-reactor" is a generic term for a system that degrades contaminants in groundwater and soil with microorganisms. The reactors can be open systems, such as a constructed wetland (described as a separate technology), or an enclosed chamber. This description only describes the latter. Unlike natural attenuation and in-situ bioremediation, bioreactors can avoid and control the frequent problems of ineffective indigenous microorganisms and/or low indigenous microbial populations. There are several types of bio-reactors.
The most common bioreactor is used in wastewater treatment. Contaminated groundwater is circulated in an aeration basin where microbes degrade organic matter, forming a sludge that is disposed of or recycled. A second type uses a rotating biological matrix. Microorganism populations grow on the matrix, which is rotated in the reactor. Another method uses a packed bed. A tank is filled with a support medium,, which provides a large surface area for microbial growth. Another system uses soil slurry bioreactor technology to degrade soil containing trinitrotoluene (TNT) and Royal Demolition Explosive (RDX).
Bioreactors can also be installed in-situ (i.e., in place). Vertically placed bio-reactors are called bio-plugs. A wall, similar to a permeable reactor barrier wall, can also be installed. Horizontally placed bio-reactors are called bio-conduits. These techniques enhance degradation as contaminated groundwater passes through the reactor. This technology has been successfully implemented in the remediation of organic compounds at several leaking underground storage tank and industrial sites.
Limitations and Concerns
Contaminated groundwater is often too dilute to support an adequate microbial population. At the other extreme, very high concentrations may be toxic to microorganisms. Also, heavy metals are not treated by this method, and they can be toxic to microorganisms.
Low ambient temperatures can decrease biodegradation rates.
If contaminants tend to volatilize, air pollution controls may be necessary.
With explosive materials or chlorinated solvents, some intermediary degradation products are more toxic than the original contaminants.
Bioreactors are prone to upset. Nuisance microorganisms can predominate and reduce treatment effectiveness.
Residuals may require treatment or disposal.
Bioreactors are used primarily to treat volatile organic compounds (VOCs) and fuel hydrocarbons in soil and groundwater. The process is less effective for pesticides. In one application, the concept was used to treat soil containing TNT and RDX. In the laboratory, it operated under aerobic and anaerobic conditions, and there was a large decrease in contaminant concentration. Intermediate byproducts were also degraded.
In-situ bioreactors can also be used to provide a cometabolite for degradation of hazardous by-products produced during the degradation process of some of the chlorinated solvents. This type of bioreactor contains adapted microbes that mineralize the organic compounds of interest. The microbes are trapped onto a biological support medium. An in-situ immobilized bioreactor system can be used in conjunction with a vapor extraction system.
Technology Development Status
Basic bio-reactors are a well-developed technology that has been used in the treatment of municipal and industrial wastewater. Bioreactors are commercially available for treating fuels. Adaptations have been recently evaluated only for treating groundwater and sohttp://clu-in.org/characterization/technologies/exp.cfm#86il containing large concentrations of chemical contaminants. Several successful pilot projects have been completed for chlorinated compounds. Laboratory experiments have been done for explosive compounds. Sequencing anaerobic/aerobic bioreactors is an innovative approach for treating halogenated VOCs, semi-volatile organic compounds (SVOCs), pesticides, polychlorinated biphenyls (PCB), and ordnance compounds.
Other Resources and Demonstrations
Louisiana State University and Envirosystems Inc. of Lafayette, La., which designed a packed-bed bioreactor, have used the concept for to treat petroleum-contaminated soils and groundwater.
See the descriptions of Constructed
and Enhanced Bioremediation.
See http://www.clu-in.org/characterization/technologies/exp.cfm for a technical description of explosives in different media and the use of some analytical techniques.