IntroductionThe term "bioreactor" in the context of soil and water bioremediation refers to any vessel or container where biological degradation of contaminants is isolated and controlled. Bioreactors can range from crude devices such as lined depressions in the ground to advanced metal containers where environmental conditions can be monitored and controlled. The essential treatment mechanism in a bioreactor is natural degredation by existing and/or added populations of microorganisms. Bioreactors have proven to be effective in remediating soil, and in some cases water, polluted with fuel hydrocarbons (oil, gasoline, diesel) and organics. The table below lists some organic compounds that are suitable for bioremediation, possibly in a bioreactor.
Reactor DesignBioreactor design is dependent on the contaminant to be remediated, the media that is contaminiated, and cost constraints. The two major types of soil reactors are dry and slurry. Dry bioreactors treat soil with no other amendments other than microbes and nutrients. Adequate moisture is maintained for microbial growth by sprinkler system or by rainfall. Physical mixing of the soil keeps it aerated. A liner can be fitted over the soil to collect vapors volatilizing from the soil. After the remediation process is complete the soil can be transported to a desired location. The drawing of a simple pile reactor shows its relative size.
This system is only applicable in highly specific situations. Only soils that are contaminated at a shallow depth are practical to treat with a soil pile reactor. It also frequently results in soil/microbe pellet formation which hinder remediation by reducing the availability of pollutants to microbes. Slurry reactors have proven more effective and efficient against a wider range of pollutants. In a slurry reactor the soil is mixed with equal or greater amounts of water and mixed with microbes and nutrients to form a soil slurry. Conditions in a slurry reactor are easier to maintain than dry reactors and result in faster treatment rates.(Nyer, 1993). This design offers many advantages such as relatively rapid treatment, reduced pellet formation, increased slurry homogenization, and increased bioavailability. Soil-water separation can become a problem, especially with high clay soils (Nyer, 1993). Also, there is a need for wastewater treatment after the soil is dewatered.
Bioreactors for groundwater treatment are usually fixed film or some form of activated sludge reactors. Fixed film reactors contain high surface area media that support microbial growth. Activated sludge reactors are aerated basins where microbes are mixed with the wastewater and nutrients. Bioreactors can be operated in batch or steady state flow regimes. A full-scale system diagram is shown below:
Bioreactors can also be designed to operate aerobic and anaerobic processes. Anaerobic degadation is useful in reducing highly halogenated compounds such as trichloroethylene to less halogenated compounds. Conversely, aerobic degradation pathways are effective against a wider range of pollutants and are the most widely implemented processes. (See table #1 above) Use of anaerobic and aerobic steps in series offers a method to treat substances that do not respond to conventional treatment. An example is highly chlorinated organic pollutants. Anaerobic organisms can dechlorinate the substance to a point where aerobic organisms can completely degrade it.
Microbes for
Degradation ProcessesMicroorganisms are the workhorses of the bioremediation process. The microorganisms responsible for pollutant degradation are usually bacteria but can also be fungi. Bacteria that are capable of degrading a wide range of substances are present in almost all subsurface materials. Microbes usually need not be added to the soil in a bioreactor since they are usually present in adequate amounts. The exception being when a toxic substance has removed all endemic microorganisms.
To survive, microorganisms require a supply of nutrients and an electron acceptor. Aerobic organisms use oxygen as the final electron acceptor and organic carbon as a carbon source. Anaerobic organisms use sulfate or carbon dioxide as the electron acceptor. Oxygen is toxic to strict anaerobes. Microbes that can survive under either conditions are termed facultative. Many of these organisms utilize nitrates, iron, and manganese as electron acceptors. This concept is demonstrated graphically in the following diagram.
Nitrogen and phosphorous are the main nutrients added to reactor mixtures. A general rule of thumb for N and P loading is five parts nitrogen and one part phosphorus. Micronutrients such as Ca, Fe, Mg, Mb, and S are usually present in sufficient amounts in the soil to adequately supply microbe metabolism.
Case Studies
The firm of
Geraghty and Miller succesfully used a bioreactor treatment
system to treat landfill leachate containing primarily toluic
acids, xylenes and ortho - methyl bensyl alcohol. Laboratory
tests were run first to determine that the leachate was suitable
for bio-treatment. Results determined that aerobic treatment had
the optimum effect of degrading the leachate. A system was then
installed that first collected the leachate and adjusted the pH
between 6 and 8. The leachate was then pumped into a 9ft. high,
6ft. diameter bio-reactor where oxygen was supplied and the
temperature kept above 60 F. The reactor was filled with
high-surface area inert plastic support media that seves as an
attachment sites for microbes. Activated carbon was used to
polish the effluent. The system has run for over a year under
automatic monitoring and regulation with only one part-time
operator. (Nyer,1993)
Several
bioreactors were used to treat a contaminated soil surrounding a
decommissioned oil refinery near Lake Ontario. The soil was
contaminated over an area of 15 acres and at a depth of
approximately one meter. The bioreactors were designed to treat
various hydrocarbons by stimulating and proliferating the growth
of hydrocarbon degrading organisms already present in the soil
matrix. Air was pulled through the bioreactors by pumps, and
liquid nutrients were applied to encourage microbial growth. The
bioreactors contained impermeable layers at the bottom for
containment of the contaminates. Penstones were added to the top
of the liner to create a permeable subdrain, and the cells were
covered with a vapor barrier to capture and treat harmful gases
that were released during the remediation processes. Moisture was
contained within the system and recirculated, creating a
bioreactor that was completely self contained. At the end of each
soil treatment, hydrocarbon levels in the soil were reduced by
approximately 97%.
This ex situ method was chosen for several reasons. Soil contamination was observed at shallow depths making this type of treatment practical and suitable. Once employed, the bioreactors clean-up time was relatively short; with times ranging between 2 and 24 months. Finally, the use of bioreactors was rather inexpensive compared to other types of possible remediation processes.
This study
involved ex situ bioremediation to remove heavy metals and
sulfate from groundwater at a zinc refinery plant in the
Netherlands. Contaminated groundwater was pumped from the
subsurface into a buffer tank to facilitate the regulation of
flow. After this process the water was pumped to mixing tank
where nutrients were added to aid future biological processes.
The water then entered an Upflow Anaerobic Sludge Blanket (USAB)
reactor where sulfate was reduced to sulfide by sulfate reducing
microrganisms. For short residence times bacteria must attach to
the sludge blanket. At this reactor site gases from the
remediation process were trapped by a series of V-shaped hoods,
and after entrapment the gases were released to a scrubber.
Effluent from the USAB was then transferred to a Submerged Fixed
Film Reactor were sulfide oxidation occurred to remove excess
sulfide. This effluent was then transferred to a tilted plate
settler where solids, such as elemental sulfur and metal
sulfides, were collected. The water was then transported back to
the aquifer from which it came.
This process was successfully demonstrated, and is still currently being used at the zinc refinery plant. However this process does not work very well if the contaminants are sorbed on the soil matrix.
Click here to see a movie about a Navy soil pile reactor!
Send comments or suggestions to:
Student Authors: Chris Lalli clalli@vt.edu, clalli@vt.edu and Marc Russell
Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998
