Peter Minting looks at the biological treatment of halogenated compounds
Many organic contaminants of groundwater are difficult to treat because they are industrial in origin and so it is hard to find microorganisms capable of breaking them down. But many organic compounds which were thought to be non-biodegradable have now been found to be biodegradable. And although it was thought biological treatment only had the potential to reduce organic compounds to the order of mg/l, researchers in the US now believe these levels could be reduced much further.
This will be of interest to water companies because as legislation has become increasingly strict a need has developed to reduce concentrations several orders of magnitude below that which is commonly achieved in biological treatment systems. At low concentrations removal is difficult because the microorganisms which can feed on organic compounds do not have enough energy to grow.
Prof Perry McCarty of Stanford University in California has been working on organic compound degradation for over 30 years. McCarty has focused on halogenated compounds as many organic chemicals found in wastewater are halogenated and resistant to biodegradation. Recent work has shown that one way to reduce levels of a substance below the order of mg/l is to provide another less-toxic substance which microorganisms can feed on at the same time. There is still a lot to discover about how chemotrophic organisms make use of pollutants but as more adaptations are discovered the range of options for bio-remediation increases.
In the absence of oxygen
One way of dealing with organic substances is reductive dehalogenation. This only occurs in anaerobic conditions. It has been known since 1967 that reductive dehalogenation can be used to deal with chlorinated pesticides such as DDT, and explains the natural attenuation of many organics which have contaminated groundwater. In the 1980s the process was found to be effective against chlorinated solvents and trihalomethanes (THMs). It was originally thought to be a cometabolic process brought about by enzymes used by organisms for other purposes. But studies carried out on the breakdown of tetrachloroethene (PCE) and trichloroethene (TCE) have now shown that reductive dehalogenation can provide chemotrophic bacteria with a direct source of energy. When organisms gain their energy for growth directly from reductive dehalogenation the process is termed ÔdehalorespirationÕ. Instead of the organic substances acting as electron donors in energy metabolism, the halogenated organic acts as an electron acceptor. By accepting two electrons from another organic compound or molecular hydrogen, a halogen atom is removed and replaced with a hydrogen atom. In this way the halogenated organic can be turned into a different substance.
In 1987 a bacterium was discovered which gained energy for growth from the dehalogenation of 3-chlorobenzoate. The hydrogen used to replace the halogen atoms was generated by other organisms which fermented benzoate to acetate and hydrogen. In 1993 another bacterium, Dehalobacter restrictus, was found to use hydrogen as an electron source and PCE as an electron acceptor. So far only PCE, TCE, hydrogen or formate have been found to support the growth of this strain so how it evolved remains a mystery.
A point to consider when designing a treatment process is that reductive dehalogenation can be non-beneficial if the product of transformation is more toxic than the parent compound. The reductive dehalogenation of PCE or TCE, for example, creates vinyl chloride which is regarded as more hazardous by the US Environmental Protection Agency than either of the first two substances. The maximum contaminant limit (MCL) for vinyl chloride in drinking water in the US is just 2µg/l compared to 5µg/l for PCE and TCE. Fortunately vinyl chloride can also be dehalogenated to produce less-toxic ethene. An organism was isolated in 1997 which converts PCE all the way to ethene. The first stage of the process is dehalorespiration but conversion of vinyl chloride in this case is by cometabolism.
Treatment by default
Breakdown by cometabolism can occur in aerobic or anaerobic conditions. For example, bacteria often use enzymes called oxygenases to oxidize methane and provide them with a primary source of energy, but the oxygenases will also oxidise other organic compounds which happen to be present. In cometabolism the organisms gain no benefit from breaking down the halogenated substances. But Prof McCarty suggests: “It is possible that what started out to be a cometabolic process in some cases may lead to an energy yielding reaction for the organism through some adaptive mechanism.” Much research has been carried out into the use of methane-oxidising bacteria to remediate groundwater polluted with TCE. Unfortunately it appears that more highly chlorinated compounds such as PCE and carbon tetrachloride cannot be broken down in this way.
The oxygenase/organic reaction is not very specific. Toluene can be broken down by five different oxygenases and it has been used to stimulate oxygenase production and degrade TCE at Edwards Air Force Base in California. A 9mg/l solution was added to groundwater along with oxygen and hydrogen peroxide to keep oxygen levels up. With a pumping rate of 25-38 l/min, 97% removal of TCE was achieved for a plume 50-60m long and 12m deep. TCE levels in the water were reduced from 1,000µg/l to 20-50µg/l. TCE can also be degraded by methane and air injection and this technique has been used at a number of sites.
Reductive dehalogenation can be cometabolic, the disadvantage being that it is slower than dehalorespiration where the organisms gain their energy directly from the substance to be degraded. But it can be very useful, for instance carbon tetrachloride is dehalogenated by Pseudomonas stutzeri in a cometabolic process producing carbon dioxide and chloride. Usually the breakdown of carbon tetrachloride leads to the formation of highly toxic chloroform.
See “Novel biological removal of hazardous chemicals at trace levels” by PL McCarty, Water Science and Technology Vol.42 No.12 p49-60.