A new bioreactor system promises to clean unprecedented levels of toxic organic chemicals classified as priority pollutants by the U.S. Environmental Protection Agency (EPA), according to researchers at Queen's University (Ontario, Canada; 202-872-4445). As the bioreactor works best on sites with high waste concentrations, it may be used to clean contaminated sites and dispose of old chemical supplies.
The clean-up process takes place in two liquid phases. The first uses a solvent that the scientists say readily dissolves high concentrations of toxic chemicals such as benzene, toluene, and p-xylene (collectively referred to as BTX). These man-made chemicals are components of gasoline and are used extensively in industrial processes. In the second phase, BTX is fed to Pseudomonas bacteria cultivated in water.
The pollutants are much more soluble in the solvent than in water. Because of this, a large amount of BTX dissolves in the solvent phase, lowering the concentration of the pollutant that reaches the water phase. This is beneficial because too much BTX can kill the bacteria population in the water. Since a strict equilibrium is maintained between the phases, BTX enters phase two only as fast as bacteria can degrade the pollutants that are already there.
"The organisms themselves determine the rate at which the BTX gets fed to them," says Andrew Daugulis, chemical engineer at Queen's University and one of the discoverers of the process. "They control the process. Therefore, we have a very efficient and self-regulating means of destroying large quantities of (toxic chemicals)." The by-products of the process are carbon dioxide and more bacteria.
To test their bioreactor, the researchers spilled BTX on a soil sample. They say their solvent was able to recover more than 99% of the pollutants from the soil. When it was then put through the bioreactor's water phase, "the bacteria readily used up all of the BTX," Daugulis says. "So we used the solvent again to recover another 'spill' and then sent it back to the bioreactor for BTX degradation." The process again cleaned all of the pollutants from the soil, he says.
This reusable system may replace current clean-up procedures for some applications. Today's methods include spraying bacteria directly on contaminated sites, removing soil and washing it with large volumes of water, and incineration. All of these systems have major drawbacks. Because the new bioreactor is still in early laboratory-testing stages, it is not yet clear what complications might occur during large-scale use.
The researchers think their system would be particularly effective at getting rid of very high concentrations of toxic organic wastes like BTX, including stores of pure chemicals that have passed their expiration dates. Unlike conventional approaches, the researchers say that the bioreactor works better on higher waste concentrations than on lower waste concentrations.
The research will be outlined in the January/February 1999 print edition of Biotechnology Progress, published by the American Chemical Society (ACS; Washington, DC).
For more information, call 202-872-4445