Vol. 6 No. 6 December 1986

Special Edition



In May of 1986 I attended a conference in Houston that focused on the biological degradation of hazardous wastes. For the last five years Jim Seiber, Wray Winterlin and I have been studying the effectiveness of biological pesticide degradation at pesticide waste disposal sites. (Jim and Wray are both faculty members in the Department of Environmental Toxicology at UC Davis). This conference provided me with the opportunity to find out what research was being performed in other areas of the country that might be applicable to our situation here in California.

The focus of the conference in Houston was industrial cleanup of petroleum hydrocarbon wastes, however two papers did focus on the cleanup of pesticide wastes. Most of the attendees were from the east coast or midwest, and most were engineers or microbiologists. I found the meeting very informative and learned a great deal about other applications of biological cleanup of contaminated soil and water. The following article is a summary of some of the information that caught my eye and ears. If you would like further information, or a copy of the program, please call my office.

The Biodegradation of Hazardous Wastes

There are two major ways that microbes can degrade chemicals; the first is to use them as a carbon source in which case the chemical can be broken down to water, carbon dioxide and mineral salts (mineralized). The second is cometabolism in which the chemical structure is modified by microbes but the chemical is not mineralized. Both processes can occur at the same time, or only one of them may occur. Different populations of microorganisms can carry out one or both processes. The use of a chemical as a carbon source is more complex than cometabolism and requires that the microbes involved have or develop enzyme systems geared to the specific chemical involved.

In the laboratory (and theoretically in the field) it is possible to acclimate naturally occurring soil microbes to specific chemicals and eventually select out strains that can utilize these chemicals as a carbon source. This process takes time during which increasing concentrations of the chemical are added slowly. At the end of the process an enriched culture of microbes results. Over 10 years ago, Dr. Dennis Hsieh in the Environmental Toxicology Department at UC Davis developed an enriched culture capable of using parathion as a carbon source.

In theory these enriched cultures would be useful to help destroy chemicals in contaminated soil and groundwater. In practice, things are a bit more complicated. For one thing, these selected microbes may not be able to survive when fed low levels of the chemical in soil or water. They may not be able to compete with other soil microbes or they may need additional nutrients to thrive. The differences in soil composition at each site make it imperative to tailor the nutrients added to these enriched cultures to achieve successful chemical breakdown.

For years it has been known that soil microbes can successfully break down petroleum hydrocarbon wastes. Land farming of these wastes is a proven technology and continues as a viable option for disposal. Two of the participants spoke of their experiences in extending this technology to cleaning up soil and water contaminated by gasoline from leaky underground storage tanks (LUST). Prior to initiating biological decontamination, physical recovery of gasoline from highly contaminated soil and water was done by excavation and pumping. Gasoline was skimmed from the surface of pumped water. Since the biological degradation of petroleum hydrocarbons (HCs) is accomplished by microbes that use the HCs as a carbon source, enriched cultures of the microbes were added to the soil along with nutrients and trace minerals (micronutrients). Petroleum hydrocarbons tend to localize at the surface of aquifer. It was once thought that the addition of detergent might help to promote removal, however it was found that detergents make it more difficult. As the petroleum hydrocarbons spread through soil, they sometimes leave "globules". Nutrients and water have to get to these globules, and then the microbes will produce their own surfactants to break up the globules and feed. Similar treatment processes can be used for chloroform, trichloroethylene, and trichloroethane, except that anaerobic conditions are most effective. By controlling redox conditions in soil and water, it is possible to promote dehalogenation of these chemicals.

At one site in Pennsylvania at which over 133,000 gallons of gasoline had leaked, over 88,000 gallons were recovered physically from groundwater by pumping and skimming. This process took one year. Because the dissolved oxygen (DO) levels in the groundwater at this site were very low, air diffusers were put down the 250 foot deep wells and air was pumped in. Another speaker mentioned that his company now uses hydrogen peroxide to raise the DO levels in groundwater. In addition, 58 tons of ammonium sulfate and 28 tons of sodium phosphate were added over a one year period. The contractor estimated that over 45,000 gallons of gasoline that had leached into the aquifer were degraded biologically. The groundwater DO was monitored continuously at different sites and when the DO rose and stayed high, it indicated that there was no more gasoline available for biodegradation. If the gasoline was in the vadose zone, they would add water to move it down to the treatment area, because the speaker said he preferred to work in the aquifer rather than the vadose zone. This is not to say that treatment could not be done in the vadose zone.

I asked about leaded gasoline and expressed concern about lead dissolution in the water. The speaker answered that he had never found lead in the groundwater samples unless free gasoline was also present. He felt that the tetraethyl lead was being tied up or metabolized into insoluble lead salts.

Another speaker talked about how a similar approach had been used to stop the movement of a plume of gasoline in groundwater in a shallow aquifer (30-50 feet depth). After doing a thorough site assessment which included drilling multiple sampling and treatment wells, they injected nutrients, oxygen and natural soil microbes capable of "eating" gasoline. This was done in areas around the front of the moving gasoline plume. They monitored the progression of the plume for a year using the monitoring wells. The treatment wells showed visible signs of biological activity (frothing) and formed a biological treatment barrier that stopped further movement of the plume. There were problems with the process and one of the biggest was the proliferation of microbes which sometimes "plugged up" the aquifer, presumably due to their organic mass. The speaker also said that the process may not work well in soils that contain a lot of clay. He stressed that it is the hydrology that is the rate limiting step in cleanup in these situations. I found this particular application exciting, considering the fact that there may be more than 10,000 LUST in California. It is going to be necessary to find effective, relatively inexpensive ways to cleanup these sites, and this process holds promise for the future.

Dr. A. Chakrabarty presented an excellent talk on the genetic and molecular basis of biodegradation. Much of his talk concerned work he had done on biodegradation of 2,4,5-T, dioxins, and related compounds. These compounds are toxic to soil microbes at high concentrations. Plasmids (small, transferrable genes) have been found that contain the genetic code for metabolism of less persistent chlorinated hydrocarbons, and these plasmids can be transferred between Pseudomonas species. By transferring these plasmids to microbes taken from hazardous waste sites, he was able to develop microbes that could completely degrade 2,4,5-T, however, these microbes could not live just on 2,4,5-T. In soil tests, this engineered microbe could degrade up to 20,000 ppm of 2,4,5-T in soil.

Another paper was presented detailing the biological cleanup of a lumber treatment facility heavily contaminated with pentachlorophenol (up to 1,000 ppm), oils, chromium, arsenic, copper and polynuclear aromatic hydrocarbons (PNAs) such as naphthalene and phenanthrene. To treat the site, micronutrients, nitrogen and phosphorus were added to the soil along with an enriched culture of microbes taken from the site. In a test plot, this treatment worked well to biodegrade the PNAs and pentachlorophenol. Such treatments can have no effect on metals, and none were reported. The entire site was treated successfully in this way, although it took quite a long time (greater than 3 months) to effect suitable degradation. The speaker stressed the need to fully evaluate the soil and make conditions right for biodegradation.

Papers were presented detailing work done to biodegrade pentachlorophenol (and other phenols), creosote, 2,4-D, 1,1,1- trichloroethane (TCA), polynuclear aromatic hydrocarbons, alachlor, and other industrial chemical wastes. In almost every case, the site cleanup process was initiated by first enriching cultures of microbes taken from the site itself. In one study, a contractor took enriched cultures from one site to another and did a controlled trial on the effect of these cultures on biodegradation. He found no effect from adding the enriched culture. Later analysis showed that the waste site already had a substantial population of these microbes, explaining the lack of effect. Every speaker stressed that because each waste site is unique, each must be evaluated and then treated according to the specific soil conditions found.

Microbes are capable of detoxifying a wide variety of human-made chemicals. This process can occur in treatment facilities and also in soil. This capacity can be enhanced, transferred, and exploited to help in detoxifying hazardous wastes. Biological degradation of hazardous wastes is certain to receive more attention in the future as an effective and also proven "technology". Along with other developing technologies, it will offer us possibilities for detoxifying hazardous wastes on-site at minimal cost. The development of such on-site treatment programs must occur, because waste relocation (taking hazardous waste from one site to another so-called secured site) will no longer be an option in the future.

Arthur L. Craigmill
Toxicology Specialist
U.C. Davis