Low-Tech Biorermediation: Microbiology for the Myriad
The longest lasting legacy of industrial society will be the tons of toxic metals, chemicals and poisons left in the environment. While some pollutants break down on their own, others persist for decades.
Many vegetables and plants absorb toxins into their edible parts, making soil contamination the largest obstacle to urban food production after gaining access to land. Metals, including lead, mercury and cadmium, are nearly ubiquitous in urban soils from the combined effects of smelters, lead paint, leaded gasoline and coal burning. Petroleum spills are common, as are the residues left from PCBs, dioxin, chlorinated compounds and other nasties.
Most polluted sites are never cleaned up. Instead, local residents are left to deal with the consequences. When governments do choose to remediate, cleanup typically involves giant environmental engineering firms with huge budgets. First, extensive soil samples are taken and tested. Next, energy-intensive excavations take tons of earth from the site and truck it to facilities where it is cleansed using high-tech procedures. This process, called ex-situ (off-site) treatment, is quite effective in determining what contaminants are present and then removing them. However, aside from causing considerable damage to the land in question, the entire operation can cost millions of dollars--not a choice for people and communities lacking in huge funds or political pull.
Fortunately, there are other options. Bioremediation is a less invasive, in-situ (on-site), lower-cost approach using naturally occurring organisms like bacteria, fungi and plants to break down or absorb toxins in soils. Having knowledge of these techniques can empower people to begin cleaning up their neighborhoods without needing a degree in chemical engineering or a massive budget. While many of these methods are experimental and cannot be guaranteed to work, many test cases have had positive results. As these methods are improved upon and made simpler, they will be used to improve the quality of our world's soils and reduce health risks.
Before proceeding, it is important to make the distinction between two different types of contamination: elemental and molecular. Elemental contaminants include what are commonly called heavy metals, and can be found on a Periodic Table: lead, cadmium, mercury, chromium, arsenic, all of the radioactive elements and others. Elements are the basic building blocks of molecules and cannot be broken down any further through natural processes. One cleanup strategy is to concentrate the metal(s) into a small area, where they can be easily removed. Another is to turn them into less dangerous compounds with other elements. When metals are removed from a site, they are commonly sent to landfills, as they cannot be made any safer. The problem has not been permanently dealt with, just shifted to another location.
Molecular contaminants, on the other hand, can be broken down into safer, elemental parts. Molecules are chains of elements strung together in different ways to create substances with different chemical properties. Some of these can be quite dangerous--PCBs, dioxin, benzenes, hydrocarbons, etc. Certain bioremediative processes, like mycoremediation, are capable of literally ripping molecular chains apart, reducing them to their less harmful, smaller parts. Bacteria are capable of eating away at hydrocarbons, breaking them down into their benign carbon and hydrogen components, which are harmless on their own. Molecular contaminants have the possibility of being permanently rendered harmless, while elemental ones do not.
Before you consider doing any type of bioremediation on a planned or existing garden site, you may want to consider having your soil tested. You may be relieved to find no dangerous levels of toxins. However, if there are, soil testing will give you an idea of the contamination level. On the downside, soil tests are expensive, and there is no single comprehensive test that can tell you everything that's bad in your soil. Each suspected contaminant must be tested for individually. Towns often keep historical records and aerial photographs of a location that can help in identifying potential hazards. Sometimes universities or agricultural extensions conduct tests for cheap or free.
The easiest method of treating contaminated soil is to simply add organic matter to it. Many toxic metals will readily form compounds with the organic particles found in compost. When they do this, the metals essentially get "locked up" and are less likely to be absorbed into the bodies of plants. The microbes living in compost will also act to break down chemical pollutants. But, while this may provide some protection, organic matter will not safeguard you from polluted soils. The best defense is to grow food in containers, like old bathtubs or pots, filled with soil that is known to be clean.
Fungi are among nature's most powerful decomposers. Secreting strong enzymes, fungi can not only break down wood and organic matter but eliminate chemicals and bacteria as well.
Experiments conducted with oyster mushrooms have demonstrated their amazing ability to break down petroleum. Piles of earth contaminated with diesel fuel were inoculated with the fungi, which then grew through them. Later testing revealed a 90 percent reduction in contaminant levels! The powerful enzymes in the fungi had effectively broken down the diesel's complex hydrocarbon chains into simpler, harmless parts of hydrogen and carbon. The mushrooms that grew out of the toxic pile showed no residual traces of diesel, yet had plants been grown in such soil, they might have become contaminated.
In coffee production, the coffee berry is the waste product. Rivers in coffee-producing regions are often polluted by caffeine that leaches from piles of discarded berries. Oyster mushrooms have been grown on these piles, successfully destroying the pollutant while creating a secondary crop. Oyster mushrooms have also been found to break down PCBs. Turkeytails, another common fungi, can degrade dioxin, pentachlorophenol and chromated copper arsenate.
It's important to understand that what is called a mushroom is only the reproductive stage of a larger organism. Most of a fungi's life occurs below ground and is visible as white, ropy threads, called mycelium. The enzymes of the mycelium are the primary decomposers.
Phytoremediation (phyto is Greek for plant) is the process of using plants to uptake toxins from contaminated soil or water. This method is used on land contaminated with heavy metals. After the plants have absorbed the metals from the ground, they are harvested and disposed of. While very much a new technology in an experimental phase, phytoremediation shows the potential of being a low-tech, low-cost method of soil cleanup that could be accessible to urban communities. In one study, two crops of Indian brassica grown on a test plot in Boston, Massachusetts, reduced lead levels in the soil by 63 percent.
However, one problematic aspect of phytoremediation is the disposal of the harvested plants, which must be treated as hazardous waste due to their high levels of toxic metals. Currently, the most common methods of plant disposal are either to incinerate them or to dump them in a landfill. Incineration creates a toxic ash, bringing the risk of accumulation to another area, and landfilling certainly concentrates toxins in another location. Another idea is to return the harvested biomass to the very mines from which the metals were extracted in the first place. For precious metals like silver and gold, it can be cost-effective to extract the metals from the plants for reuse by smelting.
Many factors contribute to the success or failure of a phytoremediation project. The most basic are contaminant and plant type. The acidity of the soil also factors in: Lead is more soluble in acidic soils, and therefore it is absorbed easier into plants.
Phytoremediation projects can be dangerous and require significant scientific monitoring and frequent soil tests. We strongly recommend conducting thorough research and small, controlled test patches.
Phytodegradation is the rapid degradation of pollutants that occurs in the area of high microbiological activity surrounding a plant's roots, known as the rhizosphere. In this area, nutrients are taken up and exchanged quickly between the many soil critters living there and the plants themselves. The increased metabolic levels mean that chemicals get broken down far more quickly than in soil without plants. One of the simplest and most effective things that can be done to combat pollution is to grow more plants.
Many of the techniques described above are new and experimental. With hope, trial and innovation, they can be made simpler, more effective and usable to communities and people, reducing the toxic legacy left to future generations and leaving a cleaner, healthier world in its place. For more information, visit Fungi Perfecti.
Stacy Pettigrew and Scott Kellogg are founding members of the Rhizome Collective, a nonprofit organization based in Austin, Texas. For more information, visit their site.
© Earth First! Journal November-December 2004