industrial wastewater through bioreactors housing special bacteria. The microbes reduce concentrations of the pollutant from over one million parts per billion to less than five parts per billion—far below the EPA's guidelines. They break down the hazardous methylene chloride into water, carbon dioxide, and salt, thereby eliminating any need to recover it, transfer it, or dispose of it.

Microbes deal with poisonous chemicals in the same way we and other animals process our food, using enzymes to convert one chemical into another, and taking energy or usable matter from the chemical change en route. The chemical conversions usually involve breaking large molecules into several much smaller molecules, much as we break down the complex carbohydrates in our food into simple sugars such as glucose. In some cases, the by-products of a bacterial banquet are not simply harmless but actually useful. Methane, for example, can be derived from a form of bacteria that degrades sulfite liquor, a waste product of paper manufacturing.

Although individual species of bacteria can carry out several different steps of chemical breakdown, most toxic compounds are degraded by groups of bacteria, called consortia. Each species in the group works on a particular stage of the degradation process, and all of them together are needed for complete detoxification.

The search for useful bacteria, fungi, or other organisms to use in bioremediation is best begun on polluted sites themselves. Anything found living there is at least resistant to the deadly chemicals, and may actually use them. After samples are grown and studied in the lab, the most effective strains are shipped back to dump sites and mixed into the chemical brew along with added organic nutrients such as nitrates and phosphates, which help the