Scavenging off toxic metals

Scavenging off toxic metals WHEN CITIZENS of Arcata, on Humboldt Bay in northern California, were faced with the need to treat the industrial waste water and sewage that had been pouring into the bay for decades, they decided against building a $30 million chemical treatment plant and chose instead to spend $5 million on creating 38 ha of wetlands and marshes.

The marshland was intended to act as a breeding ground for certain microbes that have the ability to break down toxic metals into harmless compounds. Today, the Humboldt Bay is clean enough to render fish edible for the first time in decades. And, as a bonus, the wetland has turned into a bird sanctuary (Trends In Biotechnology, Vol 11, No 8).

Gaining popularity
Bioremediation -- cleaning the environment with the help of microorganisms -- is a fast-growing and attractive alternative, especially in the US, to conventional clean-up technologies such as incinerators and chemical treatment plants.

Scientists believe that because of the low capital and running costs, bioremediation should be cheaper than conventional technologies. Incinerating hazardous wastes, for example, can cost more than $1,050 a cubic metre, as compared to about $235 for bioremediation. Moreover, bioremediation can be done on the site itself.

Impressive list
The list of bioremediation applications is impressive -- nuclear-waste sites, industrial dumps, contaminants such as greases and oils, and toxic organic compounds such as DDT. Add to this the area of pollution prevention, into which corporations across a spectrum of industries are pouring in millions of dollars.

Experts estimate that a decade hence, 40 per cent of the market for environmental remediation of hazardous wastes could be handled by microorganisms. Thanks largely to the $3.2 million biotreatment of several Alaskan beaches in the wake of the Exxon Valdez oil spill in 1989, bioremediation has become a commercially attractive technology.

There are three commonly used methods of carrying out in situ bioremediation of contaminated soil and water: introducing foreign, contaminant-degrading bacteria into the soil; adding nutrients and oxygen to the soil, so that indigenous microorganisms deficient in either or both can then metabolise the contaminants to carbon dioxide and water, and, flushing contaminants from the soil so that they can be recovered and treated in a bioreactor into which a continuous supply of nutrients, oxygen and microorganisms is fed.

Many of the technologies being developed for treating contaminated sites are also applicable to treating toxic wastes before they reach the environment. An example of pre-emptive biological treatment is the use of grease-eating bacteria in grease traps at restaurants.

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