Back to nature.

Farmers are fertilizing crops with a "new" natural resource: a treated form of the stuff you flush.

Time for lunch, at last. While flipping through the latest issue of Science World, you snarf down a burger, a salad, and some juice. Soon enough, nature calls. You make a pit stop at the restroom before heading back to class.

Question: Which items in this scene can be recycled?

Most people might guess the magazine and the food containers. But surprisingly the stuff you'd prefer to leave outta-sight, outta-mind is also a highly useful natural resource: your own bodily waste.

"Yech!" you might say. But think about it (just humor us). Every time you flush you contribute to a hefty supply of sewage sludge--human and other wastes digested by bacteria and other micromunchers. Now, thanks to sewage treatment and back-to-nature igenuity, about 45 percent of this sludge is put to good use; fertilizing America's crops, crops that eventually could feed you.

Gross-out City? Not the way Wisconsin farmer Jerry Bradley sees it. He's been "sludging" bountiful crops of corn, soybeans, and oats for the past 10 years. Before growing season, sludge-laden truck from the nearby sewage treatment plant stops by and "injects" the gunk into Bradley's soil.

IT'S NUTRITIOUS!

Like other fertilizers, such as manure or chemicals extracted from oil and rock, sludge is loaded with nutrients that are essential for plant growth--things like nitrogen, potassium, phosphorus, and other vitamins and materials.

Where do these nutrients come from? The foods you eat. After all, you only use some of the nutrients you ingest. The rest make their way out of your body to the sludge tanks at a sewage treatment plant.

When sludge is spread on fields, bacteria in the soil convert the nutrients into compounds that plants can use to grow. Leaves, stalks, and fruits made with those nutrient-rich compounds become food for grazing animals--and for you (see photos, right).

As one Colorado farmer sums up this recycling loop, "Your waste comes out here and fertilizes our wheat fields. That helps make some of the bread that finds its way back to your table."

According to soil scientist Art Peterson, sludge can revitalize cropland that's been depleted by erosion or harsh chemical fertilizers. In addition, sludge is cheaper than other fertilizers. So why waste this plentiful resource?

DOWN IN THE DUMPS

Until recently the mid-1980s, that's precisely what we did. We hauled about 75 percent of our sludge from sewage treatment plants to landfills giant manmade lagoons, or fiery incinators. Many coastal cities dumped their sludge at sea.

But studies that uncovered wildlife die-offs at ocean-dumping sites set off alarms. Even though sludge undergoes treatment to kill disease-causing microbes, other toxins often remain. These include heavy metals, such as zinc, iron, cadmium, mercury, and lead, which mainly enter the waste stream from factories. (Other sources include the shammpoo, toothpaste, household cleaners, and paint you pour down the drain, plus small amoumnts from your body's wastes.) If ingested in high amounts, these metals can cause health problems in humans and other animals, says soil scientist Dave Taylor.

To protect the waterways and prevent possible health risks to humans, Congress passed to the Ocean Dumping Ban Act, which took effect December 31, 1991. The ban forced people to come up with fresh, new ways to use the sludge on land.

RISKS vs. BENEFITS

But aren't those toxins that were hazardous at sea also bad for the land--and crops grown on it?

That question is the subject of hot debate among scientists. Toxicologist Steve Lester at the Citizens Clearinghouse for Hazardous Waste argues that metals can make their way through the food chain the same way nutrients do--with serious consequences. But other scientists maintain that sludge simply suffers from a poor "image" problem. Used properly and monitored correctly, they say, sludge is safe.

One safety measure: Many industries are new required to remove metals before they enter the sewer system. You can help too by using eco-friendly household cleaners, such as a mixture of banking soda and vinegar (instead of toxic products) to clean the toilet, say.

The U.S. Environmental Protection Agency (Environmental Protection Agency (EPA) has also devised a list of pollutant limits for metals and other sludge contaminants. Scientists regularly check the pollutant levels in sludge, soil, plant tissue, and groundwater, where harmful chemicals might seep. If levels are too high, the government could prohibit the sludge's use on that land or apply expensive treatments to remove the metals.

So far, says Bob Bastian of the EPA, these measures have paid off. "No one has ever shown evidence of someone getting sick from heating plants grown in sludge." Think about it the next time you sit dowe to a salad. Or better yet, don't!