Elizabeth Klinga, age 9, of Westport, Connecticut, for her question:

What is sludging?

Sludge sounds like something slushy and muddy. And so it is. Melting snows turn to slushy sludge. Rivers dump muddy sludge along their banks and floods dump oozy sludge on the land. The earth uses soggy sludges to help the soil to grow plants. Nowadays we are learning to use sludge to make the earth greener. Modern farmers are sludging their fields to fertilize the soil with nature's safe old recipes. And they have found some better sludges to do the job.

Mother Nature has no garbage problems and no pollution problems. This is because the waste materials from plants and animals are remodeled and reused. These wastes mix with water and powdery rock to make soil. And the soil is the earth's stomach. It digests all the waste materials it can get    and changes them into the chemicals that plants need to grow. The plants feed the animals and the people. And all living things make more waste materials to~go back into the hungry soil.

This round about is called recycling. Nowadays, we are learning to recycle our human wastes and garbage. And sludging is one of the ways to do it. Scientists are learning how to make slushy sludges from all sorts of waste plant and animal material, Modern farmers spread them on their fields to feed the soil. This is sludging.

In the fall, the ground is littered with old leaves. Some cities burn them in incinerators with other garbage. This adds smokey pollution to the air. Some cities have a better idea. Portland, Oregon, mixes its fallen leaves with moisture and farmers use this sludge to fertilize their fields. Every day, a long sludge train leaves Chicago carrying thousands of tons of sludge far out to the farmlands. This sludge is a soggy mixture of garbage.

Farm experts say that the best kind of sludge is made from human wastes. Long ago we learned that this dirty stuff is unsafe to leave around because it spreads diseases. So we flush it down the drain to a sewage plant. There they use nature's recipes to purify it. Chemicals are added to kill the germs. Beds of gravel, sand and charcoal filter out the fragments. Special bacteria are added and the mixture is sprayed with fountains. These friendly bacteria devour the dirty wastes and change them into very different material. This may be used for sludging.

Some sewage plants do a better job than others. Those that do a poor job only partly purify the dirty waste and then dump it into streams and rivers. Those that do a good job create sparkling clean water and perfectly safe sludge. Modern farmers use the soggy slush for sludging their fields. This fertilizer gives the soil as much plant food as it can get from stable manure.

This sort of sludging began when people got worried about their garbage, sewage and pollution problems. Sludging solves all these problems together, just as nature solved them. It does away with smokey incinerators that add smog to the air. It recycles all sorts of waste materials so that the hungry soil can change them into clean, safe chemicals to nourish the plants.