Have you ever stopped to think about what happens to water after it goes down the drain in your sink, bathtub, or toilet? This water is called wastewater. If you live in the country, your house probably has a septic system which separates wastewater into solid and liquid portions. The liquid portion filters through the soil underground. The solid portion collects in an underground tank, and every few years a truck comes to pump out the solids and take them to a building called a sewage treatment plant for disposal.
If your house does not have a septic system, your wastewater probably ends up eventually in a lake or stream, but first it is cleaned up so that it will not cause water pollution. Wastewater goes from your house through a series of pipes to the sewage treatment plant. There, the wastewater is separated into two parts: 1) the liquid portion, which is cleaned and sent into a lake or stream, and 2) the solid portion, called sludge.
What happens to sludge? Often it is sent to a landfill, where it is buried along with garbage and other types of wastes. In some communities it is burned in an incinerator, and in other areas it is spread on land or dumped in the ocean. Which of these do you think is the best thing to do with sludge?
The most common way of getting rid of sludge is to send it to a landfill. One problem with this method is that many landfills are filling up, and towns are having trouble finding places to put new ones.
If sludge is sent to an incinerator, most of it is burned up, but there is a part that will not burn. This part, called ash, usually is taken to a landfill. Some of the chemicals in sludge go into the air when sludge is burned, and some people are worried that burning sludge will cause air pollution.
In areas where there is plenty of land, spreading sludge on fields can be a good idea. Sludge contains nutrients, chemicals which help plants to grow, so sludge can be used to replace other kinds of fertilizers. One problem with spreading sludge on land is that some sludges contain chemicals that are toxic. That means they can injure our health if they get into the food we eat or the water we drink. Scientists can do chemical tests to measure what toxic chemicals are in the sludge and then decide whether it is a good kind of sludge to use on land.
Some cities that are near the sea send their sludge on a barge out into the ocean for dumping. This is now against the law because it causes water pollution in the ocean, and these cities will have to find new ways of getting rid of their sludge.