Sewage sludge is a thick sludge of solid materials which settle out from wastewater during the treatment process, whether the wastewater is being moved through a home septic system or a commercial sewage treatment plant. Once sludge is separated from wastewater, it undergoes processing of its own. Fully processed sludge must be disposed of or utilized in some way, and there is a great deal of debate in some regions of the world about appropriate handling for sewage sludge.
One major component of sludge is, of course, fecal material, complete with accompanying bacteria. Sewage sludge also contains everything else which ends up in a septic or sewer system, including toilet paper, tampons, and a wide variety of other materials such as food, chemical waste, and so forth. This is one of the problems with sewage sludge; pure fecal material can be processed and reused fairly easily, but sludge can be heavily contaminated, which may make it dangerous to handle.
Under normal conditions, sewage sludge will rapidly undergo anaerobic fermentation, with bacteria which thrive in an oxygen-free environment breaking down the sludge. This is sometimes used alone for processing, but sewage sludge can also be chemically treated. The treatment process also involves allowing evaporation to occur so that the sludge becomes more solid, with less liquid, making it lighter and easier to handle. Evaporated sludge may be pelletized for convenience.
One use of sludge is in agriculture. Although the use of fecal material on food crops may be restricted in some areas, sludge can be used to fertilize landscaping, and can in fact make an excellent alternative to chemical fertilizers which might otherwise be used to fertilize landscaping. Sewage sludge is, after all, rich in nutrients, and there is a long history of using human waste in agriculture throughout the world. Sludge can also be containerized, or buried, with burial involving dried sewage sludge to limit the amount of space required.
Concerns about the use of sewage sludge revolve around ingredients other than fecal material which it might contain. For example, sludge often contains traces of prescription medications such as antibiotics, which could breed antibiotic resistance if untreated sludge was introduced to the environment, along with hormones and other drugs which may be harmful. It can also contain heavy metals, toxic chemicals, and a variety of other substances which could be dangerous. Heavy treatment can potentially make sludge more dangerous by adding chemicals, and deplete it of nutrients, making it less suitable as a fertilizer.