Definition - What does Sludge mean?
Sludge is the term given to a semi-solid slurry which may include any particular variety of solid sedimentary material, mixed in with any kind of liquid. The term is used most when referring to the semi-solid material in sewage, or that which results as a product from a type of industrial or refining process of any kind. A common example of sludge is mud as it is a semi-solid containing mostly water and particulate solids.
Safeopedia explains Sludge
Sludge is also known as biosolids with regard to sewage treatment, and is removed from sewage water during primary and secondary treatment. Sludge settles to the bottom of waste water and solids can be separated and treated with bacteria. Sludge in sewage is hazardous as it contains harmful chemicals, toxic metals and pathogens. It also contains many essential nutrients which make it useful when repurposed as fertilizer.