from (1977) solid organic matter recovered from a sewage treatment process and used especially as fertilizer (or soil amendment); usually used in plural (from Merriam-Webster’s Collegiate Dictionary , 10th ed., 1998).

Note:  In this text, biosolids is used in many places (activated sludge being the exception) to replace the standard term sludge. The authors view the term sludge as an ugly, inappropriate four-letter word to describe biosolids. Biosolids is a product that can be reused; it has some value. Because biosolids has value, it certainly should not be classified as a waste product, and when biosolids for beneficial reuse is addressed, it is made clear that it is not.

Source: Handbook of water and wastewater treatment plant operations