"Residuals" are organic "wastes" that can be put to beneficial uses. They are sometimes called "by-products."
Biosolids are one kind of residual.
Another recycled organic residual is wood ash from electricity generators that burn tree wood chips; it is used in farming as a replacement for lime and in composts.
Food processing residuals, and pre- and post-consumer food scraps, are often composted and applied to soils.
Animal manures are a very common form of residual.
Most of these residuals can also be treated by anaerobic digestion before they are applied to soils; this generates biogas, a methane-rich renewable fuel.
Another residual - water treatment residual – is generated in relatively small amounts during the treatment of drinking water.