Paper mill residuals (gray, lower right and elsewhere) applied to a new corn field (central MA, 2010). The residuals enhance the organic matter and nutrient content of the soil. The corn on the right is growing on soil that was treated in the same way one year prior.
"Residuals" are organic (carbon-based) "wastes" that can be put to beneficial uses. They are sometimes called "by-products."
Other than biosolids, wastewater residuals commonly recycled in New England and eastern Canada include those generated at pulp and paper mills and septage from home septic systems.
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.