The biodecomposition of organic material, such as animal wastes, plant residues or sludges, in the presence of air, by controlled methods including mechanical mixing and aerating.

The term is used to describe a series of activities that include collecting, sorting and processing recyclables (that would otherwise considered wastes) into raw materials (such as fibres and metals) and manufacturing the raw materials into new products.

Sewage treatment is a type of wastewater treatment which aims to remove contaminants from sewage to produce an effluent that is suitable for discharge to the surrounding environment or an intended reuse application, thereby preventing water pollution from raw sewage discharges.