Substances: The substances that are broken down by biological processes, such as the action of bacteria and, other saprophytes into simpler harmless substances in due course of time are called biodegradable substances. Domestic waste products, urine and faecal matter, agricultural wastes, wood, paper, cloth, etc. are examples of biodegradable substances.

Non-biodegradable Substances: The substances that are not broken down by biological processes, but are acted upon by physical processes like heat and pressure under the ambient conditions found in the environment are called non-biodegradable substances. Plastics, glass objects, polythene bags, pesticides, metals such as mercury and lead, radioactive wastes, etc. are non-biodegradable substances.