19. Biodegradation:

  The process of biodegradation can be divided into three stages: bio deterioration, bio fragmentation, and assimilation.Biodeterioration are sometimes described as a surface-level degradation that modifies the mechanical, physical and chemical properties of the material. This stage occurs when the material is exposed to abiotic factors in the outdoor environment and allows for further degradation by weakening the material's structure. Some abiotic factors that influence these initial changes are compression (mechanical), light, temperature and chemicals in the environment. While bio deterioration typically occurs as the first stage of biodegradation, it can in some cases be parallel to bio fragmentation, Hueck, however, defined Bio deterioration as the undesirable action of living organisms on Man's materials, involving such things as breakdown of stone facades of buildings, corrosion of metals by microorganisms.