SE–Structures and Environment: Physical Properties of Composting Material: Gas Permeability, Oxygen Diffusion Coefficient and Thermal Conductivity

Composting is one of the oldest bio-technological processes used by human beings. It can be defined as the partial decomposition of heterogeneous organic matter by a mixed microbial population in a moist, warm and aerobic environment. In the organic matter, a dense population of various micro-organisms is found. The micro-organisms use organic matter, minerals, water and oxygen for their growth and metabolic activity. Each microbial species has an optimal temperature at which growth and multiplication rates are maximal. The oxygen concentration plays a dominant role in these processes. To optimize the composting process, the above-mentioned aspects must be known in detail. A simulation model offers an appropriate tool to reach this goal. Such a model must especially describe distributions of temperature and oxygen concentrations because these are considered as the most important process parameters. Reliable results can only be obtained if the physical properties of the composting material are well known under various conditions. In this paper, measurements of gas permeability, oxygen diffusivity, and thermal conductivity of the composting material are presented. Generally it is found that the gas permeability decreases as the gas velocity increases. For raw material, the gas permeability decreases with the wetness, whereas for older material there is no clear relationship. For composting material which has been turned once, the gas permeability is larger than for raw material. The oxygen diffusion coefficient is proportional to the gas-filled volume fraction to the power 1·5. There is no clear relationship between the oxygen diffusion coefficient and the age of the material. It is found that at a given temperature and for volume fractions of solid phase of 0·33 or less, the thermal conductivity increases linearly with the volume fraction of the liquid phase. The thermal conductivity is not influenced by the age of the composting material. The thermal conductivity increases with temperature.