As municipal solid waste (MSW) is a mixture of heterogeneous and diverse substances, it does not have its own specific internal structure and external characteristics as a single substance, so it does not have specific physical properties. Its physical properties often vary with the properties and proportions of its constituents.
In the process of management and disposal of municipal solid waste, three physical properties are often involved: bulk density, void fraction and moisture content.
(1) bulk density. In the natural state, the mass per unit volume of municipal solid waste is called its bulk density, expressed as kg/m3 or t/m3. The bulk density of garbage varies with the composition of garbage, compaction degree, biochemical degradation process and the way of cleaning and handling. Therefore, the bulk density of garbage can be divided into natural bulk density, loading bulk density of garbage truck and landfill bulk density. The bulk density of garbage is one of the important characteristics of garbage. It is an indispensable parameter for selecting and designing storage containers, collecting and transporting machines or pipelines, calculating and determining the size of disposal structures and landfills. By analyzing the survey data of refuse bulk density provided by 12 cities in China in 1990, it is concluded that the natural bulk density of refuse is (0.53 + 0.26) t/m3, the loading capacity of garbage truck is about 0.8 t/m3, and the landfill bulk density is 1 t/m3. Generally speaking, large cities with developed economy and high living standards have high content of light organic matter and low bulk density, which is about 0.45t/m3; while small and medium-sized cities, especially northern cities, have high content of heavy inorganic matter (mainly ash) and high bulk density, which is about 0.6-0.8t/m3. The Natural bulk density of garbage in some small and medium-sized cities in the North even reaches 1.0t/m3. The bulk density of garbage in industrial developed countries is 0.10-0.15t/m3, that in middle-income countries is 0.20-0,40t/m3 and that in low-income countries is 0.250-0.50t/m3.
(2) Void fraction. Void fraction is the proportion of void volume between materials in garbage to garbage storage volume. It is a characterization parameter of garbage ventilation capacity and is related to garbage bulk density. The void fraction of garbage with small bulk density is generally larger. The larger the void ratio, the larger the void between the material and the material, the larger the ventilation cross-section area of the material, the smaller the flow resistance of the air correspondingly, and the more conducive to the ventilation of garbage. Therefore, void fraction is widely used in compost oxygen supply ventilation and resistance calculation of forced ventilation of refuse in incinerator and determination of parameters of ventilator; the main factors affecting void fraction are material size, material strength and moisture content. The smaller the material size, the more voids; the better the material structure strength, the larger the average void volume, which will lead to the increase of the void fraction of garbage. Water will occupy the voids of materials and affect the structural strength of materials, resulting in the reduction of voids.
(3) Water content. Moisture is an important parameter in garbage disposal. Its value directly affects the normal process of landfill, refuse composting and incineration. In the process of garbage separation, moisture in garbage will also affect the screening of garbage, air separation and material transportation.
The pyrolysis and gasification process developed by Luoyang Building Materials Architectural Design and Research Institute can directly treat the original unsorted garbage, and even the garbage with high moisture content can be directly treated for power generation.
The moisture content of municipal solid waste is defined as the moisture content of the unit mass waste. It is a relatively changeable physical quantity expressed by the mass fraction (%). The moisture content of a certain component of municipal solid waste often changes with the change of humidity in the air, and is related to the moisture content of man-made mixed people. The variation range is 11% – 53% (typical value is 15%, 40%). According to the investigation, the main factors affecting the moisture content of garbage are the content of animals and plants and inorganic substances in garbage. When the content of plants and animals in garbage is high and the content of inorganic substances is low, the moisture content of garbage is high, whereas the moisture content of garbage is low, that is, the moisture content of garbage is proportional to the organic content.