Multi media filter is a process of using one or several kinds of filter media to pass the water with high turbidity through the granular or non granular materials with certain thickness under certain pressure, so as to effectively remove the suspended impurities and make the water clear. The commonly used filter materials are quartz sand, anthracite, manganese sand, etc., which are mainly used for water treatment and turbidity removal, softened water, pre-treatment of pure water, etc., and the turbidity of the effluent can reach 3 degrees following.
The common multi-media filters used in water treatment are: Anthracite quartz sand magnetite filter, activated carbon quartz sand magnetite filter, activated carbon quartz sand filter, quartz sand ceramic filter, etc.
The main factors to be considered in the design of filter layer of multi-media filter are:
1. Different filter materials have large density difference to ensure that there is no mixing phenomenon after backwashing disturbance.
2. Select filter material according to the purpose of water production.
3. The particle size of the lower filter material is required to be smaller than that of the upper filter material to ensure the effectiveness and full utilization of the lower filter material.
事实上，以三层滤床为例，上层滤料粒径 大，有密度小的轻质滤料组成，如无烟煤、活性炭；中层滤料粒径居中，密度居中，一般为石英砂组成；下层滤料由粒径 小，密度 大的重质滤料组成，如磁铁矿。由于密度差的限制，三层介质过滤器的滤料选择基本上是固定的。上层滤料起粗滤作用，下层滤料起精滤作用，这样就充分发挥了多介质滤床的作用，出水水质明显好于单层滤料的滤床。
In fact, taking three-layer filter bed as an example, the upper filter material is composed of light filter material with large particle size and small density, such as anthracite and activated carbon; the middle filter material is composed of quartz sand with medium particle size and medium density; the lower filter material is composed of heavy filter material with small particle size and large density, such as magnetite. Due to the limitation of density difference, the selection of filter material for three-layer media filter is basically fixed. The upper filter material plays a role of coarse filtration, and the lower filter material plays a role of fine filtration, so that the multi-media filter bed is fully played, and the water quality of the effluent is obviously better than that of the single-layer filter bed.
For drinking water, it is generally prohibited to use anthracite, resin and other filter materials.