Filtration is a broad concept. The process of changing the water quality caused by the passage of water through a filter bed is collectively referred to as filtration. Therefore, the filter material is different, and the function of removing impurities in water is also different. For example, to remove suspended solids in water, anthracite is generally used as filter material; iron and manganese in groundwater are removed; manganese sand is used as filter material; activated carbon is used as filter material to remove organic matters, color, odor, and residual chlorine in water. In recent years, the rapid development of synthetic fiber filter can remove the total suspended impurities of yellow, and it can also remove iron and manganese in groundwater.
In the selection of filter media, the purpose of filtration should be determined first, followed by the necessary experiments and screening of the filter media properties. The main experimental index is the mechanical strength and chemical stability of the filter media.
(1) Mechanical strength
As a filter material, it should have sufficient mechanical strength, because in the backwash process, the filter material is in a fluidized state, and the particles of the filter media continuously collide and rub against each other. If the mechanical strength is low, a large amount of filter media will be caused. Damage, particle size becomes smaller. These crushing media are carried away by backwashing water during backwashing, causing loss of filter media. If the broken filter material is not washed away and remains in the filter layer, the head loss will be increased during filtration and the filtration cycle will be shortened.
In the water treatment, the wear rate and the breakage rate are commonly used to judge the strength of the filter material. The wear rate refers to the degree of filter wear caused by the friction between the filter media particles during backwashing. The breakage ratio indicates the degree of cracking caused by particles colliding with each other during backwashing. General requirements filter material wear rate is less than 0.5%, broken rate is less than 4%.
(2) Chemical stability
The chemical stability of the filter media is one of the important reasons that affect the quality of the filtered water, and it is also the main indicator for selecting the type of filter media. In fact, the experimental method is to clean the sample after the filter was dried at 60 °C and soaked in filtered water for 24 hours. If the increase in dissolved solids in the water sample is less than 20 mg/L, the increase in oxygen consumption is less than 10 mg/L, and the increase in silicic acid is less than 10 mg/L, the chemical properties of the filter media can be considered to be stable.