Effective size and uniformity coefficient
● What is Effective Size
Sieving calculation diagram of water filter media (semi-log graph) On the lognormal distribution probability line, the size of particles whose cumulative passing mass percentage from fine particles is 10% is shown in mm.
This is because Hazen (Allen Hazen) tested a large number of sands in the natural state and found that the water permeability (easiness of water flow) of “10% from the smallest” and “90% remaining” of particle size is the same. It begins with the discovery and reading the particle size that separates these two parts as the effective diameter. In other words, the water permeability of the filter medium can be defined as the particle size that is the water permeability when assuming that the particle size of all particles is the same. Therefore, a large effective diameter means that the water permeability of the filter medium is also high, that is, water easily flows.
The effective diameter of filter sand generally used in water supply is specified as 0.30 to 0.45 mm for slow sand filter and 0.45 to 0.70 mm for rapid filter sand.
●What is Uniformity Coefficient
It is the ratio of the size of the particles to the effective diameter at which the accumulated passing mass percentage is 60% on the sieving line. Therefore, the smaller the uniformity coefficient, the more uniform the particle size, and if all have the same particle size, the uniformity coefficient will be 1.0.
Among the sands that exist in nature, we handle sand that has an equality coefficient in the range of approximately 1.5 to 3.0 as a raw material. In the case of filtered sand, fine sand is collected on the surface layer and coarse sand is collected on the lower layer by backwashing. If the uniformity coefficient is large, finer sand will collect on the surface layer, and while the suppression rate of turbidity will be extremely high, it will easily become clogged and it will not be possible to maintain a sufficient filtration duration. Therefore, the Waterworks Design Guideline (2000) stipulates that “filtered sand should be 1.70 or less”, but in reality, about 1.3 to 1.5 is used. In addition, we have a standard equality coefficient of filtered sand of 1.5 or less, and we also manufacture high quality products of 1.4 or less and 1.3 or less.
[Reference] Water Supply Facility Design Guideline (2000) JWWA Water Supply Filter Material JWWA A 103: 2006-2
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