Medium Specification For Granular Filters

The most important component of a granular filter is the medium. This medium must be of the appropriate size. Small grain sizes tend to have higher head losses, while large grain sizes, although producing comparatively smaller head losses, are not as effective in filtering. The actual grain sizes are determined from what experience has found to be most effective. The actual medium is never uniform, so the grain sizes are specified in terms of effective size and uniformity coefficient. Effective size is defined as the size of sieve opening that passes the 10% finer of the medium sample. The effective size is said to be the 10th percentile size P10. The uniformity coefficient is defined as the ratio of the size of the sieve opening that passes the 60% finer of the medium sample (P60) to the size of the sieve opening that passes the 10% finer of the medium sample. In other words, the uniformity coefficient is the ratio of the P60 to the P10. For slow-sand filters, the effective size ranges from 0.25 mm to 0.35 mm with uniformity coefficient ranging from 2 to 3. For rapid-sand filters, the effective size ranges from 0.45 mm and higher with uniformity coefficient ranging from 1.5 and lower.

Plot of a sieve analysis of a sample of run-of-bank sand is shown in Figure 7.9 by the segmented line labeled "stock sand " This sample may or may not meet the required effective size and uniformity coefficient specifications. In order to transform this sand into a usable sand, it must be given some treatment. The figure shows the cumulative percentages (represented by the "normal probability scale" on the ordinate) as a function of the increasing size of the sand (represented by the "size of separation" on the abscissa).

Let pj be the percentage of the sample stock sand that is smaller than or equal to the desired P10 of the final filter sand, and p2 be the percentage of the sample stock sand that is smaller than or equal to the desired P60 of the final filter sand. Since the percentage difference of the P60 and P10 represents half of the final filter sand, p2 - pj must represent half of the stock sand that is transformed into the final

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