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The filtration of public water-supplies / Third edition, revised and enlarged. cover

The filtration of public water-supplies / Third edition, revised and enlarged.

Chapter 15: EFFECT OF GRAIN SIZE UPON FREQUENCY OF SCRAPING.
By Allen Hazen
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About This Book

This book presents a practical, engineering-focused treatment of water filtration for municipal supplies, combining historical perspective, design principles, and operational guidance. It explains types of filters, construction of beds and underdrains, selection and grading of sands and gravels, rates of filtration, head loss, and mechanisms for regulating flow. Procedures for cleaning, sand-washing, and intermittent operation are described alongside theoretical and bacteriological considerations that bear on efficiency. Methods for measuring and removing turbidity and color, the effects of suspended mud, coagulation practices, and numerous design examples and appendices illustrate how to plan, build, and maintain effective filtration works.

EFFECT OF GRAIN SIZE UPON FREQUENCY OF SCRAPING.

The practical objection to the use of fine sand is that it becomes rapidly clogged, so that filters require to be scraped at shorter intervals, and the sand washing is much more difficult and expensive. The quantities of water filtered between successive scrapings at Lawrence in millions of gallons per acre under comparable conditions have been as follows:

  1892 1893
Effective size of sand grain 0.38 mm .... 79
Effective size of sand grain 0.29 mm .... 70
Effective size of sand grain 0.26 mm .... 57
Effective size of sand grain 0.20 mm 58 ....
Effective size of sand grain 0.14 mm 45 49
Effective size of sand grain 0.09 mm 24 14

The increase in the quantities passed between scrapings with increasing grain size is very marked.

With the fine sands, the depth to which the sand becomes dirty is much less than with the coarse sands, but as it is not generally practicable to remove a layer of sand less than about 0.6 inch thick, even when the actual clogged layer is thinner than this, the full quantity of sand has to be removed; and the quantities of sand to be removed and washed are inversely proportional to the quantities of water filtered between scrapings. On the other hand, with very coarse sands the sediment penetrates the sand to a greater depth than the 0.6 inch necessarily removed, so that a thicker layer of sand has to be removed, which may more than offset the longer interval. This happens occasionally in water-works, and a sand coarse enough to allow it occur is always disliked by superintendents, and is replaced with finer sand as soon as possible. It is obvious that the minimum expense for cleaning will be secured with a sand which just does not allow this deep penetration, and I am inclined to think that the sizes of the sands in use have actually been determined more often than otherwise in this way, and that the coarsest samples found, having effective sizes of about 0.40 mm., represent the practical limit to the coarseness of the sand, and that any increase above this size would be followed by increased expense for cleaning as well as by decreased efficiency.