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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 55: LIMITS TO THE USE OF SUBSIDENCE FOR THE PRELIMINARY TREATMENT OF MUDDY WATERS.
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.

LIMITS TO THE USE OF SUBSIDENCE FOR THE PRELIMINARY TREATMENT OF MUDDY WATERS.

When water is too muddy to be applied directly to filters, the most obvious treatment is to remove as much of the sediment as possible by sedimentation. Sedimentation-basins are considered as essential parts of filtration plants for the treatment of muddy waters. The effect of sedimentation, as noted above, is to remove principally the larger particles in the raw water. By doing this the deposit upon the surface of the filters and the cost of operation are greatly reduced.

These larger particles are mainly removed by a comparatively short period of sedimentation, and the improvement effected after the first 24 hours is comparatively slight. The particles remaining in suspension at the end of this time consist almost entirely of very fine clay, and the rate of their settlement through the water is extremely slow; and currents in the basin, due to temperature changes, winds, etc., almost entirely offset the natural tendency of the sediment to fall to the bottom.

There is thus a practical limit to the effect of sedimentation which is soon reached, and it has not been found feasible to extend the process so as to allow much more turbid waters to be brought within the range which can be economically treated by sand filtration.