Page 473 - Water and wastewater engineering

11-46 WATER AND WASTEWATER ENGINEERING

Hints from the Field
Although the rise of turbidity during ripening of the filter results in only a small fraction of the
filter run average turbidity, the potential for breakthrough of a significant dose of pathogens war-
rants significant operational consideration. This takes the form of diversion of the filtrate, that
is, “filter-to-waste,” or, preferably, “filter-to-recycle” to the head end of the plant. The major
sources of suspended solids responsible for the rise in turbidity after the filter is put back on line
are the backwash water and the material released during interparticle collisions of the media on
the closure of the backwash valve (Amirtharajah and Wetstein, 1980). The latter effect can be
ameliorated approximately 20 percent by slow closure of the backwash valve (on the order of two
minutes). The peak effluent turbidity during ripening may be in the range of three to seven times
that of the steady state operation. It may take 30 to 40 minutes of diversion of the filtered water
to waste before the steady state effluent turbidity is achieved. Real-time monitoring of turbidity
during the filter-to-waste operation provides an efficient means of achieving a safe effluent qual-
ity while minimizing the “wasting” of treated water.

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and a gallery of photos.

11-8 CHAPTER REVIEW
When you have completed studying this chapter, you should be able to do the following without
the aid of your textbooks or notes:
1. Sketch and label a rapid granular filter identifying the following pertinent features:
inlet main, outlet main, wash water outlet, gullet, support media (graded gravel), graded
filter medium, and backwash troughs.
2. Define effective size and uniformity coefficient and explain their use in designing a
rapid sand filter.
3. Compare the microscopic and macroscopic models of filtration with respect to explain-
ing the mechanisms of filtration and the ability to predict the time to turbidity break-
through and the time to reach the limiting headloss.

4. From a design point of view, explain the role of filtration rate, grain size distribution,
and porosity in controlling headloss through a granular filter.
5. Explain the role of estimating the depth of the expanded bed in designing the rapid
granular filter.
6. Compare the advantages and disadvantages in selecting the type of filter, that is, sand,
dual media, or deep bed monomedium anthracite.

7. Qualitatively compare the effectiveness of bed expansion and surface wash in back-
washing a filter.
8. Explain why it is important to provide for “filter-to-waste” or “filter to recycle” in the
filter design.