Page 451 - Water and wastewater engineering

11-24 WATER AND WASTEWATER ENGINEERING

g. An assumption of a gullet width of 0.6 m yields the plan view sketched below.

2.6 m 2.6 m

Gullet 5.5 m

0.6 m

Comments:
1. The assumed gullet width will be checked in Example 11-7 .
2. There are several lengths and widths that will meet the design criteria. The Solver tool*
in a spreadsheet can be used to define the upper and lower bounds of the width per cell.
The spreadsheet and dialog box are shown in Figure 11-9 .
3. The vertical dimensions of the filter box depends on the media, underdrain and headloss
.
through the filter. The depth of the filter box will be determined in Example 11-8
Media
The selection of the filter type implicitly specifies the media type. The grain size distribution
plays a strong role in the trade-off between headloss (larger media minimizes headloss) and
filtration efficiency (smaller media captures particles better). The primary design criteria are the
effective size ( E ) and the uniformity coefficient ( U ).
As shown in Figure 11-5 , a low uniformity coefficient results in better utilization of the filter
bed. The effective size plays a significant role in the headloss. The estimate of the clean bed headloss
can be used to evaluate alternate media specifications. For example, because clean bed headlosses
range from 0.3 to 0.6 m (Castro et. al., 2005), initial headlosses in excess of 0.6 m imply either that
the filtration rate is too high or that the media has too large a proportion of fine grain sizes.
The media in multimedia filters must be matched to ensure that all the media fluidize at the
same backwash rate so that one media is not washed out or fails to fluidize. The drag and gravi-
tational forces of the smallest denser media grain size can be balanced with the largest lighter
media grain size by equating the settling velocities of these particle sizes using Equation 10-8 ,
and solving for the ratio of the diameters (Kawamura, 2000):
.
⎡ 2 ⎤ 0 667
d 1
⎢ ⎥ (11-19)
d 2 ⎣ 1 ⎦
®
*Solver is a “tool” in Excel . Other spreadsheets may have a different name for this program.