28
The flowrate delivered to a bead filter is the principle management factor influencing suspended solids
removal. The efficiency (single pass percent reduction in TSS) of a bead filter generally increases as the
flowrate to the filter decreases; however, the capture rate (mass of TSS captured) tends to increase with
flowrate. This apparent contradiction occurs because per pass efficiency is relatively insensitive to
changes in flowrate, and so, minor drops in efficiency that occur with flow increases are more than
compensated for by enhanced solids transport to the filter. Generally, recirculating rates used with
closed or partially recycled systems should be maximized to obtain the lowest possible TSS level in the
holding tanks.
Separation of captured solids from the bead bed is accomplished by sedimentation of released sludge
after the bed is backwashed. Materials such as fats or wood chips merely float upward with the beads
and are not removed. In sufficient quantity, these materials will eventually foul the bed requiring media
replacement. Bead filters are also not well suited for the clarification of waters suffering from mineral
turbidity problems caused by fine clays or other colloidal particles. Lacking good biofilm development,
the mechanisms for the capture efficiencies are unacceptably low. Finally, the bead filters will impact
but cannot control planktonic algal blooms. Although some capture occurs as a general rule, the algae
can grow faster than they can be caught and thus little progress towards clarification is made.
Application of the bead filter technology to the problem of colloidal mineral turbidity or algal blooms
requires the use of supplemental treatments (chemical flocculation or U.V. disinfection, respectively) or
the filter will be ineffective.
1/8" Std. Bead Media Removal Efficiency
0
10
20
30
40
50
60
70
80
90
100
5-10
10-20
20-30
30-40
40-50
50-100
Particle Size [
m
m]
R
em
o
v
al
[
%
]
Figure 1. All particles above 50 microns are removed in the first pass through the filter and the
remainders are removed with multiple passes.