17 DEC 2003
PROCESS SCREENING AND SELECTION
QUICK GUIDE. Table 4-1 provides a quick guide for selecting filtration sys-
tems based on the waste stream parameters described in Chapter 3. The design pro-
fessional can use this table to identify one or two filtration system options upon which to
base his or her design. Using Chapter 5, the design professional can then become fa-
miliar with the specific filtration system and complete the design process. Where two fil-
tration systems appear to be applicable, system-specific costs can be examined to de-
termine which is more cost effective.
GENERAL. It should be noted that all filters are susceptible to upsets or foul-
ing that will manifest themselves as either blinding or poor effluent quality. For some
systems, recovery, after the problem causing the upset has been corrected, can be as
simple as replacing the filter elements (in the case of cartridge and bag filters) or back-
washing the filters for the prescribed backwash cycle (in the case of pressure and grav-
ity deep bed filters). In other cases, the fouling may have permeated the entire filter.
This is particularly the case with traveling bridge filters, which have relatively shallow
filter beds, and continuous backwash filters, which use the entire bed. Recovering from
an upset with these filters generally involves more extensive backwashing and clean-
sing and can often require that supplemental backwash water or clean process water be
provided to effectively clean the filter medium.
All systems require some manner of process control. Controls may be rela-
tively simple, such as a head loss shutoff for cartridge and bag filter systems to the
complex flow equalization and backwash controls required for traveling bridge and pres-
sure filter systems. Controls required for continuous backwash and gravity filters are
generally of moderate complexity, lying somewhere between these extremes.
As with all process equipment, manufacturers generally offer add-on controls
and system monitoring options to suit individual application needs.
Table 4-1. Filtration System Selection Guide
Advantages and Disadvantages
>2 L/s (30
1% of design
3.5 (L/s)/m2 (5
10.5 to 14
Backwash disposal required.
(L/s)/m2 (15 to
High capital cost.
(7 (L/s)/m2 (10
Medium operating and labor cost.
Not as susceptible to upsets.
< 50 mg/L TSS
Moderate complexity of control and