Page 467 - Water and wastewater engineering

11-40 WATER AND WASTEWATER ENGINEERING

time for filter effluent to be diverted to waste and to detect the onset of breakthrough of turbidity
that signals the end of a filter run.
Many very small particles can exist in the water with turbidity less than 1 NTU. This implies
that asbestos fibers, bacteria, viruses, and cysts may be passing through even with very low tur-
bidities. Particle counters are used to detect these small particles and are recommended in con-
junction with turbidimeters to address this issue (Cleasby and Logsdon, 1999).
Design Criteria
Along with Tables 11-1 , 11-3 , 11-4 , and 11-5 , Tables 11-6 through 11-13 provide a summary of
the design guidance presented in this section.

TABLE 11-6
Pretreatment conditions for different operating modes

Operating mode Pretreatment conditions
Conventional filtration Coagulation with alum or ferric chloride and
polymer followed by flocculation and
sedimentation. Can treat turbidities up to
1,000 NTU.
Direct filtration Coagulation with alum or ferric chloride and
polymer followed by flocculation, but not
sedimentation. Limited to raw water turbidities less
than 15 NTU.
In-line filtration Coagulation with alum or ferric chloride and
polymer. Flocculation is incidental. Sedimentation
is not provided. Limited to turbidities less than 10
NTU.

Source: Adapted from MWH, 2005.

TABLE 11-7
Recommended dimensions of ordinary gravity rapid filters
Reported
Parameter range of values Comment
2 2
Area of filter 25–100 m Very large plant maximum is 200 m
Width of cell 6 m For “Off-the-shelf” troughs
L:W ratio 2:1 to 4:1
Depth 4–8 m Provide space for underdrain
Depth of water 1.8 m
Gullet
Width 0.4–2 m
Depth Varies from top of media to Measured from the bottom of wash
bottom of underdrain trough
Sources: Castro et al., 2005; GLUMRB, 2003; Kawamura, 2000.