Page 266 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Coagulation 221

TABLE 9.9
Examples of Monomers That May Be Used to Synthesize Polymers
Cationic Anionic Non-ionic

CH 2 CH
C
H C CH 2
2
HC CH
CH CH
CH 2 CH 2 H C CH
3
HC CH
N + C O
C
H C CH 3 SO 3 – NH 2
3
Polydiallyldimethyl-ammonium
chloride PDADMAC (Cat-Floc) Sodium polystyrene sulfonate Acrylamide
CH 3
H C CH O
3
:NH
C O H C CH CH 2
2
CH 3 O – Cl
Dimethylamine (DMA) Acrylate Epichlorohydrin (epi)

9.9.4.3 Polymers Scientific guidelines have not been developed for practice,
As noted, polymers are composed of a sequence of mono- partly because polymers are patented, and their basic charac-
mers, and so their naming follows their composition. To give teristics are not provided sufficiently for application of any
an idea of the variation in polymer structures Tables 9.10 such guidelines, and partly because a trial and error process
through 9.12 provide examples of cationic, anionic, and non- would be involved in any case. While both are true, guidelines
ionic polymers, respectively. In addition to their structures on based upon the character of the polymer, its charge density, its
the left, the right-hand column indicates (for a given polymer molecular weight, the proposed application, and quantitative
and as data were available) the number of manufacturer’s, the knowledge (water characteristics, other coagulants used, point
number of products, the molecular weight, and the charge of application, concentration, and settled water turbidity with
density. In polymer nomenclature, the ‘‘degree of polymer- and without the polymer, filtered water turbidity with and
ization’’ is given by n in the formula (Ravve, 1995, p. 1). without the polymer, length of filter run with and without
the polymer) can aid in selection.
1. Cationic polymers: Common cationic polymers are
listed in Table 9.10. 9.9.5.1 Polymer Screening
2. Anionic polymers: The anionic polymers are gener- Screeningof agroup of selected polymers may utilize jar testing
ally of high molecular weight. Common anionic (see Section 9.8.1), which provides the flexibility to test the
polymers are listed in Table 9.11. effects of one independent variable at a time holding all others
3. Nonionic polymers: The nonionic polymers are constant. Dependentvariablesmayincludeflocsize andsettling
mostly polyacrylamide, depicted in Table 9.12, and velocity and settled water turbidity (which are all related).
have high molecular weight. Pilot plant testing, that is, after preliminary screening by jar
tests, is useful for a ‘‘final screening’’ of the coagulation
process chemicals, including polymers. Dependent variables
9.9.5 SELECTION OF POLYMERS
measured may include: effluent turbidity (or particle counts),
Thousands of polymers exist as commercial products. As rate of headloss increase, ripening time, and clean bed head-
noted, the approach to polymer selection has been based loss change after repeated filtration cycles. The independent
largely upon recommendations of manufacturer’s representa- variables to be explored are the same as for the jar test
tive and then jar testing for screening and=or confirmation. (Mangravite, 1983; Ghosh et al., 1985).