Page 265 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

220 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

9.9.3.1.2 Dewatering TABLE 9.8
The purpose of a polymer in sludge dewatering is to increase Functional Groups with Charge
the solids concentration (as in thickening). The benefits of
increased solids concentration has to do with the reducing the Anionic
costs of transport to a disposal site, for example, land spread- H 2 C CH
ing. For example, if the use of a polymer results in 24% solids
instead of 20%, the savings would be to transport 4 kg less O
C O O S O C
water per hundred kg of sludge. Sludges that are dewatered
include those from water treatment (e.g., settling, filter – – –
O O O
backwash), anaerobic digestion, and aerobic digestion. Dewa- a a b
tering methods include centrifuging, belt press processing, Carboxylate Sulfonate Arcylate
filter press processing, vacuum filtration. Other means of
Cationic
dewatering include drying beds, freezing, heating, etc.
R
9.9.4 STRUCTURE OF POLYMERS +
R N R
Several configurations are possible in polymer structure, for
example, linear, branched, dendritic. Then, to function as a R
polyelectrolyte a polymer must have incorporated functional
Quaternary amine–examples of structures
groups that may dissociate to leave charged sites. Because of
their high molecular weights, ranging from tens of thousands CH 3
to tens of millions, polymers are considered macromolecules.
To illustrate the idea of a polyelectrolyte, Figure 9.21a
C O
depicts a linear polymer with attached COOH groups. If the C O
pH is increased, consuming the H , the residual charges of O CH CH
þ
NH
the groups are negative, and the polymer is a polyelectrolyte,
that is, as in Figure 9.21b. Because the negative charges repel CH 2 CH 2 CH 2
CH 2
one another, the polymer becomes ‘‘stretched.’’
N +
CH 2 HN +
9.9.4.1 Functional Groups
+
Functional groups are a part of the structure of polyelectro- N (CH ) H 3 C CH 3 CH 3 CH 3
3 3
lytes; they have charged sites with a mobile counterion. Some a a
DMAEM-MCQ DADMAC Mannich
of the common functional groups are listed in Table 9.8.
Anionic groups include carboxylate and sulfonate, while the a May (1988).
most common cationic group is quaternary amine (see amines b
Rose (1988).
in glossary). The structure of an amine is similar to NH 3 but is
protonated to give NH 4 , but with R groups instead of H
þ
attached to the nitrogen. The ‘‘R’’ groups are any hydrocar-
bon, for example, the methyl group, CH 3 . Some of the
common structures that include quaternary amine are listed
in the lower row of Table 9.8, for example, DMAEM-MCQ, Common cationic polymers are DADMAC and DMA. Com-
DADMAC, and Mannich, respectively. mon anionic polymers include sodium polystyrene sulfonate
and acrylate. The most common nonionic polymers are acry-
9.9.4.2 Monomers lamide and epichlorohydrin (epi). Such monomers may be
Some of the monomers that comprise a polymer structure are synthesized as homopolymer or copolymers, with the myriad
categorized in Table 9.9 as cationic, anionic, and nonionic. of possible configurations.

COOH COO –
HOOC –
COOH COO –
COOH – COO
COOH COO
COOH
COO – COO –
COOH COOH – COO –
COO
(a) (b)
FIGURE 9.21 Polymers to illustrate the effect of charge on shape. (a) Polymer without charge. (b) Polymer with charged sites. (Adapted
from Black, A.P., J. Am. Water Works Assoc., 52(4), 493, April 1960.)