Page 264 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Coagulation 219

A flocculant is a polymer added just before flocculation dimethylamine quaternary amine’’ and has about
and functions the same as a coagulant aid, that is, by provid- 0.50 fraction ‘‘active’’ polymer per unit mass of
ing attachment sites for microflocs (or flocs). In practice, the liquid polymer as supplied (Dentel and Gucciardi,
1989); it is also called a ‘‘polyquaternary amine’’
terms coagulant aid and a flocculent are likely to be used
Edzwald (1985, p. 171).
interchangeably.
2. Charge density. The charge density of Magnifloc
A filter aid is a polyelectrolyte that functions by attaching
572C (an Epi=DMA polymer) was given as 7.95
to filter grains, which then provide attachment sites for coagu-
meq=g active polymer, with no effect of pH in
lated particles. The result is higher filtration efficiency (due to the range 4.0 pH 8.0, and 0.508 g ‘‘active’’
increasing the transport coefficient and perhaps by increasing polymer=g neat polymer solution (Edzwald, 1985,
the attachment coefficient, described in Chapter 12). If used pp. 173, 175; Edzwald et al., 1987, p. 172). For
improperly, a filter aid could also ‘‘gum-up’’ the filter media; these data and with a dosage of neat solution
also, if used in excess, there is potential for ‘‘mudball’’ for- given as 9.5 mg polymer product=L raw water,
mation. A filter aid is likely to be added in the pipe or channel
just before the filters. C(positive charge) ¼ (9:5 mg neat solution=L raw water)
Black (1960) calculated that a polyelectrolyte having a (0:508 g active polymer=g neat solution)
molecular weight of 100,000 at a dosage of 0.2 mg=L would (g=1000 mg)
have 120 trillion active chains per liter of water. Such a value
(7:95 meq charge=g activepolymer)
gives some appreciation of the capacity of a polymer to react
(1000 meq=meq)
with a particle of opposite charge.
¼ 38 meq=L raw water
9.9.2 CHARACTERISTICS OF POLYMERS This concentration of positive charge, that is, 38 meq=L
raw water, was the ‘‘optimum’’ concentration to
A few characteristics of polymers include molecular weight,
reduce color from about 135 Pt-Co units to about 15
structure, commercial form (e.g., solid, liquid, emulsion),
at 4 mg=L. The corresponding initial HA concentration
charge concentration, and specific gravity. As a rule, propri-
was 10 mg=L with associated DOC concentration
etary companies provide only limited information, for 4.3 mg=L.
example, the name given to a particular product.
9.9.2.1 Charge Concentration
9.9.3 POLYMERS IN WASTEWATER TREATMENT
The negative charge of NOM may be neutralized stoichiome-
trically by positive charged polymers with polymer dosage in In addition to applications in treatment of drinking waters,
terms of meq positive charge=L (Edzwald et al., 1987). polymers have found application in conditioning of waste-
Example 9.7 illustrates the calculation of charge concentration water sludges, that is, in thickening and dewatering. As with
provided by a polymer. other topics, a body of knowledge is involved, which is
reviewed here briefly.
9.9.2.2 Specific Gravity
The specific gravity for liquid polymers varies 1.0 SG 1.2 9.9.3.1 Sludge Conditioning
(Chamberlain, 1981, p. 246). Two kinds of sludge conditioning are thickening and dewa-
tering. The purpose of each is to reduce the amount of water
Example 9.7 Calculate Charge Concentration for improving the functioning of subsequent processes and
for the Cationic Polymer Magnifloc 572C reducing the cost of transport, respectively. The operative
mechanism in treatment of sludges is ‘‘inter-particle bridg-
Given ing,’’ in which a polymer molecule attaches to sites on several
Positive charge density of Magnifloc 572C ¼ 3.1 meq=g particles; important factors are as follows: mixing, Gt 10,000;
neat solution at pH ¼ 7.0; fraction ‘‘active’’ polymer dosage, about 5 mg=L; and molecular weight, MW 10 6
0.50 in neat solution (Dentel, 1988). Assume SG(Magni- (Novak, 1983).
floc 572C) 1.05.
Required 9.9.3.1.1 Thickening
Concentration of positive charge in meq=L polymer. A sludge that has more water than is desired for a subsequent
unit process may be ‘‘thickened’’ to reduce the water content.
Solution
Examples may include flotation of waste activated sludge
C(positive-charge) ¼ (3.1 meq charge=g neat solution) before anaerobic digestion (which results in less water in the
(1.05 g neat solution=L neat solution) reactor increasing the detention time), and thickening of pri-
¼ 3.3 meq=L neat solution
mary sludge prior to anaerobic digestion (again to reduce the
Discussion water content and increase of detention time). Industrial
1. Chemical formula. Magnifloc 572C is an wastes treatment often utilizes flotation for thickening of
‘‘Epi=DMA’’ polymer (i.e., an ‘‘epichlorohydrin= both organic and inorganic wastes.