Coagulation and Flocculation - Public Water Systems - page 263

• Low colloid concentration, high alkalinity . Coagulation is readily accomplished
here with a relatively high coagulant dosage by enmeshment of colloidal particles in
a sweep floc. Alternatively, a coagulant aid (such as bentonite or clay particles) may
be added to increase the colloid concentration and increase the rate of interparticle
collision. Destabilization by adsorption and charge neutralization may then be effective
at a lower primary coagulant dosage.
• Low colloid concentration, low alkalinity . Coagulation is most difficult in such
systems. Al(III) and Fe(III) salts will be ineffective if used alone, because the pH will
be depressed too low to permit the rapid formation of a sweep floc and the rate of
interparticle contacts is presumably too slow to utilize destabilization by charge neu-
tralization. Additional alkalinity, colloidal particles, or both must be added to provide
effective coagulation.
Coagulation with Polymers
Synthetic organic polymers have been shown to be effective coagulants or coagulant
aids. Polymers are long-chain molecules composed of many subunits called monomers.
A polymer that is composed of only one type of monomer is termed a homopolymer
and those comprised of different monomers are termed copolymers . The number and
type of subunits or monomers can be varied to yield a wide range of polymers having
different chemical characteristics (such as charge polarity and charge density) and
molecular weights.
A polymer is called a polyelectrolyte if its monomers consist of ionizable groups.
Polyelectrolytes having a positive charge upon ionization are referred to as cationic
polymers . Negatively charged polyelectrolytes are termed anionic polymers . Finally,
polymers that do not contain ionizable groups are called nonionic polymers .
Cationic polymers can be effective in coagulating negatively charged clay parti-
cles. 22 It has been hypothesized that electrostatic forces or ion exchange is the process
by which the polymers become attached to the clay particles, which is then followed
by bridging. Cationic polymers do not require a large molecular weight to be effective
in destabilization.
Anionic particles generally are ineffective coagulants for negatively charged parti-
cles, 22 and there is strong evidence that the presence of divalent metal ions (such as
Mg 2 ) is necessary for anionic polymers to flocculate negative colloids. However,
anionic polymers of large molecular weight or size are able to bridge the energy barrier
between two negatively charged particles, thereby effectively enhancing the coagula-
tion efficiency. The minimum polymer size depends on several factors, but limited data
indicate that the minimum size is on the order of a molecular weight of one million. 15
When anionic polymers are used in conjunction with an electrolyte such as NaCl or
CaCl 2 or another coagulant such as alum, their coagulation efficiency is increased.
Low dosages of cationic polymer (0.1 to 1.5 mg / L) are usually sufficient to achieve
coagulation. In contrast, 5 to 150 mg / L of alum is often needed to obtain similar
results. Other important differences between the use of polymers and metal ions are
sludge quantities and dosage control. The use of alum or ferric chloride can result in
copious volumes of sludge that must be handled, whereas the additional sludge quan-
tity is negligible when a polymer is used. A narrow bank exists for optimum polymer
dosage. Overdosing or underdosing from this optimum will result in restabilization of
the colloids. The control method for polymer feed systems must be precise and reliable
to give satisfactory performance.