Page 114 - Industrial Wastewater Treatment, Recycling and Reuse

88 Industrial Wastewater Treatment, Recycling, and Reuse

impossible to know the precise value of the zeta potential to get efficient
coagulation, although lower zeta potential values indicate effective coagu-
lation. As a result, carrying out coagulation in real-life operations is, by
and large, an empirical approach on the basis of laboratory data on various
coagulants and coagulation behavior under different conditions of pH, con-
centration, and other parameters.

2.2.1 Types of Coagulant
There are two distinct types of coagulant: inorganic and organic (Table 2.1).
The most common inorganic coagulants used in wastewater treatment are
simple coagulants such as alum or ferric chloride or more complex ones such
as polyaluminum chloride (PAC), while among organic coagulants the most
commonly used are polymers such as poly-DADMAC (poly diallyldimethy-
lammonium chloride) or polyacrylamide.
Organic coagulants or polyelectrolytes (also often referred to as “poly-
mers” in practice, although even in inorganic coagulants there are polymers
suchasPAC,polyaluminumsulfate(PAS)withvaryingmolecularweight)are
synthetic or organic polymeric molecules having ionizable groups or charged
groups along a polymer chain. Polyelectrolytes of natural origin are typically
derived from starch products or of biological origin such as extracts of plants/
fruits or alginate from algae, while synthetic organic coagulants are synthet-
ically polymerized from monomers (e.g., polyamine, sulfonate).

2.2.2 How Coagulants Work and How to Select Coagulant
Coagulants function broadly through dissociation, neutralization, and an
association mechanism. For example, with an inorganic coagulant, when
added to a solution the salts dissociate into ions. The cationic species (such
3+
3+
as Al ,Fe ), resulting from dissociation exist in their hydrated forms (such
3+
as [Al(H 2 O) 6 ] 3+ or [Fe(H 2 O) 6 ] ). A critical quantity of coagulant is needed

for the ions from these salts to react with the OH or bicarbonate and car-
bonate ions in solution to produce the corresponding insoluble hydroxides
(Al(OH) 3 or Fe(OH) 3 ), which then precipitate. The solubility of Al(OH) 3
or Fe(OH) 3 is a function of the pH. The cationic species neutralize negative
charges on the pollutant species, and the process requires a just-sufficient
dose corresponding to quantity of colloids and surface charge, especially
in the adsorption/charge neutralization mechanism. Any dose below or
above this optimum concentration will again stabilize the solution adversely,
affecting removal of pollutants from wastewaters. There have been various
mechanisms proposed for the coagulation process that mainly incorporate