Page 113 - Industrial Wastewater Treatment, Recycling and Reuse

Advanced Physico-chemical Methods of Treatment for Industrial Wastewaters 87

coagulation are referred to as coagulants, while those specifically promoting
the flocculation process are known as flocculants. Flocculants increase the
efficiency of the separation process through improved floc quality, which
increases the settling of flocs and thereby better removes the pollutants.
A number of pollutants that are present in industrial wastewaters are
colloidal in form and are charged species. They are:
– Organic materials
– Metals and metal oxides
– Insoluble toxic compounds
– Stable emulsions (emulsified oil)
– Material producing color, turbidity
– Other substances such as drug residues, bacteria and viruses, algae.
Some of the pollutants, although not in colloidal form, can also behave as
colloids. A significant portion of the pollutants in wastewaters, therefore,
constitute colloidal matter (up to 70% or sometimes even more). The col-
loids in wastewater are stable because of the electrical charge that they carry
that prevents them from settling or getting separated from the wastewaters.
The charges can be positive or negative. As far as wastewater treatment is
concerned, most colloidal particles in wastewater are negatively charged,
implying the necessity of cationic coagulants for neutralization. The charge
typically comes from lattice imperfections, ionizable groups that become
ionic species in water (e.g., amino or hydroxyl groups), or from ionic species
that can get adsorbed on the surface of the colloid. The size of the colloidal
particles is approximately 0.001–1 mm, representing the range between a
molecule and a bacterium. Because, for colloids, surface properties dominate
and prevent particles from coming together, the primary objective of the
coagulation process is to alter the surface properties through modification
of charges in such a way that they agglomerate to a larger size and are sep-
arated from solution. This eventually effects reduction in color, turbidity,
and COD through removal of pollutants. An example of this can be seen
from reported results on polymer waste from latex manufacture that was
coagulated with 500 mg/L ferric chloride and 200 mg/L lime at pH 9.6
resulting in COD and BOD reductions of 75% and 94%, respectively, from
initial values of 1000 and 120 mg/L (Munter, 2000).
The surface charge and charge density are critical parameters in the
removal of pollutants in colloidal form. This charge neutralization in coag-
ulation is mathematically quantified using the value of the zeta potential
parameter, which can be used for predicting stability and is pH dependent.
However, in wastewater treatment, it is to be noted that it is practically