Page 112 - Industrial Wastewater Treatment, Recycling and Reuse

86 Industrial Wastewater Treatment, Recycling, and Reuse

In general, the majority of the processes that have specific interactions
playing a role in the removal of pollutants (such as coagulation, adsorption,
ion exchange, electro-oxidation, membrane separations with ion exchange
membranes) belong to a class of charge-based separations that manipulate
electrostatic interactions with the pollutant species to effect removal. A
recent survey of patents has placed emphasis on the development of sorption
and ion exchange processes (Berrin, 2008) as well as water management
methods that can make use of the above information. The physico-chemical
methods listed above are briefly discussed in the next section.

2.2 ADVANCED COAGULATION PROCESSES
Coagulation is one of the most widely used treatment methods and is
considered a mature technology in today’s context. It has numerous appli-
cations in the area of wastewater treatment for the removal of pollutants. It is
typically employed as a primary or secondary treatment step, depending on
the effluent’s nature, either in the form of chemically induced coagulation
(conventional) or electric charge induced coagulation (electro-coagulation).
Coagulation, by default, refers to a chemically induced phenomenon and is
most commonly practiced in industrial wastewater treatment in the chem-
ical, pulp and paper, dye and textile, petrochemical, pharmaceutical, food,
mineral processing, and metal industries. In recent years, electro-coagulation
has also been used to treat wastewater in industries such as food, pulp and
paper, and dye, as well as wastewaters containing metals and oil. In commer-
cial practice, coagulation and flocculation are two common terminologies
that are, many times, interchanged and used to suggest a single process.
However, it should be understood that the principal process is coagulation,
and flocculation is an inherent part of the coagulation process as a whole.
Coagulation refers to a separation process based on charge modification
where part or full reduction of repulsive forces on colloids takes place
through the addition of specific chemicals referred to as coagulants. It can
also be termed destabilization of colloids that otherwise are in stable suspension
form. As opposed to this, flocculation refers to a process, purely physical in
nature, where partly or fully neutralized particles come in contact with each
other to form flocs, and agglomeration of the flocs takes place. The agglom-
erates are heavier than water and therefore settle and separate from water
easily. Thus, coagulation as a whole represents a physico-chemical process
where removal of colloidal charged matter takes place through charge mod-
ification/charge neutralization. The chemicals that promote the process of