Organic Polymers in the
Treatment of Industrial Wastes
JOHN T. BURKE, Product Manager
M. T. DAJANI, Research Group Leader
Nalco chemical Company
Chicago, Illinois
INTRODUCTION
A fundamental requirement common to most industrial waste treatment processes is solids-liquid separation. The objective is two-fold: 1) Removal of suspended solids from the waste water to clean it up prior to discharge or re-use within the plant; and 2) Dewatering of the concentrated solids permitting practical
methods for their recovery or disposal with a minimum of handling   problems.
Most waste systems contain both colloidal and gross (larger-than-colloidal
size) suspended solids in varying proportions. Those solids in the colloidal size
range have an enormous surface-to-volume ratio. Their behavior in the waste
system is therefore determined largely by the nature of their surface properties and
they can remain in suspension indefinitely.
Conversely, gross solids have a much smaller surface-to-volume ratio and
their surface properties exert considerably less influence. Their density is usually
sufficient to assure their ultimate settling. The problem here is the relationship
of settling rate and the time required to achieve essentially complete solids sepa -
ration, perhaps one to four or five days, or even longer.
COAGULATION AND FLOCCULATTON
Several factors influence coagulation and flocculation of colloidal systems
with chemicals. A knowledge of the individual system and the factor or factors
influencing its solids suspension stability is essential in determining the type of
treatment required (Figure 1). The three factors that influence the stability of
colloidal systems are double-layer repulsion, short-range hydration repulsion, and
the protective colloid phenomenon.
The electrical double-layer, (Figure 2) consists of a surface charge and a
compensating counter-ion charge, which is accumulated in the liquid in the
neighborhood of the particle surface. At the same time, however, these ions have
a tendency to diffuse away from the surface towards the bulk of the solution, where
their concentration is lower. The concentration of counter-ions decreases with
distance from the surface. When two particles approach each other in suspension,
their diffuse counter-ion atmospheres begin to interfere. In other words, there will
be mutual repulsion between the particles generally referred to as double-layer repulsion and measured by the magnitude of the zeta-potential of the particles. The
zeta potential is the electric potential in the double layer of a moving particle
and is determined in an electric field. It is generally computed from the electro-
phoretic mobility of the colloidal particle, however, such computation is subject to several assumptions which are difficult to evaluate quantitatively.
The fact that stable colloidal particles can be flocculated suggests the exist-
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