35    TOTAL SYSTEMS APPROACH TO FOAM CONTROL IN
INDUSTRIAL WASTEWATER
Brian S. Johnson, Senior Chemist
Nalco Chemical Company
Naperville, Illinois 60563
INTRODUCTION
Foam control is important to industrial processes. Without proper foam control, production processes are often slowed; unwanted foam is a general nuisance, and a safety hazard. Sometimes foam is
desirable, such as: ore floatation, Styrofoam, beer, fire extinguishing, etc. But, even desirable foams
need to be disposed of after use. Breaking the foam greatly reduces the volume of waste material.
Proper plant design can minimize foaming using mechanical methods. Sometimes this is not practical
or simply not thought of. Traditional approaches to foam control has concentrated on selection of the
proper antifoam chemical. How to best apply or control the antifoam chemical was a secondary
consideration. This paper covers the three aspects of foam control: antifoam selection, application,
and dosage control. Focus will be on wastewater treatment.
BACKGROUND
Foaming is an interfacial phenomenon. Foams occur because bubbles are stabilized by surfactant
molecules. In this paper, a bubble is defined as a rounded thin envelope of liquid surrounding a gas
phase, i Surfactants are molecules that are active at the air/water interface. This activity is attributed
to the dual nature of the surfactant. Part of the molecule is not water soluble. This is called the
hydrophobic or lipophilic 'tail'. The other portion is hydrophilic. The hydrophilic portion interacts
with the water molecule and is generally polar. This portion is called the head group.
Foams are formed because the bubbles' elasticity (E) is increased. This is given by:2
(I)
dlnA
where InA is the natural logarithm of the surface area (A) and 7 is the surface tension. The units of
surface tension are energy per area (ergs/cm2). As the surface tension differential (d-y) increases, the
elasticity increases. Bubbles with low surface tension differentials are less elastic, more brittle and
easily broken. Bubbles with high elasticity are readily adaptable to forces trying to break the bubble.
Increasing the bubble elasticity decreases the rate of bubble breakage, which increases foam.
Air bubbles are created by turbulence as the liquid passes through pumps, weirs, etc. Gas bubbles
are also produced by fermentation processes. The foam is created when these bubbles are stabilized by
surface active molecules. Pure liquids cannot foam. When two, pure liquid, air bubbles touch they
immediately coalesce. The pure liquid, forming the bubble, does this to minimize surface area. A
surfactant impurity can stabilize the interface by impeding the touching of the bubble interfaces.
Surface tension is lowered by the presence of a surfactant. Less energy is required to make the same
amount of extra surface area. Without a stabilizing surfactant, the generated foam quickly drains
allowing the bubbles to touch. This leads to bubble coalescence.
Foam Stabilizing Mechanisms
Foams can be stabilized by several mechanisms. The major mechanisms are: slow interfacial or
lamella drainage, and 2-D diffusion of the foaming surfactant. The lamella is the liquid region
between two air bubbles. Foam breaking is the result of lamella drainage, allowing bubbles to
coalesce. When the lamella drains, the liquid goes to a 'plateau border.'' The plateau border is
illustrated in Figure la. This plateau border is the liquid region between three bubbles. In any foam,
the plateau regions contain most of the liquid. If flow to the plateau border is impeded, then bubble
48th Purdue Industrial Waste Conference Proceedings, 1993 Lewis Publishers, Chelsea, Michigan 48118. Printed in
U.S.A.
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