Section Five
WASTE TREATMENT PROCESSES
A. PHYSICAL/CHEMICAL
34    ASSESSMENT OF WATER FRACTION METHODOLOGIES
AND THEIR ROLE IN DEFINING SLUDGE
DEWATERING CHARACTERISTICS
Amit Pramanik, Graduate Research Assistant
William R. Knocke, W. Curtis English Professor of Civil Engineering
Department of Civil Engineering
Paul S. Vail, Analytical Chemist
Department of Wood Science & Forest Products
Virginia Polytechnic Institute & State University
Blacksburg, Virginia 24061-0105
Les May, Director
Solids/Liquids Separation
Nalco Chemical Company
Naperville, Illinois 60566-1024
INTRODUCTION
Water in sludges is believed to exist in a free (or bulk or unbound) form and a bound form. The
latter has been further divided into various other forms, depending on the methods and materials
used. These may be the interstitial, capillary, physically-absorbed, chemically-bound, surface water,
bound water, or even vicinal water. There seems to be no well-distinguishing demarcation between the
various "forms" of water, as well as little agreement as to the drying (or moisture loss) mechanisms
involved. Classification of the different forms of water in solids, colloids, etc., appears to depend on
the method or technique used. The more popular methods involve demarcation of the water fractions
based on the rate of removal of water while drying (the drying rate curve method) and upon the
freezing of water (while "free" water freezes at or near 0°C, "bound" water is thought not to freeze at
temperatures as low as -20°C). Water freezing fraction methodologies include: (i) the dilatometric
method, (ii) the differential scanning calorimeter (DSC), and (iii) the differential thermal analysis
(DTA). Controlled drying methods appear to further differentiate other forms of bound water. Other
methods include the vapor pressure depression method and the nuclear magnetic resonance (NMR)
method.
Various researchers have worked on some of the above methods in a variety of disciplines including
geology and clay mineralogy, food science, agricultural sciences, pharmaceutical industry, and medicine. Table 1 is a selected summary of the various bound water determination methods as well as the
different forms of water as reported by researchers in various disciplines. As can be seen, moisture
characterization results vary in relation to the method utilized. While most methods differentiate
between two forms of water ("free" and "bound"), the drying rate method appears more popular in
bio-solids research as it seems to be able to further fractionate other forms of the "bound" water.
This paper will report on ongoing efforts to develop a more rational basis for defining the various
fractions of water present in waste sludges. Research has emphasized a drying rate method using the
controlled environment afforded by a thermogravimetric analyzer (TGA), a freezing or cooling
method using dilatometric techniques, and the freezing-thawing method of a differential scanning
calorimeter (DSC). While these methods have been well-established in different fields, they are only
recently being considered as viable tools in the characterization of moisture in sludges.
48th Purdue Industrial Waste Conference Proceedings, 1993 Lewis Publishers, Chelsea, Michigan 48118. Printed in
U.S.A.
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