Section One
PLENARY AND LUNCHEON ADDRESSES
1    SLUDGE DEWATERING: WHY THE WATER WILL WIN
EVERY TIME
P. Aarne Vesilind, Professor
Department of Civil and Environmental Engineering
Duke University, Durham, North Carolina
It has long been assumed that the water that surrounds the particles in wastewater sludges behaves
as ordinary water, and that this water has all of the chemical and physical properties of common
water. The purpose of this paper is to suggest that this may not be a safe assumption, and that there
may be different physical states of water in sludge. These different forms of water may play an
important role in determining whether or not it is easy to separate the water from the solids.
CHARACTERISTICS OF WATER IN SLUDGE
Sludge is made up of diverse solid particles suspended in an impure water continuum. Attempts to
define a sludge 'particle' have been hampered by the problem that as soon as attempts are made to
measure or even look at these particles, they will change. Within the treatment system, sludge particles
are dynamic —dispersing and reforming depending on biological, chemical, and physical conditions.
Perhaps the only means of defining a sludge "particle" is to continually dilute the sludge with water
under low shear conditions until the solids define their own size and structure. Sludge solids tend to
flocculate, and these floes include bacteria and larger animals, covered in slime and detritus, and held
together with filamentous organisms. Bacterial colonies, resembling small villages, are collected
within the structure.
Water, with its various dissolved chemicals, is the continuum within and surrounding the sludge
floe. The various water fractions associated with the sludge floe can be defined on the basis of the
sludge floe structure. Although the names given to the various water fractions have evolved over the
years, '~3 the following descriptions of different physical states of water in sludge (illustrated in Figure
1) appear useful:
Free (or Bulk) water— water not associated with and not influenced by the suspended solids particles.
-     6
S     3
* 4
9
9
w      3
ka
£
O
* 2
c
41
S     i
interstitial water
++*
vicinal  wot«r
r' rwat~tr.irhvJrot.on': ~ i
0.4 0 8 12 1.6 2 2 4
solids  concentration    (g/L)
Figure 1.    Representation of a
sludge floe with the hypothetical
types of water.
49th Purdue Industrial Waste Conference Proceedings, 1994 Lewis Publishers, Chelsea, Michigan 48118. Printed in
U.S.A.