Design Parameters for Tube ■ Settlers
JOHN W. HERNANDEZ, Professor
School of Civil Engineering
New Mexico State University
La Cruces, New Mexico
JOHN R. WRIGHT, Chief
Water and Liquid Waste Section
Health and Social Services Department
Sante Fe, New Mexico
INTRODUCTION
Many of us in the water and waste water treatment field have welcomed the
development of the tube settler with a good deal of interest and enthusiasm. One of
the principal reasons is that the tube settler represents the implementation of Allen
Hazen's 1904 theoretical derivations of design criteria for ideal sedimentation (1).
Most of us have reviewed Hazen's concepts in our studies of the fundamentals of
sedimentation and have been disappointed in our inability to incorporate these
principles in design. The tube settler now makes this possible.
There is also the satisfaction of finding that an innovative technique is being
successfully used in our field. Corporate and government press releases have led the
American public, and some of us, to anticipate some imminently forthcoming
announcement of a major breakthrough that will put the problems of water pollution
to rest. The tube settler represents at least a mini breakthrough.
BASIC DESIGN
A typical tube settler consists of a modular unit of thin wall tubes of small
cross-sectional area. The units are relatively lightweight, being PVC plastic, or other
similar materials. The cross section of the individual tubes may be circular, square,
rectangular or even hexagonal. Figures 1 and 2 are line drawings of the two basic
arrangements of tube nests in a sedimentation basin with Figure 1 showing the
system that is used with an "essentially horizontal" alignment of tube nests and
Figure 2 showing a schematic of a typical "steeply inclined" installation. In both of
these basic systems, a fluid carrying suspended solids is subjected to classification by
particle sedimentation as the fluid moves from an influent well, or distribution
chamber upward through these small tubes and into a collection gallery, clean well or
launderer, leaving a high percentage of the suspended particles in the tubes.
To prevent all of the flow from going into one section in the upper part of the
basin, one vertical row of tubes is canted in one direction and the next row of tubes
is canted at the complementary angle. This arrangement of tube rows helps to
prevent short circuiting and to limit the creation of velocity currents. The angle at
which the rows are set varies from 5 to 60 deg; the steeper the angle, the better the
self-cleaning characteristics, but the greater the depth the tube nest takes up with
respect to the length of the tubes.
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