A SIMULATION MODEL OF THE
GRAVITY SEPARATION OF
CONCENTRATED SOLIDS
A.S. ANDERSON and R.C. HANSON
Engineering-Science Inc., Arcadia, California, USA
INTRODUCTION
The separation and concentration of solids suspended in liquids is a classic process
problem and formulas which adequately predict the rate of settling of discrete particles
lave long been available (Camp 1946). This paper presents a new model for simulating
:he behavior of particles settling in close proximity to each other, in terms of hydraulics
ind consolidation theory. The model was developed specifically for the activated sludge
3rocess which is dependent upon the continuous separation of biologically active solids or
»ludges from treated wastewater and consequently the model will be described on this
framework. Most of the early work was developed empirically and did not deal
;ubstantially with the internal hydraulics and pressures which cause the thickening. A
lotable exception was the work of Dick Etal (1971) which was concerned with the
nternal phenomena associated with thickening.
The simulation model presented herein has been programmed for use with a digital
computer, and provides an effective means of dynamically simulating the settling test and
for predicting the performance of a thickener. The model is being developed in four
stages. The first stage development work resulted in a program (see Appendix A) for the
malysis of the settling test which enabled determination of an additional mathematical
xmstant which relates to some basic properties of the sludge to be separated. This
program was then modified in stage two to include the continuous addition and removal
Df solids and the program is an effective simulation of a typical separator. The second
program is presented in Appendix B. Two additional modifications are being developed,
the first of which will include provisions for varying the rate of solids introduced and the
second will include provisions for modeling the return of separated solids to the head of
the process with eventual return to the separator as is the case in the activated sludge
process. As soon as these programs have been written and verified they will be described
and published elsewhere.
MINIMUM CONCENTRATION
The minimum initial concentration of particulate matter which is suitable for analysis
by this technique is the concentration at which adjacent particles are settling at the same
velocity independent of size. When the particles are at concentrations above this
minimum, a delicate porous structure is established which collapses under its own weight
at a rate which is limited by the velocity at which water can flow through the shrinking
pores of the structure. The minimum concentration for complete hindered settling (the
concentration at which settlement is impeded by the influence of one particle on
another) is approximately one percent by volume (Scarlett 1971). The practical minimum
concentration is the concentration at which a clear interface between the liquid and the
457
GRAVITY SEPARATION OF
CONCENTRATED SOLIDS
A.S. ANDERSON and R.C. HANSON
Engineering-Science Inc., Arcadia, California, USA
INTRODUCTION
The separation and concentration of solids suspended in liquids is a classic process
problem and formulas which adequately predict the rate of settling of discrete particles
lave long been available (Camp 1946). This paper presents a new model for simulating
:he behavior of particles settling in close proximity to each other, in terms of hydraulics
ind consolidation theory. The model was developed specifically for the activated sludge
3rocess which is dependent upon the continuous separation of biologically active solids or
»ludges from treated wastewater and consequently the model will be described on this
framework. Most of the early work was developed empirically and did not deal
;ubstantially with the internal hydraulics and pressures which cause the thickening. A
lotable exception was the work of Dick Etal (1971) which was concerned with the
nternal phenomena associated with thickening.
The simulation model presented herein has been programmed for use with a digital
computer, and provides an effective means of dynamically simulating the settling test and
for predicting the performance of a thickener. The model is being developed in four
stages. The first stage development work resulted in a program (see Appendix A) for the
malysis of the settling test which enabled determination of an additional mathematical
xmstant which relates to some basic properties of the sludge to be separated. This
program was then modified in stage two to include the continuous addition and removal
Df solids and the program is an effective simulation of a typical separator. The second
program is presented in Appendix B. Two additional modifications are being developed,
the first of which will include provisions for varying the rate of solids introduced and the
second will include provisions for modeling the return of separated solids to the head of
the process with eventual return to the separator as is the case in the activated sludge
process. As soon as these programs have been written and verified they will be described
and published elsewhere.
MINIMUM CONCENTRATION
The minimum initial concentration of particulate matter which is suitable for analysis
by this technique is the concentration at which adjacent particles are settling at the same
velocity independent of size. When the particles are at concentrations above this
minimum, a delicate porous structure is established which collapses under its own weight
at a rate which is limited by the velocity at which water can flow through the shrinking
pores of the structure. The minimum concentration for complete hindered settling (the
concentration at which settlement is impeded by the influence of one particle on
another) is approximately one percent by volume (Scarlett 1971). The practical minimum
concentration is the concentration at which a clear interface between the liquid and the
457