474
Discussion
Reply to Comments of M.A. Shapiro and A.A. Sooky
The concept of "bound water" is not new but little work has been done to develop techniques for
accurately measuring this sludge parameter. The assumption that the measured dry suspended solids is
equivalent to the in-situ suspended solids is obviously incorrect in the case of organic solids and may
also be incorrect in the case inorganic particulates. Clearly a method of measuring the absolute volume
of the in-situ suspended is needed. Promising new approaches include measurement of the concentration of water soluble additive substances which cannot penetrate the bound water layer and are
not absorbed on the particulates. Another approach involves the use of a standaridized high-speed
centrifuge test to determine the packed cell volume of a known mass of dry solids.
It is agreed that additional verification is needed to fully document the model, this work has been
proposed and awaits funding.
The comment on tank volume estimation is correct, the sentence should read "The tank volume
can be estimated by multiplying the sludge feed rate in mass per unit of time by the sludge detention
time and dividing the results by the average layer concentration."
The computer program as presented in Appendix A needs two corrections to be complete. These
are the insertion of line 145 as follows:
145
VI
= .00055
and correction of line 250 as follows:
250
A K = F I * F I * * 3 / ( ( Y I * l . - F I ) * * 2 . )
C.F. Seyfried, Germany.
To what extent was the wall effect taken into consideration in the laboratory test and in the sludge
index determination?
If the diameter of a cylinder is less than 30 cm errors can arise in settlement tests due to the wall
effect when the sludge content is high. Has the wall effect been considered in these models?
Reply
We agree that wall effect must be considered in conducting laboratory settling tests and encourage
the use of large diameter'deep cylinders to minimize the wall effect on the settling test data.
R. W. Goodwin, USA
Direct application of the authors' results might lead to complications. Although a sludge can
theoretically be settled to a high degree there is a physical limitation of sludge pumpability or
handlability which should be considered. The inclusion of a sludge pumping constraint might be a
worthwhile modification to the authors' model. Voshel (a) has indicated an empirical limitation of 7%
as sludge pumpability. Could the model be expanded to include the utilization of solids loading and
overflow rate? What would be the effect upon the mathematical analogies presented of including
pickets or rakes on the scraper arms?
(a) Voshel, D., JWPCF; V. 38; No. 9 9/66.
Reply
The model does not deal with the problems of transporting the thickened sludge to the sludge
outlet nor ihe problems of pumping the concentrated sludge to another location. Clearly these are
constraints which must be considered in actual practice. We do propose to extend the model to
include solids loading and overflow rate for fixed and variable feed conditions by use of analog rather
than digital computer techniques. Mathematical treatment of the effect of pickets and rakes has not
been attempted but a picket can be visualized as a device which creates a hydraulic pressure wave in
front of it as it moves and which also creates a void space in the lee of the picket which permits
expressed liquid to exit the sludge mass. Substantial effort is needed to quantify and model these
effects. A scheme for doing this work has already been developed but no funds have been made available
for this work.
Discussion
Reply to Comments of M.A. Shapiro and A.A. Sooky
The concept of "bound water" is not new but little work has been done to develop techniques for
accurately measuring this sludge parameter. The assumption that the measured dry suspended solids is
equivalent to the in-situ suspended solids is obviously incorrect in the case of organic solids and may
also be incorrect in the case inorganic particulates. Clearly a method of measuring the absolute volume
of the in-situ suspended is needed. Promising new approaches include measurement of the concentration of water soluble additive substances which cannot penetrate the bound water layer and are
not absorbed on the particulates. Another approach involves the use of a standaridized high-speed
centrifuge test to determine the packed cell volume of a known mass of dry solids.
It is agreed that additional verification is needed to fully document the model, this work has been
proposed and awaits funding.
The comment on tank volume estimation is correct, the sentence should read "The tank volume
can be estimated by multiplying the sludge feed rate in mass per unit of time by the sludge detention
time and dividing the results by the average layer concentration."
The computer program as presented in Appendix A needs two corrections to be complete. These
are the insertion of line 145 as follows:
145
VI
= .00055
and correction of line 250 as follows:
250
A K = F I * F I * * 3 / ( ( Y I * l . - F I ) * * 2 . )
C.F. Seyfried, Germany.
To what extent was the wall effect taken into consideration in the laboratory test and in the sludge
index determination?
If the diameter of a cylinder is less than 30 cm errors can arise in settlement tests due to the wall
effect when the sludge content is high. Has the wall effect been considered in these models?
Reply
We agree that wall effect must be considered in conducting laboratory settling tests and encourage
the use of large diameter'deep cylinders to minimize the wall effect on the settling test data.
R. W. Goodwin, USA
Direct application of the authors' results might lead to complications. Although a sludge can
theoretically be settled to a high degree there is a physical limitation of sludge pumpability or
handlability which should be considered. The inclusion of a sludge pumping constraint might be a
worthwhile modification to the authors' model. Voshel (a) has indicated an empirical limitation of 7%
as sludge pumpability. Could the model be expanded to include the utilization of solids loading and
overflow rate? What would be the effect upon the mathematical analogies presented of including
pickets or rakes on the scraper arms?
(a) Voshel, D., JWPCF; V. 38; No. 9 9/66.
Reply
The model does not deal with the problems of transporting the thickened sludge to the sludge
outlet nor ihe problems of pumping the concentrated sludge to another location. Clearly these are
constraints which must be considered in actual practice. We do propose to extend the model to
include solids loading and overflow rate for fixed and variable feed conditions by use of analog rather
than digital computer techniques. Mathematical treatment of the effect of pickets and rakes has not
been attempted but a picket can be visualized as a device which creates a hydraulic pressure wave in
front of it as it moves and which also creates a void space in the lee of the picket which permits
expressed liquid to exit the sludge mass. Substantial effort is needed to quantify and model these
effects. A scheme for doing this work has already been developed but no funds have been made available
for this work.