Discussion
455
Reply
1) As to the limitations of the current meter, the author believes that modern sensitive instruments
such as hot film types are not proper to use in such a multiphase fluid as the activated sludge mixed
liquor containing air bubbles. The propeller type current meter was chosen as the first step to
approach to quantitative understanding of turbulence and of the effect of turbulence.
2) The author and his colleague conducted additional experiments comparing the viscosity of tap
water and that of supernatant liquor of boiled and cooled activated sludge. And at the same time they
measured the viscosity of various kinds of concentration of MLSS of activated sludge. So far as these
experiments are concerned, the liquid phase conditions have no effect on viscosity, but the effect of
suspended solids concentration is shown clearly. It can be concluded that the reduced ICL a and the
increased viscosity are caused primarily by the effect of suspended solids. Although the author admits
that boiling alters the condition of the sludge, his prime purpose was not to know the characteristics
of the actual activated sludges but to know the effect of suspended solids.
3) The author thinks that the increased probability or the frequency of collision may be another
explanation of the reduction of the liquid film resistance or the reduction of the clump resistance of
floe and would like to emphasize again that the reduction of specific oxygen uptake rate shown with
increasing MLSS concentrations may be caused by the structures of micro-turbulence and may not be
due to the biologically inert portion of the sludge as assumed by the previous investigators.
4) The values of %) in Table 4 rather indicate that the higher the air flow rate, the greater the
reduction because of the effect of suspended solids.
5) As to the "percentage", strictly speaking it is difficult to discuss the difference of two kinds of
sludges, because the concentration conditions are different from each other as pointed out. But
considering the different effects of two kinds of sludge on the value of kj^a and on the viscosity, the
author expects that the effect of MLSS concentrations would appear more distinctly in milk sludge
than in the glucose sludge.
6) The author believes that the mass transfer phenomena concerned with biological waste water
treatment process must have a profound relationship to the hydrodynamical mechanisms in the
boundary layer between the biotic mass and the fluid.
A.J. Englande,Jr. USA.
As Kalinske
1 has noted, turbulence scale to be influential must be of the same order of magnitude
as the size of the floe particles. It is implied by the author that the overall oxygen absorption
coefficient, Κτ a, might be a practical possibility of identifying this "micro-scale" turbulence. It should
be noted that the employment of Kj^a as a measure of turbulence intensity is not new. Its importance
has been recognised and used extensively by micro biologists concerned with submerged aerobic
fermentor reactor design and scale up.
Although the author observed negligible effects on Kj^a, by the positioning of the baffle plates, it
must be recognised that the oxygen absorption coefficient is very much dependent on the geometry of
the basin and on other pertinent physical parameters.
2
The preparation of sludges by boiling for the &Q and Οίγ^ determinations is a questionable
procedure. Boiling will certainly alter the size and nature of the floe particles and consequently their
effect on turbulence. A better method would have been to determine Kj^a with the viable organisms
present according to the method described by Eckenfelder and Ford.
3
The values of Q£) in Table 4 indicate very little effect on the suspended solids concentrations at the
higher air flow rates. Would the author comment on this?
F
With regard to the oxygen transfer mechanism Table 7 illustrates the effect of -rr-and MLSS on
maximum oxygen uptake rate for a glucose sludge. Examination reveals that for the conditions
characterizing conventional activated sludge and extended aeration operation, i.e., F/M of 0.25 and
0.1 and MLSS concentrations from 2000 mg/1 to 8000 mg/1, an extremely small effect due to solids is
observed. Only at low MLSS «2000 mg/1) and in the extreme endogenous phase is the effect more
dominant. This phenomenon could perhaps better be explained on an" active mass basis.
Two over-generalizations are made in this section. The first deals with the so-called "percentage"
value which is claimed to express the extent of solids effect and to be a meaningful characteristic of
the sludges. Since the difference in specific oxygen uptake rates for various MLSS concentrations is
not linear as illustrated in Table 7, what solids concentrations should be selected to obtain the ratio
described? Secondly, based on the data given, an extension to the premise that the rate of oxygen
uptake could be expected to correspond to the probability or frequency of the collision between
oxygen molecules or substrate molecule and the micro-organisms is somewhat obscure.
455
Reply
1) As to the limitations of the current meter, the author believes that modern sensitive instruments
such as hot film types are not proper to use in such a multiphase fluid as the activated sludge mixed
liquor containing air bubbles. The propeller type current meter was chosen as the first step to
approach to quantitative understanding of turbulence and of the effect of turbulence.
2) The author and his colleague conducted additional experiments comparing the viscosity of tap
water and that of supernatant liquor of boiled and cooled activated sludge. And at the same time they
measured the viscosity of various kinds of concentration of MLSS of activated sludge. So far as these
experiments are concerned, the liquid phase conditions have no effect on viscosity, but the effect of
suspended solids concentration is shown clearly. It can be concluded that the reduced ICL a and the
increased viscosity are caused primarily by the effect of suspended solids. Although the author admits
that boiling alters the condition of the sludge, his prime purpose was not to know the characteristics
of the actual activated sludges but to know the effect of suspended solids.
3) The author thinks that the increased probability or the frequency of collision may be another
explanation of the reduction of the liquid film resistance or the reduction of the clump resistance of
floe and would like to emphasize again that the reduction of specific oxygen uptake rate shown with
increasing MLSS concentrations may be caused by the structures of micro-turbulence and may not be
due to the biologically inert portion of the sludge as assumed by the previous investigators.
4) The values of %) in Table 4 rather indicate that the higher the air flow rate, the greater the
reduction because of the effect of suspended solids.
5) As to the "percentage", strictly speaking it is difficult to discuss the difference of two kinds of
sludges, because the concentration conditions are different from each other as pointed out. But
considering the different effects of two kinds of sludge on the value of kj^a and on the viscosity, the
author expects that the effect of MLSS concentrations would appear more distinctly in milk sludge
than in the glucose sludge.
6) The author believes that the mass transfer phenomena concerned with biological waste water
treatment process must have a profound relationship to the hydrodynamical mechanisms in the
boundary layer between the biotic mass and the fluid.
A.J. Englande,Jr. USA.
As Kalinske
1 has noted, turbulence scale to be influential must be of the same order of magnitude
as the size of the floe particles. It is implied by the author that the overall oxygen absorption
coefficient, Κτ a, might be a practical possibility of identifying this "micro-scale" turbulence. It should
be noted that the employment of Kj^a as a measure of turbulence intensity is not new. Its importance
has been recognised and used extensively by micro biologists concerned with submerged aerobic
fermentor reactor design and scale up.
Although the author observed negligible effects on Kj^a, by the positioning of the baffle plates, it
must be recognised that the oxygen absorption coefficient is very much dependent on the geometry of
the basin and on other pertinent physical parameters.
2
The preparation of sludges by boiling for the &Q and Οίγ^ determinations is a questionable
procedure. Boiling will certainly alter the size and nature of the floe particles and consequently their
effect on turbulence. A better method would have been to determine Kj^a with the viable organisms
present according to the method described by Eckenfelder and Ford.
3
The values of Q£) in Table 4 indicate very little effect on the suspended solids concentrations at the
higher air flow rates. Would the author comment on this?
F
With regard to the oxygen transfer mechanism Table 7 illustrates the effect of -rr-and MLSS on
maximum oxygen uptake rate for a glucose sludge. Examination reveals that for the conditions
characterizing conventional activated sludge and extended aeration operation, i.e., F/M of 0.25 and
0.1 and MLSS concentrations from 2000 mg/1 to 8000 mg/1, an extremely small effect due to solids is
observed. Only at low MLSS «2000 mg/1) and in the extreme endogenous phase is the effect more
dominant. This phenomenon could perhaps better be explained on an" active mass basis.
Two over-generalizations are made in this section. The first deals with the so-called "percentage"
value which is claimed to express the extent of solids effect and to be a meaningful characteristic of
the sludges. Since the difference in specific oxygen uptake rates for various MLSS concentrations is
not linear as illustrated in Table 7, what solids concentrations should be selected to obtain the ratio
described? Secondly, based on the data given, an extension to the premise that the rate of oxygen
uptake could be expected to correspond to the probability or frequency of the collision between
oxygen molecules or substrate molecule and the micro-organisms is somewhat obscure.