Discussion by E. Tellier, France,
The authors' calculations for the capacity of an activated sludge plant may be interpreted as a
supposition that Monod's curve showing the growth rate of bacterial cells is only approximate within a
certain zone. Do you not think that the doubts found in the growth rate arise in part from the
position of these experiments on the characteristic curve of Monod? It is recognised as true that by
neglecting the lag phase and considering only the exponential part of the curve in its final phase one
rapidly reaches the phase of decline of growth due to food shortage which has been taken to occur at a
linear rate by Eckenfelder in calculating the capacity of aeration tanks.
The curves for specific growth rate as a function of the soluble organic carbon to the extent that
they are in the two cases analysed, seem extraordinary, compared with each other, while one
shows termination of growth and has come to the end of its decrease the other continues to show
moderate decrease.
It seems that the requirement of at least 70 mg/1 of carbon throws some doubt on the ability to
define the rate of growth, even approximately.
It is not a question of interpreting one method of analysis or another but we think that the result
lies in a true curve - the known deviations that occur in biological situations; we believe that it is not
possible to be too sure about being able to define effluent quality on the interpretation of a favourable characteristic curve obtained using the minimum volume of aeration capacity; by the very small
discharge of organic impurity under estimating the importance of time of day.
Figure 3 and 4 show that the rate of removal of carbon is not influenced by the carbon concentration varying widely from 20 to 167 mg/1, i.e. 800% and that there is no advantage in increasing
the organisms from 640 to 850 mg/1, i.e. 30%. By using the rate of decline of growth as a function of
the bacterial mass established from several experiments (figure 6) one finds that within the range of
bacterial mass 640-850 mg/1 the rate of decrease of carbon varies from 144 to 195.
If, as we think, the parameters are the same for the volume of the model according to dissolved
oxygen concentration and that of glucose, one finds marked differences which still give a straight line
relationship for growth rate as shown in figure 5(o).
The authors' conclusions from their experimental work, linking the increase in concentration of
micro-organisms with the increase in the rate of fall of extracellular organic carbon representative of
pollution are in line with present views on the course of wastewater purification. This conclusion is
another way of stating the principle of respiratory activity as the utilization of dissolved oxygen in the
sludge, showing that each unit mass of microorganisms in unit time under ideal growth conditions is
not able to take up more than a certain maximum of oxygen.
It is perhaps unfortunate that the conditions of experiment were such as to give a low percentage
removal as well as the fact that in organic carbon the microorganisms represent only 50% of the
percentage utilised. That might explain, by the mathematical nature the authors sought to give to their
study by carrying out controlled, discontinuous tests under conditions that cannot be seen to occur in
actual treatment plant, why the regime is constantly changing and the change in nutrients is not
characteristic of that of glucose undergoing decomposition by acclimated cultures of bacteria.
Reply to E. Tellier
The empirical estimation of specific growth rates for batch biological reactors depends on the
choice of:
1. the parameter to be measured and modeled,
2. the measuring technique used, and
3. the modeling technique used.
1. Parameter Selection
Several comments have alluded to models based on microorganism growth. The function of a
biological reactor is to remove organic material, not to grow microorganisms. Efficient reactor design
and operation require a good description of the removal dynamics of soluble organic substrate. A
precise description of how microorganism levels vary is immaterial, within wide limits, as long as
organic material is removed. Estimation of the phase of classical growth characteristic of the microbial
population (a questionable classification for mixed cultures) has little relevance in this context. Even if
the growth phase could be predicted or measured, its control would be doubtful.
The degree of confidence of the mathematical description of raw data varies inversely with the
experimental error associated with the measuring technique. For the materials and methods of this
investigation, soluble organic carbon measurements possessed smaller variance than did suspended
solids measurements. Therefore, the decrease in the mass concentration of extracellular soluble organic
carbon, rather than the increase in the mass concentration of suspended solids, was used to estimate
the reactor "kinetics".
351
The authors' calculations for the capacity of an activated sludge plant may be interpreted as a
supposition that Monod's curve showing the growth rate of bacterial cells is only approximate within a
certain zone. Do you not think that the doubts found in the growth rate arise in part from the
position of these experiments on the characteristic curve of Monod? It is recognised as true that by
neglecting the lag phase and considering only the exponential part of the curve in its final phase one
rapidly reaches the phase of decline of growth due to food shortage which has been taken to occur at a
linear rate by Eckenfelder in calculating the capacity of aeration tanks.
The curves for specific growth rate as a function of the soluble organic carbon to the extent that
they are in the two cases analysed, seem extraordinary, compared with each other, while one
shows termination of growth and has come to the end of its decrease the other continues to show
moderate decrease.
It seems that the requirement of at least 70 mg/1 of carbon throws some doubt on the ability to
define the rate of growth, even approximately.
It is not a question of interpreting one method of analysis or another but we think that the result
lies in a true curve - the known deviations that occur in biological situations; we believe that it is not
possible to be too sure about being able to define effluent quality on the interpretation of a favourable characteristic curve obtained using the minimum volume of aeration capacity; by the very small
discharge of organic impurity under estimating the importance of time of day.
Figure 3 and 4 show that the rate of removal of carbon is not influenced by the carbon concentration varying widely from 20 to 167 mg/1, i.e. 800% and that there is no advantage in increasing
the organisms from 640 to 850 mg/1, i.e. 30%. By using the rate of decline of growth as a function of
the bacterial mass established from several experiments (figure 6) one finds that within the range of
bacterial mass 640-850 mg/1 the rate of decrease of carbon varies from 144 to 195.
If, as we think, the parameters are the same for the volume of the model according to dissolved
oxygen concentration and that of glucose, one finds marked differences which still give a straight line
relationship for growth rate as shown in figure 5(o).
The authors' conclusions from their experimental work, linking the increase in concentration of
micro-organisms with the increase in the rate of fall of extracellular organic carbon representative of
pollution are in line with present views on the course of wastewater purification. This conclusion is
another way of stating the principle of respiratory activity as the utilization of dissolved oxygen in the
sludge, showing that each unit mass of microorganisms in unit time under ideal growth conditions is
not able to take up more than a certain maximum of oxygen.
It is perhaps unfortunate that the conditions of experiment were such as to give a low percentage
removal as well as the fact that in organic carbon the microorganisms represent only 50% of the
percentage utilised. That might explain, by the mathematical nature the authors sought to give to their
study by carrying out controlled, discontinuous tests under conditions that cannot be seen to occur in
actual treatment plant, why the regime is constantly changing and the change in nutrients is not
characteristic of that of glucose undergoing decomposition by acclimated cultures of bacteria.
Reply to E. Tellier
The empirical estimation of specific growth rates for batch biological reactors depends on the
choice of:
1. the parameter to be measured and modeled,
2. the measuring technique used, and
3. the modeling technique used.
1. Parameter Selection
Several comments have alluded to models based on microorganism growth. The function of a
biological reactor is to remove organic material, not to grow microorganisms. Efficient reactor design
and operation require a good description of the removal dynamics of soluble organic substrate. A
precise description of how microorganism levels vary is immaterial, within wide limits, as long as
organic material is removed. Estimation of the phase of classical growth characteristic of the microbial
population (a questionable classification for mixed cultures) has little relevance in this context. Even if
the growth phase could be predicted or measured, its control would be doubtful.
The degree of confidence of the mathematical description of raw data varies inversely with the
experimental error associated with the measuring technique. For the materials and methods of this
investigation, soluble organic carbon measurements possessed smaller variance than did suspended
solids measurements. Therefore, the decrease in the mass concentration of extracellular soluble organic
carbon, rather than the increase in the mass concentration of suspended solids, was used to estimate
the reactor "kinetics".
351