Discussion
365
removal of organic material in the waste without producing proportionately greater quantities of
sludge to handle. This is consistent with the accelerated growth/death concept and allows for a greater
mass of organic carbon to be oxidised completely (to CO2). The second reason why this operational
variation is significant is that nitrification, which is a function of net growth rate (of Nitrosomonas
spp. and Nitrobacter spp.), can be made independent of removal rate. Thus the high rate removal
process which has not previously yielded a nitrified effluent can now be induced so to do. This in itself
of course is not highly desirable, but with a nitrified waste it is possible to induce denitrification which
of course is a further step of operational alternatives. Previous studies (Jones & Downs 1968) indicated
that significant quantities of nitrogen were being lost from a full-scale municipal plant employing the
contact stabilization process. The authors concluded that simultaneous nitrification-denitrification was
occurring as postulated by Wurhmann (Wurhmann, 1963). All of these findings can be fitted to the
concept postulated in this paper.
REFERENCES
CHASE, E.S., "High rate activated sludge treatment of sewage," Sewage Works Journal, 16, 5, 878
(Sept. 1944).
JONES, P.H., "A mathematical model for Contact Stabilization - modification of the activated sludge
process," 5th Int. Conf. Wat. Poll. Res. (1970) San Francisco.
JONES, P.H. and BROWN, R.E., "The effect of the ratio of soluble to total COD on biological waste
treatment processes," Proc. 2nd Canadian Symp. on Wat. Poll. Res. (1967).
JONES, P.H. and DOWNS, A.T., "Some nitrogen removal aspects of a contact stabilization plant,"
Proc. 3rd Canadian Symp. on Wat. Poll. Res. (1968).
ULLRICH, A.H. and SMITH, M.W., "The biosorption process of sewage & waste treatment," Sew. and
Ind. Wastes, 23,1248 (1951).
WURHMANN, K., "Effect of oxygen tension in sewage purification plants," Proc. 3rd Conf. on Biol.
Waste Treatment, Pergamon Press (1963).
K. Wuhrmann, Switzerland
True death rate, i.e. death of cells by metabolic breakdown is a very slow process in water and
relatively low temperatures as in sewage plants. Half life periods at 15 C are in days. It is difficult to
conceive how this reaction can have a significant influence on a process which lasts a few hours.
With regard to the concept of "storage" of impurities in cells, which later will then be metabolised,
it has to be made clear that no such reaction exists. Substrate resorption (i.e. substrate transfer from
the external to the internal medium of a cell) is a highly controlled and severely regulated process.
I think that the main mechanism of control of the activated sludge cell population is by predation.
This process (mostly by protozoans) is largely favoured in a "stabilisation barrier" due to the long
sludge retention time, and hence a low wash out of the organisms. The main provision to be
considered is sufficient oxygen supply.
The figures on "viable" cell decrease in activated sludge as a function of loading rate is also
difficult to understand in terms of "viability" as stated in the figure. If the figures really represent true
cell numbers (which I doubt for microbiological reasons) there might also be a "dilution" of new cells
by accumulation of non degradable solids in the stabilization unit.
Reply by D. Orhon and D. Jenkins to discussion by K. Wuhrmann
Professor Wuhrmann is not correct in stating that "true death rate" or the rate of loss of viability
of bacterial cells is a very slow process. Data presented by Postgate and Hunter show that death rates
of the starved bacterium Aerobacter Aerogenes vary with the growth rate of the organism. These death
rates are of the same order of magnitude and vary in the same way with growth rate as do those rates
obtained for viable activated sludge "cells" in this study.
The idea suggested by Professor Wuhrmann that predation is the main mechanism of activated
sludge cell population control cannot apply to these studies because some 85-95% of the mass of the
activated sludge was accounted for by viable bacteria. Protozoans at most, therefore, could account
for some 5-15% of the sludge mass. In view of this and of the previously stated agreement of activated
sludge death rates and pure bacterial culture death rates (in the absence of protozoa) only a secondary
role is proposed for protozoa.
REFERENCE
1 Postgate and Hunter, (1962). "The Survival of Starved Bacteria." /. Gen. Microbiol, 29, 233.
365
removal of organic material in the waste without producing proportionately greater quantities of
sludge to handle. This is consistent with the accelerated growth/death concept and allows for a greater
mass of organic carbon to be oxidised completely (to CO2). The second reason why this operational
variation is significant is that nitrification, which is a function of net growth rate (of Nitrosomonas
spp. and Nitrobacter spp.), can be made independent of removal rate. Thus the high rate removal
process which has not previously yielded a nitrified effluent can now be induced so to do. This in itself
of course is not highly desirable, but with a nitrified waste it is possible to induce denitrification which
of course is a further step of operational alternatives. Previous studies (Jones & Downs 1968) indicated
that significant quantities of nitrogen were being lost from a full-scale municipal plant employing the
contact stabilization process. The authors concluded that simultaneous nitrification-denitrification was
occurring as postulated by Wurhmann (Wurhmann, 1963). All of these findings can be fitted to the
concept postulated in this paper.
REFERENCES
CHASE, E.S., "High rate activated sludge treatment of sewage," Sewage Works Journal, 16, 5, 878
(Sept. 1944).
JONES, P.H., "A mathematical model for Contact Stabilization - modification of the activated sludge
process," 5th Int. Conf. Wat. Poll. Res. (1970) San Francisco.
JONES, P.H. and BROWN, R.E., "The effect of the ratio of soluble to total COD on biological waste
treatment processes," Proc. 2nd Canadian Symp. on Wat. Poll. Res. (1967).
JONES, P.H. and DOWNS, A.T., "Some nitrogen removal aspects of a contact stabilization plant,"
Proc. 3rd Canadian Symp. on Wat. Poll. Res. (1968).
ULLRICH, A.H. and SMITH, M.W., "The biosorption process of sewage & waste treatment," Sew. and
Ind. Wastes, 23,1248 (1951).
WURHMANN, K., "Effect of oxygen tension in sewage purification plants," Proc. 3rd Conf. on Biol.
Waste Treatment, Pergamon Press (1963).
K. Wuhrmann, Switzerland
True death rate, i.e. death of cells by metabolic breakdown is a very slow process in water and
relatively low temperatures as in sewage plants. Half life periods at 15 C are in days. It is difficult to
conceive how this reaction can have a significant influence on a process which lasts a few hours.
With regard to the concept of "storage" of impurities in cells, which later will then be metabolised,
it has to be made clear that no such reaction exists. Substrate resorption (i.e. substrate transfer from
the external to the internal medium of a cell) is a highly controlled and severely regulated process.
I think that the main mechanism of control of the activated sludge cell population is by predation.
This process (mostly by protozoans) is largely favoured in a "stabilisation barrier" due to the long
sludge retention time, and hence a low wash out of the organisms. The main provision to be
considered is sufficient oxygen supply.
The figures on "viable" cell decrease in activated sludge as a function of loading rate is also
difficult to understand in terms of "viability" as stated in the figure. If the figures really represent true
cell numbers (which I doubt for microbiological reasons) there might also be a "dilution" of new cells
by accumulation of non degradable solids in the stabilization unit.
Reply by D. Orhon and D. Jenkins to discussion by K. Wuhrmann
Professor Wuhrmann is not correct in stating that "true death rate" or the rate of loss of viability
of bacterial cells is a very slow process. Data presented by Postgate and Hunter show that death rates
of the starved bacterium Aerobacter Aerogenes vary with the growth rate of the organism. These death
rates are of the same order of magnitude and vary in the same way with growth rate as do those rates
obtained for viable activated sludge "cells" in this study.
The idea suggested by Professor Wuhrmann that predation is the main mechanism of activated
sludge cell population control cannot apply to these studies because some 85-95% of the mass of the
activated sludge was accounted for by viable bacteria. Protozoans at most, therefore, could account
for some 5-15% of the sludge mass. In view of this and of the previously stated agreement of activated
sludge death rates and pure bacterial culture death rates (in the absence of protozoa) only a secondary
role is proposed for protozoa.
REFERENCE
1 Postgate and Hunter, (1962). "The Survival of Starved Bacteria." /. Gen. Microbiol, 29, 233.