Formal discussion by D. Orhon, Turkey.
Recent reviews
1 ,
2 have indicated that almost all engineering studies to verify the existence of
substrate storage in sewage treatment processes have been carried out on synthetic substrates, mostly
nutrient deficient totally different from domestic sewage in their nutritional balance. In experiments
on domestic sewage, however, Walters
1 observed that " . . . the major portion of the synthesis and
subsequent degradation of poly-hydroxy butyrate, (PHB-2 storage product) occurred in the contact
tank" of a contact stabilization unit. The total amount of stored PHB was insignificant. Miller
2
reached the conclusion that adsorption or storage were not significant in the activated sludge process.
This concept was later confirmed by Patterson
3 and recent studies at Berkeley, USA
4 , where the
immediate growth response of the stabilized activated sludge upon admixture with settled sewage in
the contact basin, as indicated by significant viability and activity increases, cast doubt on the belief
that associated the substrate removal phase with physical and non-growth oriented biological
processes.
The authors conceived activated sludge as being composed of active microbial mass containing
reserves of dissolved and undissolved origin. The mathematical analysis is based upon a model in which
the incoming volatile suspended matter, built up within the floe as reserves of undissolved origin, is
directly converted into active mass, while soluble BOD s follows a two step mechanism where it is
transformed into reserves of dissolved origin, before being incorporated into active mass. The general
consensus of the literature on this subject
4 tends to favor the hypothesis that particle incorporation
into the activated sludge floe cannot be associated with a single mechanism, but it appears to be the
result of a number of physical and biological phenomena such as adsorption, particle entrapment,
enzymatic hydrolysis, enzyme complexation, etc., that take place simultaneously to a different extent.
The numerical interpretation of the model shows that reserves of dissolved origin within the
activated sludge mass are of negligible magnitude and thus, appears to confirm the hypothesis that
biological oxidation of the substrate within the floe is largely growth-oriented, resulting in the increase
of the microbial mass.
The maximum removal rate of soluble BOD s , reported as 0.2 day"
1
, is estimated to be quite low as
compared to those achieved in high rate activated sludge treatment of soluble wastes.
The activity data presented by the authors are reasonably consistent with the results of Weddle
5 ,
Washington,
6 and Downing
7 , for what is called standard or conventional rate of activated sludge
operation. Weddle showed that dissolved oxygen uptake rates increased from 8 to 40 mg02/gVSS-hr
for a range of net growth rates from 0.03 to 0.6 day'
1 .
The active biomass is evaluated to vary from 78 to 94% of the activated sludge mass for the loading
rates ranging from 0.2 to 1.8 kg BOD s /kg SS/day. The reported values are not in agreement with the
experimental work recently concluded at Berkeley
4 , where the viable fraction of activated sludge was
found to decrease sharply as an hyperbolic function of decreasing net growth rates, in a manner
consistent with previous work of Postgate and Hunter on pure cultures of Aerobacter Aerogenes , and
of Weddle and Jenkins on activated sludge.
5 The latter found that standard rate activated sludge viable
fraction was 10-20% and they attributed their results to the concept of energy of maintenance
together with the decreasing activity of predators as the net growth rate increases.
The loading rate evaluated on the basis of the influent waste stream is expected to differ from the
removal rate, due to dispersion and metabolic activities within the aeration tank. In cases where
transients of load are experienced, shifts between peaks on the loading and removal rate curves are
observed and normally attributed to the changing growth response of the activated sludge biomass. It
is believed that a comparison of the removal rate profile with change in activity would be a better
indication of a possible storage phenomenon. It would be of great interest if any information regarding
the relationship between removal rate and activity profile would be provided. This information would
also reflect changes in the effluent BOD s concentration, a parameter that is essential in the control of
the process.
As pointed out by the authors, efforts leading to the mathematical expression of the complex
phenomena taking place during substrate removal, are bound to be based upon a number of
assumptions. However, significant differences among experimental results on activated sludge kinetics,
make the choice of assumptions the most crucial part in any attempt to improve the description of the
process. In view of this consideration, the authors are asked to elaborate on their assumptions with
special reference to the choice of the same yield value for particulate and soluble organic matter and
to the possible effect of neglecting sedimentation phase in a storage/metabolism model.
REFERENCES
1. WALTERS, C , "Biochemical Storage Phenomena in Activated Sludge." Ph.D. Thesis, University of
Illinois, 1966.
2. MILLER, W.J. "Kinetics of COD removal from Domestic Wastewaters by the Activated Sludge
373
Recent reviews
1 ,
2 have indicated that almost all engineering studies to verify the existence of
substrate storage in sewage treatment processes have been carried out on synthetic substrates, mostly
nutrient deficient totally different from domestic sewage in their nutritional balance. In experiments
on domestic sewage, however, Walters
1 observed that " . . . the major portion of the synthesis and
subsequent degradation of poly-hydroxy butyrate, (PHB-2 storage product) occurred in the contact
tank" of a contact stabilization unit. The total amount of stored PHB was insignificant. Miller
2
reached the conclusion that adsorption or storage were not significant in the activated sludge process.
This concept was later confirmed by Patterson
3 and recent studies at Berkeley, USA
4 , where the
immediate growth response of the stabilized activated sludge upon admixture with settled sewage in
the contact basin, as indicated by significant viability and activity increases, cast doubt on the belief
that associated the substrate removal phase with physical and non-growth oriented biological
processes.
The authors conceived activated sludge as being composed of active microbial mass containing
reserves of dissolved and undissolved origin. The mathematical analysis is based upon a model in which
the incoming volatile suspended matter, built up within the floe as reserves of undissolved origin, is
directly converted into active mass, while soluble BOD s follows a two step mechanism where it is
transformed into reserves of dissolved origin, before being incorporated into active mass. The general
consensus of the literature on this subject
4 tends to favor the hypothesis that particle incorporation
into the activated sludge floe cannot be associated with a single mechanism, but it appears to be the
result of a number of physical and biological phenomena such as adsorption, particle entrapment,
enzymatic hydrolysis, enzyme complexation, etc., that take place simultaneously to a different extent.
The numerical interpretation of the model shows that reserves of dissolved origin within the
activated sludge mass are of negligible magnitude and thus, appears to confirm the hypothesis that
biological oxidation of the substrate within the floe is largely growth-oriented, resulting in the increase
of the microbial mass.
The maximum removal rate of soluble BOD s , reported as 0.2 day"
1
, is estimated to be quite low as
compared to those achieved in high rate activated sludge treatment of soluble wastes.
The activity data presented by the authors are reasonably consistent with the results of Weddle
5 ,
Washington,
6 and Downing
7 , for what is called standard or conventional rate of activated sludge
operation. Weddle showed that dissolved oxygen uptake rates increased from 8 to 40 mg02/gVSS-hr
for a range of net growth rates from 0.03 to 0.6 day'
1 .
The active biomass is evaluated to vary from 78 to 94% of the activated sludge mass for the loading
rates ranging from 0.2 to 1.8 kg BOD s /kg SS/day. The reported values are not in agreement with the
experimental work recently concluded at Berkeley
4 , where the viable fraction of activated sludge was
found to decrease sharply as an hyperbolic function of decreasing net growth rates, in a manner
consistent with previous work of Postgate and Hunter on pure cultures of Aerobacter Aerogenes , and
of Weddle and Jenkins on activated sludge.
5 The latter found that standard rate activated sludge viable
fraction was 10-20% and they attributed their results to the concept of energy of maintenance
together with the decreasing activity of predators as the net growth rate increases.
The loading rate evaluated on the basis of the influent waste stream is expected to differ from the
removal rate, due to dispersion and metabolic activities within the aeration tank. In cases where
transients of load are experienced, shifts between peaks on the loading and removal rate curves are
observed and normally attributed to the changing growth response of the activated sludge biomass. It
is believed that a comparison of the removal rate profile with change in activity would be a better
indication of a possible storage phenomenon. It would be of great interest if any information regarding
the relationship between removal rate and activity profile would be provided. This information would
also reflect changes in the effluent BOD s concentration, a parameter that is essential in the control of
the process.
As pointed out by the authors, efforts leading to the mathematical expression of the complex
phenomena taking place during substrate removal, are bound to be based upon a number of
assumptions. However, significant differences among experimental results on activated sludge kinetics,
make the choice of assumptions the most crucial part in any attempt to improve the description of the
process. In view of this consideration, the authors are asked to elaborate on their assumptions with
special reference to the choice of the same yield value for particulate and soluble organic matter and
to the possible effect of neglecting sedimentation phase in a storage/metabolism model.
REFERENCES
1. WALTERS, C , "Biochemical Storage Phenomena in Activated Sludge." Ph.D. Thesis, University of
Illinois, 1966.
2. MILLER, W.J. "Kinetics of COD removal from Domestic Wastewaters by the Activated Sludge
373