436
Discussion
Author's Reply
The present work was undertaken with a practical bias and thus the magnitude of the
auto-oxidation of sludge was assessed in terms of the bio-degradation of the volatile suspended solids,
VSS, in the sludge. Despite all the shortcomings, it is probably the only standard and convenient
parameter used widely by the researchers to assess biodegradation of biological solids. The excellent
summary of work on aerobic digestion reported by BURD (1968) clearly brings out this fact. Also, in
the design of aerobic digestion systems, the average retention time of the micro organisms closely
approximates the average retention time of the mixed VSS in the system as stated by PFEFFER ET
AL. (1967). The use of VSS in activated sludge treatment and aerobic digestion as an operational
parameter is also recommended by ECKENFELDER and FORD (1970).
Aerobic digestion of sludge involves the aerobic oxidation of organic carbon to carbon dioxide.
The rate of oxygen uptake shows the rate of activity of the organisms and is an important parameter
in aerobic digestion study. However, for the total mass of organics degraded, the total amount of
oxygen will have to be known and this will require continuous and accurate monitoring of oxygen
uptake for the entire duration of digestion. Since the primary interest is to know the reduction in the
ultimate volume and weight of sludge resulting due to digestion, with and without blending, it may
not be convenient to use oxygen uptake rate to measure it. Consequently, VSS measurement was
adopted to determine the concentration of sludge while settleability parameter was adopted to
determine the volume of digested sludge for disposal.
The authors agree with the drawbacks pointed out by Dr. Mudrack in the test for total plate count.
That is why the results of the total plate counts were not used for any numerical comparison. They
have been used solely to show the trends in the variation of bacterial numbers for blended and control
sludges under similar conditions.
During this study, the nitrogen in the aeration jars was not measured. It is therefore difficult to
support or contradict Dr. Mudrack's interpretation for the changes in pH values of the two sludges.
It is evident from Figure 2 of the paper, that the rate of auto-oxidation, after 4 to 6 days, for the
blended sludge had become almost equal to that for the control. The settling characteristics and the
oxygen uptake rates showed a similar trend. Similar results were observed in many of these
experiments. Therefore, Dr. Mudrack's interpretation, that there was a steadily increasing difference
between VSS of both sludges while the oxygen uptake rates had become the same, does not appear to
be correct.
Dr. Mudrack has suggested that (i) during the first four days biologically degradable but
undissolved organic matter still in the sludge is solubilized due to blending, and (ii) small particles
resulting due to blending are lost in the supernatant showing higher degradation of VSS. It should be
noted that the VSS were determined by filtering through Whatman Filter Paper No. 1, and the samples
used were taken directly from the aeration jars and not from the settled sludge. Therefore, there is a
little possibility of any organic matter getting lost in the supernatant. It is difficult to believe that,
even after one day, there would be any organic matter loosely associated with the bacterial floes. By
that time, the soluble and colloidal organic matter would have been synthesized into biomass or
converted into bacterial storage products, inside bacterial cells, and subsequently the process of
endogenous respiration will predominate in the aeration jars. Blending the sludge merely enhances this
endogenous process. A simply experiment was conducted to determine the amount of decomposable
solids present in the filtrate obtained by filtering the digested sludge through Whatman Filter Paper
No. 1. Each day, for the first five days of the experiment, 5-Day 20°C biochemical oxygen demand
BOD, suspend solids, SS, and volatile suspended solids, VSS, were tested on filterates from both
control and blended sludges. The results are shown in Fig 1A and Table 1A. It is quite evident that
both BOD and BSS in the filtrate(s) for blended sludge are always lower than that for the control.
This rules out any possibility of more organic matter getting lost in the filtrate(s) of blended sludge
as compared to the control.
The suggestion about the effectiveness of blending for the stabilization of sludge with a large sludge
age is interesting and would need further studies. The floe size is certainly decreased due to blending;
in fact it was observed from the photographs taken that higher the intensity of blending, lower was the
size of the resulting floe.
Some work was done on the drainability aspect of the digesting sludge. The drainability of the
sludge was measured as the time required to drain 50 ml of sludge under a constant vacuum pressure.
Fig. 2A shows the drainability characteristics of the same sludge, wherein one sample was washed
initially with 0.05M phosphate buffer and blended daily; the second sample was worked once,
Discussion
Author's Reply
The present work was undertaken with a practical bias and thus the magnitude of the
auto-oxidation of sludge was assessed in terms of the bio-degradation of the volatile suspended solids,
VSS, in the sludge. Despite all the shortcomings, it is probably the only standard and convenient
parameter used widely by the researchers to assess biodegradation of biological solids. The excellent
summary of work on aerobic digestion reported by BURD (1968) clearly brings out this fact. Also, in
the design of aerobic digestion systems, the average retention time of the micro organisms closely
approximates the average retention time of the mixed VSS in the system as stated by PFEFFER ET
AL. (1967). The use of VSS in activated sludge treatment and aerobic digestion as an operational
parameter is also recommended by ECKENFELDER and FORD (1970).
Aerobic digestion of sludge involves the aerobic oxidation of organic carbon to carbon dioxide.
The rate of oxygen uptake shows the rate of activity of the organisms and is an important parameter
in aerobic digestion study. However, for the total mass of organics degraded, the total amount of
oxygen will have to be known and this will require continuous and accurate monitoring of oxygen
uptake for the entire duration of digestion. Since the primary interest is to know the reduction in the
ultimate volume and weight of sludge resulting due to digestion, with and without blending, it may
not be convenient to use oxygen uptake rate to measure it. Consequently, VSS measurement was
adopted to determine the concentration of sludge while settleability parameter was adopted to
determine the volume of digested sludge for disposal.
The authors agree with the drawbacks pointed out by Dr. Mudrack in the test for total plate count.
That is why the results of the total plate counts were not used for any numerical comparison. They
have been used solely to show the trends in the variation of bacterial numbers for blended and control
sludges under similar conditions.
During this study, the nitrogen in the aeration jars was not measured. It is therefore difficult to
support or contradict Dr. Mudrack's interpretation for the changes in pH values of the two sludges.
It is evident from Figure 2 of the paper, that the rate of auto-oxidation, after 4 to 6 days, for the
blended sludge had become almost equal to that for the control. The settling characteristics and the
oxygen uptake rates showed a similar trend. Similar results were observed in many of these
experiments. Therefore, Dr. Mudrack's interpretation, that there was a steadily increasing difference
between VSS of both sludges while the oxygen uptake rates had become the same, does not appear to
be correct.
Dr. Mudrack has suggested that (i) during the first four days biologically degradable but
undissolved organic matter still in the sludge is solubilized due to blending, and (ii) small particles
resulting due to blending are lost in the supernatant showing higher degradation of VSS. It should be
noted that the VSS were determined by filtering through Whatman Filter Paper No. 1, and the samples
used were taken directly from the aeration jars and not from the settled sludge. Therefore, there is a
little possibility of any organic matter getting lost in the supernatant. It is difficult to believe that,
even after one day, there would be any organic matter loosely associated with the bacterial floes. By
that time, the soluble and colloidal organic matter would have been synthesized into biomass or
converted into bacterial storage products, inside bacterial cells, and subsequently the process of
endogenous respiration will predominate in the aeration jars. Blending the sludge merely enhances this
endogenous process. A simply experiment was conducted to determine the amount of decomposable
solids present in the filtrate obtained by filtering the digested sludge through Whatman Filter Paper
No. 1. Each day, for the first five days of the experiment, 5-Day 20°C biochemical oxygen demand
BOD, suspend solids, SS, and volatile suspended solids, VSS, were tested on filterates from both
control and blended sludges. The results are shown in Fig 1A and Table 1A. It is quite evident that
both BOD and BSS in the filtrate(s) for blended sludge are always lower than that for the control.
This rules out any possibility of more organic matter getting lost in the filtrate(s) of blended sludge
as compared to the control.
The suggestion about the effectiveness of blending for the stabilization of sludge with a large sludge
age is interesting and would need further studies. The floe size is certainly decreased due to blending;
in fact it was observed from the photographs taken that higher the intensity of blending, lower was the
size of the resulting floe.
Some work was done on the drainability aspect of the digesting sludge. The drainability of the
sludge was measured as the time required to drain 50 ml of sludge under a constant vacuum pressure.
Fig. 2A shows the drainability characteristics of the same sludge, wherein one sample was washed
initially with 0.05M phosphate buffer and blended daily; the second sample was worked once,