Discussion by K. MUDRACK, Technische Universitδt, Hannover, Fed. Rep. Germany
Significant research of aerobic stabilization of sludge are the applied methods of characterizing the
development of stabilization (1,2). The methods used may be divided in three groups.
1 A convenient
method is by metering 0 2 uptake rates, because it gives evidence of the biologically decomposable
substances (exogenous and endogenous) of sludge. 2. Less convenient are the measurements of
suspended solids (SS) and volatile suspended solids (VSS), because of the impossibility of
differentiating between biological decomposable and undecomposable substances. 3. Inconvenient are
the determination of settleability, of total plate count and of pH-value. The settleability depends, on
the sludge structure. If this is changed by comminution, the settling velocity changes.
The determination of the plate count of sludge is of questionable value because it is nearly
impossible to break the floes into single bacteria. The value is dependent upon the subdivision of the
floes. Under such conditions, different test methods give strikingly different results. 0 2 -uptake rate
(see fig. 6 of the paper) doubles by comminution from nearly 1st to 3rd day. From 4th day
02-uptake rate of both sludges is adequate and has the same level as the rate of a stabilized slude
(1,2,3). The percentage decrease of volatile suspended solids (VSS) see fig. 2 and 4 of the paper -
however, shows a steadily increasing difference between both sludges during the duration of
stablization which is intensified by sludge elutriation.
These results are to be interpreted as follows: Initially biological decomposable organic substances
undissolved in the sludge are dissolved by comminution and made available as metabolites to the micro
organisms. So O2-uptake-rate increases until exhaustion of the metabolites. At the same time
undecomposable substances in the sludge may become so comminuted that they escape with the
supernatant liquor. This process takes place during stabilization. It is intensified by elutriation and
results in a faster percentage diminution of suspended solids and volatile suspended solids.
I see the essential importance of sludge comminution during stabilization as a more rapid
mobilization of organic substances in the sludge and in an increase of the sludge floes into non-settling
solids in the supernatant liquor. An acceleration of biological oxidation by increasing the surface of
active micro organisms, a decrease in the diffusional resistance and a more frequent renewal of contact
between the surface of active microorganisms and the surrounding medium will be of less importance.
The same tendency was shown in investigations at the Technische Universitδt Hannover. We tested
the influence of sludge comminution on raw sludge (sludge of primary settling tank and excess
activated sludge) from a sewage work
4 , and the influence on activated sludge from a pilot plant
treating synthetic sewage.
5 Raw sludge containing much undissolved organic substance showed an
increase in 0 2 -uptake-rate (see fig. 1). The pilot plant sludge consisting only of bacterial floes, showed
no increase of O2-uptake-rate (see fig. 3) in spite of strong comminution of the floes (see fig. 2). Floe
size alone has no significant effect on metabolism velocity. According to LAUBENBERGER
6 "an old
stabilized sludge will have a smaller elimination rate in spite of a possibly larger specific surface than a
large-flocculent young sludge." Because of the relation between stabilization and increasing age of
sludge acceleration of the metabolic processes by floe comminution accompanied by decreasing sludge
activity with increasing age of sludge, or alternatively acceleration is only seen at the beginning of the
stabilization.
Another question is the need for additional comminution of sludge in aerobic stabilization. By
observing the variation of floe size during stabilization, a destruction of the floes to nearly adequate
particles is seen to occur especially at the beginning of stabilization
6 7 8 and destruction of the floes is
connected with increasing sludge age.
9 Mechanical sludge comminution therefore has a significant
influence only at the beginning of the stabilization phase.
A further question is, do inconvenient secondary actions result apart from desired stabilization the
comminution of the sludge floe? Test results and practice do not show the expected improvement of
sludge drainability by aerobic stabilization. The cause therefore might be the content of fine floes in
sludge. They produce compact sludge cakes during drainage which reduce the drainability. This
disadvantage could be eliminated by washing out the fine floes or by coagulation to larger floes again
by coagulant aids
10 . So BENEDEK
11 improved drainability (measured by the specific loss of head of
a filter) by nearly one order of magnitude by sludge elutriation. HESSE
8 i 0 improved the specific loss
of head of a filter 4-to 5-fold by washing once with a redoubled volume of water and by addition of a
coagulant aid (Sedipur KA, 30 ml 1% solution pro 1) by 250-fold. These results show that sludge
drainability is worsened by the fine floes.
The investigation shows the possibility of acceleration of digestion of organic substances by floe
comminution at the beginning of stabilization but further investigation is required to indicate whether
this is due to genuine digestion or to loss of material in the supernatant liquor. Further work is also
required to see if the stabilization phase may be shortened, and if the sludge drainability is influenced.
433
Significant research of aerobic stabilization of sludge are the applied methods of characterizing the
development of stabilization (1,2). The methods used may be divided in three groups.
1 A convenient
method is by metering 0 2 uptake rates, because it gives evidence of the biologically decomposable
substances (exogenous and endogenous) of sludge. 2. Less convenient are the measurements of
suspended solids (SS) and volatile suspended solids (VSS), because of the impossibility of
differentiating between biological decomposable and undecomposable substances. 3. Inconvenient are
the determination of settleability, of total plate count and of pH-value. The settleability depends, on
the sludge structure. If this is changed by comminution, the settling velocity changes.
The determination of the plate count of sludge is of questionable value because it is nearly
impossible to break the floes into single bacteria. The value is dependent upon the subdivision of the
floes. Under such conditions, different test methods give strikingly different results. 0 2 -uptake rate
(see fig. 6 of the paper) doubles by comminution from nearly 1st to 3rd day. From 4th day
02-uptake rate of both sludges is adequate and has the same level as the rate of a stabilized slude
(1,2,3). The percentage decrease of volatile suspended solids (VSS) see fig. 2 and 4 of the paper -
however, shows a steadily increasing difference between both sludges during the duration of
stablization which is intensified by sludge elutriation.
These results are to be interpreted as follows: Initially biological decomposable organic substances
undissolved in the sludge are dissolved by comminution and made available as metabolites to the micro
organisms. So O2-uptake-rate increases until exhaustion of the metabolites. At the same time
undecomposable substances in the sludge may become so comminuted that they escape with the
supernatant liquor. This process takes place during stabilization. It is intensified by elutriation and
results in a faster percentage diminution of suspended solids and volatile suspended solids.
I see the essential importance of sludge comminution during stabilization as a more rapid
mobilization of organic substances in the sludge and in an increase of the sludge floes into non-settling
solids in the supernatant liquor. An acceleration of biological oxidation by increasing the surface of
active micro organisms, a decrease in the diffusional resistance and a more frequent renewal of contact
between the surface of active microorganisms and the surrounding medium will be of less importance.
The same tendency was shown in investigations at the Technische Universitδt Hannover. We tested
the influence of sludge comminution on raw sludge (sludge of primary settling tank and excess
activated sludge) from a sewage work
4 , and the influence on activated sludge from a pilot plant
treating synthetic sewage.
5 Raw sludge containing much undissolved organic substance showed an
increase in 0 2 -uptake-rate (see fig. 1). The pilot plant sludge consisting only of bacterial floes, showed
no increase of O2-uptake-rate (see fig. 3) in spite of strong comminution of the floes (see fig. 2). Floe
size alone has no significant effect on metabolism velocity. According to LAUBENBERGER
6 "an old
stabilized sludge will have a smaller elimination rate in spite of a possibly larger specific surface than a
large-flocculent young sludge." Because of the relation between stabilization and increasing age of
sludge acceleration of the metabolic processes by floe comminution accompanied by decreasing sludge
activity with increasing age of sludge, or alternatively acceleration is only seen at the beginning of the
stabilization.
Another question is the need for additional comminution of sludge in aerobic stabilization. By
observing the variation of floe size during stabilization, a destruction of the floes to nearly adequate
particles is seen to occur especially at the beginning of stabilization
6 7 8 and destruction of the floes is
connected with increasing sludge age.
9 Mechanical sludge comminution therefore has a significant
influence only at the beginning of the stabilization phase.
A further question is, do inconvenient secondary actions result apart from desired stabilization the
comminution of the sludge floe? Test results and practice do not show the expected improvement of
sludge drainability by aerobic stabilization. The cause therefore might be the content of fine floes in
sludge. They produce compact sludge cakes during drainage which reduce the drainability. This
disadvantage could be eliminated by washing out the fine floes or by coagulation to larger floes again
by coagulant aids
10 . So BENEDEK
11 improved drainability (measured by the specific loss of head of
a filter) by nearly one order of magnitude by sludge elutriation. HESSE
8 i 0 improved the specific loss
of head of a filter 4-to 5-fold by washing once with a redoubled volume of water and by addition of a
coagulant aid (Sedipur KA, 30 ml 1% solution pro 1) by 250-fold. These results show that sludge
drainability is worsened by the fine floes.
The investigation shows the possibility of acceleration of digestion of organic substances by floe
comminution at the beginning of stabilization but further investigation is required to indicate whether
this is due to genuine digestion or to loss of material in the supernatant liquor. Further work is also
required to see if the stabilization phase may be shortened, and if the sludge drainability is influenced.
433