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Metabolic Selection in Activated Sludge...
Metabolic selection can affect the composition of activated sludge biocenoses whenever the
cells can obtain energy by means of more than one metabolic process. For metabolic selection,
the most important processes are:
Nitrate respiration and dissimilative nitrate reduction (denitrification)
Enhanced biological phosphorus removal (EBPR)
The direct role of nitrification in the competition between floc - formers and filaments can
be neglected because of low energy yield in comparison with the metabolism of carbonaceous
compounds. In addition, nitrifying bacteria apparently are not involved in floc formation.
On the contrary, they need well - established flocs to which they can attach. However,
nitrification can affect the composition of activated sludge biocenoses indirectly. The
nitrifiers, as slow - growing microbes, require an increased biomass retention time in the
system, and nitrification is most effective when the organotrophic bacteria are in the
endogenous metabolism phase. These specific conditions favor the growth of starvation resistant
The metabolism of sulphur compounds is important only when there are elevated concentrations
in wastewaters. Metabolic selection can be expected in all nutrient removal systems in which
the activated sludge is alternately exposed to anaerobic / anoxic and oxic conditions. The
alternating exposure of activated sludge microorganisms to different cultivation conditions
can also occur in fully oxic activated sludge systems with high substrate concentration
The principle of metabolic selection first formulated for systems with anaerobic zones can be
generalized for all nutrient removal activated sludge systems; In nutrient removal activated
sludge systems, the microorganisms are equipped with such an enzymatic apparatus enabling them
to obtain energy by all possible metabolic mechanisms more efficiently than the other
microorganisms will be selected.
As the decisive fraction of available substrate is removed under anaerobic and / or anoxic
conditions, the biocenoses of activated sludges from nutrient removal systems will be dominated
by microorganisms utilizing substrates under these conditions. The question of practical
importance is which filamentous microorganisms can be selected according to the above principle.
Metabolic Selection in Anaerobic / Oxic Conditions
The metabolic selection of activated sludge microorganisms in alternating anaerobic and oxic
conditions is connected with the mechanism of enhanced biological phosphorus removal, especially
with the capability of transferring energy chemically bound in polyphosphates and intracellular
storage products between anaerobic and oxic conditions. As building blocks for the synthesis
of organic storage products are scarce only in municipal wastewaters, the poly - P bacteria
should be accompanied by fermentive bacteria which can generate low - molecular weight compounds
from wastewater organics. Both fermentive and poly - P bacteria generally contribute to a better
flocculation of activated sludges. Thus, the more available substrate is removed in an anaerobic
zone of the EBPR activated sludge systems, the better should be the activated sludge settling
Substrate removal under anaerobic conditions has a positive effect on activated sludge
settleability, a phenomenon which has been known for a long time. However, the phenomenon
of metabolic selection was not clearly distinguished from the kinetic selection. For
instance, an author who demonstrated the improvement of settling properties in systems with
an anaerobic zone, used an anaerobic zone divided into three compartments. Also, some
researchers studied the suppression of filamentous microorganism growth in an anaerobic
cascade with substrate concentration gradient. Some authors found that a remarkable suppression
of filamentous microorganism growth was achieved when an anaerobic period was included in the
cycle of SBR (sequencing batch reactor). All these studies concluded that the improvement in
settling properties was reached after the mechanism of EBPR had been established. Unfortunately,
the types of suppressed filamentous microorganisms were not specified in these studies.
Some authors proved that no substrate gradient was necessary in anaerobic conditions for
suppressing the growth of such filamentous microorganisms as Sphaerotilus natans and
Type 021N. They operated a lab - scale activated sludge system which consisted of an anaerobic
completely mixed tank followed by an oxic tank. The ratio between the anaerobic and oxic
volumes was gradually increased. The anaerobic volume was changed from an initial 8 % to 16 %
and finally to 33 % of the total system volume which remained unchanged.
Table given below shows the relationship between the sludge volume index and the COD value
remaining in the effluent from the anaerobic completely mixed tank. It can be seen that the
increased anaerobic volume helped to stabilize the population of poly - P bacteria, which was
reflected in the increase of specific phosphorus release rate under anaerobic conditions
rx,Prel. The stabilization of the EBPR mechanism in the system was accompanied by
a complete removal of available substrate (both initially present in the treated wastewater
and generated by fermentative bacteria) in the anaerobic zone (the effluent value of about
50 mg / L from the anaerobic zone corresponds to refractory organics), so that no readily
biodegradable substrate was left for the growth of filamentous microorganisms under oxic
conditions. This was the reason for the rapid disappearance of filaments from the bulk liquid.
For the rest of the experiment, the filamentous microorganisms were found only sporadically
as short filaments protruding from compact flocs.
Anaerobic volume (%)
Effluent COD from anaerobic reactor (mg / L)
SVI (mL / g)
rx,Prel (mg / g . h)
400 - 500 *
180 - 350
* : CODin 1,000 mg / L
** : Dominant filament is Type 021N
These experiments verified that some filamentous microorganisms can be eliminated from activated
sludges just by means of metabolic selections, i.e., without any simultaneous supporting kinetic
selection. Nevertheless, further experiments with the compartmentalized anaerobic zone proved
how the combination of metabolic and kinetic selective pressures can increase the effectiveness
of filamentous bulking control measures and contribute to the stability of the activated sludge
Metabolic selection in anaerobic conditions, especially in combination with substrate
concentration gradient arrangements, is no longer a toy for researchers but an effective
practical method of controlling the growth of filamentous microorganisms. The reports about
the A / O process, consisting of an anaerobic cascade followed by an oxic one, confirm that
these activated sludge systems usually produce well - settling activated sludges, provided
such extravagant filaments as Microthrix parvicella are not present.
Two researchers described a successful case in Fayetteville, Arkansas where the compartmentalized
anaerobic zone introduced for achieving the EBPR mechanism also solved the bulking problems. The
tested system consisted of a six - compartment anaerobic zone followed by a four - compartment
oxic zone. The mixed liquor hydraulic retention times in individual zones averaged 1.2 h and
8.3 h, respectively. The system was operated at a biomass retention time of about 10 days
(winter) and 5 days (summer). The conditions in the anaerobic compartmentalized zone were
truly anaerobic during the evaluation period, because there was no internal recycle of
nitrified mixed liquor from the oxic to the anaerobic zones. Once the population of Poly - P
bacteria was stabilized in the biocenoses of this activated sludge, the SVI - values stabilized
in a range of 60 - 80 mL / g.
In his surveys of bulking control technologies, an author gave another example of the positive
effect of an anaerobic zone on activated sludge settling properties. The original wastewater
treatment plant in Newark, Ohio suffered from long - term poor performance due to frequent
bulking and foaming problems. In 1988, a new plant was brought on - line in which a three -
compartment anaerobic / anoxic zone was included. The SVI values in the new plant decreased
from the initial 100 - 400 mL / g to 80 - 200 mL / g with an average of 120 mL / g.
Metabolic selection in anaerobic / oxic cultivation conditions, however, does not seem to be
effective in suppressing, all known filamentous microorganisms in activated sludges. Examples
from the Netherlands and from South Africa (Johannesburg Northern Works), when the anaerobic
zones failed to control the growth of Microthrix parvicella, were given in the section
of morphology of filaments. From the survey of properties of most common Eikelboom types
presented in section given at the morphology, at least two other filamentous organisms seem
to be capable of competing with floc - formers under alternating anaerobic / oxic conditions,
namely Type 0092 and Nostocoida limicola.
Metabolic Selection in Anoxic Conditions
Contrary to the previous selective mechanism, metabolic selection under anoxic conditions does
not necessarily require alternation between anoxie and oxic conditions. For balanced growth,
the substrate is taken up and metabolized directly in anoxic conditions. Of course, balanced
growth under anoxic conditions has no practical meaning, but was used as an experimental proof
of this phenomenon. In actual activated sludge plants, the alternation between anoxic and oxic
conditions is unavoidable when nitrification and denitrification are required. In addition,
most denitrifiers exhibit accumulation / storage capacity during unbalanced growth under anoxic
conditions, so that the subsequent oxic conditions are necessary for restoring this capacity.
Indications that the removal of available substrate under anoxic conditions may support the
growth of floc - forming microorganisms can be traced back to the 1970s. This is similar to
discoveries about metabolic selection under anaerobic / oxic conditions - at first the
phenomenon was neither clearly separated from the simultaneous effect of kinetic selection,
nor properly explained.
For instance, an author recommended the installation of an anoxic premixing tank ahead of the
main aeration basin as one measure to prevent and control filamentous bulking. In terms of his
correlation between phosphorus content in activated sludge and SVIs, he explained the positive
effect of the anoxic premixing tank by an increased content of phosphorus. The reason for this
increase is supposed to be a local increase in pH due to denitrification which supports the
production of calcium phosphate in activated sludge flocs.
Also, in England, two researchers recommended the use of anoxic premixing zones in systems
which nitrify for the control of bulking problems. However, they connected the positive effect
of these zones with an increase of substrate concentration gradient in the whole activated
At this time it is uncertain why an anoxic environment brings about an improvement in sludge
settling characteristics. Some effect is probably due to the decrease in longitudinal mixing.
The hydraulic retention time in the anoxic zones used in the reported experiments was about 25
to 30 minutes. During this period it was found that a substantial proportion of the nitrate in
the recycled sludge was utilized as an oxygen source by denitrifying organisms. It is possible
that the selection of microbial flora is affected by the denitrification reactions which occur,
or by the adsorption of substrate by organisms in the absence of normal aerobic metabolism.
The effect of anoxic conditions on on "the selection of microbial flora" supposed by an author
was fully verified in the 1984 - 1985 Prague experiments, the results of which were published
later in "Water Research". The clearest evidence was obtained in that part of the experiments
when the balanced growth in completely mixed reactors was compared for anoxic and oxic conditions.
Both anoxic and oxic completely mixed systems were inoculated with a bulking activated sludge in
which Type 021N dominated.
To verify the effect of anoxic conditions, the activated sludges from both systems were mutually
interchanged on a day. Thus, the bulking sludge with an excessive abundance of Type 021N filaments
was exposed to anoxic conditions while the well - settling sludge started to be cultivated in oxic
condition from that day. Two characteristic features of the metabolic selection under anoxic
conditions were observed during this set of experiments:
Extremely fast response
Complete elimination of filaments from activated sludge
After the change of sludges, the sludge volume index dropped below 100 mL / g within only two
weeks. The filaments of Type 021N disappeared from the bulk liquid, because they formed a basis
for irregular, but very compact and firm, flocs. The reason for such striking results was found
in specific denitrification rates for bulking and well - settling activated sludges which
differed by one order of magnitude (denitrification rate for bulking sludge was less than 2.3
mg / g . h; well - settling sludge averaged 27 mg / g . h, NO-3 - N).
Thus, the filamentous microorganisms were actually cut off from the source of carbon and energy
as they were not able to use the present electron acceptor.
Two researchers mentioned the results obtained by Alleman in his Ph.D. dissertation. Alleman
operated a lab - scale SBR with a 2 h aerated fill period. Activated sludge in the system gained
extremely poor settling properties caused by long filaments. When the conditions during the fill
period were switched from oxic to anoxic, a dramatic improvement in settling properties occurred
within 2 days. This is another proof of the strength of the selective pressure based on metabolic
An author found in well - controlled experiments with compartmentalized activated sludge systems
that anoxic conditions significantly intensify kinetic selective pressure. Two researchers
observed that the presence of an anoxic contact zone may impart to the activated sludge the
same features by the term selector effect. The similarity of kinetic and metabolic properties
between activated sludges developed in systems with substrate concentration gradient, regardless
of oxic or anoxic cultivation conditions, led Jenkins to the formulation of floc - forming
microorganisms FF2. These floc - formers are characterized with high rates (both in oxic and
in anoxic conditions) of:
Accumulation and storage under unbalanced growth
These features distinguish them from Jenkins' floc - former FF1, which are found predominantly
in oxic completely mixed reactors, and in systems with alternating anaerobic / oxic conditions.
As a typical representative of the FF2 floc - formers, Zoogloea ramigera is mentioned.
Floc - formers FF1 are not specified, mostly floc - former isolates from activated sludges are
given as examples. The inclusion of polyphosphate - accumulating bacteria in this group is
questionable, because they form clusters, not true flocs.
The fact that zoogloeal bacteria are an important component of the consortium of floc - forming
microorganisms under both oxic and anoxic conditions is important for the design of nutrient
removal systems. There is a possibility that the zoogloeal bacteria can dominate the entire
mixed culture of activated sludge when the selector effect under anoxic conditions is too
strong, which could lead to viscous bulking problems. For instance, some researchers experienced
bulking and foaming due to enormous biopolymer production by zoogloeal colonies in a system
with a four - compartment anoxic contact zone. When the number of compartments was reduced to
two by keeping the total volume of the anoxic zone, the number of slimy zoogloeal colonies
The phenomenon of metabolic selection under anoxic conditions was fully recognized and approved ,
by two authors. They concluded that the ability to oxidize organic carbon by utilizing nitrate
and / or nitrite nitrogen as an electron acceptor must represent a selective mechanism in
addition to the difference in substrate utilization and growth rates (kinetic selection). The
authors consider the relative amount of substrate removed in the anoxic zone of anoxic / oxic
activated sludge systems to be the decisive parameter determining the microbial composition of
activated sludges in such systems. The higher the fraction of substrate removed by anoxic
processes, the better settling properties can be expected.
Recent worldwide wastewater treatment practice confirms the conclusions from lab - scale and
pilot plant experiments. When filamentous microorganisms from Group S and Group C caused the
bulking troubles, the introduction of anoxic cultivation conditions always resulted in very
fast and effective improvement of settling properties. A survey of successful case histories
in the US was prepared by Albertson and by Daigger and Nicholson. The most typical example was
the Tri - City wastewater treatment plant in Oregon. The activated sludge tank there is operated
in two principally different regimes, depending on the period of the year:
Fully oxic, step - feed operational mode resulting in completely mixed conditions in the
Wastewater and return activated sludge mixed together in a completely mixed anoxic zone
formed in the head end of the activated sludge tank
Two main conclusions can be drawn:
The low SVIs were achieved in an anoxic zone without substrate concentration gradient
When the anoxic zone was partially aerated, the SVIs immediately started to deteriorate
An author reported about good settling properties in his modification of the sequencing batch
reactor in which the SBR itself is preceded by a nonaerated mixing zone where the influent is
mixed with nitrified return activated sludge. However, when Microthrix parvicella was
present, this arrangement did not lead to a complete elimination of this filamentous
microorganism. This evidence indicates that not all filamentous microorganism are susceptible
to metabolic pressure in systems with alternating anoxic and oxic cultivation conditions. The
filaments resistant to the metabolic selection in activated sludge systems with anoxic zones
can be classified as all zones growers (group A) or as filamentous organisms FO2.
To develop a control strategy for all zones growers, it would be beneficial to know the exact
mechanism which enables the filamentous microorganisms to proliferate in anoxic / oxic activated
sludge systems. In the description of Microthrix parvicella, this question has already
been touched. There are two possible explanations for the proliferation of all zones growers in
All zones grower filaments are able to utilize substrate with nitrate nitrogen as a final
electron acceptor with a rate comparable to that of floc - formers (especially of floc - formers
FF2 in Jenkins' terminology. In such a case the metabolic selection is ineffective.
The alternation of anoxic and oxic cultivation conditions creates some selective advantage
for all zones growers over floc - formers.
The second possibility was hypothesized by Casey in South Africa. The hypothesis is based on
Floc - formers are inhibited under oxic conditions by nitric oxide or to a lesser extent
by nitrite as denitrification intermediates accumulated in their cells during the preceding
Filamentous microorganisms denitrify nitrate only to nitrite and thus NO cannot be
accumulated in their cells.
This hypothesis can be termed "inverse metabolic selection". The first part of the hypothesis
seems to be valid and experimentally well - verified at present. In addition to the results
presented by Casey and his co - workers, other proofs for the inhibition of oxic respiration
under the presence of nitric oxide can be found in the literature. Nitric oxide is one of the
intermetabolites of dissimilative nitrate reduction. Its production has not been studied in
activated sludges, simply because it has not been considered as important. However, this
phenomenon is well - known for soil bacteria, for which it has been studied in the connection
with emissions of nitric and nitrous oxides as photochemically active trace gases from soil.
The production of NO is significantly stimulated by an elevated concentration of nitrite in the
Recently, investigations have been made into the production of nitric and nitrous oxides as a
possible explanation for nitrogen losses in anoxic / oxic activated sludge systems. Kugelman
and Spector observed the production of denitrification intermetabolites at such high dissolved
oxygen concentrations that the denitrification in the floc interior could be neglected. Since
oxic denitrification is frequently mentioned in microbiological literature, they concluded that
concurrent nitrification and denitrification occurred in their systems in which the production
of denitrification intermetabolites was observed under oxic conditions. They suggested that
oxic denitrification served as a detoxification mechanism for nitrite in anoxic / oxic activated
sludge systems. The internally formed nitrite by nitrification inhibited oxic respiration, which
could not continue until the nitrite had been removed by oxic denitrification. Regardless of
whether this mechanism is possible or not, Kugelman and Spector verified in their experiments
the results by Casey et at. on the inhibition of oxic respiration by nitrite (both artificially
added to activated sludge, or formed from nitrate reduction).
The second point of the above hypothesis awaits direct experimental proof. Casey et at. carried
out denitrification batch tests with well - settling activated sludges, and with activated sludges
containing many low F / M filaments. While the samples with few low F / M filaments produced
nitrogen gas with no nitrite accumulation, the samples with many low F / M filaments showed an
accumulation of nitrite. Nevertheless, the authors themselves admit that these tests showed
only a qualitative tendency of floc - formers and low F / M filaments to denitrify to nitrogen
gas and nitrite, respectively. More detailed metabolic studies with all zones growers are
needed to positively verify the hypothesis. The first responses to the hypothesis by Casey and
his coworkers confirm that better knowledge of all zones growers' metabolic abilities is the
key to understanding bulking in nutrient removal plants.
One finding from the South African experiments is of great practical importance. The presence
of readily biodegradable substrates increased the rate of oxic denitrification, which shortened
the period of floc - former inhibition by toxic NO and N0-2. These results
lend strength to the hope that inverse metabolic selection will not have a great impact on floc -
formers in anoxic / oxic systems with high concentration gradient. Accumulated substrates or
storage products synthesized under anoxic conditions may play the role of readily biodegradable
compounds required for a rapid reduction of toxic NO and N0-2 after the
transfer of activated sludge to oxic conditions. Thus, the duration of the hypothesized selective
inhibition of floc - formers could be minimized even without any addition of external readily
biodegradable substrates at the end of the anoxic period.
The proper understanding of kinetic and metabolic selection requires a comparison of the
characteristic features of both selective mechanisms.
The characteristic features of kinetic selection in an activated sludge system under oxic
cultivation conditions can be summarized:
Selective pressure based on different growth strategies is rather low
Consequently, response to the application of kinetic selection to bulking activated sludges
is slow, and takes weeks or months
Filamentous microorganisms are always present in the biocenoses of activated sludges -
kinetic selection does not result in total ultimate elimination of filaments
Contrary to a general belief, the main purpose of kinetic selection is not to suppress
filaments, but to support the growth of floc - formers
Recent experiments performed at Prague Institute of Chemical Technology confirmed that even
when the high selector effect is established in activated sludge, filamentous microorganisms
cannot be completely suppressed. An activated sludge system with a four - compartment contact
zone was designed and operated exactly according to the latest literature recommendations.
There was a strong substrate concentration gradient between individual compartments of the
contact zone, and between the contact zone and the main aeration, tank which was operated as
a completely mixed reactor. In spite of this, filamentous microorganisms of Sphaerotilus
natans and Type 021N had been present in the activated sludge biocenoses for all 100 days
of the experiment, with the frequency of occurrence ranging from 3 - 6 (Jenkins' scale).
However, the adverse effect of an abundant filamentous population was counterbalanced by the
presence of heavy compact flocs which developed in the system due to the existence of a high
selector effect. The same effect of the compensation of the filamentous population in activated
sludge by heavy and compact flocs was observed also in another set of experiments with an SBR.
Metabolic selection, at least in the competition between floc - formers FF2 and filamentous
microorganisms from Groups S and C, can result in a very rapid improvement of settling
properties. The growth of filamentous microorganisms can be completely stopped in this way.
Eikelboom expressed exactly this feature when he described the metabolic selection as
"selection by cutting out filamentous bacteria" while kinetic selection he termed "selection
by favoring floc - forming bacteria".
Eckenfelder and Musterman illustrate different ways of achieving the same result with an
example from industrial wastewater treatment. They used pure oxygen activated sludge systems
for treatment of pulp and paper mill wastewaters. The main reaction tanks were equipped with
contact zones differing in hydraulic retention times and cultivation conditions:
Oxic, two - compartments, hydraulic retention time = 0.5 h
Anaerobic / anoxic, one compartment, hydraulic retention time = 0.9 h
Anaerobic / anoxic, one compartment, hydraulic retention time = 2.5 h
The first two systems produced activated sludges with nearly the same settling properties.
However, while the filamentous microorganisms were commonly present in activated sludge from
the oxic two - compartment contact zone, they were completely absent in activated sludge from
the system with the anaerobic / anoxic contact zone with a hydraulic retention time = 2.5 h.
The example given by Eckenfelder and Musterman also shows a difference between kinetic and
metabolic selection which is especially important for design purposes, namely the different
reaction times required for performing the selection.
This difference has recently been observed in the experiments aimed at the comparison of
kinetic and metabolic selective mechanisms. Two activated sludge systems with contact zones
had been operated simultaneously for 110 days. One contact zone was aerated while the contact
zone of the other system was only mixed, and nitrate was dosed as an electron acceptor. The
initial volumes of the contact zones were the same, 0.25 L, and the contact zones were followed
with the oxic completely mixed reactors, each of 2.75 L volume. Both systems were fed with a
synthetic wastewater based on a mixture of glutamic acid, glucose, and ethanol (1 : 2 : 2
according to COD) and balanced with nutrients. The volumetric loading was adjusted to
1 g / L . day (COD), and the target biomass retention time was 7 days. Although the time trends
in SVIs measured in both systems were rather similar, the final effect - decreased SVIs - was
achieved by means of quite opposite actions in the systems.
System with Oxic Contact Zone
In the first half of the experiment, the sludge volume indices were unstable and oscillated
from 200 - 500 mL / g. The dominant type Sphaerotilus natans was replaced by Type 021N /
Thiothrix in the second wave of high SVIs after day 30. Saprochaete saccharophila
was observed as a secondary filament for the whole period of the experiment, probably due to
the composition of the synthetic wastewater. The reason for the proliferation of filamentous
microorganisms and poor and unstable settling properties was in the size and construction of
the contact zone. More than 90 % of readily biodegradable substrate (as COD) was removed in the
oxic contact zone. The substrate concentration gradient between the contact zone and the
subsequent completely mixed reactor was negligible (about 10 mg / L COD). The oxic contact zone
was apparently too large and, in fact, acted as a completely mixed reactor which favored the
growth of filaments. Thus, on the 58th day of operation, the oxic contact zone was
divided into two compartments, each with half the initial volume. Twenty days after this change,
a decreased occurrence of Type 021N / Thiothrix was observed. The decrease in filament
abundance was accompanied by a slow decrease in SVIs.
System with Anoxic Contact Zone
The course of SVIs during the first half of the experiment was simllar to the previous system.
Also, the shift between Sphaerotilus natans and Type 021N / Thiothrix occurred
in the system, but the growth of Saprochaete saccharophila was not observed here. However,
the reasons for bulking problems were quite different from the system with an oxic contact zone.
Because of a lower anoxic substrate uptake rate, 200 - 300 mg / L of readily biodegradable
substrate (as COD) penetrated from the anoxic contact zone to the main oxic completely mixed
reactor. The anoxic contact zone proved to be too small. Filamentous microorganisms grew in the
main reactor, not the contact zone. On day 58, the volume of the anoxic contact zone was doubled
at the expense of the volume of the main completely mixed reactor (total volume of the system
was not changed) but the contact zone was not compartmentalized. The response to this change
was very rapid. The sludge volume indices dropped from nearly 600 mL / g to 100 mL / g within
20 days and remained very low for the rest of the experiment. After this rapid response, Type
021N / Thiothrix filaments were only occasionally observed in the biocenoses. The reason
for this dramatic improvement was that with the increase in anoxic zone volume, the retention
time there matched the denitritication capacity of the activated sludge in the system. The
difference in COD values in effluents from the anoxic contact zone and from the main completely
mixed reactor was only about 10 mg / L at the end of the experiment.
It can be concluded from the comparison of kinetic and metabolic selection that when the
anoxic contact zone is the only measure for controlling the growth of Group S and Group C
filaments in the activated sludge system, all readily biodegradable substrate has to be
removed in this zone. On the other hand, the oxic contact zone will fail to control the
growth of filamentous microorganisms if there is no substrate gradient between the zone
and the main reactor. If not compartmentalized, the oxic contact zone in this case acts
as a small completely mixed reactor simply placed ahead of the main reactor. This could
also explain some unsuccessful applications of noncompartmentalized oxic contact zones
which can be found in the literature.