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Applied and Environmental Microbiology, March 1999, p. 1161-1167, Vol. 65, No. 3
0099-2240/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
The Role of Benzoate in Anaerobic Degradation
of Terephthalate
Robbert
Kleerebezem,*
Look W. Hulshoff
Pol, and
Gatze
Lettinga
Subdepartment of Environmental Technology,
Department of Agricultural, Environmental and Systems Technology,
Wageningen Agricultural University, 6703 HD Wageningen, The
Netherlands
Received 8 June 1998/Accepted 15 December 1998
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ABSTRACT |
The effects of acetate, benzoate, and periods without substrate on
the anaerobic degradation of terephthalate
(1,4-benzene-dicarboxylate) by a syntrophic methanogenic culture were
studied. The culture had been enriched on terephthalate and was
capable of benzoate degradation without a lag phase. When incubated
with a mixture of benzoate and terephthalate, subsequent
degradation with preference for benzoate was observed. Both
benzoate and acetate inhibited the anaerobic degradation of
terephthalate. The observed inhibition is partially
irreversible, resulting in a decrease (or even a complete loss)
of the terephthalate-degrading activity after complete degradation of benzoate or acetate. Irreversible inhibition was characteristic for terephthalate degradation only
because the inhibition of benzoate degradation by acetate could well be
described by reversible noncompetitive product inhibition.
Terephthalate degradation was furthermore irreversibly inhibited by
periods without substrate of only a few hours. The inhibition of
terephthalate degradation due to periods without substrate
could be overcome through incubation of the culture with a mixture of
benzoate and terephthalate. In this case no influence of a
period without substrate was observed. Based on these observations it
is postulated that decarboxylation of terephthalate,
resulting in the formation of benzoate, is strictly dependent on the
concomitant fermentation of benzoate. In the presence of higher
concentrations of benzoate, however, benzoate is the favored substrate
over terephthalate, and the culture loses its ability to
degrade terephthalate. In order to overcome the inhibition
of terephthalate degradation by benzoate and acetate, a
two-stage reactor system is suggested for the treatment of wastewater
generated during terephthalic acid production.
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INTRODUCTION |
With an annual production of 14.4 million tons in 1993, purified terephthalic acid (PTA) is among the top
50 chemicals manufactured in the world (23). PTA is used in
the production of polyethylene terephthalate (PET) bottles,
polyester films, and textile fibers. Production of PTA is based on the
well-established process developed by the American Amoco group (1,
6). The process consists of two steps. In the first step, crude
terephthalic acid (CTA) is produced through "wet oxidation" of
p-xylene with air, and in the second step CTA is upgraded
through hydrogenation of impurities during formation of PTA
(1). During both steps, wastestreams are generated with a
high level of organic contamination. The main components in these
wastestreams are, in decreasing order of concentration, terephthalic
acid, acetic acid, benzoic acid, and para-toluic acid
(3, 18, 21).
Wastewater generated during the production of terephthalic acid is
traditionally handled by aerobic treatment technologies (12). Due to the lower nutrient and energy requirements and lower surplus biomass production, anaerobic pretreatment
may represent an attractive alternative for or contribution to
conventional aerobic treatment. Therefore, several technological
studies have been conducted to assess the feasibility of anaerobic
pretreatment of terephthalic acid wastewater (11, 15, 18),
and approximately 10 full-scale treatment systems are currently in
operation or under construction (3, 16, 21). Results
obtained during these studies indicate that most wastewater
constituents are biodegradable under methanogenic conditions
(9) and are hardly toxic to methanogenic organisms (5,
11). However, the degradation rates found in anaerobic
bioreactors are low (3, 11, 18), and lag phases prior to
degradation of the phthalic acid isomers are long, ranging from 1 to 3 months in batch studies (9) to more than 1 year in full-scale reactors (3, 21).
In anaerobic bioreactors, organic compounds are converted into a
mixture of methane and carbon dioxide in a complex network of several
types of bacteria. These metabolic networks have been studied
extensively for the anaerobic degradation of important agroindustrial
wastewater constituents, such as volatile fatty acids and alcohols
(24, 26). Combined with the development of high-rate
anaerobic bioreactors, which have the ability to uncouple the solid
retention time and the hydraulic retention time (13, 14),
this knowledge contributed to the successful implementation of
high-rate anaerobic bioreactors for the treatment of concentrated,
noncomplex wastewaters. In contrast to these relatively noncomplex
substrates, hardly any information is currently available about the
kinetic properties of the methanogenic degradation of aromatic
pollutants. Furthermore, the influence of rapidly degradable substrates
on the anaerobic degradation of aromatic substrates is poorly
documented. This lack of information seriously hampers the successful
introduction and application of anaerobic treatment methods for the
more-complex wastewaters, such as those generated in the petrochemical
industries. With respect to PTA- wastewater, it has been shown that
terephthalate and para-toluate are the rate-limiting
substrates in anaerobic bioreactors (3, 11, 17, 21). Taking
into account that terephthalate is the main polluting compound
in PTA-wastewater, we focused our work on the anaerobic degradation of
this compound. In a related paper (10), we describe the
kinetic properties of the different types of bacteria involved in the
methanogenic degradation of terephthalate, as well as its
ortho- and meta-oriented isomers. These
experiments were conducted with enrichment cultures obtained from
methanogenic granular sludge or digested sewage sludge. We postulated
in that study that the anaerobic degradation of the phthalate isomers and benzoate proceeds according to the reaction scheme shown in Fig.
1.
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FIG. 1.
Proposed degradation scheme for anaerobic degradation of
terephthalate by a syntrophic culture consisting of
terephthalate-decarboxylating and benzoate-fermenting organisms
(Ferm), acetoclastic methanogens (AcM), and hydrogenotrophic
methanogens (HyM).
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The present study focuses on the following characteristics of a
terephthalate-grown, methanogenic enrichment culture: (i) the
influence of acetate and benzoate on terephthalate degradation, as well as the influence of acetate on benzoate degradation, and (ii)
the influence of periods without substrate on benzoate and terephthalate degradation. Based on the results obtained, the specific role of benzoate in the anaerobic degradation of
terephthalate is discussed, and the practical implications for
anaerobic treatment of PTA-wastewater are presented.
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MATERIALS AND METHODS |
Biomass.
The terephthalate-grown enrichment culture
used in the experiments was obtained from granular biomass from a
laboratory-scale anaerobic hybrid reactor as previously described
(10). In order to cultivate a large amount of biomass, a
continuously stirred 5-liter batch reactor was operated with the
enrichment culture. The temperature of the polyacrylate reactor was
controlled at 37 ± 1°C by a thermostat bath circulator (Haake
D1-L) connected to the double wall of the reactor. Prior to inoculation
of the 5-liter cultivation reactor, cultures were transferred into
serum bottles with increasing volume (up to 2 liters, liquid volume of
500 ml) in order to obtain a sufficient amount of biomass for inoculation. The cultivation reactor was operated in a fed-batch mode:
approximately once a week, 1 liter of the culture was removed, and the
reactor was replenished with a mixture of substrate and nutrients. By
using this approach it was possible to grow a large amount of
terephthalate-degrading biomass with a relatively constant volumetric conversion rate of 1 to 2 mM · day
1.
Due to the low growth rate of the terephthalate-degrading
enrichment culture, it took approximately 6 months to obtain a stable
culture in the 5-liter reactor.
Kinetic analyses.
Experimental data were analyzed by using
the previously described mathematical model, with the same set of
experimentally determined or estimated parameter values for the
different trophic groups in the terephthalate-degrading mixed
culture (10).
Noncompetitive product inhibition of benzoate degradation by acetate
was modelled by using the following rate equation for
benzoate
degradation:
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(1)
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where R, µ, Y, and C stand for volumetric conversion rate
(mol · liter
1 · day
1),
maximum specific growth rate (day
1), biomass yield
(g · mol
1), and concentration (mol · liter
1), respectively. The subscripts X
Ferm,
BA, and C2 stand for benzoate-fermenting
biomass, benzoate, and acetate
respectively. Ki
C2 is a noncompetitive
inhibition
coefficient.
Online measurement of the CH4 production rate.
Methane production in the 5-liter cultivation reactor was
measured through liquid displacement with a Mariotte bottle. Prior to
the use of the Mariotte bottle, carbon dioxide was washed from the biogas by leading it over a 20% NaOH solution and a column filled
with soda lime pellets for removal of water vapor and traces of carbon
dioxide. The liquid displaced was collected in a container placed on a
pressure sensor (DS-Europe model QB745) to detect the weight increase
of the container. The pressure sensor was connected to a data logger
(Campbell CR10), and weights were recorded every 30 min. The data
logger was connected to a personal computer for continuous monitoring
of the methane production.
Degradation of mixed substrates.
In order to study the
degradation of mixtures of two of the three substrates (acetate,
benzoate, and/or terephthalate), batch experiments were
performed in 300-ml serum bottles. Media were prepared with mixtures of
substrates at the desired concentrations as described previously
(10). All organic substrates were dosed as sodium salts from
stock solutions and, if necessary, sodium concentrations were corrected
through dosage of NaCl. Serum bottles were sealed with butyl rubber
stoppers and aluminum screw caps. The headspace was flushed with a
mixture of N2 and CO2 (70:30 [vol/vol]), and
Na2S · 7 to 9H2O was dosed from a
concentrated stock solution to obtain a final concentration of 150 mg · liter1. Serum bottles were preincubated at
37 ± 1°C in an orbital-motion shaker and, after temperature
equalization, inoculated by syringe with the
terephthalate-grown enrichment culture. Samples for inoculation (10 to 20 ml) were taken from the cultivation reactor at the end of the
exponential-growth phase. The total liquid volume in the serum bottles
amounted to 50 to 70 ml. Throughout the experimental period, serum
bottles were sampled at least once a day for analyses of the
concentration of terephthalate and benzoate by high-pressure liquid chromatography, and volatile fatty acids, molecular hydrogen, and methane were analyzed by gas chromatography. A detailed description of the analytical methods applied can be found elsewhere
(10). Measured concentrations in the headspace were
corrected for the reduction in liquid volume due to sampling. All
experiments were performed in duplicate.
Influence of periods without substrate on benzoate and
terephthalate degradation.
To study the influence of short
periods without substrate on the terephthalate-degrading
activity of the culture, 20-ml samples were regularly taken from the
cultivation reactor. The samples were incubated with
terephthalate (5 mM) in 117-ml serum bottles with a
N2-CO2 gas-phase mixture for determination of
the specific terephthalate-degrading activity of the biomass.
The experimental procedure was basically the same as that described
above, except no additional nutrients were included. Terephthalate
degradation in the serum bottles was monitored by repeated measurement
of the methane concentration in the headspace for 1 to 2 days. The volumetric terephthalate conversion rate
(mol-terephthalate · 1-inoculum1 · day1) was calculated from the measured methane production
by using linear regression techniques. Using this approach, the
specific terephthalate-degrading activity, as measured at high
terephthalate concentrations in the serum bottles, could be
related to the terephthalate concentration and consequently the
terephthalate conversion rate in the cultivation reactor.
In order to compare the influence of a short period of a few hours
without substrate on the degradation of terephthalate and
a
benzoate-terephthalate mixture, a slightly different procedure
was used. Serum bottles (117 ml) with a N
2-CO
2
gas-phase mixture
were inoculated by syringe with a mixture of biomass
and terephthalate
from the cultivation reactor. Terephthalate,
or a mixture of benzoate
and terephthalate (final concentration
of both substrates, 5 mM),
was dosed to four bottles (in duplicate),
while no substrate was
dosed to four other bottles. After 1 day,
benzoate or a benzoate-terephthalate
mixture was dosed to the
bottles that had not received any substrate
at the moment of sampling.
With this approach, the latter bottles
were exposed to a period without
substrate due to the depletion
of the terephthalate in the
inoculum. In time the concentrations
of benzoate and
terephthalate, the volatile fatty acids, and the
methane
content of the headspace were
measured.
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RESULTS |
Mutual influence of acetate, benzoate, and
terephthalate.
The terephthalate-grown
culture had the ability to degrade benzoate without a lag period, and
specific conversion rates obtained with either benzoate or
terephthalate as substrate were comparable (Fig.
2). When the culture was incubated
with a mixture of terephthalate and benzoate, a
sequential conversion of both substrates was obtained (Fig.
3). From this figure it can be seen that
benzoate is the preferred substrate over terephthalate. Since
hardly any terephthalate is degraded in the presence of
benzoate, the degradation of a mixture of benzoate and
terephthalate approaches diauxic degradation.
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FIG. 2.
Degradation of terephthalate (TA,  ) or
benzoate (BA,  ) by the terephthalate-grown enrichment
culture.
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FIG. 3.
Degradation of a mixture of terephthalate (TA,
) and benzoate (BA, ) and concomitant methane production
(CH4,  ) by the terephthalate-grown enrichment
culture. Calculated lines indicate that the terephthalate
degradation rate after complete removal of benzoate is approximately
31% lower than the initial benzoate degradation rate.
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From Fig.
3 it can be seen that the calculated terephthalate
conversion rate after complete removal of benzoate is 31% lower
than
the initial benzoate conversion rate, suggesting that part
of the
terephthalate-degrading capacity is lost during the degradation
of benzoate. At higher initial concentrations of benzoate and
lower
biomass concentrations, this loss in terephthalate-degrading
activity is more pronounced, as shown in Fig.
4. Even though all
of the benzoate is
degraded within 6 days, a lag phase prior to
terephthalate
degradation of approximately 20 days is observed.
Terephthalate
conversion rates, after complete conversion of benzoate,
are
significantly lower compared to the experiment with
terephthalate
as the sole carbon and energy source.
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FIG. 4.
Terephthalate (TA) degradation by the
terephthalate-grown enrichment culture incubated with () and
without () 12 mM benzoate (BA). In the experiment in which the
culture was incubated with a mixture of benzoate and
terephthalate, benzoate was completely degraded after 6 days.
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A similar effect is observed in cultures incubated with a mixture of
acetate and terephthalate (Fig.
5). Despite complete
degradation of
acetate within 7 days, no degradation of terephthalate
is
observed in a culture incubated with a mixture of terephthalate
and acetate within 38 days.
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FIG. 5.
Terephthalate (TA) degradation by the
terephthalate-grown enrichment culture incubated with () and
without () 50 mM acetate (C2). In the experiment incubated with a
mixture of terephthalate and acetate, the acetate was
completely degraded after 8 days.
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The loss in degrading capacity is characteristic for
terephthalate degradation because no loss in activity was
observed when
the terephthalate-grown culture was incubated
with a mixture of
benzoate and acetate (Fig.
6). Fully reversible product inhibition
of benzoate degradation by acetate could accurately be described
by a simple noncompetitive inhibition model (equation 1, above).
For
calculating the drawn lines in Fig.
6, a noncompetitive
inhibition
coefficient of acetate (Ki
C2) of 33 mM was used.
It can furthermore
be seen that at equal acetate concentrations,
terephthalate degradation
was completely inhibited, while
benzoate conversion still proceeded.
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FIG. 6.
Inhibition by acetate (C2, upper graph) of benzoate (BA,
bottom graph) degradation by the terephthalate-grown enrichment
culture. Markers correspond to measured concentrations, and drawn
lines were calculated by using a noncompetitive inhibition
model (equation 1) with a value for KiC2 of 33 mM.
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Influence of substrate depletion on benzoate and
terephthalate degradation.
During cultivation of the
terephthalate-grown enrichment culture, it was observed
that if substrate was dosed after complete conversion of
terephthalate, a long lag-phase (up to more than 1 month)
occurred prior to terephthalate degradation. In order to
quantify this inactivation due to substrate depletion, samples were
regularly taken from the cultivation reactor and incubated with 5 mM
terephthalate in 117-ml serum bottles. The specific terephthalate conversion rate was measured in the serum bottles for a period of 1 to 2 days. By this approach, the specific
terephthalate-degrading activity of the culture was measured
shortly before and after depletion of terephthalate in the
cultivation reactor. From Fig. 7 it can
be seen that the volumetric terephthalate conversion rate in
the first sample, taken after approximately 2 days, is highly
comparable to the rate in the 5-liter cultivation reactor. This result
shows that no loss in activity occurred due to transfer of the culture.
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FIG. 7.
Terephthalate (TA, ) degradation and cumulative
methane production (CH4) in the cultivation
reactor (top graph) and the volumetric rates of methane production in
the cultivation reactor () compared to the volumetric
terephthalate conversion rate of biomass sampled from the
cultivation reactor and incubated with 5 mM terephthalate in
serum bottles ( ) (bottom graph). Lines showing terephthalate
degradation and methane production in the cultivation reactor were
calculated with a half-saturation constant for terephthalate
fermentation (KTA) of 0.8 mM.
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In time the rate of methane production in the cultivation reactor
decreases due to substrate depletion. Kinetically, this
decrease in the
methane production rate is expressed with the
apparent half-saturation
constant (K
TA), which was estimated to
be 0.8 mM for
terephthalate fermentation. From Fig.
7 it can be
seen that the
volumetric conversion rate of terephthalate degradation
in the
serum bottles (where sufficient substrate is present) proceeds
parallel
with the decrease in the conversion rate in the cultivation
reactor.
This observation evidently shows that the
terephthalate-degrading
culture almost completely loses its
capacity to degrade terephthalate
during short periods of
starvation.
A similar experiment was performed with both terephthalate and
a mixture of benzoate and terephthalate as substrates. The
specific objective of this experiment was to determine if the
culture
only lost its ability to degrade terephthalate during
short
periods without substrate or if the benzoate-degrading activity
was
lost as
well.
The data shown in Fig.
8 confirm that due
to short periods without substrate, the initial terephthalate
degradation rate is
negligible during the first 2 days after
terephthalate is dosed.
Partial recovery of the
terephthalate-degrading activity is obtained
during the
following days, but the terephthalate degradation rate
remains
distinctly lower than in experiments without a period
of starvation.
The initial increase in the methane concentration
observed in the serum
bottles that received no substrate at time
zero is due to the presence
of terephthalate in the inoculum.
From this initial increase in
the methane concentration, the length
of the period without substrate
was estimated to be approximately
4 h.
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FIG. 8.
Terephthalate degradation (TA, bottom graph) and
concomitant methane production (CH4, top graph) with
(dashed lines, solid markers) and without (solid lines, open markers) a
4-h period without substrate.
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The results are clearly different in experiments where a mixture of
terephthalate and benzoate was incubated (Fig.
9). First
of all, it can clearly be seen
that the period without substrate
hardly affects the conversion of
benzoate. It is furthermore evident
that terephthalate
degradation is not affected by the period without
substrate if a
mixture of benzoate and terephthalate is used.
These
observations were confirmed by the highly parallel methane
production
curves.
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FIG. 9.
Terephthalate (TA, bottom graph) and benzoate (BA,
middle graph) degradation and concomitant production of methane
(CH4, top graph) by the terephthalate-grown
enrichment culture incubated with a mixture of BA and TA with (dashed
lines, solid markers) and without (solid lines, open markers) a 4-h
period without substrate.
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DISCUSSION |
Substrate competition during degradation of mixtures of benzoate
and terephthalate.
If benzoate and terephthalate
are fermented by the same organism, the observed preference for
benzoate degradation can be attributed to substrate competition.
Fermentation of terephthalate is energetically more favorable
than benzoate fermentation because decarboxylation of
terephthalate is an exergonic process (
G0' 
20 kJ · mol1). Despite this energetic advantage
of terephthalate fermentation, benzoate is the preferred substrate.
Based on the literature information (
7,
8,
19,
20,
25), the
initial steps in the degradation of terephthalate
and benzoate
likely proceed according to the pathway shown in
Fig.
10. From this figure it can be seen
that both terephthalate
and benzoate degradation were proposed
to converge at benzoyl
coenzyme A (benzoyl-CoA), a central intermediate
in the anaerobic
degradation of aromatic compounds. We suggest that
kinetic differences
between these limited numbers of steps in the
formation of benzoyl-CoA
have to determine the preference for benzoate
conversion because
the preference for benzoate degradation over
terephthalate is
observed immediately after benzoate addition.
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FIG. 10.
Schematic representation of the proposed initial steps
in terephthalate and benzoate degradation, both converging at
benzoyl-CoA (19, 25).
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From the conversion steps shown in Fig.
10, the rate of substrate
uptake across the microbial membrane may represent the rate-determining
step in benzoyl-CoA formation. Since the pK
a value for
terephthalate
(pK
a1,2 = 3.5) is lower than for
benzoate (pK
a = 4.2), the flux
of terephthalate
across the cytoplasmic membrane will be lower
compared to benzoate, if
both terephthalate and benzoate are activated
at comparable
rates. In the presence of both benzoate and terephthalate,
this
may result in higher concentrations of benzoyl-CoA from benzoate
compared to terephthalate and, consequently, benzoate
conversion
will proceed faster. The activation rate may not be the
rate-determining
step in the overall conversion of benzoate and
terephthalate,
and therefore comparable maximum specific
conversion rates for
terephthalate and benzoate can still be
obtained (Fig.
2). Aromatic
ring reduction steps were proposed to be
rate limiting in anaerobic
benzoate degradation by
Rhodopseudomonas palustris (
22).
Product inhibition by acetate of benzoate and terephthalate
degradation.
Benzoate and terephthalate degradation are
both inhibited by acetate. Benzoate inhibition by acetate could be well
described with a noncompetitive inhibition model, with an inhibition
constant (KiC2) of 33 mM. This value for KiC2
is in the same order of magnitude as the value of 40 mM determined in a
defined coculture consisting of the benzoate degrader BZ-2 and
Methanospirillum sp. strain PM-1 (2). In a
coculture consisting of the benzoate degrader SB with
Desulfovibrio sp. strain G-11, 50% inhibition of the
benzoate degradation rate was obtained at an acetate concentration of
approximately 10 mM (27).
In contrast to the inhibition of benzoate degradation by acetate,
terephthalate degradation was found to be irreversibly
inhibited
by acetate, resulting in long lag phases prior to
terephthalate
degradation after complete degradation of acetate
(Fig.
5). The
reasons for the apparent loss in
terephthalate-degrading capacity
are discussed
below.
Deactivation of the terephthalate-degrading enrichment
culture.
The terephthalate-degrading enrichment culture
lost a large part of its capacity to degrade terephthalate when
(i) the culture had been incubated with a mixture of acetate and
terephthalate, (ii) the culture had been degrading benzoate for
a prolonged period of time (several days), or (iii) the culture
had been exposed to a short period (hours) of starvation. The extent of
deactivation due to incubation with a mixture of benzoate and
terephthalate appears to be related to the time needed for
complete degradation of benzoate. At low biomass concentrations and/or
high benzoate concentrations, long lag phases prior to
terephthalate degradation were observed after complete removal
of benzoate. At higher biomass and/or lower benzoate concentrations,
the inhibition of terephthalate degradation by benzoate
appeared to be partially reversible. Partially reversible
inhibition of terephthalate degradation by benzoate has
previously been observed with biomass from anaerobic bioreactors treating terephthalate-containing wastewater (5,
11). Irreversible inhibition of terephthalate degradation
was observed in experiments incubated with a mixture of
terephthalate and glucose (5). The inhibition in the
latter case can probably be attributed to the accumulation of
intermediates of glucose degradation (acetate and hydrogen).
Short periods without substrate resulted in an almost complete loss of
the terephthalate-degrading capacity. As long as the
periods
without substrate were short (a few hours), part of the
terephthalate-degrading activity could be recovered
within a few
days. However, when cultures were kept unfed for
a period of several
days, recovery of the
terephthalate-degrading activity took more
than 1 month (data
not shown). The time periods without substrate
leading to deactivation
of the culture were too short to be explained
by bacterial decay. Lag
phases prior to growth due to periods
without substrate have previously
been reported for butyrate-degrading
syntrophic cocultures
(
4).
The observation that the degradation of a mixture of benzoate and
terephthalate is unaffected by a short period without substrate
(Fig.
9) suggests that (one of) the first steps in the degradation
pathway of terephthalate as shown in Fig.
10 are highly
dependent
on the latter steps (fermentation of benzoyl-CoA) in a
coupled
"chain reaction." It may be speculated that the organism
needs
the energy generated during fermentation of benzoyl-CoA
(approximately
60 kJ/mol) to initiate the decarboxylation of
terephthalate or
to maintain gradients across the bacterial
membrane, as may be
needed for the active uptake of
terephthalate. If the conversion
of benzoyl-CoA is interrupted
due to a feedless period, the chain
is broken and one of the initial
steps in terephthalate degradation
may not be possible
anymore.
In summary, it is suggested here that benzoate plays a peculiar double
role in the degradation of terephthalate: benzoate
(i)
stimulates the degradation of terephthalate when supplied
in a
low concentration after a short period of starvation, (ii)
inhibits
terephthalate degradation when both substrates are present,
and
(iii) may cause a loss in terephthalate-degrading activity
after benzoate degradation for a prolonged period of
time.
Practical implications.
The results described here have clear
implications for anaerobic reactor technology for PTA-wastewater
treatment. Due to the presence of both acetate and benzoate in the
wastewater, the anaerobic degradation of terephthalate
will be strongly inhibited in well-mixed reactors. Only if the reactor
concentrations of acetate and benzoate can be kept low can growth on
terephthalate be expected. Taking this into account, as well as
the measured low growth rates on terephthalate of the
methanogenic enrichment culture (10), we suggest that this
type of wastewater should be treated in a staged bioreactor fashion. In
the first stage of such a system, acetate and benzoate can be removed
at high rates, while in the later stages terephthalate can be
removed at lower volumetric conversion rates and maximized solid
retention times. As a result of the preremoval of acetate and benzoate,
anaerobic mineralization of terephthalate in the latter stages
can be optimized.
It should furthermore be emphasized that wastewater needs to be
fed to the anaerobic bioreactors continuously in order to
avoid inactivation of the terephthalate-degrading
biomass. Since
the industrial production of terephthalic acid is
accomplished
in a continuous process, continuous operation of the
anaerobic
reactors will normally not represent a problem. However,
terephthalic
acid production plants are normally stopped once or twice
a year
for a period of 1 to 2 weeks for maintenance purposes. It is
clear
that during these periods, measures should be taken to prevent
a
dramatic loss of the terephthalate-degrading capacity of the
system due to feed interruption. If no sufficient measures are
taken, a
renewed startup procedure of several months may be required
to
regain the terephthalate-degrading capacity. If the
terephthalate-degrading
biomass in the latter stages of a
staged anaerobic bioreactor
is deactivated due to periods without
substrate, it may be beneficial
to direct a part of the
benzoate-containing raw wastewater to
the later stages of the process
to enhance the recovery of the
terephthalate-degrading
activity.
| |
ACKNOWLEDGMENTS |
This study was supported through IOP Milieubiotechnologie
(Innovative Research Program Environmental Biotechnology, The Netherlands).
R.K. wishes to thank Alfons J. M. Stams for critical review of the manuscript.
| |
FOOTNOTES |
*
Corresponding author. Mailing address: Wageningen
Agricultural University, Department of Agricultural, Environmental and
Systems Technology, Subdepartment of Environmental Technology,
"Biotechnion" Bomenweg 2, 6703 HD Wageningen, The Netherlands.
Phone: (31-317) 483798. Fax: (31-317) 482108. E-mail:
robbert.kleerebezem{at}algemeen.mt.wau.nl.
| |
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Applied and Environmental Microbiology, March 1999, p. 1161-1167, Vol. 65, No. 3
0099-2240/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
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Top
Abstract
Introduction
