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Optimization of Anaerobic Digestion of Sewage Sludge Using Thermophilic AnaerobicPre-Treatment
Lu, Jingquan
Publication date:2007
Document VersionPublisher's PDF, also known as Version of record
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Citation (APA):Lu, J. (2007). Optimization of Anaerobic Digestion of Sewage Sludge Using Thermophilic Anaerobic Pre-Treatment.
Biological and thermal effects of thermophilic anaerobic pre-treatment on the
hydrolysis of organic solids in sewage sludge
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Jingquan Lu and Birgitte Kiær Ahring*
BioScience and Technology Group, BioCentrum-DTU
Technical University of Denmark, 2800 Lyngby
Denmark
*Corresponding author
Telephone: +45 4525 6183
Fax: +45 4588 3276
Email: [email protected]
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ABSTRAT
Biological and thermal effects involved in thermophilic anaerobic pre-treatment of
primary sludge and waste activated sludge were investigated at temperatures of 60oC,
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oC and 80oC, respectively. The study revealed that solubilization of lipids, proteins
and carbohydrates were improved by both biological and thermal effects. However,
the solubilization of lipids and proteins was more dependent on microbial impact
while the solubilization of carbohydrates more dependent on thermal impact. At 60oC,
70oC when biological activity was high, significantly larger fractions of lipids and
proteins were solubilized than at 80oC. Batch experiment for a period of 72 hours
shown that, due to biological effect, for primary sludge, the production of d-COD
could be increased by 49.5% and 48.3% at 60oC and 70oC, respectively; and for waste
activated sludge, it could be increased by 50.7% and 46.0% at 60oC and 70oC,
respectively. At 80oC, biological effect was negligible at 80oC, and the thermal
hydrolysis of carbohydrates was the dominant mechanism of the primary sludge
degradation, and the hydrolysis of both proteins and carbohydrates was the dominant
mechanism for waste activated sludge, due to the release of the inner cellular material
by thermal lysis of microbial cells.
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Key words—biological, thermal, effect, thermophilic, anaerobic, pre-treatment,
primary sludge, waste activated sludge, lipids, proteins, carbohydrates
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Introduction
Organic material in sewage sludge, for both primary and waste activated sludge, is in
the form of particulates, and this makes hydrolysis of these particulates the obstacle
when anaerobic digestion (AD) process is employed for treatment (Ghosh et al., 1975;
Eastman and Fergusson, 1981; Li and Noike, 1992). By using various kinds of pre-
treatment methods, such as mechanical, thermal, chemical, biological and the
combination of these methods, hydrolysis of the organic particulates can be enhanced
and thus make the AD process more efficient both in organic material degradation and
in biogas production (Müller, 2001).
In our previous study (Lu and Ahring, 2005), it has been found that thermophilic
anaerobic pre-treatment, which is anaerobically conducted at the temperatures of 55-
80oC for a short retention time not longer than 3.0 days, has a very promising feature
in treating sewage sludge in a sustainable way. This is because this pre-treatment
method, at the same time of enhancing hydrolysis as the other pre-treatment methods
do, also carries out a significant pathogen reduction effect (PRE) that makes it
possible to re-use the anaerobic digestate of sewage sludge as fertilizer and soil
conditioner on the farmland without in fear of spreading diseases.
Although effects of high volatile fatty acids (VFA) concentration on the reduction
pathogenic microbes such as Salmonella typhi, Shigella dysenteriae and Vibrro
cholerae, have been reported (Kunte et al., 1998; 2000; 2004), PRE is normally
regarded as the contribution mainly from thermal effect (Huyard et al., 2000;
Carrington, 2001). Besides for the purpose of PRE, another reason for conducting the
pre-treatment under thermophilic anaerobic conditions was to take full advantage of
both thermal effect and biological effect on hydrolysis enhancement simultaneously
(Lu and Ahring, 2005). Hiraoka et al. (1984) demonstrated that thermal pre-treatment
at temperatures of 60-100 oC for a holding time within 2 hours was efficient for the
hydrolysis of waste activated sludge and at higher temperatures from 120oC to 175oC
thermal effect can be carried out at RTs as short as 30 minutes according to Li and
Noike (1992). Biological effect for sludge treatment is by way of enzymes secreted by
the microorganisms (Wiegel, 1991; Stetter, 1998). Since different microbes may have
different optimal temperature for growth, the biological effect at different
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temperatures can be different as well. In addition, methanogens are active at
thermophilic temperatures (Kristjansson and Stetter, 1991; Ahring, 2003). Running
the pre-treatment reactor with the presence of methanogenesis to remove H
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in favor of more efficient hydrolysis of lipids and proteins (Novak and Carlson, 1970;
Palenzuela-Rollon, 1999). However, to maintain the reactor to be a biological system
rather than a simple thermal process, more sophisticated control system is needed to
keep proper anaerobic environment, pH level, organic loading rate, feeding, and
agitation and so on. The contributions of biological effect and thermal effect to the
hydrolysis should be clarified in order to evaluate the advantages of thermophilic
anaerobic pre-treatment over a simple thermal pre-treatment.
In this study, we used anaerobic cultures that had been enriched in and acclimatized to
primary sludge and waste activated sludge, respectively, at thermophilic temperatures
of 60oC, 70oC and 80oC, respectively, to determine how biological activity and
thermal impact contribute to the solubilization of organic solids in primary sludge and
waste activated sludge, and to investigate how hydrolysis of organic compounds such
as lipids, proteins and carbohydrates are affected.
MATERIAL AND METHODS
The sludge used in the study
Both of the primary sludge and the waste activated sludge used in this study were
obtained from Lundtofte Wastewater Treatment Plant, Lyngby, Denmark.
Immediately after sampling, the sludges were dispensed into 500 ml plastic bags, and
then stored at -20oC. Each day, a required portion of the sludge was thawed at room
temperature and used in the experiments.
Continuous reactor experiment
Continuous reactor experiments were carried out in two series by employing six 1-
litter completely stirred tank reactors (CSTR), the working volume of which was 800
ml for each. In Series I, three of the reactors running at 60oC, 70oC and 80oC,
respectively, were fed with primary sludge as feedstock. In Series II, the other three
reactors running also at 60oC, 70oC and 80oC, respectively, were fed with waste
activated sludge as feedstock. The hydraulic retention time (HRT) of the reactors was
set to be the same, i.e. 2.0 days, and the feeding frequency four times per day. The
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reactors were intermittently mixed by magnetic stick and stirrer combinations at a
frequency of 1min per hour.
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The inoculum used to start up the reactors was taken from an anaerobic reactor
running at 73oC and with HRT of 2.0 days. Before the inoculum was taken, this
reactor had been adapted to the mixture of the primary and secondary sludges as feed
and reached the steady-state for more than one year.
Biogas flow was quantified by liquid displacement technique using paraffin oil as
liquid in a 10 ml U-tube. The concentrations of CH4 and H2 in the biogas, and pH,
VFA, COD, carbohydrates, lipids and proteins in the effluents were monitored since
the start-up of the reactors. When the reactors reached their steady-states, three
successive sets of sampling were carried out for the determination of the above-
mentioned parameters.
Batch experiment
To compare efficiency of the thermophilic anaerobic pre-treatment (with both
biological and thermal impacts) with that of the thermal pre-treatment (with only
thermal impact) at identical temperatures in solubilizing the organic particulates in the
sludge, batch experiments were carried out. The inoculum used in the batch setups
were taken from the continuous reactors, respectively, under their steady-states.
Beside the test vials, in which both inoculum and raw primary or waste activated
sludge were contained, three kinds of controls were employed. To quantify the
contributions of the biological impact coming from the inoculum and the raw sludge,
Control 1 and Control 2 were involved, in which only inoculum and raw sludge was
contained in the vials, respectively. To quantify the thermal impact when biological
impact was inhibited, NaN3 was dosed to the vials in Control 3 with a final
concentration of 0.1% to prevent the biological activity of the anaerobes that had
existed in the raw sludge (Slanetz and Bartley, 1957). Since the ratio of biomass to
inculum and the medium composition could affect the biodegradation test, the same
amount of inoculum was used and the final volume in all of the control vials was
adjusted to the same as the test vial by adding BA medium (Moreno et al., 1999;
German, 2002). After pH had been adjusted to the respective inoculum pH and the
headspace was degassed with N2/CO2, (20%/80%), the vials were closed with butyl
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rubber stoppers and aluminum crimps, and finally put in the water bath that had been
set at 60oC, 70oC and 80oC, respectively. The composition of batch setups is shown in
Table 1.
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To avoid these drawbacks such as adherence of the material to the vial wall,
insufficiency of mixing during sampling and blockage of sampling needles, ‘multiple
flasks’ method (Sanders, 2002) was followed. At each sampling time, three vials from
each kind of the vial setups were taken out of the water bath and immediately
quenched in a –20oC freezer. At the end of the experiment, all of the vials were taken
out of the freezer and thawed at room temperature for immediate analysis.
Analytical methods
The content of total organic solids in the sludges was measured as volatile suspended
solids (VSS) and COD, and lipids, proteins and carbohydrates were determined as the
main organic compounds, as done in many of the other similar studies (Li & Noike,
1992; Miron et al., 2000; Mahmoud et al., 2004). Soluble COD and VFA were
measured for the soluble components. Because there existed simultaneous
acidogenesis and methanogenesis during the pre-treatment, biogas volume and the
concentrations of CH4 and H2 were also measured.
To separate the solid and the soluble components in the sludge samples, the
previously described method (Eastman and Ferguson, 1981) was improved. A 40-ml
sludge sample was first centrifuged at 4,000 rev/min for 20 minutes to separate the
bulk of the suspended solids. The supernatant was centrifuged again at 13,000 for 10
minutes. The second supernatant was then filtered by 0.45 mμ membrane filter, and
the filtrate was used for soluble COD and VFA analysis. All of the centrifugation- and
filtration-captured solids were washed by distilled water for two times so that there
was no soluble organic matter remained. This was done by re-suspending with
distilled water and repeating the above-mentioned centrifugations and filtration
(Hasegawa et al., 2000). The contents of VSS, COD, lipids, proteins and
carbohydrates were determined for the finally obtained solid sample.
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VSS, COD, lipids, Kjeldahl-N and ammonia-N were analyzed according to the
standard methods (APHA, 1995). The content of proteins was obtained the difference
of Kjeldahl-N and ammonia-N according to Mahmoud et al. (2004).
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Since lipids, proteins and carbohydrates account for more than 95% of the total
volatile suspended solids for both primary sludge and secondary sludge (Eastman and
Ferguson, 1981; Elefsiniotis and Oldham, 1994; Hiraoka et al., 1984), the content of
carbohydrates was estimated by subtracting the lipids and proteins from VSS.
For the measurements of VFA in the filtrate and CH4 and H2 concentrations in the
biogas, methods as previously described (Sørensen et al., 1991; Mladenovska and
Ahring, 1997) were followed.
Conversion factors
For simplification, methanogenic and acidogenic products were converted to COD
basis according to the following factors:
• 1g of volatile fatty acids, such as acetic, propionic, butyric and valeric acids is
equivalent to 1.07, 1.51, 1.86 and 1.04g COD, respectively;
• 1 l CH4 (Standard T & P) is equivalent to 2.86g COD;
• 1 l H2 (Standard T & P) is equivalent to 0.71g COD.
The main organic components were converted to COD basis according to the
following factors:
• 1g lipids is equivalent to 2.91g COD (Mahmoud et al., 2004)
• 1 g proteins (assumed as ( ) is equivalent to 1.5g COD (Eastman and
Ferguson, 1981)
xOHC )( 2.11.64
• 1g carbohydrates (assumed as C6H12O6) is equivalent to 1.07g COD (Miron et al.,
2000)
Calculations
Solubilization rates of carbohydrates, lipids, proteins and VSS and the total soluble
COD production for the continuous experiments were calculated by the following
equations:
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• %100/)( 00 ×−= VSSVSSVSS tVSSη (1)
• %100/)( ,0,,0 ×−= lipidlipidtlipidlipid XXXη (2)
• %100/)( ,0,,0 ×−= proteinproteintproteinprotein CCCη (3)
270 • %100/)( ,0,,0 ×−= carbhcarbhtcarbhcarbh CCCη (4)
• 240,,, HCHSSS CODtCODmeasuredCOD ++−= (5)
• 07.1)(5.1)
(88.2)(
,0,,
0,,0,,
×−+×−
+×−=
tcarbhcarbhtprotein
proteintlipidlipidcalculatedCOD
XXX
XXXS (6)
In the batch experiment, the production of soluble COD in the testing vials came from
six parts: A. the biological hydrolysis of the biomass in the inoculum by the microbes
in the inoculum; B. the biological hydrolysis of the biomass in the raw sludge by the
microbes in the raw sludge; C. the biological hydrolysis of the biomass in the raw
sludge by the microbes in the inoculum; D. the thermal hydrolysis of the biomass in
the raw sludge; E. the biological hydrolysis of biomass in the inoculum by the
microbes in the raw sludge, and F. the thermal hydrolysis of the biomass in the
inoculum. Since the amount of thermally and biologically hydrolysable biomass in
inoculum is much smaller than that in the raw sludge, and the microbial activity in
raw sludge was much smaller than that in the inoculum, so Part E and Part F could be
ignored. Part A could be obtained by Control 1. Part B could be obtained by
subtracting Control 2 from Control 3. Finally, soluble COD produced from the
biomass in the raw sludge by both biological and thermal effects (i.e. Part C and Part
D, respectively) could be obtained by subtracting Part A and Part B from the soluble
COD measured in the testing vial.
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The total soluble COD yields with both biological and thermal impacts and with
thermal effect only were calculated by the following equations:
)/)
(
0,0,,,2,
,1,24,,..,
VSStestCODtcontCOD
tcontCODttestCODtherbiosCOD
MSSSHCHSY
−−
−++=
−
−+ (7)
(8) )/)( 0,0,1,,3,., VSScontCODtcontCODthersCOD MSSY −− −=
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The index ‘‘0’’ and ‘‘t’’ refers the influent and the effluent, respectively.
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RESULTS AND DISCUSSION
Characteristics of the primary sludge and waste activated sludge
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Table 2 shows the characteristics of the primary sludge and waste activated sludge
used in this study. In primary sludge, the order of the component contents is
carbohydrates>lipids>proteins. Carbohydrates were the dominant organic compound,
accounting for 59.7% of the total VSS, while lipids and proteins had comparable
contents, accounting for 22.1% and 18.1% of the total VSS, respectively. In waste
activated sludge, the order is proteins>carbohydrates>lipids. Proteins are the
dominant organic compound, accounting for 54.8% of the total VSS, and
carbohydrates for 40.6%, while lipids are negligible, only accounting for less than
4.6% of the total VSS.
Continuous reactor experiment
Hydrolysis, acidogenesis, methanogenesis and VSS solubilization After start-up, it took about two months for all of the continuous reactors to reach
steady stage, indicated the stable biogas production and composition and stable
concentrations of soluble COD and VFA. Properties of the pre-treatment of primary
sludge waste active sludge during steady stage are summarized in Table 3 and 4,
respectively.
Figure 1 depicts the production of CH4, VFA and soluble COD and the solubilization
rate of VSS as a function of temperature for primary sludge and secondary sludge,
respectively. It can be seen that, even though the operation condition was not
favorable for CH4 production, 9.2% and 11.4% of the total solid COD was produced
when the reactors were run at 60oC for primary sludge and waste activated sludge,
respectively. At 70oC, the production was 6.2% and 8.5%, respectively. The
production of CH4 at 80oC was rather low, only account for 0.3 % and 0.4% for
primary sludge and waste activated sludge, respectively. The decrease of CH4
production indicates that methanogenesis activities decrease along with the increase
of temperature in thermophilic temperature range.
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Besides CH4, trace amount of H2 was also detectable in the 70oC reactor fed with
primary sludge and in the 80
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oC reactor fed with waste activated sludge from time to
time. Since the production only accounted for 0.2% of the total solid COD, it was
ignored in calculating the solubilization of organic solids.
Similar to the production of CH4, the production of VFA was also decreasing along
with the increase of temperature for both primary sludge and waste activated sludge.
At 80oC, the production was significantly lower than at 60oC and 70oC. Since VFA
production in relation to the biological activities of fermentation and/or β oxidation
(Novak and Carlson, 1970; McInernery, 1988), it can be concluded that at 80oC the
activities of thermophilic bacteria for hydrolysis and acidogenesis were very low even
though it had been expected.
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For both primary sludge and waste activated sludge, the decrease of total soluble
COD was not as sharp as the decrease of CH4 and VFA. It seems that the decrease of
biological activity did not seriously affect the production of soluble. The measured
soluble COD in the pre-treated was actually increased along with the increase of
temperature.
For primary sludge, the decrease of VSS solubilization was not parallel with the
production of soluble COD. Especially at 80oC, the decrease of soluble COD
production was much faster than the decrease of VSS even though soluble COD was
from the VSS. This phenomenon was not shown for the waste activated sludge.
Degradation of lipids, proteins and carbohydrates Organic compounds such as lipids, proteins and carbohydrates can be biologically
degraded. Under anaerobic conditions, carbohydrates are hydrolyzed by cellulase or
xylanase to simple sugars and subsequently fermented to volatile acids (Cohen, 1982).
Proteins are hydrolyzed by protease to amino acids and further degraded to VFA
either via anaerobic oxidation linked to hydrogen production or via fermentation
according to Stickland reaction (McInerney, 1988). The former is dependent on the
presence of hydrogen-utilizing methanogens while the latter is independent on the
methanogenic activities in the anaerobic ecosystem (Nagase and Matsuo, 1982). The
polymeric lipids, such as glycerol esters, are first hydrolyzed with the release of free
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fatty acids. Glycerol, galactose, choline and other non-fatty acid components released
by the hydrolysis are fermented mainly to volatile fatty acids by the fermentative
bacteria. Free fatty acids including oils and greases are further oxidized via oxidation
to acetate or propionate by H
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2-producing syntrophic bacteria (Bryant, 1979). The
presence of H2 indicated that the activity of H2-utilizing methanogens was not high
enough to convert the H2 produced during fermentation of the hydrolysis products to
CH4.
As it can be seen in Figure 2, for both primary sludge and waste activated sludge, the
degradation rate of carbohydrates, the theoretical COD potential of which is 1.07 g-
COD/g-dw, increased along with the elevation of temperature, while the degradation
of lipids and proteins, the COD potential of which is 2.88 g-COD/g-dw and 1.50 g-
COD/g-dw, respectively, decreased along with the elevation of temperature. So, even
though relatively less VSS reduction was obtained at lower temperatures, higher d-
COD yields could still be obtained due to the degradation of compounds with high
COD potential. It can be seen that the degradation of carbohydrates is more dependent
on thermal impact than the degradation of proteins and lipids, while the degradation
of proteins and lipids is more dependent on the microbial activities than that of
carbohydrates. By using the individual measurements on the abovementioned organic
compounds and their corresponding COD potential, the calculated VSS degradation
rates and the d-COD yields for both primary sludge and waste activated sludge were
obtained, which dovetail very well with the direct measurements as plotted in Figure
2.
It should be also noticed the decrease in the degradation rate of the proteins in
primary sludge is much faster than in waste activated sludge, and the degradation rate
at all temperatures are much lower than that in waste activated sludge. The
degradation rate of the proteins in waste activated sludge at 80oC is even higher than
that in primary sludge at 60oC. The VSS reduction rates at all the temperatures in
waste activated sludge are much higher than those in primary sludge. All of these
phenomena can be explained by the fact that the proteins in waste activated sludge
were different from those in primary sludge. The microbial cells in waste activated
sludge contained a large amount of easily degradable proteins. When the cells were
lysed by heat and/or enzyme attack, large amount of easily degradable substance
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including proteins was released, resulting higher degradation rate of proteins and
higher VSS reduction rate.
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Differentiation of biological effect and thermal effect
After calculation to omit the interferences from the biomass in inoculum and from the
microbial activity in the raw sludge, the soluble COD yield from biological effect and
thermal effect on the biomass of raw sludge in the test vial, indicated as inoculated
sludge and the soluble COD yield from thermal effect in Control 3, indicated as
inhibited sludge, were plotted in Figure 3. For primary sludge, it can be seen that due
to the introduction of biological activities, the yield of soluble COD can be
significantly increased by 49.5% and 48.3% at 60oC and 70oC, respectively, while at
80oC, due to negligible biological activity involved, the increase in d-COD yield is
not so significant between the inoculated sludge and the inhibited sludge. To word
this quantitively, during thermophilic anaerobic pre-treatment of primary sludge in
batch at 60 oC for a period of 72 hours, 33.1% of the hydrolysate was caused by
biological activity and the rest, 69.9%, is caused by thermal effect. At 70 oC for the
same period, 32.6% is caused by biological activity, and 67.4% is caused by thermal
effect. At 80oC, almost the entire soluble COD yield was from thermal effect.
For waste activated sludge, the increase of soluble COD yield due to biological
activity was 50.7% and 46.0% for 60oC and 70oC, respectively. Also, it can be said
that during thermophilic anaerobic pre-treatment of waste activated sludge in batch at
60oC for a period of 72 hours, 33.6% of the hydrolysate is caused by biological
activity and the rest, 66.4%, was caused by thermal effect. At 70 oC for the same
period, 31.5% was caused by biological activity, and 68.5% is caused by thermal
effect. At 80oC, almost the entire soluble COD yield was from thermal effect.
From Figure 3, it can be noticed that the yield of d-COD from waste activated sludge
is higher than that from primary sludge at each of the corresponding temperatures, and
at 80oC, the decrease in d-COD yield from waste activated sludge was not so
significant compared with the counter part from primary sludge. These findings also
prove that the additional mechanism, thermal lysis of microbial cell, did exist during
thermophilic anaerobic pre-treatment of waste activated sludge, by which soluble or
easily hydrolyzed substances were released from the microbial cells.
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CONCLUSION
Thermophilic anaerobic pre-treatment is an efficient pre-treatment method. By
running completely stirred tank reactors for a retention time of 2 day at 60
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oC, 70 oC
and 80oC, respectively, and using primary sludge and waste activated sludge,
respectively, it was confirmed that both biological and thermal effects contribute to
the hydrolysis of organic particulates. For waste activated sludge, the additional
mechanism, thermal lysis of microbial cell, was proved.
For the hydrolysis of different types of organic material, i.e. lipids, proteins and
carbohydrates, their dependency on thermal effect and biological effect were
different. The hydrolysis of lipids and proteins was more dependent on biological
activity than the hydrolysis of carbohydrates, while the hydrolysis of carbohydrates
was more dependent on thermal effect than lipids and proteins. For both of primary
sludge and waste activated sludge, at 60oC and 70oC, i.e., at the temperatures of which
microbial activity was high, larger percent of lipids and proteins were hydrolyzed than
at 80oC. For primary sludge, at 80oC, the biological effect is negligible, and high
percent of carbohydrates are hydrolyzed by thermal effect. For waste activated sludge,
at 80oC, thermal lysis of microbial cell was the dominant mechanism resulting in the
high degradation rate of proteins released from the microbial cells.
In differentiating the contributions to hydrolysis from biological effect and thermal
effect, batch experiment for a period of 72 hours shown that, for the pre-treatment of
primary sludge, an increase of 49.5% and 48.3% of hydrolysis product could be
obtained due to biological activity at 60oC and 70oC, respectively. At 60 oC, 33.1% of
the hydrolysate was caused by biological activity and the rest, 69.9%, is caused by
thermal effect. At 70oC, 32.6% is caused by biological activity, and 67.4% is caused
by thermal effect.
For the pre-treatment of waste activated sludge, an increase of 50.7% and 46.0% of
hydrolysis product could be obtained due to biological activity at 60oC and 70oC,
respectively. At 60 oC, 33.6% of the hydrolysate was caused by biological activity and
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the rest, 66.4%, is caused by thermal effect. At 70oC, 31.5% is caused by biological
activity, and 68.5%is caused by thermal effect.
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At 80oC, for the pre-treatment of both primary sludge and waste activated sludge,
almost all of the hydrolysis is caused by thermal effect. Biological effect is negligible.
The weak biological effect might be due to the failure in enriching the microbes that
can growth at this high temperature or might be the microbes were enriched, but with
low activity at this high temperature. This temperature can be recommended as the
upper limit for thermophilic anaerobic pre-treatment for sewage sludge.
ACKNOWLEGDEMENTS
The authors wish to thank the Danish Technical Research Council (STVF) for the
financial support of this work under the Framework program 26-01-0119
“Optimisation of biogas processes”.
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TABLES Table 1 Composition in the experimental batch set-ups Vials Composition Test vials 60 ml sludge + 20 ml Inocula Control vials
Control 1 20 ml inocula + 60 ml BA medium Control 2 60 ml sludge + 20 ml BA medium Control 3 60 ml sludge + 20 ml BA medium + 0.1% NaN3
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625
Table 2 Properties of the primary sludge and waste activated sludge
Type of sludge Parameter Units Primary sludge Waste activated sludge Components in solid form
TSS g/l 24.54± 0.22a 17.06 0.70 ± VSS g/l 17.39± 0.17 9.61± 0.35 Lipids g/l 3.85± 0.27 0.44± 0.10 Proteins g/l 3.15± 0.06 5.27± 0.12 Carbohydrates g/l 10.39± 0.21 1.46± 0.14 COD g/l 27.91± 1.25 14.09 0.86 ±
Components in soluble form COD g/l 1.02± 0.03 0.64± 0.04 VFA g/l 0.55± 0.01 0.23± 0.01
a. Values are average of three measurements with standard deviation
630
635
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640
645
650
Table 3 Properties of the primary sludge pre-treated at different temperatures
Temperature of reactor (oC) Parameter Units 60 70 80
pH ---- 6.53± 0.02 a 6.50± 0.01 6.67 0.01 ±
Biogas production Volume ml/l-influent 2756± 120 2101± 158 834 63 ± H2 concentration % ND b 0.91± 0.01 ND CH4 concentration % 32.51± 1.21 28.66± 1.56 3.12 0.55 ±
Components in soluble form Soluble COD g/l 7.72± 0.32 7.99± 0.20 8.18 0.35 ± VFA g/l 2.55± 0.05 2.06± 0.09 1.21 0.03 ±
Components in solid form VSS g/l 12.15± 0.01 12.40± 0.01 12.60 0.01 ± Lipids g/l 2.96± 0.12 3.05± 0.09 3.60 0.12 ± Proteins g/l 2.15± 0.04 2.30± 0.01 2.80 0.05 ± Carbohydrates g/l 7.04± 0.12 7.05± 0.07 6.20 0.06 ±
a. Values are average of three measurements with standard deviation; b Not detectable.
655 Table 4 Properties of the waste activated sludge pre-treated at different temperatures
Temperature of reactor (oC) Parameter Units 60 70 80
pH ---- 6.60± 0.01a 6.64± 0.01 6.70 0.01 ±
Biogas production Volume ml/l-influent 1538± 404 1157± 350 418 201 ± H2 concentration % NDb ND 0.85 0.01 ± CH4 concentration % 36.66± 1.23 36.20± 2.12 4.23 1.30 ±
Components in soluble form Soluble COD g/l 4.12± 0.23 4.48± 0.19 5.54 0.25 ± VFA g/l 2.05± 0.00 1.83± 0.00 0.87 0.00 ±
Components in solid form
VSS g/l 5.96± 0.21 5.99± 0.19 6.03 0.32 ± Lipids g/l 0.33± 0.07 0.34± 0.09 0.40 0.04 ± Proteins g/l 2.51± 0.15 2.91± 0.06 3.10 0.02 ±
± ± Carbohydrates g/l 3.12 0.07 2.74 0.11 2.53 0.10 ± a. Values are average of three measurements with standard deviation; b Not detectable.
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665
670
675
680
685
690
FIGURE LEGENDS
Figure 1. Percentage of hydrolysis, acidogenesis and methanogenesis products (i.e. s-
COD, VFA and CH4, respectively) on account of total solid form COD in the influent
sludge, and the solubilization rate of VSS after pre-treatment at different temperatures
for primary sludge (a) and waste activated sludge (b). Methanogenesis products are
included in acidogenesis products, and the acidogenesis products are included in
hydrolysis products.
Figure 2. Solubilization rates of the main organic compounds in comparison with the
production of total soluble COD for primary sludge (a) and waste activated sludge (b).
Figure 3. Comparison of pre-treatment with and without biological effects on soluble
COD yield. Soluble COD converted to CH was included. (a , a4 1 2 and a3 are for
primary sludge at 60o o oC, 70 C and 80 C, respectively; b , b and b1 2 3 are for waste
activated sludge at 60oC, 70o oC and 80 C, respectively).
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740
Figure 1
CH4 VFA d-COD VSSCH4CH4 VFAVFA d-CODd-COD VSSVSS
0
10
20
30
40
50 60 70 80 90Temperature (oC)
%
0
10
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40
50 60 70 80 90Temperature (oC)
%
a b
Figure 2
0
20
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60
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100
50 60 70 80 90Temperature (oC)
Solu
biliz
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te (%
)
0
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8
COD
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n (g
/l)
0
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Solu
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)
0
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COD
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cent
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n (g
/l)
Lipids Proteins Carbohydrates
Calculated soluble COD Measured soluble COD
LipidsLipids ProteinsProteins CarbohydratesCarbohydrates
Calculated soluble COD Calculated soluble COD Measured soluble COD Measured soluble COD
a b
22
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775
785
790
795
Figure 3
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c2c1
Inoculated sludgeInoculated sludgeInoculated sludge Inhibited sludgeInhibited sludgeInhibited sludge
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June 5, 2007 Re: Biological and thermal effects of thermophilic anaerobic pre-treatment on the hydrolysis of organic solids in sewage sludge To Whom It May Concern: Attached is our paper for possible publication in Water Research. Thank you for your attention, and I am looking forward to receiving your positive reply. Sincerely yours, Jingquan Lu Bioprocess Science and Technology BioCentrum-DTU, Building 227, st. Technical University of Denmark DK-2800 Kgs. Lyngby Denmark Tel.: +45 4525 6187 Fax: + 45 4588 3276 E-mail: [email protected] www.biocentrum.dtu.dk
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