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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

Link back to DTU Orbit

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: bka@biocentrum.dtu.dk

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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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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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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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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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Figure 1

CH4 VFA d-COD VSSCH4CH4 VFAVFA d-CODd-COD VSSVSS

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Figure 2

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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

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Figure 3

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Inoculated sludgeInoculated sludgeInoculated sludge Inhibited sludgeInhibited sludgeInhibited sludge

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Editorial Office

Water Research

Radarweg 29, 1043 NX Amsterdam

The Netherlands

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: jql@biocentrum.dtu.dk www.biocentrum.dtu.dk

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