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Wat. Res. Vol. 35, No. 8, pp. 20032009, 2001# 2001 Elsevier Science Ltd. All rights reserved

Printed in Great Britain0043-1354/01/$ - see front matterPII: S0043-1354(00)00468-1

ULTRASONIC WASTE ACTIVATED SLUDGEDISINTEGRATION FOR IMPROVING ANAEROBIC

STABILIZATION

A. TIEHM1, K. NICKEL, M. ZELLHORN and U. NEIS*

Department of Sanitary and Environmental Engineering, Technical University of Hamburg-Harburg,AB Gewa sserreinigungstechnik, Eissendorfer Str. 42, D-21073 Hamburg, Germany

(First received 7 March 2000; accepted in revised form 18 September 2000)

Abstract}The pretreatment of waste activated sludge by ultrasonic disintegration was studied in order toimprove the anaerobic sludge stabilization. The ultrasound frequency was varied within a range from 41 to

3217 kHz. The impact of different ultrasound intensities and treatment times was examined. Sludgedisintegration was most significant at low frequencies. Low-frequency ultrasound creates large cavitationbubbles which upon collapse initiate powerful jet streams exerting strong shear forces in the liquid. Thedecreasing sludge disintegration efficiency observed at higher frequencies was attributed to smallercavitation bubbles which do not allow the initiation of such strong shear forces. Short sonication timesresulted in sludge floc deagglomeration without the destruction of bacteria cells. Longer sonicationbrought about the break-up of cell walls, the sludge solids were disintegrated and dissolved organiccompounds were released. The anaerobic digestion of waste activated sludge following ultrasonicpretreatment causing microbial cell lysis was significantly improved. There was an increase in the volatilesolids degradation as well as an increase in the biogas production. The increase in digestion efficiency wasproportional to the degree of sludge disintegration. To a lesser degree the deagglomeration of sludge flocsalso augmented the anaerobic volatile solids degradation. # 2001 Elsevier Science Ltd. All rightsreserved

Key words}anaerobic digestion, sludge stabilization, biogas, ultrasound, cavitation, disintegration

INTRODUCTION

Anaerobic digestion is the most applied technique for

sewage sludge stabilization resulting in the reduction

of sludge volatile solids and the production of biogas.

The anaerobic stabilization is a slow process. There-

fore, long residence times in the fermenters and large

fermenter volumes are required. Anaerobic degrada-

tion of particulate material and macromolecules is

considered to follow a sequence of four steps:

hydrolysis, acidogenesis, acetogenesis, and methano-

genesis. In the case of sewage sludge digestion, the

biological hydrolysis has been identified as the rate-

limiting step (Eastman and Ferguson, 1981; Shimizu

et al., 1993). Therefore, the pretreatment of sewage

sludge by mechanical, chemical, or thermal disinte-

gration can improve the subsequent anaerobic

digestion (Chiu et al., 1997; Doha nyos et al., 1997;

Hiraoka et al., 1984; Mueller et al., 1998). However,

there is a lack of information as to how different

degrees of sludge disintegration impact on the

digestion process.

Ultrasonic disintegration is a well-known method

for the break-up of microbial cells to extract

intracellular material (Harrison, 1991). The impact

of ultrasound waves on a liquid causes the periodical

compression and rarefaction of the medium. Cavita-

tion occurs above a certain intensity threshold, when

gas bubbles are created which first grow in size before

violently collapsing within a few microseconds. Theviolent collapse produces very powerful hydrome-

chanical shear forces in the bulk liquid surrounding

the bubble. It has been shown that macromolecules

with a molar mass above 40,000 are disrupted by the

hydromechanical shear forces produced by ultrasonic

cavitation. The mechanical forces are most effective

at frequencies below 100 kHz (Portenla nger, 1999).

The temperature and pressure inside the collaps-

ing cavitation bubbles rise up to about 5000 K and

several hundred atmospheres. These extreme condi-

tions can lead to the thermal destruction of

compounds present in the cavitation bubbles and to

the generation of very reactive hydroxyl radicals

(Mason, 1991; Young, 1989). In this way sonochem-

ical reactions can degrade volatile pollutants by1Present address: Water Technology Center, Karlsruher Str.

84, D-76139 Karlsruhe, Germany.

*Author to whom all correspondence should be addressed.

Tel.:+49-40-42878-3107; fax: +49-40-42878-2684; e-mail:[email protected]

2003

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pyrolytic processes inside the cavitation bubbles and

non-volatile pollutants by hydroxyl radical reactions

in the bulk liquid (Pe trier and Francony, 1997;

Tiehm, 1999). While sonochemical degradation

processes can occur in a broad ultrasound frequency

range from 20 kHz up to about 1 MHz the highest

efficiency of sonochemical reactions was observed at

more than 100 kHz (Hua and Hoffmann, 1997;Pe trier and Francony, 1997). Both the hydromecha-

nical shear forces and the sonochemical effects can

contribute to the ultrasonic disintegration of sewage

sludge.

Our previously published studies demonstrated

that ultrasonic pretreatment of raw sludge (Tiehm

et al., 1997) or waste activated sludge (Neis et al.,

1999) allowed for a significant increase in digester

through-put without losses in sludge stabilization.

This study was done (i) to obtain more insight into

the mechanisms of ultrasonic sludge disintegration,

(ii) to study the effect of changing the ultrasoundfrequency, and (iii) to examine the effect of different

degrees of sludge disintegration on the anaerobic

sludge stabilization process.

MATERIALS AND METHODS

Examination of sludge disintegration

Our experiments were done with waste activated sludge(WAS) obtained from the municipal full-scale treatmentplant of Bad Bramstedt, Germany. This treatment plantserves about 85,000 P.E. (35% domestic and 65% industrialwaste water) and is operated with 16 d sludge age.

The oxygen utilization rate (OUR) was measured toassess the overall microbiological activity of WAS. TheOUR was determined at 258C after a 5 min aeration of thesludge samples and an addition of acetate as rapidlyavailable substrate (final concentration 1 g l1 Na-acetate).After stopping the aeration, the decrease of the dissolvedoxygen concentration was measured by an electrode,recorded on a plotter, and corrected for oxygen consump-tion by the electrode itself.

The degree of sludge disintegration was assessed bydetermining the chemical oxygen demand (COD) in thesludge supernatant. A reference (100%) was defined as theaqueous phase COD obtained by chemical sludge disin-tegration in 0.5 moll1 sodium hydroxide for 22 h at 208C.

The degree of disintegration (DDCOD) is calculated as theratio of COD-increase by sonication to the COD-increaseby the chemical disintegration:

DDCOD CODUltrasound COD0=CODNaOHCOD0

100% 1

where CODUltrasound is the COD in the supernatant of the

sonicated sample (mgl1

), COD0 is the COD in thesupernatant of the untreated sample (mg l1), CODNaOH isthe COD in the supernatant of the reference sample(mgl1).

The particle size distribution of sludge samples wasdetermined by the laser light obscuration method based onthe time-of-transition principle. A description of the methodand the laser instrument (CIS 100; Galai; Israel) waspublished previously (Neis and Tiehm, 1997). Since repeatedmeasurement of the same sludge sample exhibited signifi-cantly different results, five replicate determinations weredone. Results are given as median particle size and standarddeviation.

The turbidity of sludge samples was measured aftercentrifugation for 30 min at 40,000 rcf with a nephelometer.

Anaerobic sludge stabilization

The anaerobic digestion of WAS was studied in fivestirred tank fermenters (Fig. 1) at 378C. Each fermenter hada total volume of 1 l and contained 800 ml of WAS. At thebeginning of the digestion experiments, the fermenters werefilled with digested sludge from the full-scale Bad Bramstedtdigester. The fermenters were operated with 8 dayshydraulic sludge retention time in a semi-continuous mode.Three times a week appropriate volumes of WAS werereplaced. The produced biogas was collected in calibratedglass cylinders. The cylinders were filled with A. deion.acidified with HCl to pH 0 to avoid losses of CO2 due tothe formation of carbonate. The sonication of freshWAS was done immediately before it was fed to thefermenters.

During the fermentation experiments, the concentrationof volatile solids (VS), production of biogas and pH wererecorded three times a week at each sludge replacement. Theaqueous phase COD, NH4-N, and total phosphorus weredetermined once a week. The concentrations of volatilefatty acids (VFA) and the biogas composition weredetermined at the end of each experiment.

Analytical procedures

In order to determine the volatile solids (VS), the sampleswere first dried at 1058C for 24 h to obtain the concentrationof dry solids. Next, the dry solids were incinerated at 5508C

Fig. 1. Experimental setup for anaerobic sludge digestion. The 1 l fermenters were operated semi-continuously with 800 ml waste activated sludge.

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for 2.5 h. The residues after incineration representthe inorganic dry solids. The difference between the drysolids and the inorganic dry solids represents the volatilesolids.

For the analysis of aqueous phase supernatants, theparticulate sludge material was removed by high-speedcentrifugation (30 min at 40,000 rcf ) followed by filtrationthrough 0.45mm pore size cellulosenitrate membrane filters.

The chemical oxygen demand (COD) was determined byoxidation of the organic compounds with K2Cr2O7. TheCr3+ produced thus was analysed colorimetrically. Re-agents and equipment were purchased from Dr. LangeGmbH, Du sseldorf, Germany.

The concentration of NH4-N was determined afterreaction with salicylic acid and hypochlorite. The colouredreaction product was measured colorimetrically at 660 nm.The concentration of total phosphorus in sludge super-natants was determined after acidic hydrolysis of theorganic phosphates and poly-phosphates to ortho-phos-phate and subsequent reaction of ortho-phosphate withmolybdate and ascorbic acid. The resulting blue complexwas determined colorimetrically at 600 nm.

The biogas composition was determined by a gas

chromatograph equipped with a heat conductivitydetector. The concentration of the volatile fatty acids(VFA) was determined after acidification of the samples,head space sampling, and determination by a gaschromatograph equipped with a flame ionisation detector(FID).

Ultrasound application

Sonication was done with an ultrasound reactor equippedwith disk transducers (Allied Signal, Kiel, Germany)operating at 41, 207, 360, 616, 1068, and 3217 kHz. Thedisk transducers (area 25 cm2) were fixed at the bottom ofthe cylindrical reactor (Fig. 2). During sonication, sludgesamples were stirred and the temperature was maintained at25 38C by thermostated jackets. The ultrasonic power

input was determined by calorimetric measurement (Mason,1991). In this study, ultrasonic parameters are used asdefined by Hua and Hoffmann (1997):

* ultrasonic intensity relates to the power supplied pertransducer area, unit (W cm2),

* ultrasonic density relates to the power supplied persample volume, unit (W l1),

* ultrasonic dose relates to the energy supplied per samplevolume, unit (Ws l1).

RESULTS AND DISCUSSION

Ultrasonic sludge disintegration

The effect of ultrasound frequency on sludge

disintegration was studied in order to find out the

preferential pretreatment conditions. The results of

our relevant experiments are presented in Fig. 3(a)

and (b). Within the range of explored frequencies

between 41 and 3217 kHz, the disintegration of WAS

was most effective at the lower end, i.e. 41 kHz. This

is demonstrated by the most pronounced reduction

of the median sludge particle size as well as the

largest increase in turbidity of the sludge samples at

low frequency (Fig. 3(a)). Obviously particulate

sludge material was broken down into smaller pieces.We also measured the highest degree of disintegra-

tion (DDCOD) at 41 kHz (Fig. 3(b)). The significant

increase of the DDCOD was attributed to the break-

up of microbial cells leading to the release of

intracellular material. The efficiency of sludge disin-

tegration decreased with increasing frequency (Fig.

3). Hence we would expect the best disintegration

results with the lowest ultrasound frequency of

20 kHz. However such a frequency could not be set

with the device available.

As we have outlined before, two cavitation

phenomena might be responsible for the destructionof solid cell matter: powerful hydromechanical shear

forces and sonochemical reactions. Kuttruff (1991)

described cavitation as a good example of a physicalFig. 2. Sketch of the ultrasound reactor.

Fig. 3. Effect of ultrasound frequency on (a) WAS medianparticle size and aqueous phase turbidity and (b) degree ofsludge disintegration, DDCOD. Error bars represent thestandard deviations of five replicate measurements of themedian particle size. Ultrasonic treatment was done for 4 h

at 1.8 W cm2.

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chaos. This becomes even more relevant when

dealing with cavitation in an environment like sewage

sludge which may be the least suitable example for a

well-defined aqueous system. Nevertheless, theoreti-

cal considerations are useful to understand the

decrease in disintegration efficacy with increasing

ultrasound frequency.

Cavitation bubble collapse occurs when theexpanding bubbles have reached their resonant

radius. The resonant cavitation bubble radius is a

function of the ultrasound frequency. In pure water,

it can be calculated by the following equation

(Young, 1989):

ro2r R

2r 3gPo 2

where r is the density of water, or is the resonance

angular frequency, Rr is the resonant bubble radius,

Po is the pressure exerted on the liquid, and g is

the ratio of the specific heats of gases. g correlates to

the heat released upon gas compression (Hua andHoffmann, 1997) and varies from 1.66 to 1.4 and 1.33

for monoatomic, diatomic and triatomic gases,

respectively. Equation (2) is valid for pure water

and low surface tension.

Taking the case of air bubbles in water at

atmospheric pressure, the ultrasonic cavitation bub-

ble radius can be approximated as

Rr % 3:28f1

r 3

where the resonant bubble radius Rr is expressed in

millimetres and fr is the resonance frequency in

kilohertz (Young, 1989). The bubble radius is

inversely proportional to the ultrasound frequency.

The application of low frequencies creates larger

cavitation bubbles. Upon bubble collapse, hard

mechanical jet streams are produced that are

responsible for many cavitation effects observed on

solid surfaces. A valid assumption might be that

the energy released by a jet stream is a function

of the bubble size at the moment of collapse.

The number and size of cavitation bubbles in a

sludge media may certainly be different to a

pure water system due to the presence of a high

number of solids, different density of the liquid, and

the presence of dissolved gases. However, the degreeof sludge disintegration could be related to the

theoretical bubble size calculated by using equation

(3). As it appears, starting at a point where R is about

4 mm, the degree of cell disintegration increases

proportionally to the logarithm of the bubble radius

(Fig. 4).

The theoretical approach gives evidence that the

hydromechanical shear forces produced by ultrasonic

cavitation are more important for sewage sludge

disintegration than sonochemical processes. This

finding is supported by another study using similar

ultrasonic devices. Mark et al. (1998) reported thatsonochemical processes, i.e. production of hydroxyl

radicals, were most significant at frequencies between

200 to 1000 kHz but not at 41 kHz.

The effect of ultrasound intensity on sludge

disintegration was examined at 41 kHz. Interestingly,

we could already observe disintegration phenomena

at a rather low intensity of 0.1W cm2. This is well

below the cavitation threshold for water which is

reported to be about 0.4 W cm2 (Lorimer, 1990). A

lower cavitation threshold for sludge seems reason-

able due to the presence of a high number of small

particles and gas bubbles acting as cavitation nuclei.Figure 5 shows that the degree of sludge disintegra-

tion is directly related to the specific energy input

expressed as the amount of energy consumed related

to the sludge dry solids content. When dealing with

sludge disintegration, expressing the energy con-

sumption this way is useful because the sludge dry

solids concentration influences the disintegration

efficiency (Neis et al., 1999). Since the concentration

of dry solids remained constant in our experiments,

the same linear correlation would result if the

DDCOD had been presented as function of the

ultrasonic dose. In this study, the specific energyrequired to achieve a certain degree of disintegration

was higher as compared to a high performance

reactor operated with 31 kHz (Neis et al., 1999).

Fig. 4. Degree of sludge disintegration (DDCOD) as func-tion of the theoretical resonant cavitation bubble size. Thebubble sizes increase with decreasing ultrasound frequencyand were calculated for the frequencies applied in the

experiments (413217 kHz).

Fig. 5. Degree of sludge disintegration (DDCOD) as func-

tion of the ultrasonic specific energy input. Waste activatedsludge samples (dry solids=25.9 g kg1) were sonicated at41 kHz with different ultrasound intensities.

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Taking into consideration the results presented in

Figs 3 and 4, the lower operating frequency is

expected to be one factor contributing to a higher

disintegration efficiency.

Anaerobic sludge digestion following ultrasonic

disintegrationThe sonication frequency in these experiments was

41 kHz and the sonication time was varied from 7.5

to 150 min in order to obtain different degrees of

disintegration. At the shortest sonication time

(7.5 min) no increase in soluble COD was observed:

there was no cell lysis. The oxygen utilization rate of

the sample however increased by 10% as compared

to the control (Table 1). Thus we observed an

improved microbial activity by dispersing the floc

agglomerates into smaller units or even single

bacteria. When the sonication time was raised to

30, 60, and 150 min, the degrees of disintegrationDDCOD were augmented to 4.7, 13.1, and 23.7%,

respectively (Table 1).

The disintegration pretreatment resulted in a better

anaerobic degradation of the WAS. In Fig. 6, this is

shown by comparing the volatile solids concentration

of the digested sludges (Fig. 6(a)) to the biogas

production (Fig. 6(b)) of sonicated and control

samples. Table 1 summarizes the results obtained at

different pretreatment times. The VS reduction of the

control fermenter was 21.5%. It was 22.7% in the

fermenter operated with the sludge that was soni-

cated for only 7.5 min causing floc deagglomeration

and not cell disintegration. The VS reduction was

highest (33.7%) in the digester which was fed with

sludge of the highest degree of disintegration.

The total biogas production was slightly reduced

when the WAS sonicated for only 7.5 min was

digested. In the other fermenters fed with disinte-

grated WAS the biogas production increased sig-

nificantly with an increasing degree of disintegration.

The percentage of methane in the biogas also

increased with increasing degree of disintegration.

Another case where this has been observed is in

thermal sludge disintegration (Hiraoka et al., 1984).

The specific biogas production, i.e. the biogas

production related to the mass of VS degraded, was

slightly lower for the disintegrated WAS as comparedto the untreated control. This effect might be due to

changes in the biochemical fermentation process.

Because of the higher methane content in the biogas

of the disintegrated WAS, the resulting specific

methane production remained almost constant

(Table 1).

Table 1. Effect of ultrasound treatment time on waste activated sludge disintegration and subsequent anaerobic digestion. The appliedultrasonic frequency was 41 kHz

Disintegration time (min)

Control 7.5 30 60 150

DisintegrationDegree of disintegration DDCOD (%) 0.0 0.0 4.7 13.1 23.7Oxygen utilization rate (%) 100 110 80 65 23Volatile solids degraded (%) 21.5 22.7 27.3 31.4 33.7

Gas productionTotal biogas (l) 2.93 2.79 3.39 3.83 4.15Biogas/VS degraded (l kg1) 483 434 441 433 436CH4 (%) 62.8 63.5 65.9 67.3 68.9CH4/VS degraded (l kg

1) 303 276 291 291 300

Digester effluent supernatantCOD (mg l

1) 215 202 240 250 622Acetic acid (mg l1) 11 21 9 11 11

C3C6 VFA (mg l1

) nda

nd nd nd ndNH4-N (mg l

1) 516 471 558 560 638P total (mg l1) 19.0 19.0 19.3 24.7 20.7

and=not detectable (53mg l1).

Fig. 6. Enhanced anaerobic digestion after waste activ-ated sludge disintegration (degree of disintegrationDDCOD=23.7%) as demonstrated by (a) a better volatilesolids reduction and (b) an increased production of biogas.


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Due to the better biodegradation of disintegrated

WAS the concentration of ammonium and phos-

phorus of the sludge supernatants was higher as

compared to the control. There was no direct

correlation of the increased ammonium and phos-

phorus concentrations to the volatile solids degrada-

tion since the concentration of dissolved phosphorus

especially is strongly affected by precipitation pro-

cesses like the formation of calcium phosphates.

A second set of digestion experiments was done

after ultrasound pretreatment with different frequen-

cies: 41, 207, 360, and 1068kHz. As was expected

from the preceding experiments, the application of

higher frequencies resulted in lower degrees of

disintegration and also in lower volatile solids

reduction in the digestion process (Table 2).

Figure 7 presents a summary of the results of this

study and also integrates a value from a pilot-scale

digestion experiment with sonicated WAS and thesame sludge retention time of 8 days (Neis et al.,

1999). The graph shows that anaerobic sludge

stabilization is intensified with increasing degrees of

disintegration. A regression analysis of the data

indicates that the function is linear. The slope of the

linear function is 1.9 (Fig. 7). By increasing the

degree of disintegration to, say 10%, the anaerobic

volatile solids degradation can be improved by 19%.

What may seem amazing at a first glance is the

section of the ordinate. As we have shown earlier,

short sonication times cause floc deagglomeration

but no cell destruction. Our digestion experiments(Table 1) revealed that the VS degradation was

slightly improved. The observed increase was 6%

which is quite well in agreement with the section of

the ordinate in the statistical analysis (10.4%). The

positive effect of the floc deagglomeration on the

digestion process can be explained by a better

availability of single bacteria cells to hydrolysing

extracellular enzymes as compared to cells which are

embedded and protected in sludge floc agglomerates.

The effect of floc deagglomeration on the VS

degradation might change with respect to the sludge

floc strength and structure (Morgan and Forster,

1992).

CONCLUSIONS

In this study, ultrasonic disintegration of waste

activated sludge was examined in order to improve

the anaerobic stabilization process. The most im-

portant results are:

* Ultrasonic sludge disintegration is most effective

at low ultrasound frequencies.* Hydromechanical shear forces produced by ultra-

sonic cavitation are predominantly responsible forsludge disintegration.

* Ultrasonic pretreatment enhances the subsequent

anaerobic digestion resulting in a better degrada-

tion of volatile solids and an increased production

of biogas.

At short ultrasound application times, sludge floc

agglomerates are dispersed while no cell destruction

occurs. Floc deagglomeration already improves the

anaerobic digestion process. At longer treatment

times or higher ultrasound intensity, the microbial

cell walls are broken and intracellular material is

released to the liquid phase. The increase in volatile

solids reduction in the anaerobic digester is propor-

tional to the degree of sludge cell disintegration.

Acknowledgements}This study was supported by the Ger-man Ministry for Education and Research (BMBF; grantNo. 02WS9460/7).

REFERENCES

Chiu Y.-C., Chang C.-N., Lin J.-G. and Huang S.-J. (1997)Alkaline and ultrasonic pretreatment of sludge beforeanaerobic digestion. Water Sci. Technol. 36(11), 155162.

Doha nyos M., Zabra nska J. and Jencek P. (1997)Enhancement of sludge anaerobic digestion by using ofa special thickening centrifuge. Water Sci. Tech. 36(11),145153.

Table 2. Effect of ultrasound frequency on waste activated sludge disintegration and subsequent anaerobic digestion. At all frequencies,treatment was done with identical ultrasound densities for 60 min

Ultrasound frequency (kHz)

Control 41 207 360 1068

Degree of disintegration DDCOD (%) 0.0 13.9 3.6 3.1 1.0Volatile solids degraded (%) 23.5 32.2 28.9 26.3 25.2

Fig. 7. Enhanced volatile solids degradation as function ofthe degree of sludge disintegration (DDCOD). The sludge

retention time in the digesters was 8 days.

A. Tiehm et al.2008

7/31/2019 586646

7/7

Eastman J. A. and Ferguson J. F. (1981) Solubilization ofparticulate organic carbon during the acid phase ofanaerobic digestion. JWPCF 53(3), 352366.

Harrison S. T. L. (1991) Bacterial cell disruption: a key unitoperation in the recovery of intracellular products.Biotechnol. Adv. 9, 217240.

Hiraoka M., Takeda N., Sakai S. and Yasuda A. (1984)Highly efficient anaerobic digestion with thermal pre-

treatment. Water Sci. Technol. 17, 529539.Hua I. and Hoffmann M. R. (1997) Optimization of

ultrasonic irradiation as an advanced oxidation technol-ogy. Environ. Sci. Technol. 31, 22372243.

Kuttruff H. (1991) Ultrasonics Fundamentals and Applica-tions. Elsevier Science Publishers Ltd., Essex, England.

Lorimer J. P. (1990) Sonochemistry}the general principles.In Sonochemistry: the Uses of Ultrasound in Chemistry,ed. T. Mason. The Royal Society of Chemistry, Cam-bridge, UK.

Mark G., Tauber A., Laupert R., Schuchmann H.-P.,Schulz D., Mues A. and von Sonntag C. (1998) OH-radical formation by ultrasound in aqueous solution}Part II. Terephthalate and Fricke dosimetry and theinfluence of various conditions on the sonolytic yield.

Ultrasonics Sonochem. 5, 4152.Mason T. (1991) Practical Sonochemistry: Users Guide toApplications in Chemistry and Chemical Engineering. EllisHorword Ltd., Chichester, UK.

Morgan J. W. and Forster C. F. (1992) A comparative studyof the sonication of anaerobic and activated sludges.J. Chem. Tech. Biotechnol. 55, 5358.

Mueller J., Lehne G., Schwedes J., Battenberg S., Na vekeR., Kopp J., Dichtl N., Scheminski A., Krull R. andHempel D. C. (1998) Disintegration of sewage sludges

and influence on anaerobic digestion. Water Sci. Technol.38(89), 425433.

Neis U., Nickel K. and Tiehm A. (1999) Enhancement ofanaerobic sludge digestion by ultrasonic disintegration.In: Disposal and Uutilisation of Sewage Sludge: TreatmentMethods and Application Modalities, IAWQ SpecialisedConference on Sludge Treatment, 1315 October 1999,Athens, Greece, pp. 129136.

Neis U. and Tiehm A. (1997) Particle size analysis inprimary and secondary waste water effluents. Water Sci.Technol. 36(4), 151158.

Pe trier C. and Francony A. (1997) Incidence of wave-frequency on the reaction rates during ultrasonic waste-water treatment. Water Sci. Technol. 35(4), 175180.

Portenla nger G. (1999) Mechanical and radical effects ofultrasound. In Ultrasound in Environmental Engineering,TU Hamburg-Harburg Reports on Sanitary Engineering,eds A. Tiehm and U. Neis, Vol. 25, pp. 139151. GFEU-Verlag (ISBN 3-930400-23-5).

Shimizu T., Kudo K. and Nasu Y. (1993) Anaerobic waste-activated sludge digestion a bioconversion and kineticmodel. Biotechnol. Bioengng. 41, 10821091.

Tiehm A. (1999) Combination of ultrasound and biode-

gradation: enhanced bioavailability of polycyclic aro-matic hydrocarbons. In Ultrasound in environmentalengineering, TU Hamburg-Harburg Reports on SanitaryEngineering, eds A. Tiehm and U. Neis, Vol. 25, pp. 167180. GFEU-Verlag (ISBN 3-930400-23-5).

Tiehm A., Nickel K. and Neis U. (1997) The use ofultrasound to accelerate the anaerobic digestion of sewagesludge. Water Sci. Technol. 36(11), 121128.

Young F. R. (1989) Cavitation, pp. 4076. McGraw-HillBook Company, Maidenhead, UK.


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