2284 © IWA Publishing 2012 Water Science & Technology | 66.11 | 2012

Effects of ultrasonic pre-treatment on sludge

characteristics and anaerobic digestion

Lise Appels, Sofie Houtmeyers, Floriaan Van Mechelen, Jan Degrève,

Jan Van Impe and Raf Dewil

ABSTRACT

In this work, the influence of an ultrasonic pre-treatment on anaerobic digestion of waste activated

sludge is studied. Attention is paid to the solubilisation of the main organic (proteins, carbohydrates)

and inorganic (heavy metals) sludge components during ultrasonic treatment and the influence of

the dry solids content (DS) on the degree of solubilisation. The second part of the paper focuses on

the relationship between the applied specific energy of the ultrasonic treatment and methane

production. In general, a higher specific energy and a higher DS content are beneficial for the release

of organic matter, resulting in an increased methane production. The efficiency of the subsequent

anaerobic digestion is similar for both sludge types (2.1 and 3.2% DS). However, at lower DS contents

(2.1%), the methane production increase was more significant.

doi: 10.2166/wst.2012.415

Lise Appels (corresponding author)Sofie HoutmeyersFloriaan Van MechelenJan DegrèveJan Van ImpeRaf DewilChemical and Biochemical Process Technology

and Control Section,Department of Chemical Engineering,KU Leuven (University of Leuven),Willem De Croylaan 46,3001 Heverlee,BelgiumE-mail: lise.appels@cit.kuleuven.be

Key words | anaerobic digestion, dry solids content, ultrasonic pre-treatment, waste activated

sludge

INTRODUCTION

The treatment of wastewater generates a huge amount ofbiological excess sludge, which needs further processing

and disposal, resulting in high treatment costs (Appelset al. ).

Anaerobic digestion as a sludge treatment step is widely

applied due to the increasing trend towards energy recoveryof this waste stream. Its main advantage is the production ofan energy-rich biogas (55–70% methane), which can be

valorised energetically in a CHP (combined heat andpower) to generate heat and electricity. The presence ofrigid and persistent structures, however, results in limitedconversion rates (ca. 25–40% on dry solids (DS) basis) and

long retention times (ca. 20 days) (Demirel & Scherer; Appels et al. ).

It is generally accepted that the first step of anaerobic

digestion, the hydrolysis, is rate-limiting for the process(Ghyoot & Verstraete ; Thiem et al. ). Here, com-plex macromolecular structures (such as extracellular

polymeric substances (EPS)) are converted into simple, sol-uble organics. The application of pre-treatment techniquescan facilitate this step, resulting in higher degradation effi-

ciencies and shorter retention times. In the literature,various mechanical, thermal, chemical and biological

methods are described (as summarised in Appels et al.). These methods cause a partial disintegration of the

sludge, resulting in the release of soluble, readily availableorganic compounds for the micro-organisms. The digestionefficiency and biogas production can be enhanced due to

the faster and more complete degradation of the sludge. Thetype and structure of the sludge will greatly determine theeffects and the efficiency of those disintegration methods

(Bougrier et al. ).Previous research (e.g. Dewil et al. ; Show et al.

) demonstrates a positive influence on the efficiency ofthe ultrasonic disintegration by a higher dry solids content.

With higher DS content, more particles will be in contactwith hydromechanical shear forces and, furthermore, ahigher DS content creates more nuclei for cavitation. It

can be assumed that with a higher DS content, an implosionof a cavitation bubble results in a better disintegration bycolliding particles, hence resulting in smaller particles.

This is caused by an acceleration of the particles becauseof an implosion of a cavitation bubble near the particle.Khanal et al. () identified an increase of soluble chemi-

cal oxygen demand (sCOD) both for increasing energy inputand DS content. Moreover, the ultrasonic pre-treatment

2285 L. Appels et al. | Effects of ultrasonic pre-treatment on sludge Water Science & Technology | 66.11 | 2012

influences the hydrolysis by releasing easily biodegradable

organic components which ensures a faster and more com-plete degradation of the sludge during digestion.According to Thiem et al. (), a longer treatment time

of the sludge will result in more degradation of organicdry solids (ODS) and an increased production of biogas.

This paper studies the influence of an ultrasonic pre-treatment on the sludge composition (solubilisation) and

subsequent anaerobic digestion. The influence of a varyingenergy input and sludge dry solids content on the particlesize distribution of the sludge flocs and the release of various

components such as COD, carbohydrates and proteins willbe examined. Next, the influence of the ultrasonic pre-treat-ment on the degradation efficiency and the biogas

(methane) production during digestion will be studied.

METHODS

Sludge characteristics

Sludge samples originating from the full-scale wastewater

treatment plant (WWTP) of Antwerp-South (Belgium)were used in the experiments. Seven samples were takenfrom the sludge buffer, located before the digester. Thesludge was stored at 4 WC in the laboratory prior to the exper-

iments (max 4 h). All seven samples were pre-treated andtwo were subsequently digested. For the digestion exper-iments, digested sludge obtained from the anaerobic

mesophilic digester of the same WWTP was used as inocu-lum. Prior to the experiment, the inoculum is degassed inorder to deplete the present biodegradable organic material,

as indicated by Angelidaki et al. ().Sludge with a DS content between 9.12 and 33.12 g/kg

sludge were used in this study. The concentrations of Cd,

Cr, Cu, Hg, Ni, Pb and Zn were respectively 4.2, 53.1,444.5, 0.8378, 31.6, 171.7 and 987.8 mg/kg DS.

Pre-treatment conditions

Waste activated sludge (WAS) was pumped through a tubu-lar ultrasound treatment reactor (Bandelin SONOREXTECHNNIK Sonobloc® SB 5.1–1002) at a fixed flow rateof 10 L/min. The energy-input, expressed in kJ/kg DS, was

determined by the applied power and the exposure time ofthe sludge together with the DS content of the sludgevolume under scrutiny. A frequency of 25 kHz was applied

in accordance with literature recommendations (Dewilet al. ), and the power was varied between 0 and

1,000 W. The rise in temperature due to the ultrasonic treat-

ment was limited by using a heat exchanger, to avoid effectsof thermal disintegration.

Digester set-up

Fifteen anaerobic digesters, operating as mesophilic singlestage batch reactors, were built in parallel. The temperature

of the digesters was kept at 37 WC throughout the experimentby submerging them in a temperature-controlled water bath.The reactors, each of 1 L content, were filled with 500 mL

WAS and 100 mL inoculum (having a DS content of 89 g/kg sludge). The reactors were not flushed with nitrogenprior to the experiments because of the very limited

volume of the headspace. The contents of the digesterswere agitated manually once a day. The digestion exper-iments were run for three weeks, after which the biogas

production had stopped. The experiments were carried outin triplicate. The biogas was collected in calibrated glasscylinders filled with acidified water (0.05 M H2SO4) to pre-vent CO2-solubilisation.

Component analysis

The following components were analyzed: DS, ODS and(in)organic components, including COD, carbohydrate con-centration, protein concentration and the concentration of

seven heavy metals (Cu, Ni, Zn, Cd, Cr, Hg and Pb). TheDS was determined as the residue after drying a sludgesample at 105 WC to constant weight. Further heating at605 WC (to constant weight) drives off the ODS. These

methods are described in Standard Methods (APHA ).The COD was determined by a HACH DR/890 Colorimeterusing a HACH COD reactor. Carbohydrates were analyzed

according to the Anthrone method (Gerhardt et al. ).The amount of proteins present in the sludge and the super-natant was measured using the bicinchoninic acid (BCA)

method (Lowry ; Smith ). Heavy metals, sulphurand phosphorus were measured by inductively coupledplasma atomic emission spectroscopy (ICP-AES), according

to the method described in Dewil et al. ().The particle size distribution was determined using a

Malvern 3601 Particle Sizer (applying laser diffraction).The breakdown of organic components during ultra-

sonic treatment and anaerobic digestion was characterisedby measuring the total concentrations before and after ultra-sonic treatment and anaerobic digestion. The solubilisation

of the components was determined by measuring the solublefraction of the components (concentration in liquid phase

2286 L. Appels et al. | Effects of ultrasonic pre-treatment on sludge Water Science & Technology | 66.11 | 2012

after filtration over a 1.3 μm glass microfibre filter paper

(Merck Eurolab)).All analyses were carried out following the procedures

previously described in Appels et al. ().

Figure 2 | ΔsCOD for various specific energies and different DS contents of the sludge.

RESULTS AND DISCUSSION

Physical observations after the ultrasonic treatment ofthe sludge

The cumulative particle size distribution of the sludge flocs

as a function of applied ultrasonic energy is depicted inFigure 1. The horizontal line shows the d50 of the differenttreated sludge samples as a measure for the average flocsize. d50 is the diameter of the sludge particle for which

50% of the particles are smaller and 50% of the particlesare larger than the d50-value. The more energy is transferredto the sludge, the greater the disintegration effect will be,

resulting in smaller particles and hence a lower d50. Thed50 decreases from 80.20 μm for the blank (untreated)sludge to 46.60 μm for the sludge subjected to the highest

specific energy (3,244.46 kJ/kg DS).The results show that application of ultrasonic disinte-

gration results in a break-up of sludge flocs, hencedecreasing the average floc size.

Influence of the dry solids content on sludgesolubilisation

The dry solids content of the sludge is an important factor

influencing the efficiency of the ultrasonic treatment, as illus-trated in Figure 2. This figure depicts the increase in sCODdue to the ultrasonic treatment, denoted by ΔsCOD (defined

as the difference between sCOD before and after the

Figure 1 | Cumulative particle size distribution (sludge 25.24 g DS/kg sludge).

treatment). It is observed that the sCOD-release increases lin-early with increasing applied energy. The higher the drysolids content, the higher the ΔsCOD for a fixed specific

energy. The observation that the slope of the curve increaseswith increasing DS content, hence indicating that the effec-tiveness of the treatment increases with increasing DScontent, is clear. This is in agreement with previous obser-

vations (e.g. by Dewil et al. ; Khanal et al. ; Zhanget al. ). For a higher DS content, more particles will besubjected to the hydromechanical shear forces and more

nuclei are created for cavitation. It can be assumed that animplosion of a cavitation bubble affects and acceleratesmore adjacent particles in the presence of a higher DS con-

tent. In this case, sludge disintegration will be promoted bythe collision between particles.

Zhang et al. () found a decrease in the release oforganic solids for DS contents in excess of 3.08%. Show

et al. () identified an optimal DS range between 2.3and 3.2%.

When, based on the slopes of the curves depicted in

Figure 2, ΔsCOD is calculated for a fixed specific energy, alinear relationship between ΔsCOD and DS is obtained (asillustrated, for instance in Figure 3 for a specific energy of

1,000 kJ/kg DS).

Solubilisation of sludge components

The solubilisation of different classes of components was

examined for the sludge with a DS content of 23.75 g DS/kgsludge. The release of organic material (COD, carbohydrates

Figure 3 | ΔsCOD for different DS contents of the sludge subjected to a specific energy

(SE) of 1,000 kJ/kg DS.

2287 L. Appels et al. | Effects of ultrasonic pre-treatment on sludge Water Science & Technology | 66.11 | 2012

and proteins) is depicted in Figure 4. For all classes of

components, a linear increase of soluble concentration forincreasing specific energy is observed. The slope of thecurve is the smallest for the carbohydrates. This means

that, relatively speaking, more proteins than carbohydratesare released due to the treatment. This observation indicatesthat the treatment specifically opens the cell membranesof the micro-organisms, hence releasing intracellular

fluid, as more proteins are present in the intracellular waterthan in the extracellular polymeric substances, whichsurround the cell. This is an important observation regarding

the working mechanism of the ultrasonic treatment.Extracellular polymeric substances offer many adsorp-

tion sites for pollutants such as heavy metal ions (Neyens

et al. ). During the ultrasonic pre-treatment, EPS aredegraded and therefore heavy metals are released into thewater phase of the sludge. When the concentration of

these heavy metals exceeds a certain limit, inhibition ofanaerobic digestion may occur. The release from the heavymetals is only a temporary effect, as they are capturedagain during the digestion by the sludge particles. Figure 5

Figure 4 | Concentration of carbohydrates and proteins in the liquid phase of the sludge (23.7

depicts the concentration of Cu, Ni and Zn in the water

phase for varying specific energies.The results follow a logical trend: the more energy that is

supplied to the sludge, themore heavymetals will be released

into the soluble phase. As for Zn and Cu, it seems that athreshold value of the specific energy is required before a sig-nificant amount is released. In this case, a steep increase forboth metals is observed starting from 2,400 kJ/kg DS. The

release of Ni was only limited for the entire range of SEtested. Also, the concentration of Cr remained stable between0.008 and 0.011 mg/L (not included in the figure). The con-

centration of Cd (<0.004 mg/L), Hg (<0.0004 mg/L) andPb (<0.013 mg/L) were below the detection limit of theICP-AES. They were present in the water phase, but not in

the amount that inhibition would occur.Trace amounts are known to stimulate the activity of

bacteria. When the concentration of heavy metals is toohigh, it can induce an inhibitory or toxic effect for certain

enzymes and bacteria. In particular, Cd, Cr, Cu, Hg, Ni,Pb and Zn can disturb the digestion if the concentration istoo high. According to Deublein & Steinhauser (), Cu

(75.6 mg/L) and Zn (167.9 mg/L) can exert inhibition ofthe digestion process. The inhibition can take place if theconcentration of Cu is between 5–300 mg/L and the concen-

tration of Zn between 3–400 mg/L.

Influence of ultrasonic pre-treatment on anaerobicdigestion

In the literature, various studies report a positive impact ofultrasonic pre-treatment on anaerobic digestion (i.e. on the

biogas (methane) production, e.g. Bougrier et al. ()).

5 g DS/kg sludge).

Figure 5 | Concentration of Cu, Ni and Zn for different specific energy.

2288 L. Appels et al. | Effects of ultrasonic pre-treatment on sludge Water Science & Technology | 66.11 | 2012

Two tests were carried out to investigate the effects ofultrasound on the sludge, each with a different DS content.

In Table 1, the specifications of the sludge used in bothexperiments can be found. The samples of sludge used inexperiments A and B were taken at different times at the

same wastewater treatment plant.Figure 6 depicts the evolution of the methane production

for the blank and pre-treated sludges with a DS content of

2.1%. An increase in methane production of 20% wasobserved for the treated sludge (326.53 kJ/kg DS) comparedwith the blank. The sludge subjected to the highest ultrasonic

energy produced the highest amount of methane. The

Table 1 | Specifications of the sludge

Experiment A Experiment B

DS (g DS/kg sludge) 16.13 33.03

ODS (%) 66.86 65.04

DS (500 mL WASþ inoculum)(g DS/kg sludge)

21.44 31.83

COD (mg O2/L) �22,000 �31,000

Carbohydrates (mg/L) �1,800 �2,800

Proteins (mg/L) �21,000 �21,000

Figure 6 | Cumulative methane production during digestion (DS of 2.1%).

differences in methane production between the various trea-

ted samples were, however, relatively small. The treatmentwith 1,967.43 kJ/kg DS only results in 7% extra methane pro-duction compared with the lowest treatment.

In contrast to the results for the sludge with a DS con-tent of 2.1%, different effects were noticed for theultrasonic treatment of the more concentrated sludge(DS¼ 3.2%). As depicted in Figure 7, no significant differ-

ence in biogas production was observed between the blank(untreated) sludge and the various ultrasonically treatedsludges. This finding is unexpected, as in most literature

sources, a positive result is reported. This observation canpossibly be explained by the already high concentration ofreadily digestible organic material in the sludge. During

the digestion, these organics are preferentially degraded,resulting in a direct biogas production. In the meantime,hydrolysis of more rigid structures will take place, leadingto a solubilisation and the further availability for the sub-

sequent digestion stages. This mechanism diminishes theeffects of the pre-treatment. On the other hand, the solubil-isation of organic material by the pre-treatment may have

an inhibiting effect on the anaerobic digestion in the batchtests, hence levelling out the advantages of the treatment.It is clear that this observation and its underlying mechan-

isms need to be cleared in future research.Table 2 presents the degradation of the various classes of

organic compounds during digestion. The values given for

COD, carbohydrates and proteins are the differencesbetween before and after anaerobic digestion (described by‘Δ’). The degradation of COD, carbohydrates and proteinsincreases if more energy is added to the sludge.

The biogas production rate of experiment B is remark-ably slower: after day 5, only 30 mL/g ODS (cf. Figure 7)was produced, compared with 70 mL/g ODS of experiment

A (cf. Figure 6).After 23 days of digestion (Figure 6), a methane pro-

duction of 126 mL/g ODS was noticed. The energy value

of the total methane production was about 1.014 kWh/kgDS. The energetic content of the surplus biogas byultrasonic treatment equals 0.195 kWh/kg DS. Comparing

this value with the required ultrasonic energy input(i.e. 0.127 kWh/kg DS) leads to the conclusion that there isa positive net energy production due to ultrasonic treatment.

CONCLUSION

The present paper studied the application of an ultrasonicpre-treatment prior to anaerobic digestion of waste activated

Figure 7 | Cumulative methane production during digestion (DS¼ 3.2%).

Table 2 | Concentration and degradation of the organic components of untreated and ultrasonically treated sludge for both experiments

Experiment A (2.1%DS)0 (Blank) 326.5 818.0 1967.3

SE (kJ/kg DS) mg/L % mg/L % mg/L % mg/L %

Δ COD (mg O2/L) 7,058.3 32.2 8,358.3 38.1 8,513.9 38.8 8,136.1 37.1

Δ Carbohydrates (mg/L) 419.0 23.2 562.3 31.2 477.5 26.5 620.8 34.4

Δ Proteins (mg/L) 14,402.9 67.8 13,201.5 62.2 15,017.4 70.7 14,034.8 66.1

Experiment B (3.2%DS)

SE (kJ/kg DS) 0 (Blank) 172.5 431.8 1037.9

mg/L % mg/L % mg/L % mg/L %

Δ COD (mg O2/L) 7,708.3 25.0 8,508.3 27.6 10,800.0 35.0 8,483.3 27.5

Δ Carbohydrates (mg/L) 901.6 33.0 812.8 29.8 820.9 30.1 855.3 31.3

Δ Proteins (mg/L) 8,784.6 40.7 9,397.9 43.5 9,004.6 41.7 8983.3 41.6

2289 L. Appels et al. | Effects of ultrasonic pre-treatment on sludge Water Science & Technology | 66.11 | 2012

sludge. It was seen that organic and inorganic compounds

are efficiently solubilised during ultrasonic treatment. Ingeneral, a higher specific energy and a higher dry solids con-tent are beneficial for the release. The efficiency of the

subsequent anaerobic digestion stays about the same fortreated sludge with a higher DS content (3.2%). At lowerDS contents (2.1%), the methane production increased sig-nificantly due to the treatment.

ACKNOWLEDGEMENTS

The authors would like to thank the Agency for Innovationby Science and Technology in Flanders (IWT/063374 andIWT/100196) and the Research Council of the Katholieke

Universiteit Leuven (PDMK/10/121) for the financialsupport. This work was supported in part by Project

KP/09/005 (SCORES4CHEM Knowledge Platform) of the

Industrial Research Council of the Katholieke UniversiteitLeuven, Project PFV/10/002 Center of ExcellenceOPTEC-Optimization in Engineering and the Belgian Pro-

gram on Interuniversity Poles of Attraction initiated by theBelgian Federal Science Policy Office. Jan Van Impe holdsthe chair Safety Engineering sponsored by the Belgianchemistry and life science federation, essenscia.

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First received 25 February 2012; accepted in revised form 1 June 2012