Computational Water, Energy, and Environmental Engineering, 2013, 2, 7-11
doi:10.4236/cweee.2013.23B002 Published Online July 2013 (http://www.scirp.org/journal/cweee)
Effect of Combined Microwave-Ultrasonic Pretreatment
on Anaerobic Biodegradability of Primary, Excess
Activated and Mixed Sludge
Anteneh Mesfin Yeneneh1, Tushar Kanti Sen1, Siewhui Chong1, Ha Ming Ang1, Ahmet Kayaalp2
1Department of Chemical Engineering, Curtin University, GPO Box U1987, Perth 6845, Australia
2Water Corporation of Western Australia, West Leederville, 6007, Australia
[e-mail], Ahmet.
ReceivedApril, 2013
ABSTRACT
This work deals with the effect of combined microwave-ultrasonic pretreatment on the anaerobic biodegradability of
primary, excess activated and mixed sludge. The characteristics, biodegradability and anaerobic digester performance
for untreated primary, excess activated and mixed sludge were compared to combined microwave-ultrasonic pretreated
primary, excess activated and mixed sludge. All sludge samples were subjected to Microwave treatment at 2450 MHz,
800 W and 3 min followed by ultrasonic treatment at a density of 0.4 W/mL, amplitude of 90%, Intensity of 150 W,
pulse of 55/5 for 6min. Methane production in pretreated primary sludge was significantly greater (11.9 ml/g TCOD)
than the methane yield of the untreated primary sludge (7.9 ml/g TCOD ). Cumulative methane production of pretreated
Excess Activated Sludge (EAS) was higher (66.5 ml/g TCOD) than the methane yield from pretreated mixed sludge
(44.1 ml/g TCOD). Furthermore, digested EAS showed significantly higher dewaterability (201 s) than digested pri-
mary sludge (305 s) or mixed sludge (522 s). The average Methane: Carbondioxide ratio from EAS (1.85) was higher
than that for mixed untreated sludge (1.24). VS reduction was also higher for EAS than the other two sludge types.
However, pretreatment of EAS resulted in significant reduction in dewaterability due to higher percentage of fine floc
particles in the pretreated EAS.
Keywords: Biodegradability; Combined Microwave-Ultrasonic; Dewaterability; Primary; Excess Activated Sludge;
Mixed Slu dge
1. Introduction
Anaerobic digestion is a very effective sludge treatment
technology applied in municipal and industrial wastewa-
ter treatment plants to stabilize organic matter [1]. The
process comprises four major microbiological degrada-
tion steps of hydrolysis, acidogenesis, acetogenesis and
methanogenesis. One of the disadvantages of anaerobic
digestion technique is the slow degradation or hydrolysis
of microorganisms that accounts for 70% of excess
sludge which is the primary degradation step in the an-
aerobic digestion process. The microorganisms in the
excess sludge contain Extracellular Polymeric Sub-
stances (EPS) that are resistant to biodegradation which
in turn limits the rate of the whole anaerobic digestion
process [2,3]. Different pretreatment technologies were
found to enhance sludge hydrolysis and anaerobic diges-
tion performance [4]. Pretreatment of sludge through
ultrasonic, mechanical, chemical or thermal techniques
result in bacterial cell wall disruption, disintegration of
EPS and release of enzymes enhance the rate of hydroly-
sis and biodegradation [5,6]. Ultrasonic, micro- wave,
oxidative, and thermal pretreatment techniques are well
documented in literature as viable methods to enhance
biodegradability, h ydrolysis rate and digester peformance
[7]. It was reported that ultrasonic pretreatment results in
disruption of cells and large sized macromolecules by the
hydro-mechanical shear forces produced by ultrasonic
cavitation [8]. Sonication density of 0.5 W/mL and soni-
cation intensity of 4.8 W/cm2 resulted in significant in-
crease in soluble COD and 24.6% increase in VS reduc-
tion [9]. Microwave (MW) irradiation is another efficient
sludge pre-treatment technology that involves high fre-
quency electromagnetic radiation which interacts with
the dipolar molecules in the sludge [6]. Microwave pre-
treatment helps to enhance rate of anaerobic digestion
and dewaterability [10]. Microwave pretreatment in-
creased SCOD up to 4 fold, soluble protein concentration
up to 1.8 fold and soluble carbohydrate concentration up

A. M. YENENEH ET AL. 8
to 14 fold [11]. The application of more than one treat-
ment also resulted in improved sludge biodegradation,
floc destruction, cell wall disruption and release of or-
ganics due to the complementary synergy between the
treatment techniques that are combined [12-14]. Micro-
wave enhanced-oxidative pretreatment with H2O2 re-
sulted in 11% - 34% TS, TCOD reduction and total bio-
polymer solubilisation [15]. Combined ultrasonic-alkali
pretreatment of waste activated sludge resulted in 60%
VS solubilisation. The use of NaOH weakens the cells
walls increasing the disintegration effect of ultrasonica-
tion or other lysis techniques [5]. Very few researchers
have reported that the microwave combined with ultra-
sonic would be a rapid and economical method of sludge
pre-treatment for enhanced biogas production. Combined
microwave-ultrasonic pretreatment resulted in significant
improvement in gas production, solid removal and de-
waterability of municipal sludge compared to the indi-
vidual ultrasonic or microwave pretreatment approaches
[16]. Primary sludge, excess activated sludge and mixed
sludge have distinctively different biochemical composi-
tion, rheological property, response to pretreatment, bio-
degradability and methane potential, floc size and de-
waterability. Studying effect of pretreatment technolo-
gies and biodegradability of each of the sludge types is
beneficial for the selection of appropriate pretreatment
technology and pretreatment condition, better d esign and
operation of sludge treatment units [17]. This research
aims at understanding the effect of combined microwave-
ultrasonic pretreatment on biodegradability, methane
potential, dewaterability and characteristics of primary,
excess activated and mixed sludge systems.
2. Materials and Methods
2.1. Sampling and Characterization
Primary sludge was collected from primary gallery un-
derflow lines particularly from primary sedimentation
tank No. 4 of Beenyup Waste Water Treatment Plant
(BWWTP), Perth, Western Australia. Mixed sludge was
collected from Beenyup anaerobic digesters feed mixed
sludge sampling point and the Excess Activated Sludge
(EAS) was collected from Module 4 of the secondary
treatment section of BWWTP. Primary and Excess acti-
vated sludge samples were mixed with 75:25 ratio to
prepare the mixed sludge before all the samples were
charged to the jacketed digesters. Samples were with-
drawn from each anaerobic digester for characterization
purpose. The characteristics of sludge fed to the digesters
are presented in Table 1.
2.2. Analytical Methods
Total solids (TS), Volatile solids (VS), pH, conductivity,
chemical oxygen demand and dewaterability were meas-
Table 1. Characteristics of the sludge fed to the reactors.
Parameter TS (%)VS (% TS) COD (g/l)pH
Raw primary sludge2 88.8 30.5 7.2
Primary pretreated
sludge 2 88.8 32.8 7.1
Excess activated
sludge 1 90 18.9 6.9
Pretreated thickened
excess activated
sludge 2.7 83 39.6 7
Untreated Mixed
Sludge 1.5 87.5 22.9 7.1
Mixed Pretreated
Sludge 1.5 87.5 24.9 7.1
ured and analysed for all sludge samples. Chemical oxy-
gen demand (COD), Total solid s (TS) and volatile solids
(VS) of the feed and digested sludge were determined
according to the standard methods [18]. Methane, carbon
dioxide and oxyg en content of the biogas was determined
by Gas analyser (Thermo Fisher SCIENTIFIC GA 2000
plus). pH and conductivity/total dissolved solids for the
feed and digested sludge were measured using WP-90
and WP-81 conductivity/TDS-pH/temperature meter.
The dewaterability (filterability) of each of the digested
sludge samples was measured using capillary suction
timer (Type 304 CST equipment).
2.3. Combined Microwave-Ultrasonic
Pretreatment
Primary excess activated and mixed sludge samples were
pretreated according to th e conditions shown in Table 2 .
Initially each of the sludge samples was homogenized
and pretreatment was carried out in the sequence of mi-
crowave treatment first followed by ultrasonic pretreat-
ment at the condition s specified in Tab le 2. Pretreatment
conditions were selected based on the treatment power
and time optimization tests carried out earlier.
2.4. Experimental Setup for Methane Potential
and Sludge Biodegradability Tests
The tests for methane potential were conducted in batch
continuously stirred 1 L jacketed digesters. All the di-
gesters were kept at a mesophilic temperature of 36.5℃
by means of a water bath heater. 50 ml of digested sludge
was introduced to each of the digesters for acclimation.
The digesters were inoculated with the digested sludge
for a period of 3 days and sludge feeding to the reactors
was carried out after adjusting the pH and purging the
digesters with nitrogen gas. The effective digester vol-
ume was 500 ml for each of the reactors after charging
the feed sludge. The pH was maintained between 6.8 -
7.3 using sodium hydroxide and hydrochloric acid. The
biogas generated was allowed to pass through buffer
tanks to remove any condensate before the gas volume

A. M. YENENEH ET AL. 9
Table 2. Different conditions of pre-treatment.
Pre-treatment
method Conditions
Microwave-
ultrasonic treatment
(MU)
Microwave: 2450 MHz, 800 W, 3 min,
Ultrasonic: 0.4 W/mL, 48,000 Joules, 90%
amplitude, 55/5 pulse , 6 min
was measured in inverted cylinders by water displace-
ment technique. The biogas composition and other pa-
rameters were continuously monitored until biogas gen-
eration reached SRT of 25 days.
3. Result and Discussion
3.1. Methane Potential of Different Kinds of
Sludge
Methane production in pretreated primary sludge (11.9
ml/g TCOD) was 33.6% greater than the methane yield
of the untreated primary sludge (7.9 ml/g TCOD) as
shown in Figure 1. SCOD/TCOD ratio for pretreated
primary sludge was 48% less than the ratio for untreated
primary sludge as it is consumed due to increased or-
ganic disintegration and methanogenic activity in the
anaerobic digestion process. In case of untreated primary
sludge, the biopolymers and organics are dominantly
present in the solid phase than in the soluble liquid phase.
Pretreatment enhances destruction of complex floc
structure of secondary sludge and biopolymers in pri-
mary sludge and promotes the transfer of organics to the
solu- ble phase [19]. Specific methane yield of pretreated
mixed sludge was 12.6% greater than untreated mixed
sludge as shown in Figure 2. Excess Activated Sludge
(EAS) showed less methane production (20.7 ml/g
TCOD) as compared Pretreated Excess Activated Sludge
(PEAS) (66.5 ml/g TCOD) as shown in Figure 3. The
thickening process in the dissolved air floatation tank
(DAFT) has significantly increased the solid concentra-
tion and the pretreatment further enhanced the methane
production an d the kinet i cs of the digest i o n p roces s.
3.2. Effect of Pretreatment on the Dewaterability
of Different Kinds of Sludge
The dewaterability of Excess activated sludge was sig-
nificantly better than primary or mixed slud ge as th e total
solid in EAS was less than the other two sludge types
Figure 4. However, combined microwave-ultrasonic pre-
treatment resulted in the de terioration of the dewaterabil-
ity of excess activated or slight improvement in case of
mixed sludge. Dewaterability is a function of particle
size of the flocs and the hydrophilicty of biopolymers
released due to the disintegration of microbial cells. Pre-
treatment decreases average size of flocs and increases
release of biopolymers which may trap water and limit
the dewaterability. The change in floc structure and col-
Figure 1. Specific methane yield from pretreated and un-
treated primary sludge.
Figure 2. Specific methane yield from untreated and pre-
treated mixed sludge.
Figure 3. Specific methane yield from untreated and pre-
treated excess activated sludge.
Figure 4. Dewaterability of different sludge samples.

A. M. YENENEH ET AL. 10
loidon
3.3. Effect of Pretreatment on Biogas
Th e ma4 21.85 and pre-
ve-ultrasonic pretreatment improve
al charge may have also contributed to the reducti
in dewaterability. Ultrasonication is known to have ef-
fects of changing the surface charge.
Composition and CH4/CO2 Ratio
xi mum C H/CO ratio for EAS was
treatment enhanced the quality of the biogas by 7.5%.
The CH4/CO2 ratio for mixed untreated sludge was 1.24
and the enhancement in gas quality due to combined mi-
crowave-ultrasonic pretreatment was 18.9 %. The effect
of pretreatment on biogas quality was much greater in
mixed sludge system than excess activated sludge. In the
initial phase of the digestion process, the CH4/CO2 ratio
was relatively lower for all sludge types; it progressively
increased due to the conversion of CO2 to CH4 through
hydrogenotrophic methanogenesis reaching the maxi-
mum CH4/CO2 ratio after 25 days of SRT as shown in
Figure 5.
4. Conclusions
Combined microwad
sludge solubilisation, biogas production and anaerobic
digester performance and biodegradability of primary,
EAS and mixed sludge. This Combined pretreatment
technique disintegrates the complex floc structure of
EAS and macromolecules in primary sludge. The degree
of sludge solubilisation after pretreatment for different
sludge types was different. The combined Pretreatment
resulted in comparatively greater improvement of meth-
ane production and biogas quality (CH4/CO2 ratio) and
VS destruction in EAS. The increase in digestion effi-
ciency is proportional to the degree of sludge disintegra-
tion. Sludge disintegration and increased biodegradabil-
ity and methane production is due to rapid internal heat-
ing of microwave radiation and the floc destruction achi-
CH
4
/CO
2
ratio
eved by ultrasonic treatment. EAS also showed better
dewaterability compared to other sludge types. How-
ever, dewaterability deteriorated with pretreatment is due
to higher percentage of fines and greater availability of
biopolymers which increased the amount of bound water.
Generally, combined ultrasonic and microwave treatment
at the optimum cond itions will play great role in reducing
sludge treatment handling and disposal expenses and
enhances efficiency and profitability.
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