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Anaerobic digestion of waste activated sludgepretreated by a combined ultrasound and chemicalprocessBunrith Seng a , Samir Kumar Khanal b & Chettiyappan Visvanathan ca Interdisciplinary Graduate School of Medicine and Engineering , University of Yamanashi ,Kofu, Japanb Department of Molecular Biosciences and Bioengineering , University of Hawai’i atMänoa , Honolulu, HI, 96822, USAc Environmental Engineering and Management , Asian Institute of Technology , Pathumthani12120, ThailandPublished online: 24 Feb 2010.

To cite this article: Bunrith Seng , Samir Kumar Khanal & Chettiyappan Visvanathan (2010) Anaerobic digestion of wasteactivated sludge pretreated by a combined ultrasound and chemical process, Environmental Technology, 31:3, 257-265, DOI:10.1080/09593330903453236

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Environmental Technology

Vol. 31, No. 3, March 2010, 257–265

ISSN 0959-3330 print/ISSN 1479-487X online© 2010 Taylor & FrancisDOI: 10.1080/09593330903453236http://www.informaworld.com

Anaerobic digestion of waste activated sludge pretreated by a combined ultrasound and chemical process

Bunrith Seng

a

*, Samir Kumar Khanal

b

and Chettiyappan Visvanathan

c

a

Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, Kofu, Japan;

b

Department of Molecular Biosciences and Bioengineering, University of Hawai’i at M

[amacr ]

änoa, Honolulu, HI 96822, USA;

c

Environmental Engineering and Management, Asian Institute of Technology, Pathumthani 12120, Thailand

Taylor and Francis

(

Received 7 June 2009; Accepted 29 October 2009

)

10.1080/09593330903453236

Waste activated sludge (WAS) requires a long digestion time because of a rate-limiting hydrolysis step – the firstphase of anaerobic digestion. Pretreatment of WAS facilitates the hydrolysis step and improves the digestibility.This study examined the effects of ultrasonic, chemical, and combined chemical–ultrasonic pretreatments on WASdisintegration and its subsequent digestion at different solids retention times (SRTs). The efficient conditions foreach pretreatment were evaluated based on per cent soluble chemical oxygen demand (%SCOD). The resultsshowed that the combined chemical–ultrasonic pretreatment resulted in better WAS disintegration, based on%SCOD release, compared with individual chemical and ultrasonic pretreatments. At the optimum operatingconditions of the combined chemical–ultrasonic pretreatment (NaOH dose of 10 mg g

−

1

TS (total solids) andspecific energy input of 3.8 kJ g

−

1

TS), the %SCOD release was 18.1%

±

0.5%, whereas 13.5%

±

0.9%, 13.0%

±

0.5% and 1.1%

±

0.1% corresponded to individual chemical (50 mg g

−

1

TS) and ultrasonic (3.8 kJ g

−

1

TS)pretreatments and control (without pretreatment), respectively. The anaerobic digestion studies in continuousstirred tank reactors showed an increase in methane production of 23.4%

±

1.3% and 31.1

±

1.2% for digesters fedwith WAS pretreated with ultrasonic and combined chemical–ultrasonic, respectively, with respect to controls atthe effective SRT of 15 days. The highest total solids removal was achieved in the digester fed with ultrasonicpretreated WAS (16.6%

±

0.3%), whereas the highest volatile solids removal was achieved from the digester fedwith combined chemical–ultrasonic pretreated WAS (24.8

±

0.4%). The findings from this study are a usefulcontribution to new pretreatment techniques in the field of sludge treatment technology through anaerobicdigestion.

Keywords:

anaerobic digestion; sludge disintegration; pretreatment; waste activated sludge; ultrasound

Introduction

Aerobic biological wastewater treatment plantsgenerate a significant amount of biological sludgeknown as waste activated sludge (WAS). The treatmentand disposal of WAS accounts for nearly 60% of thetotal wastewater treatment plant (WWTP) operatingcosts [1]. The WAS is rich in organic content and has asignificant potential for bioenergy production throughanaerobic digestion. It can reduce the sludge volume by40–50% with simultaneous recovery of biogas as arenewable energy [2]. The biological cells in WAS,however, are slowly digestible because of the rate-limiting hydrolysis. As a result, a long solids retentiontime (SRT) of 30 to 60 days is needed for effectivedigestion [3]. Various pretreatment options such asphysical, chemical and biological methods have beenexamined to improve digestibility so as to shorten therequired SRT and enhance the biogas production rate.

Alkaline pretreatment using sodium hydroxide(NaOH) was found to be effective for sludge disinte-gration [2,4]. Alkaline dosage is directly related to pH.Kim

et al

. [5] carried out alkaline pretreatment ofWAS in the pH range of 9–12 using 2 M NaOH solu-tion at a constant holding time. The results showed thata pH of 11 was critical for effective sludge disintegra-tion. Lin

et al

. [6] found that the released solublechemical oxygen demand (SCOD) increased from 830to 1190 mg L

−

1

(the SCOD of untreated samples was70 mg L

−

1

), when the NaOH dosage was increasedfrom 20 to 40 meq L

−

1

. In another study, Li

et al

. [4]reported that a chemical dosage of 0.05 mol L

−

1

waseffective for cell disruption.

Ultrasonic pretreatment has been widely studied forWAS disintegration [7–14]. The efficiency of sonica-tion depends on many factors such as sonication time,ultrasonic frequency, energy input, total solids, and the

*Corresponding author. Email: g08dea02@yamanashi.ac.jp

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type of ultrasound system among others [3]. Energyinput has been a major concern with ultrasound technol-ogy. A mild chemical pretreatment with acid or alkaliprior to ultrasound pretreatment may weaken the cellwalls, and thus has a potential to enhance cell disinte-gration at a lower energy input.

Studies on the effects of chemical pretreatment onultrasonic disintegration of WAS and subsequentdigestion of pretreated WAS are very limited. Chiu

et al

. [7] investigated WAS solubilization with a NaOHdose of 40 meq L

−

1

for 24 h followed by sonication for24 s mL

−

1

(288 kJ g

−

1

total solids (TS)). The presentstudy was carried out to compare the WAS disintegra-tion under different pretreatment conditions, e.g. ultra-sound, chemical and combined chemical–ultrasound,and to examine the subsequent digestibility ofpretreated WAS at different SRTs.

Materials and methods

Waste activated sludge

Waste activated sludge samples were collected from thesludge conditioning tank (prior to polymer and limeaddition) at a local municipal WWTP in Bangkok, Thai-land. The sludge was shipped immediately and stored ina temperature-controlled room at 4

°

C prior to use. Thecollected sludge had a TS content of around 1%, whichwas increased to 3% through centrifugation (HettichZentrifugen 320R, D-78532, Tuttlingen, Germany) at4500 rpm. The important characteristics of the WAS areshown in Table 1.

Sludge disintegration evaluation

Ultrasonic, chemical and combined chemical–ultrasonic pretreatments disintegrate biological cells,including organic debris and extracellular polymericsubstances (EPS), which contribute to SCOD. The%SCOD release, therefore, was considered as aparameter to evaluate the sludge disintegrationefficiency in this study. The %SCOD is defined asfollows:

where %SCOD

soluble

is per cent soluble COD (%),SCOD

after pre

is soluble COD measured after pretreat-ment (mg L

−

1

), and TCOD is total COD measured afterpretreatment (mg L

−

1

).

Ultrasonic pretreatment

A bench-scale ultrasonic unit (VC750 model,Newtown, CT, USA) was used for sludge disintegra-tion. The maximum power output was 750 W with anindependent amplitude of 20–100%. Its frequency wasconstant at 20 kHz. The horn diameter was 3.8 cm andthe horn was immersed 2 cm into the sludge duringsonication. A steel sonication chamber with a totalvolume of 600 mL was used for sludge disintegration asshown in Figure 1.

Figure 1. Ultrasonic system for waste activated sludge disintegration.

One hundred millilitres of WAS were sonicated atthe power input of 50, 100, 150 and 190 W for a dura-tion of 30, 60 and 120 s under each power level. TheSCOD of each run was measured and used to evaluatethe effective sonication conditions. The sonication testswere carried out at ambient temperature (25

±

2

°

C)with no temperature control.

Chemical pretreatment

An alkaline pretreatment was carried out at an ambienttemperature with NaOH (commercial grade). Threehundred and fifty millilitres of WAS were placed into abatch-mixed container together with NaOH doses of 10,25, 50 and 75 mg g

−

1

TS. The experiment was conductedat different treatment durations of 5, 10, 30 and 60 min.The pH and SCOD of the pretreated WAS were measuredat each run (three runs for each treatment time).

Combined chemical–ultrasonic pretreatment

Combined chemical–ultrasonic pretreatment is thecombination of chemical treatment followed by

%.

SCODSCOD

TCODxso le

after prelub = 100

Table 1. Characteristics of raw sludge and centrifuged sludge.

Parameters Raw sludge Centrifuged sludge

TS (%) 1.01

±

0.08 3.00

±

0.10VS (%) 0.84

±

0.07 2.62

±

0.01TCOD (mg L

−

1

) 11,700

±

402 42,196

±

875SCOD (mg L

−

1

) 77

±

14 458

±

35TKN (mg-N L

−

1

) 814

±

104 3,136

±

150NH

3

(mg-N L

−

1

) 26

±

03 245

±

10pH 6.67

±

0.20 6.97

±

0.24

Mean

±

standard deviation of a minimum of three samples.

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sonication. The chemical–ultrasonic pretreatment wascarried out based on the findings of individual experi-ments for ultrasonic and chemical pretreatments. Theoptimum pH found (pH 11, corresponding to NaOHdosage of 50 mg g

−

1

TS) was high, which may affect themethanogenic bacteria activities during digestion. Thus,NaOH doses of 10, 15, 20 and 25 mg g

−

1

TS wereinvestigated at a constant treatment time of 6 min(based on the effective duration found from chemicalpretreatments) and an ultrasonic power supply of 190 W(based on the effective power input found fromultrasonic pretreatments) for a duration of 30, 60 and120 s.

Biochemical methane potential

A biochemical methane potential (BMP) test wasconducted for the combined chemical–ultrasoundpretreated WAS prior to semi-continuous digestionstudies. The test was conducted to select a suitablechemical dose of NaOH so as to avoid toxicity ofsodium ions to methanogenic bacteria. Therefore,pretreated WAS with a NaOH dose of 10, 15, 20 and25 mg g

−

1

TS at a constant specific energy input of3.8 kJ g

−

1

TS (based on the effective specific energyinput found from individual ultrasonic pretreatments)were used for this experiment. A series of 120 mL

serum bottles with butyl rubber stoppers was used forthe BMP test. The total working volume was 30 mL,which consisted of 5 mL of inoculum (anaerobicdigested sewage sludge from a local full-scaleanaerobic digester) and 25 mL of substrate (pretreatedWAS). Before purging nitrogen gas to remove oxygenfrom the headspace, sodium bicarbonate (NaHCO

3

) wasadded, which as a result contributed alkalinity up to4000 mg L

−

1

as CaCO

3

. The BMP test was conducted at37

±

1

°

C, and the produced biogas was removed dailyusing a 25 mL syringe. Blank (inoculum alone) andcontrol (non-pretreated WAS alone) digesters were setup for each series of experiments to compute the netbiogas production.

Semi-continuous digestion studies

Figure 2 shows the experimental set-up of the one-phaseanaerobic digester. The anaerobic digestion studieswere conducted in three continuous stirred tank reactors(CSTRs) at a mesophilic temperature (37

±

1

°

C); onewas fed with non-pretreated WAS (control), one was fedwith ultrasonic pretreated WAS (at an effectivecondition of ultrasonic pretreatment) and another wasfed with combined chemical–ultrasonic pretreated WAS(at the optimum operating condition found from theBMP test). Each digester had a total volume of 5.6 Lwith a working volume of 3 L. The CSTRs were startedup by feeding anaerobically digested sewage sludge,collected from a local full-scale anaerobic digester, asseed inoculums. The CSTRs were then gradually fedwith WAS until a full design loading rate was achieved.The digesters were operated at three different SRTs of25, 15 and 10 days in semi-continuous mode. Feedingand withdrawal (120, 200 and 300 mL d

−

1

at the respec-tive SRTs of 25, 15 and 10 days) were carried out manu-ally twice a day. Biogas was collected in a 3 L samplingbiogas bag (SKC sampling bag, SKC Gulf Coast Inc.,Houston, Texas, USA), and biogas was removed with a100 cc glass cylinder syringe.

Figure 2. Experimental set-up for anaerobic digesters.

Analytical procedure

Total solids and volatile solids (VS) were determined asper

Standard Methods

[15]. For SCOD determination,the sludge sample was centrifuged for 30 min at 5000rpm, followed by filtration through a 0.45

µ

mmembrane filter. The filtrate was collected and analysedfor SCOD according to

Standard Methods

[15].Alkalinity was also measured following the procedureoutlined in

Standard Methods

[15] using a pH meter.The biogas composition was measured by a gaschromatograph equipped with a thermal conductivitydetector using helium as a carrier gas (ShimadzuGC14A, column SUS, WG-100). After 15 min of

Figure 1. Ultrasonic system for waste activated sludgedisintegration.

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sample injection, the percentage concentrations of fivecomponents (H

2

, CO

2

, O

2

, N

2

and CH

4

) were derived.

Results and discussion

Optimization of ultrasonic pretreatment

The efficacy of sludge disintegration was investigatedbased on SCOD release at a power input of 50, 100, 150and 190 W and a duration of 30, 60 and 120 s. Withsuch short sonication times, the temperature of thepretreated WAS was not increased significantly. Hence,temperature change of pretreated WAS is not addressedin this discussion. The SCOD release per unit weight ofTS at different specific energy inputs is presented inFigure 3a.

Figure 3. SCOD release of pretreated WAS: (a) ultrasonic pretreated WAS, (b) chemical pretreated WAS, (c) chemical–ultrasonic pretreated WAS.

The results showed that the increase in SCOD releasewas in direct proportion to the specific energy input. TheSCOD release per unit weight of TS increased ratherslowly at a specific energy input lower than 1 kJ g

−

1

TS,but increased significantly at a higher energy input. Thislow SCOD release was due to a low energy input, whichwas insufficient to disrupt the cell membranes. Bougrier

et al

. [11] reported that the minimum specific energyinput of 1 kJ g

−

1

TS is needed to break the microbial cells.Of the four power levels tested, the highest power inputof 190 W, with a specific energy input range of 2–3.8 kJg

−

1

TS, was found to contribute significant SCOD release(%SCOD was 13.1%

±

0.5% at 3.8 kJ g

−

1

TS). Foreconomic considerations, the highest SCOD release at

the lowest specific energy input should be adopted forsludge disintegration. Therefore, sonicating at a specificenergy input of 3.8 kJ g

−

1

TS with a power level of190 W was considered as the best operating condition foreffective sludge disintegration. Dewil

et al

. [16] foundthe minimum energy requirement to be 1.5 kJ g

−

1

TS fordisintegrating microbial cells. Khanal

et al

. [13],however, found an optimum specific energy input of 35kWs g−1 TS for effective disintegration, which wassignificantly higher than many reported values. It isimportant to point out that different ultrasound systemshave different efficiencies [3].

Optimization of chemical pretreatment

The disintegration of WAS using NaOH was carried outwith four different doses and four different holdingtimes at an ambient temperature of 25 ± 2 °C. Theresults indicated that the SCOD of pretreated WASincreased with an increase in chemical doses. For allchemical doses, the SCOD release steadily increasedwithin the 6 min of holding time. The increase inSCOD release slowed down when the holding time waslonger, as shown in Figure 3b. At alkali dosages of 10and 25 mg g−1 TS, the %SCOD release improvedslightly, whereas it nearly doubled at dosages of 50 and75 mg g−1 TS at all holding times. The %SCOD at50 mg g−1 TS, however, was 13.5% ± 0.9%, whichwas significantly higher than that at 25 mg g−1 TS (4.7%

Figure 2. Experimental set-up for anaerobic digesters.

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Figure 3. SCOD release of pretreated WAS: (a) ultrasonic pretreated WAS, (b) chemical pretreated WAS, (c) chemical–ultrasonic pretreated WAS.

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± 0.3%) and slightly lower than that at 75 mg g−1 TS(17.6% ± 0.4%) during the 5 min holding time. Theincreasing trends remained fairly constant at otherpretreatment durations of 10, 30 and 60 min. The slowlyincreasing trend after 10 min was not expected tochange with longer holding times because of the factthat the initial rapid stage of alkaline treatment occurswithin 30 min [4,5]. Kim et al. [2] reported up to 43.5%COD solubilization at a chemical dose of 7 g L−1 NaOH(or 180 mg g−1 TS). Similarly, Li et al. [4] found anoptimum chemical dose of 0.05 mol L−1 (160 mg g−1

DS), which is three-fold higher than that found in thisstudy. The reason for such discrepancy could be due tothe different sludge characteristics such as the TScontent and pretreatment duration, as the current studyemployed a 3% TS content with 5 min of pretreatment.In contrast, Li et al. [4] used only 2% TS with 30 minof pretreatment duration.

In this study, a chemical dose of 50 mg g−1 TS corre-sponding to a pH of around 11 was considered to be thebest operating condition for effective sludge disintegra-tion. The optimum pH of 11 was also reported by Kimet al. [5].

Optimization of combined chemical–ultrasonic pretreatment

The results from combined chemical–ultrasonicpretreatment indicated that the SCOD release increasedwith an increase in chemical dose and sonication dura-tion. At all sonication durations, the %SCOD releasesof pretreated sludge for all chemical doses were compa-rable. As is apparent from Figure 3c, 10 mg g−1 TS wasthe optimal chemical dose for a combined chemical–ultrasonic pretreatment. At this dosage, the %SCODrelease was significant compared with non-pretreatedsludge and was relatively comparable to high doses of15, 20 and 25 mg g−1 TS.

The %SCOD release of pretreated sludge graduallyincreased even at very low specific energy inputs. Thisis due to the usage of chemical, which weakened themicrobial cell wall and enhanced the disruption ofthe microbial cells during sonication. At 10 mg g−1 TS,the %SCOD release significantly increased from 1.1%± 0.1% (non-treated WAS) to 18.1% ± 0.5% at aspecific energy input of 3.8 kJ g−1 TS, and then theincreasing trend slowed down with a maximum releaseof 24.1% at a nearly two-fold input of specific energy(7.3 kJ g−1 TS). The %SCOD release found in this studywas significantly lower than that reported by Chiu et al.[7]. These authors reported a SCOD release of 89.3% ata chemical dose of 40 meq L−1 NaOH (160 mg g−1 TS)during 24 h and sonication at a specific energy input of288 kJ g−1 TS. The higher SCOD release in the abovestudy is primarily attributed to a difference in operating

conditions, e.g. sludge characteristics, chemical dosage,power input and pretreatment duration. Longer pretreat-ment duration with NaOH enhances the disruption ofthe microbial cell membrane and facilitates better disin-tegration during long sonication. At a longer sonicationtime, the collapse of microbubbles, generated throughcavitation, produces extremely high heat which contrib-utes significantly to higher COD solubilization as aresult of thermal effects.

Batch anaerobic digestion of combined chemical–ultrasonic pretreated WAS

A biochemical methane potential (BMP) test wascarried out to select an effective chemical dose for thesemi-continuous digestion studies without causingsodium ion inhibition. Figure 4 shows the cumulativemethane production under different digestion times andchemical doses. After five days of digestion, the meth-ane production from WAS pretreated with chemicaldosages of 10, 15 and 25 mg g−1 TS progressivelyincreased in comparison with non-pretreated WAS.This was due to enhanced hydrolysis of the pretreatedWAS, which was facilitated by the chemical–ultrasonicpretreatment. Hence, high methane production waspossible even at a short digestion time. Although allchemical doses resulted in high methane production, themethane production at a chemical dose of 20 mg g−1 TSwas lower than that for non-pretreated sludge owing toloss of some biogas from the serum bottle.Figure 4. Cumulative methane production of BMP test at different chemical doses along with digestion time.The removal of TS and VS was also investigated foreach run. The per cent removal was calculated based onthe difference between the initial and final TS or VSconcentration compared with the initial TS or VSconcentration before fermentation. The TS removalincreased from 12.5% (control digester) to 17% with achemical dose of 15 mg g−1 TS and then continuedincreasing to around 18% when the chemical doseincreased to 25 mg g−1 TS. The results revealed that theTS removal considerably increased at chemical dose of15 mg g−1 TS, but slightly increased at chemical dosehigher than 15 mg g−1 TS. The removal efficiency wasexpected not to increase further when higher chemicaldoses (higher than 25 mg g−1 TS) were used. This lowTS removal efficiency at a higher chemical dose was dueto the contribution of dissolved solids by NaOH addition.The VS removal increased from 25.6% (control digester)to 28.8% with a chemical dose of 10 mg g−1 TS and thencontinued increasing to a maximum of 33.8% with achemical dose of 25 mg g−1 TS. The VS removal ofpretreated WAS was similar for all chemical doses.

Cumulative methane yield was found to decreasewith an increase in NaOH dosage. With a chemicaldose of 10 mg g−1 TS, cumulative methane yield was0.51 m3 CH4 kg−1 VSremoved, and it was down to 0.43 m3

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CH4 kg−1 VSremoved at chemical dose of 25 mg g−1 TS.This is due to the fact that the Na ion inhibits methano-gens during digestion [17]. Although the combinedchemical–ultrasonic pretreatment with 10 mg g−1 TS(pH = 7.03) gave slightly lower TS and VS removalsthan the 15 mg g−1 TS pretreatment, the methaneproduction was comparable to other chemical doseswith relatively high cumulative methane yield. Thus, achemical dose of 10 mg g−1 TS was selected for subse-quent semi-continuous studies.

Semi-continuous anaerobic digester performances

A series of CSTRs, namely control (digester fed withnon-pretreated WAS), Du (digester fed with ultrasonicpretreated WAS) and Dcu (digester fed with combinedchemical–ultrasonic pretreated WAS), were operated atthree different SRTs under a semi-continuous feedingmode. All CSTRs were initially operated at an SRT of25 days for 54 days to reach quasi-steady state, andthe SRTs were then lowered to 15 and 10 days follow-ing 57 and 40 days of operation, respectively. Table 2

Figure 4. Cumulative methane production of BMP test at different chemical doses along with digestion time.

Table 2. Total solids (TS) and volatile solids (VS) removals, and methane gas properties.

TS removed (%)

VS removed (%)

Methane production (mL d−1)

Gas yield (m3 CH4 kg−1 VSremoved)

Methane content (%)

SRT 25 days:Control 3.6 ± 0.19 19.6 ± 0.42 358.4 ± 04 0.56 ± 0.01 58.0 ± 0.34Ultrasonic 12.5 ± 0.38 22.2 ± 0.17 404.4 ± 09 0.56 ± 0.02 57.2 ± 0.85Chemical–ultrasonic 2.4 ± 0.02 24.8 ± 0.20 420.4 ± 10 0.52 ± 0.03 58.2 ± 0.57SRT 15 days:Control 10.9 ± 0.62 17.8 ± 0.81 477.6 ± 08 0.52 ± 0.03 58.5 ± 0.55Ultrasonic 16.6 ± 0.29 21.5 ± 0.34 589.2 ± 06 0.54 ± 0.01 58.0 ± 0.19Chemical–ultrasonic 7.7 ± 0.69 24.8 ± 0.37 626.3 ± 06 0.49 ± 0.01 58.1 ± 0.66SRT 10 days:Control 7.4 ± 0.28 17.5 ± 2.19 485.5 ± 12 0.35 ± 0.05 58.9 ± 0.64Ultrasonic 16.3 ± 0.54 19.7 ± 1.29 635.7 ± 10 0.40 ± 0.03 58.8 ± 0.80Chemical–ultrasonic 8.6 ± 0.41 22.1 ± 1.56 688.4 ± 22 0.39 ± 0.02 59.2 ± 0.46

Mean ± standard deviation of a minimum of three samples.

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summarizes the overall performance of the CSTRs ateach SRT.

At all SRTs, TS removal was in the range 3.6–10.9% for the control digester, 12.5–16.6% for Du and2.4–8.6% for Dcu. The results showed significantlyhigher TS removal from Du than the control digester,whereas there was a slight or no improvement from Dcu(Table 2). At the longest SRT, i.e. 25 days, the TSremoval efficiency for Du was relatively low (12.5% ±0.4%), but reached a maximum value of 16.6% ± 0.3%at the SRT of 15 days (the removal efficiency was52.5% ± 6.2% higher than that of the control digester).This indicates that ultrasound is an effective techniquefor breaking down the microbial cells or particulatematter. Thus, it facilitated the digestion of WAS. Thelow TS removal for chemical–ultrasound pretreatedWAS was due to the addition of NaOH, which contrib-uted to the TS content.

The VS removal was in the range 7.5–19.6% for thecontrol digester, 19.7–22.2% for Du, and 22.1–24.8%for Dcu. The VS removal for Dcu was significant,among all the digesters, at each SRT (one way ANOVAat p < 0.05). The VS removal improved by 26.6% ±3.7%, 39.2% ± 4.7% and 31.4% ± 9.6% compared withthe control at the SRT of 25, 15 and 10 days, respec-tively, whereas, for Du, the respective improvementswere 13.3% ± 1.5%, 20.7% ± 3.% and 16.% ± 8.2%compared with the control. This finding apparentlyshows that the combined chemical–ultrasonic pretreat-ment was more effective than the other pretreatmentsfor stabilization of WAS at all SRTs.

In terms of methane content in the biogas, there wasno significant difference between the digesters at allthree SRTs. Methane production of Du, however,increased by 12.9% ± 2.8%, 23.4% ± 1.3% and 31.3%± 3.4%, compared with the control, at SRTs of 25, 15and 10 days, respectively, whereas for Dcu theincreases were 17.3% ± 3.9%, 31.1% ± 1.2% and 42.1%± 3.9%, at the respective SRTs, with respect to thecontrols. By comparing the methane production at thethree SRTs, methane production at 15 days SRT signif-icantly increased compared with that of 25 days SRT.However, there was a very small improvement when theSRT was shortened to 10 days, e.g. at 25 days SRT,methane production of Du was 404.4 mL d−1 andincreased to 589.2 and 635.7 mL d−1 at 15 and 10 daysSRTs, respectively. With regards to methane yield,there was almost no difference between the digesters ateach SRT; it increased with an increase in the SRT.Digester Du, however, gave a slightly better yield thanthe other two digesters, at each SRT: at 25 days SRT, itwas 0.56 m3 CH4 kg−1-VSremoved; at 15 days SRT, it was0.54 m3 CH4 kg−1 VSremoved; and, at 10 days SRT, it was0.40 m3 CH4 kg−1 VSremoved. According to the resultsfound in this study, ultrasonic pretreatment was an

effective pretreatment for subsequent anaerobic diges-tion, in term of biogas yield and TS removal. In addi-tion, an SRT of 15 days is the optimum operatingcondition for anaerobic digestion of both the ultrasonicpretreated WAS and the chemical–ultrasonic pretreatedWAS, in terms of methane production.

Conclusions

Of the three pretreatment options for sludge disintegra-tion, the combined chemical–ultrasonic pretreatmentgave better results compared with individual chemicaland ultrasonic pretreatments. At a relatively low NaOHdose (10 mg g−1 TS) and low energy input (3.8 kJ g−1

TS), the combined chemical–ultrasonic pretreatmentgave a %SCOD release of 18.1% ± 0.5%, whereas thecorresponding values for the chemical and ultrasonicpretreatments were 13.5% ± 0.9% and 13.0% ± 0.5%,respectively, under the best operating conditions.

At all three SRTs investigated, an SRT of 15 dayswas the most effective digestion time for anaerobicdigestion with pretreated WAS. It provided an optimaloperating condition with high output products in term ofbiogas production, and TS and VS removal.

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