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Synergetic pretreatment of sewage sludge by microwaveirradiation in presence of H2O2 for enhanced anaerobicdigestion

Cigdem Eskicioglua,*, Audrey Prorotb, Juan Marinc, Ronald L. Drostec, Kevin J. Kennedyc

aSchool of Engineering, University of British Columbia Okanagan, Kelowna, BC V1V 1V7, CanadabEcole Nationale Superieure d’Ingenieurs de Limoges, Universite de Limoges, FrancecDepartment of Civil Engineering, University of Ottawa, Ottawa, ON K1N 6N5, Canada

a r t i c l e i n f o

Article history:

Received 21 April 2008

Received in revised form

7 August 2008

Accepted 8 August 2008

Published online 19 August 2008

Keywords:

Microwave

Hydrogen peroxide

Pretreatment

Oxidation

WAS

Anaerobic biodegradability

* Corresponding author. Tel.: þ1 250 807 854E-mail address: cigdem.eskicioglu@ubc.ca

0043-1354/$ – see front matter ª 2008 Elsevidoi:10.1016/j.watres.2008.08.010

a b s t r a c t

A microwave-enhanced advanced hydrogen peroxide oxidation process (MW/H2O2-AOP)

was studied in order to investigate the synergetic effects of MW irradiation on H2O2 treated

waste activated sludges (WAS) in terms of mineralization (permanent stabilization), sludge

disintegration/solubilization, and subsequent anaerobic biodegradation as well as dew-

aterability after digestion. Thickened WAS sample pretreated with 1 g H2O2/g total solids

(TS) lost 11–34% of its TS, total chemical oxygen demand (COD) and total biopolymers

(humic acids, proteins and sugars) via advanced oxidation. In a temperature range of

60–120 �C, elevated MW temperatures (>80 �C) further increased the decomposition of H2O2

into OH� radicals and enhanced both oxidation of COD and solubilization of particulate

COD (>0.45 micron) of WAS indicating that a synergetic effect was observed when both

H2O2 and MW treatments were combined. However, at all temperatures tested, MW/H2O2

treated samples had lower first-order mesophilic (33� 2 �C) biodegradation rate constants

and ultimate (after 32 days of digestion) methane yields (mL per gram sample) compared to

control and MW irradiated WAS samples, indicating that synergistically (MW/H2O2-AOP)

generated soluble organics were slower to biodegrade or more refractory than those

generated during MW irradiation.

ª 2008 Elsevier Ltd. All rights reserved.

1. Introduction biphenyls, polycyclic aromatic hydrocarbons or dioxins. New

In Canada, approximately 670,000 tons of dry sewage sludge

(biosolids) is produced yearly and existing wastewater treat-

ment plants (WWTPs) are expanding to serve the demands of

growing cities, which will result in an increase of sludge

production in the future (Renzetti, 2005). Furthermore, the

quality of waste sludge is worsening as a result of the presence

of anthropogenic micropollutants, such as heavy metals,

personal care products, endocrine disrupters, polychlorinated

4; fax: þ1 250 807 9850.(C. Eskicioglu).

er Ltd. All rights reserved

and practical WWTP sludge reduction approaches would not

only reduce the operational and capital costs associated with

sludge management and disposal, but also result in the

production of less biosolids and fewer deleterious environ-

mental effects when sent to landfill or agricultural land.

Concomitantly, there is world-wide interest in three main

sludge reduction strategies, applied: (a) in the wastewater line

(energy uncouplers and alternating stream exposure to oxic

and anoxic environments), (b) in the sludge line [physical,

.

w a t e r r e s e a r c h 4 2 ( 2 0 0 8 ) 4 6 7 4 – 4 6 8 2 4675

chemical, thermal or combination pretreatments for

enhanced hydrolysis before anaerobic digestion (AD)], and (c)

in the final waste line (incineration and pyrolysis).

Mesophilic AD (MAD) of excess sewage sludge is an

economic and environmentally friendly approach for meeting

the legislative requirements and to reduce the volume of

sludge to be disposed or land applied. However, MAD has

several limitations: (a) reduction of fecal coliforms does not

meet required standards, and (b) the extent of solids

destruction is limited. Many different pretreatment technol-

ogies were previously investigated to overcome these limita-

tions. Previous studies indicated that sludge line pretreatment

processes such as mechanical (Muller et al., 2003), ultrasonic

(Bougrier et al., 2006), steam explosion (Dereix et al., 2006),

microwave (MW) irradiation (Hong, 2002; Eskicioglu et al.,

2008), chemical (Chiu et al., 1997) and enzymatic (Barjenbruch

and Kopplow, 2003) methods could disrupt extracellular

polymeric substances (EPS) and divalent cation network and

increase the extent of WAS biodegradability through

enhanced hydrolysis. Unfortunately, some of these tech-

niques require intense operational conditions of high pressure

and temperature making the process expensive and

increasing safety concerns. However, several studies sug-

gested that combining the techniques mentioned above can

achieve better or similar results under more acceptable

operating conditions.

In the past, several oxidants with disinfecting power such

as ozone, hydrogen peroxide, chlorine, and chlorine dioxide

have been applied to disintegrate the sewage sludge, although

only few have been combined with anaerobic post-treatments

(Weemaes et al., 2000; Bougrier et al., 2006). Hydrogen

peroxide (H2O2) is one of the most powerful oxidizers known.

From the thermodynamic point of view, it is stronger than

chlorine (Cl2) and chlorine dioxide (ClO2) as standard redox

potential of H2O2 is greater than Cl2 and ClO2. However, its

activity or rate of decomposition is dependent on the

temperature and at room temperature is usually catalyzed by

transition materials such as Ag or Mo; therefore it is much

slower than other two oxidants. H2O2 is also used as a geno-

toxic agent which causes breakage of DNA strands by

generation of hydroxyl radicals (OH�) close to a DNA molecule

according to the Fenton reaction (Imlay et al., 1988). Exposure

of DNA to oxidative stress leads to more than 20 different

types of base damage, producing oxidized and ring-fermented

nitrogen bases (Slupphaug et al., 2003) as well as causing

permeabilization of cell membranes (Alvarez et al., 1987) and

changes in membrane fluidity and induction of apoptosis

(Griveau and LeLannou, 1997). Through catalysis, H2O2 can be

converted into OH� radicals (with an oxidation potential of

2.8 V) that are much more reactive than H2O2 and O3 with

oxidation potentials of 1.8 and 2.1 V, respectively. Therefore,

the oxidation power of H2O2 can be enhanced via a number of

treatment scenarios including O3/H2O2, UV/H2O2, H2O2/ultra-

sound and H2O2/thermal process. When H2O2 is added to the

system with O3, the decomposition of O3 into OH� radicals is

accelerated, shifting the process entirely to an advanced

oxygen process (AOP). Similarly, when H2O2 and UV are

combined, H2O2 absorbs the UV light and breaks down into

OH� radicals which degrade the contaminants in the sample

(Rosenfeldt et al., 2006). Furthermore, heating will also

increase decomposition of H2O2 into OH� radicals and there-

fore enhance the oxidation process when H2O2 is applied

simultaneously with conventional or microwave (MW)

heating.

In the field of bacteriology, synergistic killing of bacterial

spores by UV or MW radiation combined with H2O2 has been

previously observed (Hartman and Eisenstark, 1978; Bayliss

and Waites, 1979; Kuchma et al., 1990). In the field of envi-

ronmental engineering, the application of MW irradiation

(a closed-vessel MW digestion system, 2450 MHz, 1000 W) in

combination with H2O2 for sewage sludge treatment has been

recently developed as a novel microwave-enhanced advanced

oxidation process (MW/H2O2-AOP) (Wong et al., 2006a,b; Yin

et al., 2007; Chan et al., 2007). Their research focused on the

investigation of combined effects of MW/H2O2-AOP and MW/

H2O2/O3-AOP to solubilize and then recover the nutrients

(ammonia, ortho-phosphate) from a secondary aerobic sludge

for the purpose of magnesium ammonium phosphate (stru-

vite) crystallization. In a temperature range of 60–120 �C,

addition of H2O2 [2.6 and 5.4 g H2O2/g total solid (TS)]

enhanced ammonia release and resulted in a significant

decrease in PO4:NH3 ratio, an important parameter controlling

the struvite formation (Wong et al., 2006b). In terms of COD,

ammonia and ortho-phosphate solubilization, H2O2 was

a better oxidant than O3 when combined with MW alone (MW/

H2O2-AOP or MW/O3-AOP) and there was an advantage of

using a combination of three agents (MW/H2O2/O3-AOP) (Yin

et al., 2007). None of these studies included anaerobic post-

treatment of sludge after MW/H2O2-AOP before final disposal;

however, it is postulated that attaining very high COD solu-

bilization would greatly improve the efficiency of subsequent

AD with enhanced methane production.

This is the first study that evaluates the synergetic effects

of a MW/H2O2-AOP process for a thickened waste activated

sludge (TWAS) in terms of: (a) oxidation, (b) disintegration and

solubilization, (c) MAD and (d) dewaterability after MAD.

2. Materials and methods

2.1. Thickened waste activated sludge (TWAS)

Raw TWAS was obtained from the thickener centrifuge at the

Robert O. Pickard Environmental Center (ROPEC) located in Glou-

cester, (ON, Canada). ROPEC has preliminary and primary

treatment followed by a conventional aerobic activated sludge

unit operated at an average sludge retention time (SRT) of 5 d.

Ferric chloride is added to WAS for P removal prior to WAS

thickening. TWAS and primary sludge (PS) are blended in

a 58:42 (v/v) ratio and undergo mesophilic anaerobic sludge

digestion to produce a stabilized biosolids product for

disposal. For feed characterization, raw TWAS was sampled

from ROPEC and its general characteristics are displayed in

Table 1 along with the changes in its characterization during

the pretreatments before the anaerobic tests. In the labora-

tory, the TWAS collected from ROPEC in different buckets was

mixed a few times in a larger bucket to ensure homogeneity

and each sample used for pretreatments or characterization

was obtained upon thorough mixing a bucket with a stirring

rod. Raw TWAS was characterized as young sludge based on

Table 1 – General characteristics of raw and pretreated TWAS

Parameters TWAS(control)

H2O2

(treated)T¼ 60 �C T¼ 80 �C T¼ 100 �C T¼ 120 �C

MW MW/H2O2 MW MW/H2O2 MW MW/H2O2 MW MW/H2O2

pH 6.4 6.1 6.2 6.5 6.3 6.5 6.3 6.6 6.2 6.7

TS [%, w/w] 6.4 (0.0)a 5.2 (0.0) 6.8 (0.0) 5.2 (0.0) 6.4 (0.0) 5.3 (0.0) 6.4 (0.0) 5.3 (0.0) 5.8 (0.0) 5.1 (0.0)

VS [%, w/w] 4.7 (0.0) 3.8 (0.0) 5.1 (0.0) 3.8 (0.0) 4.7 (0.0) 3.9 (0.0) 4.6 (0.0) 3.8 (0.0) 4.7 (0.0) 3.7 (0.0)

VS/TS [%] 73.4 (0.0) 72.8 (0.0) 74.7 (0.0) 73.0 (0.0) 73.7 (0.0) 72.9 (0.0) 72.9 (0.0) 72.8 (0.0) 72.8 (0.0) 72.8 (0.0)

NH3-N [mg/L] 106 (1) 284 (0) 451 (0) 318 (0) 341 (0) 323 (0) 304 (0) 301 (0) 185 (0) 373 (0)

Total fraction [g/L]

COD 77.9 (0.7) 63.6 (3.6) 81.4 (1.4) 74.3 (5.7) 73.6 (3.6) 68.6 (0.0) 80.7 (2.1) 62.9 (0.0) 82.1 (3.6) 55.7 (1.4)

Proteins 12.3 (0.3) 10.9 (0.3) 14.8 (0.2) 7.0 (1.9) 15.0 (0.0) 9.5 (0.0) 16.0 (0.3) 10.2 (1.0) 17.7 (0.1) 8.0 (0.0)

Sugars 16.9 (0.4) 11.2 (0.5) 10.8 (0.7) 11.7 (0.4) 10.5 (0.4) 9.2 (0.4) 11.8 (0.9) 7.9 (0.0) 12.7 (0.1) 5.4 (0.0)

Humic acids 8.3 (1.1) 6.9 (0.2) 4.4 (0.1) 7.7 (2.1) 6.2 (0.1) 6.1 (0.0) 6.0 (0.2) 4.5 (0.0) 5.4 (0.1) 3.9 (0.1)

Soluble fraction (<0.45 micron) [g/L]

COD 2.7 (0.1) 10.6 (0.3) 10.0 (0.1) 10.8 (0.1) 10.9 (0.2) 12.5 (0.1) 13.2 (0.5) 13.2 (0.3) 12.1 (0.3) 13.6 (0.3)

Proteins 0.1 (0.0) 1.0 (0.1) 0.4 (0.1) 1.1 (0.1) 0.6 (0.0) 1.4 (0.0) 1.0 (0.0) 2.3 (0.1) 2.0 (0.0) 2.5 (0.0)

Sugars 0.2 (0.0) 1.6 (0.1) 0.7 (0.1) 1.6 (0.1) 1.7 (0.0) 1.8 (0.0) 2.3 (0.1) 2.6 (0.1) 2.3 (0.0) 1.9 (0.1)

Humic acids 0.3 (0.0) 1.5 (0.0) 1.4 (0.0) 1.6 (0.0) 1.9 (0.0) 1.9 (0.0) 1.9 (0.0) 1.7 (0.0) 1.1 (0.0) 1.4 (0.0)

Volatile fatty acids (VFA) [g/L]

Acetic 1.5 0.9 0.8 1.9 1.0 1.3 0.7 1.0 0.7 1.1

Propionic 0.9 0.6 0.6 1.1 0.7 0.9 0.6 0.8 0.7 0.8

Butyric 0.5 0.0 0.0 0.4 0.0 0.2 0.0 0.4 0.0 0.2

a Data represent arithmetic mean of duplicates (absolute difference between mean and duplicate measurements).

Table 2 – Summary of experimental conditions forpretreatment

Set Treatment Temperature(�C)

Ramptime(min)

Heatingtime(min)

Totaltime(min)

1 MW 60 2 5 7

2 MW 80 3 5 8

3 MW 100 4 5 9

4 MW 120 5 5 10

5 MW/H2O2 60 2 5 7

6 MW/H2O2 80 3 5 8

7 MW/H2O2 100 4 5 9

8 MW/H2O2 120 5 5 10

9 H2O2 – – – –

10 (Control) – – – – –

w a t e r r e s e a r c h 4 2 ( 2 0 0 8 ) 4 6 7 4 – 4 6 8 24676

the 5-d SRT as supported by the relatively high volatile solids

(VS)/TS� 100 ratios that was close to 74% (Table 1).

2.2. Microwave accelerated reaction system (MARS-5)

A closed-vessel microwave accelerated reaction system

(MARS-5; CEM Corporation, Quebec, Canada) which operates

at 2450 MHz with a maximum power, temperature and

pressure of 1200 W, 260 �C and 33 bars, respectively was

used. MARS-5 is equipped with fiber optic temperature and

pressure probes within the cavity and a turning carousel

with a maximum of 14 pressure sealed vessels of 100 mL

capacity each.

The primary goal of this study was to use the synergetic

disinfection and disintegration features of a MW/H2O2 system

in order to enhance the MAD of TWAS without loosing the

methane potential of the sample via oxidation; therefore,

a lower H2O2 dose (1 g H2O2/g TS) was applied compared to

those (2.6 and 5.4 g H2O2/g TS) used in earlier MW/H2O2-AOP

studies; however, identical temperature profiles (heating and

holding times and temperatures) were achieved by both

MARS-5 and Ethos TC Digestion Lab Station MW used by Wong

et al. (2006b). Ten sets of experimental conditions were carried

out as displayed in Table 2. Pretreatments were applied on

TWAS within 3 h of sampling from ROPEC. The sludge was

placed in a 2-L volumetric flask and H2O2 (30% v/v) was slowly

added (1 g H2O2/g TS) by a burette while TWAS was gently

mixed. After peroxide addition, a total of 500 g of H2O2 con-

taining TWAS was heated in 14 vessels (35 g TWAS per vessel)

rotating on the MW carousel. In order to achieve comparable

results at different temperatures, a uniform rate of ramping

(increase of 20 �C per min of heating) was used (Table 2). At the

end of MW heating, samples were cooled to room temperature

in the sealed MW vessels to avoid evaporation of organics and

then stored at 4 �C.

2.3. Determination of anaerobic biodegradability

The anaerobic degradabilities of control (untreated), H2O2,

MW and MW/H2O2 treated samples were determined by batch

mesophilic biochemical methane potential (BMP) assays in

125 mL serum bottles sealed with butyl rubber stoppers. The

BMP tests (total of 22 bottles including blanks, duplicates and

controls) were performed for the various treatment conditions

given in Table 2 with an acclimated mesophilic inoculum

based on Owen et al. (1979).

For BMP assays, inoculums (15 mL) were placed into serum

bottles and then sludge samples (70 mL) were added. Nitrogen

w a t e r r e s e a r c h 4 2 ( 2 0 0 8 ) 4 6 7 4 – 4 6 8 2 4677

sparging was applied to batch reactors when TWAS and

inoculum were mixed to prevent exposure to air. Serum

bottles (120 mL) were sealed after addition of a mixture con-

taining equal parts of NaHCO3 and KHCO3 to achieve an

alkalinity of 4000 mg/L (as CaCO3). Serum bottles were kept in

a darkened temperature controlled incubator shaker moving

at 90 rpm in a temperature controlled room at 33� 2 �C until

they stopped producing biogas. Daily biogas produced was

measured by inserting a needle attached to a manometer.

2.4. Mesophilic inoculum acclimation

Mesophilic inoculum used in BMP assays was originally taken

from the effluent line of the anaerobic sludge digesters (SRT of

15–20 d) treating a mixture of TWAS and PS [58:42 (v/v)] at

ROPEC. Before setting up the BMP assays, ROPEC inoculum

was acclimated to MW irradiated TWAS. For acclimation, one

5-L anaerobic semi-continuous reactor with ROPEC inoculum,

fed with MW irradiated ROPEC-TWAS, was run at an SRT of

approximately 20 d over a year. MW toxicity effect on

secondary sludge digestion was expected to increase with MW

temperature (Hong, 2002; Eskicioglu et al., 2007a); therefore,

a severe MW pretreatment temperature (room temperature to

175 �C by MARS-5 @20 �C/min with zero hold time) was used to

pretreat the sludge fed to the acclimation reactor. The accli-

mated inoculum had the following characteristics: TS: 1.8%

(w/w), VS: 0.9% (w/w), VS/TS�100: 51.5%, pH: 7.72, total vola-

tile fatty acids (summation of acetic, propionic and butyric

acids): 397 mg/L.

2.5. Analysis

All sample analyses were performed for control and pre-

treated samples at room temperature. Total and volatile solids

were determined based on Standard Methods procedure

2540G (APHA, 1995). For ammonia-N determination, centrifu-

gation [for 20 min at 5856 relative centrifugal force (RCF) in

a Dupont instruments Sorvall SS-3 automatic centrifuge] was

used and ammonia-N measurements were performed using

an ORION Model 95-12 ammonia gas sensing electrode con-

nected to a Fisher Accumet pH meter model 750. The analysis

was conducted according to Standard Methods 4500D proce-

dure (APHA, 1995) and reported as ammonia-N. Colorimetric

COD measurements were performed based on Standard

Methods procedure 5250D (APHA, 1995) with a Coleman Per-

kin–Elmer spectrophotometer Model 295 at 600 nm light

absorbance. Before soluble COD determination, sludge

samples were centrifuged (for 20 min at 5856 RCF) and filtered

through membrane disc filters first with 1.2 mm (Whatman-

Glass Microfiber, Catalog no: 1822 070) and then with 0.45 mm

(Fisher-Nitrocellulose, Catalog no: 09-719-555) pore sizes.

Reactor pH, total volatile fatty acids (VFAs; summation of

acetic, propionic and butyric acids) and biogas composition

(nitrogen, methane and carbon dioxide percentage) were

monitored weekly during the batch AD. Total VFAs were

measured by injecting supernatants into an HP 5840A GC with

glass packed column (Chromatographic Specialties Inc.,

Brockville, ON, Canada, Chromosorb 101, packing mesh size:

80/100, column length� ID: 3.05 m� 2.1 mm) and a flame

ionization detector (oven, inlet and outlet temperatures: 180,

250 and 350 �C, respectively, carrier gas flowrate: 25 mL

helium/min) equipped with HP 7672A autosampler (van

Huyssteen, 1967). Biogas composition was determined with an

HP 5710A GC with metal packed column (Chromatographic

Specialties Inc., Brockville, ON, Canada, Porapak T, packing

mesh size: 50/80, column length�OD: 3.05 m� 6.35 mm) and

thermal conductivity detector (oven, inlet and outlet temper-

atures: 70, 100 and 150 �C, respectively) using helium as the

carrier gas (flowrate: 25 mL/min) (Ackman, 1972). The protein

and humic acid concentrations in both total and soluble

(<0.45 mm) fractions were measured by the modified Lowry

method (Frolund et al., 1995) using bovine serum albumin

(BSA) and humic acid sodium salt (H16752, Sigma–Aldrich

Canada Ltd.) as protein and humic acid standards, respec-

tively. Concentrations of total and soluble sugars were

measured according to the method of Dubois et al. (1956) with

glucose stock solution used as the sugar standard.

Dewaterability of WAS at the end of digestion was tested by

a Capillary Suction Timer [Model 440, Fann Instrument

Company, TX, USA] based on Standard Methods Procedure

2710G (APHA, 1995). The method consists of injecting a sludge

sample to a small cylinder placed on a sheet of chromatog-

raphy paper. While the paper extracts liquid from the sludge

by capillary suction, water released from sludge travels

between two contact points on the chromatography paper and

then the travel time or capillary suction time (CST in seconds)

is recorded by a digital timer. In this project, sludge temper-

ature and volume were kept constant (22� 1 �C and 5 mL,

respectively) since variations in temperature and sample

volume can affect CST results. At the end of experiments, CST

values indicated by the timer were divided by TS concentra-

tion of sludge samples in order to prevent bias among samples

with different solid concentrations.

3. Results and discussion

3.1. Oxidation and disintegration effects of MW, H2O2

and MW/H2O2 treatments

Hydrogen peroxide, as a strong oxidizer, converts some of the

sensitive organic compounds to CO2 and water in addition to

many others oxidized to other organics containing stable

functional groups; therefore it will provide some level of

permanent stabilization. Table 1 indicates the effects of MW

and MW/H2O2 treatments on total organic matter composition

of ROPEC-TWAS in terms of TS, total COD, total proteins, total

sugars, total humic acids as well as supernatant NH3–N

concentrations. In this study, hydrogen peroxide oxidized

a portion of sludge in H2O2 and MW/H2O2 treated samples and

caused reductions in the total organic solids and biopolymers.

The TWAS sample, treated with H2O2 (1 g H2O2/g TS), lost

19� 0, 18� 5, 11� 3, 34� 3 and 16� 2% of TS, COD, proteins,

sugars and humic acid concentrations, respectively, through

oxidation compared to controls (Table 1). The concentration of

organic compounds present in the TWAS samples decreased

further when H2O2 was combined with MW irradiation espe-

cially at temperatures above 80 �C. At a MW temperature of

120 �C, MW/H2O2 samples experienced additional 2, 12, 27, 52

and 45% reductions in their TS, COD, proteins, sugars and

w a t e r r e s e a r c h 4 2 ( 2 0 0 8 ) 4 6 7 4 – 4 6 8 24678

humic acids concentrations, respectively, compared to the

H2O2 treated sample. The level of permanent stabilizations in

similar samples (MW/H2O2 at 120 �C) were 20� 0%, 28� 2%,

35� 0%, 68� 0% and 53� 1% for TS, COD, proteins, sugars and

humic acids compared to the original raw TWAS (control)

sample. It is possible that the elevated MW temperatures

increased the decomposition of H2O2 into OH� radicals and

enhanced the oxidation process when both MW and H2O2 are

applied. Among organic substances studied, the sugars and

humic acids experienced the highest level of reductions after

H2O2 (16–34%) and MW/H2O2 (53–68%) treatments at 120 �C

(Table 1).

The concentration of NH3–N in the supernatant of TWAS

before and after MW, H2O2 and MW/H2O2 treatments were

also shown in Table 1. All pretreatment scenarios contained

higher NH3–N in their supernatants compared to that of

control. Both H2O2 treatment and MW irradiation resulted in

significant (2–4 fold) ammonia release into solution over the

control. These results are in agreement with previous studies

which concluded that both MW and MW/H2O2 treatments

enhance the release of ammonia depending on the MW

temperature and H2O2 concentration (Wong et al., 2006b; Yin

et al., 2007). Among the pretreatment temperatures tested, the

TWAS sample pretreated at 60 �C contained the highest

ammonia concentration which was 451 mg/L. The other

TWAS samples pretreated at 80, 100 and 120 �C had NH3–N

concentrations of 341, 304 and 185 mg/L, respectively (Table 1).

The reason for the reductions in the NH3–N concentrations of

TWAS samples near boiling point MW temperatures (without

presence of H2O2) is currently unknown but has been

previously observed in other studies by both household

(Eskicioglu et al., 2007a) and closed-vessel type MW units

(Wong et al., 2006b; Eskicioglu et al., 2007b).

The combined effects of MW/H2O2 to disintegrate the

complex floc structure of WAS and to solubilize the intra- and

Fig. 1 – Particulate organics solubilization achieved on TWAS s

humic acids after MW and MW/H2O2 treatments [data represent

absolute difference between the mean and the sample values].

extracellular polymeric compounds (proteins, sugars and

humic acids) were evaluated by the percentages of soluble to

total biopolymer concentration ratios (Fig. 1a–d). It was

expected that by using a combination of MW and H2O2, poly-

meric network disintegration could be increased. As it can be

observed from Fig. 1a–d that all MW, H2O2 and MW/H2O2

treatments increased the amount of soluble COD, soluble

proteins, soluble sugars and soluble humic acids fractions in

the TWAS samples compared to those of control. The level of

COD solubilizations without the presence of H2O2 were 3� 0,

12� 0, 15� 0, 16� 0 and 15� 1% for control, MW-60, MW-80,

MW-100 and MW-120 samples, respectively. Furthermore,

H2O2, MW-60/H2O2, MW-80/H2O2, MW-100/H2O2 and MW-120/

H2O2 samples achieved 17� 2, 15� 1, 18� 0, 21� 1 and

24� 1% COD solubilizations, respectively, indicating that

a synergistic disintegration effect was observed when both

treatments were combined and resulted in smaller organic

compounds (Fig. 1a). Both proteins and humic acids can be

found in polymeric network as extra- and intra-cellular

compounds. The increase of concentration of these

compounds in the soluble phase of WAS after both MW and

MW/H2O2 treatments is compelling evidence of floc structure

of secondary sludge being disintegrated and solubilized as

MW temperature increased. In general, the COD solubilization

trend observed in this study was consistent with the data

obtained in other MW/H2O2 studies on secondary sludge

(Wong et al., 2006b; Yin et al., 2007). The results presented in

Table 1 and Fig. 1a–d revealed that H2O2 and MW/H2O2

treatments did not only reduce the amount of total

organic residues in TWAS, but also reduced the particulate

fractions by converting them into soluble (<0.45 mm) organics.

Among the organic substances studied, higher level of

solubilizations were achieved in humic acids and sugars

which were 37� 1% and 35� 2% for the MW/H2O2 sample at

120 �C (Fig. 1c and d).

amples in terms of: (a) COD, (b) proteins, (c) sugars, and (d)

arithmetic mean of duplicates and error bars represent the

w a t e r r e s e a r c h 4 2 ( 2 0 0 8 ) 4 6 7 4 – 4 6 8 2 4679

It is also necessary to emphasize that so far, different MW

pretreatment studies achieved different soluble to total COD

ratios at similar MW temperatures depending on the ramp,

heating/holding times, MW units used (kitchen or closed-

vessel) as well as the characteristics of secondary sludge

being pretreated (moisture content, SRT of the activated

sludge process, and chemicals added for thickening or

nutrient removal at the WWTP). In another study, for an

anaerobically digested biocake sample, in a ramp range of

1.95–7.8 �C/min, increasing the ramp rate (shorter exposure

Fig. 2 – Cumulative biogas productions (mL per 1.4 g volatile sol

(a) control and H2O2 samples, (b) MW-60 and MW-60/H2O2 samp

MW-100/H2O2 samples, and (e) MW-120 and MW-120/H2O2 samp

bars represent the absolute difference between the mean and t

to MW irradiation) to achieve desired temperatures of 120,

150 and 175 �C consistently decreased the improvements in

sludge solubilizations at all temperatures, with both 30 and

15% TS concentrations (Eskicioglu et al., 2008). These results

suggest that MW temperature achieved or H2O2 dose used are

not the only factors which need to be taken into account for

an ideal comparison among the different MW, H2O2 and MW/

H2O2 studies. Temporal heating profiles in a MW system can

be more important than the final pretreatment temperatures

reached.

ids added) and first-order biodegradation constant (k) from:

les, (c) MW-80 and MW-80/H2O2 samples, (d) MW-100 and

les [data represent arithmetic mean of duplicates and error

he sample values].

w a t e r r e s e a r c h 4 2 ( 2 0 0 8 ) 4 6 7 4 – 4 6 8 24680

3.2. Anaerobic biodegradability and dewaterability afterMW, H2O2 and MW/H2O2 treatments

Anaerobic biodegradability or BMP of control and pretreated

WAS samples was tested at mesophilic temperature

(33� 2 �C) in 125-mL serum bottles. The BMP assays were

designed in a way to reveal valuable information on both

initial methane production (biodegradation rate and toxicity

relative to control) and cumulative final methane production

(ultimate biodegradability). Comparing the sample charac-

terizations after pretreatments indicated significant

reductions in the total organics remaining after H2O2 and MW/

H2O2 treatments (Table 1); therefore, organic loading adjust-

ments were required during the initial BMP set-up. To be able

to compare the biodegradation rates in different samples

properly, initial organic loadings were adjusted to 1.42 g VS in

each BMP bottle and daily biogas productions were then

monitored. Fig. 2a–e indicates the first-order anaerobic

biodegradation constants (k, d�1) as well as the coefficient of

determination (R2), which is used as the primary discriminator

to evaluate the adequacy of fit between measured and pre-

dicted cumulative biogas productions. It was interesting to

observe that although H2O2 and MW/H2O2 treated samples

contained significant amounts of higher soluble organics

(proteins, sugars, humic acids, etc.) compared to control and

MW treated samples (Table 1 and Fig. 1a–d), these soluble

fractions were not metabolized more rapidly than others, and

therefore did not yield conclusive differences in the anaerobic

biodegradation rates (k). In fact, H2O2 and MW/H2O2 samples

yielded slightly lower k constants at all tested MW tempera-

tures compared to control and MW irradiated samples indi-

cating that some products were formed during H2O2 addition

and digested at a slower rate (Fig. 2a–e). Toxicity of both

conventionally heated (CH) and MW irradiated samples at 45,

65 and 100 �C have been previously studied and compared to

untreated samples by other researchers as well. In another

study (Hong, 2002), among three different sludge samples

Table 3 – Ultimate methane yields from raw andpretreated TWAS samples

Methane yield

[mL CH4/g VSadded] [mL CH4/gTWASadded]

Raw TWAS (Control) 308.4 (0.5)a 14.6 (0.0)

H2O2 treated 304.3 (7.0) 11.7 (0.3)

T¼ 60 �C MW 289.6 (0.6) 14.7 (0.1)

MW/H2O2 290.8 (3.5) 11.2 (0.0)

T¼ 80 �C MW 313.3 (2.6) 14.9 (0.6)

MW/H2O2 302.0 (15.3) 11.6 (0.1)

T¼ 100 �C MW 396.8 (2.8) 18.2 (0.1)

MW/H2O2 308.1 (2.5) 11.8 (0.1)

T¼ 120 �C MW 310.4 (1.2) 14.2 (0.2)

MW/H2O2 316.8 (4.2) 11.9 (0.1)

a Data represent arithmetic mean of duplicates (absolute differ-

ence between mean and dup. measurements).

[anaerobically digested sludge (ADS), PS and WAS] tested, the

highest MW toxicity was observed in WAS followed by ADS

and PS, while CH sludge samples did not indicate any toxicity.

Hong’s study implied that toxicity was caused by the

substances leaked from sludge to soluble phase after MW

pretreatment which was supported by other studies on

soluble phase (<0.45 mm) only (Eskicioglu et al., 2006) except

the fact that in the later study, supernatants from both MW

and CH (at 96 �C) WAS samples were being digested at a slower

rate than those of controls.

In addition to initial biogas production rate, the overall

shapes of the cumulative biogas curves were also interesting.

Regardless of the pretreatment type and intensity, all biogas

curves had three distinct phases (from 1–12, 12–20 and 20–32

digestion days) which may indicate three different substrates

(or one substrate hydrolysed and transformed to others) being

metabolized at different rates. Among the BMP conditions

tested, only MW-100 sample indicated a different overall

cumulative biogas shape (Fig. 2d) with only two distinct

phases.

Ultimate (at the end of 32 days) methane yields from the

BMP assays indicated that only the MW-100 sample achieved

significant enhancement in methane yields which was

29� 3% higher than that of the control sample (Table 3) and

the rest of the digesters had very similar ultimate methane

yields in terms of mL CH4 per gram VS added. However, the

ultimate methane yields per gram of sample added were signifi-

cantly different among MW and MW/H2O2 samples (Table 3).

At all temperatures, MW/H2O2 samples achieved lower (by 20–

54%) ultimate methane yields (mL per g sample added) than

those of MW samples. Similarly, H2O2 treated samples had

25% lower methane yield than the control sample indicating

that H2O2 treatment (1 g H2O2/g TS) decreased the rate of

methane production and the ultimate methane potential of

WAS via advanced oxidation. As it was previously stated, in

this study, the choice of using 1 g H2O2/g TS was somewhat

arbitrary at a low dose where the potential amount of oxida-

tion itself would be sparingly small. Based on the results from

this study, it is suggested that the future studies can try even

Table 4 – Dewaterability results at the end of the BMP test

Capillary suction time [sec per TS(%, w/w)]

Raw TWAS (Control) 180.7 (9.0)a

H2O2 treated 160.7 (5.9)

T¼ 60 �C MW 171.9 (6.5)

MW/H2O2 171.5 (6.3)

T¼ 80 �C MW 166.4 (10.3)

MW/H2O2 158.7 (4.8)

T¼ 100 �C MW 193.3 (9.5)

MW/H2O2 154.8 (6.2)

T¼ 120 �C MW 158.2 (6.5)

MW/H2O2 157.9 (9.3)

a Data represent arithmetic mean of six replicates (standard

deviation).

w a t e r r e s e a r c h 4 2 ( 2 0 0 8 ) 4 6 7 4 – 4 6 8 2 4681

a lower dose (w0.5 g H2O2/g TS) in order to minimize further

loss of methane potential via advanced oxidation.

As a first MW/H2O2-AOP study to enhance WAS biodegra-

dation, these results are in agreement with other sludge

pretreatment studies which combined conventional heating

(in a mechanically stirred autoclave at 140 �C) with alkali

addition (in a range of 0–26 g NaOH/L) and observed reduc-

tions in the biodegradation performance when NaOH

concentrations exceeded 10 g/L (Penaud et al., 1999). Similarly,

Valo et al. (2004) studied a thermo-chemical (150 mmol H2O2/

dm3 dose and autoclave heating at 90 �C) pretreatment on

WAS and compared the results with those from thermally

pretreated WAS only at 130 �C. In their research, although

both samples achieved similar level of COD solubilization

(w30%), the biogas production from thermo-chemically

treated sample (H2O2þ 90 �C) was lower than that obtained

after thermal at 130 �C, indicating that a portion of the soluble

organics generated after thermo-chemical treatment was

refractory. On the other hand, Kim et al. (2003) reported that

among four different pretreatment methods [autoclave

(121 �C for 30 min), chemical (7 g NaOH/L), ultrasonic (42 kHz

for 120 min) and thermo-chemical (121 �C for 30 min, 7 g

NaOH/L)] tested, the thermo-chemical method achieved the

highest level of biogas production which was w38% higher

than the control. It seems that initial sludge characteristics,

dose, type and intensity of chemical and heating applied may

influence final pretreatment outcomes so that general state-

ments of performance cannot always be made.

The capillary suction time (CST)-dewaterability method

provides a quantitative measure, reported in seconds, of how

readily a sludge releases its water. The method requires

a minimum of five replicate injections from each sample

(APHA, 1995). In this study, the CSTs of three replicates from

each BMP bottle (six replicates for each pretreatment scenario)

were measured and reported in Table 4 along with standard

deviations of six replicate injections. Although, the results did

not exhibit any district trends among MW treated samples, in

general H2O2 and MW/H2O2 treated samples resulted in

slightly faster dewaterabilities compared to control and MW

treated samples.

4. Conclusions

Based on the experimental data and analysis the following

conclusions are drawn.

(1) In a temperature range of 60–120 �C, the microwave-

enhanced advanced hydrogen peroxide process (MW/

H2O2-AOP) tested on WAS for enhanced solubilization and

methane production caused significant level of permanent

stabilization via oxidation before digestion.

(2) Elevated MW temperatures (>80 �C) increased the

decomposition of H2O2 into OH� radicals and enhanced

both oxidation and particulate COD disintegration of WAS

samples.

(3) However, at all temperatures tested, MW/H2O2 treated

samples had lower first-order biodegradation rate

constants (k) and ultimate methane yields (mL per gram

sample added ) compared to control and MW irradiated

WAS samples, indicating that synergistically (MW/H2O2-

AOP) generated soluble organics were slower to biodegrade

than those generated during MW irradiation.

Acknowledgments

The authors thank NSERC, BIOCAP Canada and Environ-

mental Waste International Corporation for financial support.

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