enhancement of thermophilic anaerobic digestion of thickened waste activated sludge by combined...

Journal of Environmental Sciences 2011, 23(8) 1257–1265

Enhancement of thermophilic anaerobic digestion of thickened waste activatedsludge by combined microwave and alkaline pretreatment

Yongzhi Chi1, Yuyou Li1,2,∗, Xuening Fei1, Shaopo Wang1, Hongying Yuan1

1. Tianjin Key laboratory of Aquatic Science and Technology, Department of Environmental and Municipal Engineering, Tianjin Institute of UrbanConstruction, Tianjin 300384, China. E-mail: [email protected]

2. Department of Environmental Science, Graduate School of Environmental Studies, Tohoku University, Sendai 980-8579, Japan

Received 16 October 2010; revised 29 May 2011; accepted 02 June 2011

AbstractPretreatment of thickened waste activated sludge (TWAS) by combined microwave and alkaline pretreatment (MAP) was studied to

improve thermophilic anaerobic digestion efficiency. Uniform design was applied to determine the combination of target temperature

(110–210°C), microwave holding time (1–51 min), and NaOH dose (0–2.5 g NaOH/g suspended solids (SS)) in terms of their effect

on volatile suspended solids (VSS) solubilization. Maximum solubilization ratio (85.1%) of VSS was observed at 210°C with 0.2

g-NaOH/g-SS and 35 min holding time. The effects of 12 different pretreatment methods were investigated in 28 thermophilic batch

reactors by monitoring cumulative methane production (CMP). Improvements in methane production in the TWAS were directly related

to the microwave and alkaline pretreatment of the sludge. The highest CMP was a 27% improvement over the control. In spite of the

increase in soluble chemical oxygen demand concentration and the decrease in dewaterability of digested sludge, a semi-continuous

thermophilic reactor fed with pretreated TWAS without neutralization (at 170°C with 1 min holding time and 0.05 g NaOH/g SS)

was stable and functioned well, with volatile solid (VS) and total chemical oxygen demand (TCOD) reductions of 28% and 18%,

respectively, which were higher than those of the control system. Additionally, methane yields (L@STP/g-CODadded, at standard

temperature and pressure (STP) conditions of 0°C and 101.325 kPa) and (L@STP/g VSadded) increased by 17% and 13%, respectively,

compared to the control reactor.

Key words: thickened waste activated sludge; microwave pretreatment; alkaline pretreatment; thermophilic anaerobic digestion

DOI: 10.1016/S1001-0742(10)60561-X

Citation: Chi Y Z, Li Y Y, Fei X N, Wang S P, Yuan H Y, 2011. Enhancement of thermophilic anaerobic digestion of thickened waste

activated sludge by combined microwave and alkaline pretreatment. Journal of Environmental Sciences, 23(8): 1257–1265

Introduction

The activated sludge process is the most widely used

process for biological wastewater treatment, but it results

in the generation of a considerable amount of waste ac-

tivated sludge (WAS) that requires disposal. In China, for

example, 4.54×106 tons of dry WAS was produced in 2007

(Chi et al., 2009). The handling of WAS represents 30%–

40% of the capital cost and about 50% of the operating

cost of many wastewater treatment facilities (Vlyssides

and Karlis, 2004). Before disposing of WAS, it has to be

stabilized sufficiently to reduce its organic content, odor

problems, and pathogen contamination.

Although different possibilities exist for WAS treatment,

anaerobic digestion plays an important role as it can pro-

duce energy-rich biogas, destroy pathogens, and stabilize

the sludge (Yang, 2010). Anaerobic digestion techniques

traditionally reduce the volume and weight of sludge and to

produce biogas even though the process has limitations in

* Corresponding author. E-mail: [email protected]

terms of long retention times and low overall degradation

efficiency of the organic matter. As most organics in WAS

are present within the slowly biodegradable cell walls and

extracellular polymeric substances (EPS), the rate-limiting

step in WAS digestion is the hydrolysis of organic matter

(Li and Noike, 1992). Thus, various pretreatment processes

have been studied to better lyse the cells in WAS to readily

biodegradable low-molecular-weight compounds.

Many different pretreatment technologies have been in-

vestigated to overcome these limitations. Previous studies

have indicated that various pretreatment processes such

as mechanical (Muller et al., 2007), ultrasonic (Jin et

al., 2009), thermal (Li and Noike, 1992; Wang et al.,

2009), chemical (Shao et al., 2010), and enzymatic (Ayol

et al., 2008) methods could disrupt the cell walls and

EPS and increase WAS biodegradability by enhancing the

hydrolysis process.

Compared with other methods, alkaline treatment has

the advantage of being a simple device with high ef-

ficiency (Jin et al., 2009). The mechanism of alkaline

pretreatment, which is commonly the subject of investiga-

1258 Journal of Environmental Sciences 2011, 23(8) 1257–1265 / Yongzhi Chi et al. Vol. 23

tions, is to induce the swelling of particulate organics at

high pH, making the cellular substances more susceptible

to enzymatic reaction (Feng et al., 2009b; Yuan et al.,

2006). Alkaline treatment becomes especially effective

when combined with thermal hydrolysis (Neyens et al.,

2004). Tanaka et al. (1997) studied chemical and ther-

mochemical pretreatment methods. For optimal alkaline

pretreatment, 0.5–0.6 g NaOH/g volatile suspended solids

(VSS) were suggested because with higher alkali doses,

VSS solubilization stays constant at around 15%. For

the thermochemical method, 130°C with 0.3 g NaOH/g

VSS was suggested, since 45% of VSS solubilization was

achieved and methane production was 2.2 times of the

control. However, the addition of too many chemicals

makes this treatment less economically attractive.

Microwave pretreatment, an alternative method to con-

ventional thermal pretreatment, conserves energy, destroys

pathogens, and increases VSS destruction (Jones et al.,

2002). Microwave irradiation can rapidly produce focused

direct heat, which lowers energy losses during transmis-

sion. Additionally, the changing dipole orientation of polar

molecules that takes place during microwave irradiation

causes athermal (non-thermal) effects which, when com-

bined with thermal effects, may cause the breakage of

hydrogen bonds and the unfolding and denaturing of com-

plex biological molecules. The existence of the microwave

athermal effect on WAS solubilization and concomitant

improvements in VSS destruction and biogas production

have recently been reported (Eskicioglu et al., 2007b,

2009; Qiao et al., 2008).

Since microwave and alkaline treatment are based on

different mechanisms of sludge dissolution, the combina-

tion of these two methods means the advantages of both

methods can be attained and better treatment efficiency

can be achieved. Qiao et al. (2008) studied the effects

of microwave treatment on WAS with alkali addition

using single factor experiments, with both solubilization

of organic matters and settleability of WAS investigated.

It was found that most organic fractions dissolved into

a solution within 5 min. Adding NaOH increased the

VSS dissolution ratio above 20% and the suspended solids

(SS) dissolution ratio above 10%. After 1 min microwave

treatment (170°C) with alkali (0.2 g NaOH/g total solids

(TS)), the treated WAS showed settleability improvement.

Chan et al. (2010) investigated the effects of pH and

microwave irradiation intensity on nutrients release, solids

destruction, particle size distribution and dewaterability

of WAS using microwave and alkaline treatment. The

higher irradiation levels tended to be more effective in

the solubilization of nutrients and had more pronounced

effects in the dewaterability of WAS. In contrast to the

study of Qiao et al. (2008), in treatments under alkaline

conditions, particle size range increased and more small

particles were formed, thereby significantly deteriorating

the dewaterability of WAS treated in alkaline conditions.

Dogan and Sanin (2009) used microwave-NaOH

(160°C) for the pretreatment of WAS (TS=10954 mg/L)

and evaluated its effect on the mesophilic anaerobic di-

gestion process (MADP) operated at a hydraulic retention

time (HRT) of 15 days and compared their results to

those of the control reactors. For combined microwave-

NaOH pretreatment (pH = 12), the ratio of soluble

chemical oxygen demand/total chemical oxygen demand

(SCOD/TCOD) of WAS increased from 0.005 (the control)

to 0.34. After the pH of the pretreated WAS was neutral-

ized, it was fed into the MADP. The results showed that

TCOD reductions and daily biogas production improved

by 30.3% and 43.5%, respectively.

However, few studies have determined the combination

of NaOH dose (0–2.5 g NaOH/g SS), target temperature

(110–210°C), and microwave holding time (1–51 min).

Moreover, few studies have evaluated the effect of high

temperature microwave and alkaline pretreatment (MAP)

on the thermophilic anaerobic digestion process (TADP)

with the pH of the pretreated thickened waste activated

sludge (TWAS) non-neutralized. Therefore, the main ob-

jective of this study was to investigate the effect of the

combination of a chemical method, alkaline pretreatment

and a physical method (microwave irradiation) on ther-

mophilic anaerobic digestion efficiency of TWAS. The

study consisted of three parts: MAP, biochemical methane

potential (BMP) tests, and semi-continuous TADP exper-

iments. The aim of the MAP test was to investigate the

effect of combined (alkaline + microwave) pretreatments

on TWAS disintegration using a uniform design method.

Three factors (target temperature, microwave holding time,

and NaOH dose) were used in the uniform design at six

levels. After obtaining and checking the quadratic poly-

nomial model describing the solubilization ratio of VSS,

twelve kinds of pretreated TWAS were used to evaluate

the effect of combined pretreatments on the cumulative

methane production (CMP) in BMP tests. The appropri-

ate pretreatment was then screened from the MAP and

BMP tests in accordance with solubilization ratio of VSS,

inhibition of methane production, and treatment costs.

Finally, the raw TWAS and the TWAS pretreated by the

screened pretreatment were used as substrates to evaluate

the effect of combined pretreatment on thermophilic anaer-

obic digestion performances in semi-continuous TADP,

respectively.

1 Materials and methods

1.1 Thickened waste activated sludge (TWAS) and seedsludge

Raw TWAS was obtained from a wastewater treatment

plant (WWTP) in Sendai City, Japan, and was stored at

4°C in a feed tank. The sludge was sequentially passed

through a 5 × 5 mm square mesh screen and a 1 mm sieve

to prevent clogging problems during transfer from the feed

tank to the digesters. Screening resulted in a minor loss

of sludge solids. Based on element analysis, the simulated

chemical formula of TWAS was C5.85H9.75O3.96N. The

chemical composition of the TWAS was similar to the

general characteristics of bacteria (Rittman and McCarty,

2001). Characteristics of raw TWAS are given in Table 1.

The original seeds used for BMP tests and TADP

No. 8 Enhancement of thermophilic anaerobic digestion of thickened waste activated sludge by combined microwave······ 1259

Table 1 Characteristics of raw thickened waste activated sludge (TWAS)

Sludge TS (mg/L) VS (mg/L) SS (mg/L) VSS (mg/L) TCOD (mg/L)

Raw TWASa 40900 ± 500b 31300 ± 301 33000 ± 249 27700 ± 266 47500 ± 1634

Raw TWASc 51100 ± 752 38900 ± 453 44500 ± 135 37000 ± 355 59300 ± 1971

TS: total solids; VS: volatile solids; SS: suspended solids; VSS: volatile suspended solids; TCOD: total chemical oxygen demand.a which was used in the BMP tests; b mean ± standard deviation (n � 3); c which was used in the TADP experiments.

experiments were also taken from the same WWTP’s full-

scale sewage sludge digesters operated at thermophilic

conditions. The seed sludge was also screened with a 1 mm

sieve to remove impurities, and after analysis of the initial

characteristics, it was used as the inocula for BMP tests

and TADP experiments.

1.2 Microwave and alkaline pretreatment

Uniform design was applied to determine the relation-

ship of TWAS solubilization to environmental conditions

(target temperature, microwave holding time, and NaOH

dose). As seen in Table 2, the experiment was based on

a uniform design with three factors (Fang and Ma, 2001).

The ranges of independent variables were 110–210°C, 1–

51 min, and 0–2.5 g NaOH/g SS. A sequential procedure

of collecting data, estimating polynomials, and checking

the adequacy of the model was used. Least squares were

used to estimate the parameters in approximating the

polynomials. Minitab 15 (Minitab Inc., USA) was used

to establish and to test complex polynomials to model the

data.

We used a Microwave Digestion System (Speedwave�

MWS-3+, Berghof, Germany, 0–1450 W microwave pow-

er output, 2450 MHz frequency, 4.053 MPa maximum

pressure, temperature range 100–230°C) equipped with

unique optical temperature and pressure monitoring of

each individual sample during digestion. In the system,

there were 12 Teflon vessels each with a capacity of 100

mL. An alkali dose of 0–0.25 g NaOH/g SS was mixed

by 50 mL TWAS with NaOH solution (15 mol/L). A

staged temperature profile was applied for pretreatment.

To achieve comparable results at different temperatures,

a uniform ramp time of 9 min was used for each target

temperature. Holding time was varied from 1 to 51 min

depending on time required (Table 2). After the designated

holding times were reached, samples were removed from

the microwave system immediately and placed and cooled

in air at room temperature. According to the target tem-

perature, the cooling ramp time fluctuated in the range

of 10–30 min. After samples were cooled to room tem-

perature in the sealed vessels to avoid the evaporation of

organics, the samples were stored at 4°C in a feed tank.

The cooling method was the same as described by other

studies (Qiao et al., 2008; Toreci et al., 2009).

1.3 Biochemical methane potential (BMP) tests

The anaerobic degradability of the control (untreated)

and MAP pretreated samples were determined by batch

thermophilic (55 ± 1°C) BMP tests in 120 mL serum

bottles sealed with butyl rubber stoppers. The BMP tests

(a total of 28 bottles including blanks, duplicates and

controls), without the addition of nutrients in the vials,

were performed for the various pretreatment conditions

given in Table 2 with a thermophilic inoculum based on Li

and Noike (1992). To evaluate the effect of pretreatment

on anaerobic digestion of TWAS, CMP of 30 days and

lag-phase time were applied as parameters in this study

(Table 2). Lag-phase time, which was a useful indicator for

monitoring the acclimation condition of methanogens with

one specific substrate in the BMP tests, was calculated in

accordance with the modified Gompertz equation (Lay et

al., 1998)

1.4 Semi-continuous TADP experiments

The experiments were conducted under thermophilic

(55 ± 1°C) conditions at the same HRT (30 days) for

more than 90 days (more than three times the time for

HRT) using two semi-continuous flow completely mixed

reactors, with effective volumes of 5 L. One was fed

with raw TWAS (control), and the other with pretreated

TWAS without neutralization. The schematic diagram of

the thermophilic anaerobic digestion system used for the

Table 2 Uniform design and experimental data

Trial Target Holding NaOH dose Solubilization CMP of 30 days Lag-phase

temperature (°C) time (min) (g NaOH/g SS) ratio of VSS (%) (mL@STP/g VSadded) time (days)

1 110 11 0.1 45 307 0.0

2 190 31 0.25 76 319 4.0

3 190 21 0 26 304 0.0

4 170 51 0.2 72 312 2.6

5 130 31 0 10 291 0.0

6 150 51 0.05 32 311 0.0

7 150 1 0.2 65 307 1.4

8 110 41 0.15 52 299 0.2

9 210 11 0.15 77 327 2.3

10 210 41 0.1 74 331 1.6

11 170 1 0.05 39 315 0.0

12 130 21 0.25 67 295 2.6

CMP: cumulative methane production; STP: standard temperature and pressure (0°C and 101.523 kPa).


Gas-liquid separator

Time

controller

Feed tank

Pump

Cooler Wastewater tank Heater

Digester

Wet gas meter

Fig. 1 Schematic diagram of thermophilic anaerobic digestion system.

experiments is shown in Fig. 1. A steady-state of digester

operation was assumed when the daily effluent properties

and biogas production fluctuated in a narrow range (10%),

which is typically achieved in the three HRT periods (Li

and Noike, 1992).

1.5 Analytical methods

The TS, VS, SS, VSS, chemical oxygen demand (COD),

ammonia nitrogen (NH4+-N), pH, and alkalinity were

determined following procedures outlined in Standard

Methods (APHA et al., 1998). The VSS compositions

of TWAS was analyzed by an elemental analyzer (Vario

EL III, Elementar Analysensysteme GmbH, Germany)

(Akutsu et al., 2009). Carbohydrates were measured by the

Phenol-H2SO4 method (DuBois et al., 1956), and proteins

were measured by the Lowry method (Lowry et al., 1951).

Biogas production was measured with a wet gas meter

(W-NK-0.5A, Sinagawa, Japan). Gas composition was

detected by a gas chromatograph (GC 8A, Shimadzu Corp.

Japan). Volatile fatty acids (VFAs, C2 to C5) were deter-

mined by gas chromatography with FID (GC 6890, Agilent

Technologies, Inc., USA) following methods described by

Akutsu et al. (2009).

Samples used for the measurement of SCOD, soluble

proteins, soluble carbohydrates, VFAs and NH4+-N were

prepared by centrifugation at 15,000 r/min for 30 minutes

and filtration through 0.45 μm membrane filters (Puradisc�

25 PEC, Whatman International Ltd., UK).

2 Results and discussion

2.1 Combined microwave and alkaline pretreatment

For evaluation of TWAS solubilization in the pre-

treatment, the following expression was used.

η = (VSS1−VSS2)/VSS1 (1)

where, η (%) is solubilization ratio of VSS, VSS1 is

VSS concentration before pretreatment, and VSS2 is VSS

concentration after pretreatment.

Since the thermal effect of microwave can cause the

temperature increment of WAS, the process of microwave

pretreatment is similar to that of thermal pretreatment and

includes four procedures: (1) disintegration of WAS floc,

(2) cell rupture and cellular substances leak, (3) hydrolysis

of organics, and (4) Maillard reaction (Xiao et al., 2009).

When WAS was disintegrated by microwave irradiation,

microwave irradiation would disrupt the complex WAS

floc structure. During the process, it led to a remarkable

increase in protein and polysaccharide levels (Yu et al.,

2010). At the same time, since cell membranes are a

selectively permeable lipid bilayer and lipids absorb mi-

crowaves, it is possible that microwave irradiation can

cause substantial damage on cell membranes and result

in the release of intracellular material (Eskicioglu et al.,

2007a). After the floc structure of the sludge and cell

membrane were destroyed, the organics released from

extracellular and intracellular biopolymers began to hy-

drolyze. The organics hydrolysis pathway was assumed

as follows: the lipids hydrolyzed to palmitic acid, stearic

acid, and oleic acid; protein to a series of saturated and

unsaturated acids, ammonia, and some carbon dioxide; the

carbohydrate to polysaccharides with a smaller molecu-

lar weight and possibly, even to simple sugars (Qiao et

al., 2008). With the increase in the temperature and the

production of hydrolysates, Maillard reaction occurred,

which is known to be responsible for the formation of

refractory dissolved organic compounds (Bougrier et al.,

2007; Eskicioglu et al., 2006).

To date, several studies have reported that alkaline

conditions and microwave irradiation tend to disrupt the

complex WAS floc structure and release extracellular and

intracellular biopolymers such as proteins, carbohydrates,

and lipids from the floc structure into the soluble phase,

as well as enhance the solubilization of particulate organic

matter (Eskicioglu et al., 2008; Feng et al., 2009a; Yin et

al., 2008). The degree of solubilization of the substrate

can be estimated from the solubilization ratio of VSS. In

Trial 9, the solubilization ratio of VSS was highest, at

an average of 77% among 12 trials (Table 2). Qiao et al.

(2008) showed that approximately 72% VSS of the mixture

of primary and secondary sludge dissolved into aqueous

phase at 170°C with 0.2 g NaOH/g SS and 30 min holding

time. The results indicated there was a strong effect of

MAP on sludge solubilization.

To find the maximum solubilization degree of TWAS,

a quadratic equation was tested to model the data ob-

tained from the trials in Table 2. When the data was

analyzed using the model, a stepwise regression using the

forward method was used when alpha-to-enter was 0.1.

The quadratic model (Eq. (2)) was then used to describe

the solubilization degree of TWAS.

y = 64.7 + 0.258(x1−160) + 208(x3−0.125)+

0.00344(x1−160)(x2−25) − 0.498(x1−160)(x3−0.125)

− 0.00869(x2−25)2 − 1269(x3−0.125)2

(2)


where, y (%) is experimental value of the solubilization

ratio of VSS, and xi is independent variable i (i = 1 for

target temperature, 2 for holding time, and 3 for NaOH

dose). 160, 25, and 0.125 in Eq. (2) were the average value

of target temperature (°C), holding time (min), and NaOH

dose (g NaOH/g SS), respectively. The analysis of variance

(ANOVA) and the significance of each coefficient for

the solubilization ratio of VSS, important in determining

the adequacy and significance of a predictive model, are

shown in Table 3 and Table 4, respectively. The p-value

of regression was significant at the 0.1% α-level for the

quadratic model. The R2 (multiple regression coefficient

squares) and S (standard error of fitted value) were 0.998

and 1.58, respectively. An analysis of variance using Eq.

(2) showed that the effects of all terms in Eq. (2) were

significant at the 5% α-level. According to the p-value,

the variable with the largest effect was the NaOH dose

(x3), followed by the target temperature (x1), the NaOH

dose × NaOH dose (x3x3), the holding time × holding

time (x2x2), the interaction effects of target temperature

and holding time (x1x2), and the interaction effects of

target temperature and NaOH dose (x1x3). The residual

plots for the model and the experimental data set showed

no patterns or trends (data not shown). Therefore, this

equation was used to determine the conditions that would

maximize the solubilization degree of TWAS. Within the

design boundaries, Eq. (2) estimated a maximum VSS

solubilization degree of TWAS (85.1%) at 210°C with 0.2

g NaOH/g SS and 35 min holding time. The adequacy

of Eq. (2) for predicting the optimum response values of

VSS solubilization ratio was verified effectively by the

experiment, and the six experimental mean values agreed

with the predicted values of Eq. (2) with a 1.8% deviation

(data not shown).

2.2 Biochemical methane potential (BMP) tests

Biodegradability and potential toxicity of MAP pre-

treated TWAS relative to the control were studied in

duplicate batch thermophilic BMP tests. The first 10 days

of incubation are critical since maximum substrate utiliza-

tion generally occurs in the first 5–10 days. Therefore, a

Table 3 Analysis of variance for the solubilization ratio of VSS

Source Degree of Sum of Mean F-Value P-Value

freedom squares square

Regression 6 3766.34 627.72 327.41 0.000

Residual error 5 9.59 1.92

Total 11 3775.93

Table 4 Significance of regression coefficient for the solubilization

ratio of VSS

Predictor Standard error t-Value P-Value

Constant 1.158 55.85 0.00

(x1–160) 1.333 × 10−2 19.34 0.00

(x3–0.125) 5.400 38.50 0.00

(x1–160)(x2–25) 9.925 × 10−4 3.47 0.02

(x1–160)(x3–0.125) 0.196 –2.54 0.05

(x2–25)2 2.108 × 10−3 –4.12 0.01

(x3–0.125)2 84.310 –15.05 0.00

comparison of the methane production rates of pretreated

TWAS samples at the beginning of the BMP tests to the

control likely provides valuable information on the possi-

ble toxicity effects of MAP at different conditions (Table 2)

corresponding to different levels of floc solubilization and

disruption.

The CMP results are presented in Fig. 2. Daily methane

production from the blank was subtracted from daily

methane production from the mixtures to obtain methane

from TWAS samples only. As shown in Fig. 2, starting with

pretreated TWAS not only affected the rate of methane

production but also affected the extent of TWAS digestion.

In the pretreatment ranges (Table 2), all digesters fed with

pretreated TWAS improved methane production compared

to the control system, but the MAP pretreated TWAS of

Trial 9 (at 210°C with 0.15 g NaOH/g SS and 11 min

holding time) produced the highest CMP (319 mL@STP/g

VSadded (at standard temperature and pressure (STP) con-

ditions of 0°C and 101.523 kPa), which was 30% higher

than the control after 18 days of digestion. At the end of

30 days, in Trial 10 (at 210°C with 0.1 g NaOH/g SS and

41 min holding time) the highest CMP (331 mL@STP/g

VSadded) was achieved, which was 27% more than the

control, followed by Trial 9, Trial 2, and Trial 11.

Results also indicated that within a practical time of

0–10 days, the CMPs of Trials 2, 4, 7, 8, 9, 10 and 12

were lower than that of the control (Fig. 2). The lag-phase

time of Trials 2, 4, 7, 8, 9, 10 and 12 were greater than

zero (Table 2). This implied that some product or products

were formed in these trials, which resulted a mild short-

term microbial inhibition. According to several authors,

the optimal temperature of thermal treatment is around

170–200°C (Li and Noike, 1992; Neyens and Baeyens,

2003). Indeed, at higher temperatures, the biodegradability

of sludge does not improve but may actually decrease due

to the formation of refractory compounds linked to “burnt

sugar” reactions and the Maillard reaction caused by high

pretreatment temperatures (Bougrier et al., 2007; Eski-

cioglu et al., 2006). Among other refractory compounds,

it has been suggested that melanoidin is produced (Penaud

et al., 2000), resulting in a dark brown liquor. Melanoidins

are formed during the final stage of the Maillard reaction.

0 5 10 15 20 25 30 35 40

400

350

300

250

200

150

100

50

0

Time (days)

Cum

ula

tive

met

han

e pro

duct

ion

(mL

@S

TP

/gV

Sad

ded

)

1#2#3#4#5#6#

8#9#10#11#12#

7#Control

Fig. 2 Cumulative methane production for the control and pretreated

sludge with time. The curve codes correspond to the trial number in Table

2.


The primary mechanism of color formation during the

Maillard reaction is the polymerization of low molecular

weight intermediates, such as carbohydrates and amino

compounds, with the formation of compounds with a

molecular weight of 50–70 kDa (Dwyer et al., 2008).

Relative to control, toxicities of soluble fractions (<0.45

μm) from microwave pretreatment (at 45°C, 65°C, 100°C)

anaerobically digested sludge, primary sludge, and WAS

have been previously studied and compared by Hong et

al. (2006). Hong’s study implied that the toxicity resulted

from the substances leaking from the sludge to the solu-

ble phase after microwave pretreatment, which has been

supported by other soluble phase studies (Eskicioglu et al.,

2006).

In this study, acute inhibition appeared during the first

10 days in Trials 2, 4, 7, 8, 9, 10 and 12 (Fig. 2), which

is in line with results of lag-phase time (Table 2), when

the TWASs were pretreated at 210°C (Trials 9, 10) or

with 0.15–0.25 g NaOH/g SS (Trials 2, 4, 7, 8, 9 and 12).

This suggests that to reduce the formation of the refractory

compounds in MAP, the target temperature should be

lower than 210°C and the alkali dose should be lower than

0.15 g NaOH/g SS. In Trials 1, 3, 5, 6, and 11, CMPs

were not inhibited acutely by the addition of NaOH. This

indicated that the addition of NaOH (at 0–0.10 g NaOH/g

SS) did not cause the acute inhibition of the succeeding

methane production process. For alkaline pretreatment,

dewaterability of WAS deteriorated with increasing pH

(Dogan and Sanin, 2009). If we use the high-dose NaOH,

the pretreatment costs will increase and pretreated TWAS

will need to be neutralized before anaerobic digestion.

To overcome the disadvantages of high-dose NaOH and

utilize the synergetic advantages of combined MAP, 0.05 g

NaOH/g SS was suggested for MAP of TWAS.

At the end of 30 days, the methane content of biogas

produced in the BMP tests fluctuated in the range of

65%–80%. The extent of CMP was limited in the range

of 291–331 mL@STP/g VSadded for the vials fed with

pretreated TWAS. Therefore, CMPs from the BMP tests

were within the range determined by other pretreatment

studies (Eskicioglu et al., 2007b). Compared with the

control, the highest CMP was improved by 27%. Since the

ultimate CMP of Trial 11 was 315 mL@STP/g VSadded,

which only decreased by 5% compared with the highest

CMP among all trials.

According to Eq. (2), the target temperature was a

very influential parameter on VSS solubilization. Wang

and Wang (2005) concluded by single factor experiment

that the more the target temperature increased, the more

the solubilization ratio of WAS increased in the thermal

pretreatment of WAS. To increase the solubilization ratio

of WAS and avoid producing more refractory compounds,

some researchers suggested the optimum pretreatment

temperature should be 170°C. Moreover, TWAS pretreated

by Trial 11 resulted in the maximum CBP among the Trials

1, 3, 5, 6, and 11, in which CMPs were not inhibited

acutely. Therefore, 170°C was chosen as the appropriate

target temperature.

Since holding time (x2) was not included in the optimal

regression equation, Eq. (2), the holding time (x2) did not

creat a statistically significant effect on TWAS solubiliza-

tion as measured by the solubilization ratio of VSS at the

95% confidence level. This implies that VSS hydrolysis

took place within a short holding time. The finding of this

study was in agreement with the results obtained by single

factor tests (Qiao et al., 2008). For the sake of reducing the

reactor volume and cutting treatment cost, 1 min holding

time was chosen for the further studies.

With respect to the following three considerations: (1)

improving anaerobic biodegradability, (2) reducing for-

mation of the refractory compounds, and (3) reducing

pretreatment operation costs and not degrading the dewa-

terability of TWAS, 170°C with 0.05 g NaOH/g SS and 1

min holding time were suggested for MAP of TWAS.

2.3 Comparison of semi-continuous TADP

Table 5 summarizes the steady state data for the process-

es fed with the raw and pretreated TWAS. For subsequent

discussions, microwave-TADP denotes the operation of the

digester fed with the MAP pretreated TWAS at 170°C with

0.05 g NaOH/g SS and 1 min holding time.

The two processes were evaluated with regard to the

following: (1) process stability and inhibition, (2) organic

matter reduction and methane production, (3) water quality

of effluent supernatant, and (4) flocculation efficiency of

thermophilic digested sludge.

Since the raw and pretreated TWAS were used as the

substrate in this study, the VS level may have been high,

therefore ammonia inhibition needed to be assessed. It

have been reported that methane production is inhibited by

2000 mg/L NH4+-N at 55°C (Han et al., 2007). As shown

Table 5 Steady state data for the control TADP and microwave-TADP

Parameter Control TADP Microwave-TADP

Organic matter reduction

VS (%) 40 51

TCOD (%) 44 52

Methane production

Methane yield 11.48 ± 0.10a 14.97 ± 0.10

(L@STP/L TWASadded)

Methane content (%) 60.97 ± 0.61 65.80 ± 0.30

CH4/VSadded 0.23 ± 0.01 0.27 ± 0.01

(L@STP/g VSadded)

CH4/CODadded 0.15 ± 0.01 0.17 ± 0.01

(L@STP/g CODadded)

Effluent supernatant

pH 7.49 ± 0.01 7.88 ± 0.01

SCOD (mg/L) 5100 ± 967 7700 ± 43

Soluble carbohydrates (mg/L)b 800 ± 174 1300 ± 125

Soluble proteins (mg/L)b 2100 ± 310 2200 ± 42

VFAs (mg/L)b 163 ± 16 435 ± 36

NH4+-N (mg/L) 1900 ± 36 2000 ± 35

Alkalinity (mg CaCO3/L) 5600 ± 41 7900 ± 170

Flocculation efficiency

Flocculant dose (g/kg TS) 13.1 23.4

Dewatered supernatants

SCOD (mg/L) 1940 ± 40 1540 ± 30

TADP: thermophilic anaerobic digestion process; VS: volatile solids;

TCOD: total chemical oxygen demand; STP: standard temperature and

pressure; TWAS: thickened waste activated sludge; COD: chemical

oxygen demand; SCOD: soluble chemical oxygen demand; TS: total

solids.a mean ± standard deviation (n � 3); b as COD.


in Table 5, the levels of NH4+-N were 1900 and 2000 mg/L

for the control system and microwave-TADP, respectively.

This demonstrated that the amount of ammonia was within

a safe level in this study. The VFAs to alkalinity ratio for

the two systems were monitored to compare the buffering

capacities. It has been reported that buffering capacity is

sufficient when the VFAs to alkalinity ratio is maintained

below 0.4 (Sachez et al., 2005; Schoen et al., 2009). In this

study, the VFAs to alkalinity ratios of both processes were

below 0.1, which was considered to be favorable operating

conditions without the risk of acidification. This indicates

that the buffering capabilities of the control system and

microwave -TADP were sufficient at HRT 30 days.

For organic matter reduction, the VS and total TCOD

reductions for the microwave-TADP were 28% and 18%,

respectively, higher than those of the control system. These

reductions were calculated based on the constant values

of VS and TCOD fed to the reactors daily. Meanwhile,

the methane yield (L@STP/g-CODadded) and (L@STP/g

VSadded) were 17% and 13% higher than those of the

control system, respectively. Therefore, organics inside

the cells were released as a result of cell lysis by MAP,

thereby increasing their biodegradability. In other words,

the pretreated sludge was more susceptible to microbial

attack primarily by acidogens than the control sludge,

thus providing a better substrate for methanogens and

enhancing overall methane yield.

Toreci et al. (2009) studied the effect of microwave

pretreatment (175°C) on semi-continuous MAD efficiency

at different sludge retention times (SRTs) (20, 10, and 5

days). It was reported that the improvement in biogas yield

(L@STP/kg CODadded) of WAS microwave pretreated at

175°C compare to the control (untreated WAS, 20 days

SRT) increased by 5.4%, –0.5%, and –22.8%, as SRT

decreased from 20 to 10 and 5, respectively. A similar

trend was observed for COD removal efficiency. The im-

provement in COD removal efficiency of WAS microwave

pretreated at 175°C compare to the control (untreated

WAS, 20 days SRT) increased by 19.2%, –16.5%, and

–42.5%, as SRT decreased from 20 to 10 and 5, respec-

tively.

The minimum generation time of methanogens is longer

than that of the acidogenic bacteria. When the SRT of

the anaerobic digestion reactors decreased, the number of

methanogens in the reactors decreased too. Therefore, the

biogas yield and organics removal efficiency decreased. To

increase the rate of digestion and the extent of organics

destruction of WAS microwave pretreated, the SRT of the

anaerobic digestion reactors in the range of 20 and 30 days

was suggested.

As discussed above, VS reduction is expected to occur

in the total mass of sludge. Therefore, with the assumption

of 70% organic matter present in sludge prior to pretreat-

ment and digestion, and with 51% of organics destroyed

in microwave-TADP and 40% of organics destroyed in

control reactors, it is possible to reduce the total sludge

mass by about 36% in microwave-TADP and 28% in the

control system. This kind of reduction would be reflected

in savings in transportation and in reduced disposal costs

of sludge in a full scale system.

To understand the bioconversion process of organic

matter in both digesters, the COD mass balance, which

is based on the feed and effluent COD and methane

COD values, is illustrated in Fig. 3. The microwave-

TADP effluent had a ratio of particulate COD (PCOD)

to influent TCOD of 35%, which was less than that in

the control effluent (45%). The methane to the feed COD

ratio in the microwave-TADP was about 14% higher than

that of the control system. This implies that microwave-

TADP improved the degradability of organics and the

conversion ratio of organics to methane. Furthermore, the

COD balance indicated good agreement between these

measurements and further demonstrates that the COD

destroyed was converted to methane instead of being

consumed via alternative routes (i.e., sulfate reduction).

Hydrogen sulfide was not detected in the gas phase of these

reactors.

As seen in Table 5, the pH of the microwave-TADP was

higher than the control. A relative effluent SCOD improve-

ment of 51% was achieved in the microwave-TADP. This

is in line with previous findings that pretreatment methods

increase the SCOD. Since the typical application is to

recycle the digester supernatant to the head of the treatment

plant rather than to dispose of it directly, this input needs

to be carefully investigated to determine the suitability of

the plant for such an application.

The flocculation efficiency was 23.4 g flocculants/kg

TS in microwave-TADP and 13.1 g flocculants/kg TS in

TADP, respectively. This indicated that the dewaterability

of digested sludge from microwave-TADP was lower than

the control. As shown in Table 5, the concentrations of

soluble proteins and carbohydrates in microwave-TADP

were higher than those in the control reactor. It was also

possible that MAP creates a higher amount of interme-

diate molecular size organic materials (such as, colloids)

between soluble (< 0.45 μm) and particulates (> 0.45

μm) (Eskicioglu et al., 2007b). Sludge dewaterability was

affected by both colloid and soluble organic matters. An

increase of colloid and soluble organic matters results

in the deterioration of sludge dewaterability (Shao et al.,

0

20

40

60

80

100

120

Raw TWAS Control

TADP

Pretreated

TWAS

Microwave

-TADP

Rec

over

y f

rom

infl

uen

t (

%)

86

45

64

35

14

11

36

13

4349

PCOD SCOD Methane

Fig. 3 COD mass balance comparison of raw TWAS, pretreated TWAS,

control TADP, and microwave-TADP.


2009). Dewatered supernatant was obtained by 10 min

centrifugation (3,000 r/min) of dewatered sludge from

the flocculation efficiency test. No significant difference

between microwave-TADP and the control in terms of

the water quality of dewatered supernatant was observed.

According to flocculation efficiency, the dewaterability of

digested sludge from microwave-TADP was lower than

that of the control.

It is not possible to draw a conclusion regarding the

economic viability of MAP as a pretreatment method from

this study. This is because it was a lab scale experiment

conducted with unfocused microwave generators. Industri-

al scale microwave generators are generally more efficient

than lab scale units. Additional factors such as plant

capacity, sludge characteristics, plant energy requirements,

disposal costs and regulatory requirements are major fac-

tors that would affect the economic analyses. To feasibly

incorporate MAP into a system, it is very important to use

the excess heat that is produced after microwave heating.

3 Conclusions

The results of this study indicated that MAP has the po-

tential to damage TWAS floc structure and cell membranes

and to release extracellular and possibly intracellular com-

pounds with high solubility. The MAP also increased the

bioavailability of TWAS components under thermophilic

BMP tests. Results related to the performances of the semi-

continuous TADP experiments implied, in spite of the

increase in effluent SCOD and decrease in dewaterability

of digested sludge, that MAP at 170°C with 1 min holding

time and 0.05 g NaOH/g SS has the potential to increase

the biodegradability of TWAS in full-scale, continuous-

flow thermophilic digestion digesters. The findings of this

study contribute to the development of new pretreatment

techniques in the field of TWAS treatment technology

through thermophilic anaerobic digestion.

Acknowledgments

This work was supported by the Natural Science

Foundation of Tianjin, China (No. 08JCYBJC13200) and

the National Natural Science Foundation of China (No.

50808128). We would like to thank the staff for their

continual assistance in preparing and carrying out this

research in the Department of Civil and Environmental

Engineering, Tohoku University.

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