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Enhancement of waste activated sludge aerobic digestionby electrochemical pre-treatment

Li-Jie Song a, Nan-Wen Zhu a,*, Hai-Ping Yuan a, Ying Hong b, Jin Ding a

a School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR Chinab 4208 Belle Grove Ct, Orlando, FL 32812, United States

a r t i c l e i n f o

Article history:

Received 15 March 2010

Received in revised form

26 May 2010

Accepted 29 May 2010

Available online 10 June 2010

Keywords:

Waste activated sludge (WAS)

Electrochemical pre-treatment

Aerobic digestion

Scanning electron microscope (SEM)

Infrared (IR) spectra

* Corresponding author. Tel.: þ86021 5474281E-mail address: nwzhu@sjtu.edu.cn (N.-W

0043-1354/$ e see front matter ª 2010 Elsevdoi:10.1016/j.watres.2010.05.052

a b s t r a c t

Electrochemical technology with a pair of RuO2/Ti mesh plate electrode is first applied to

pre-treat Waste Activated Sludge (WAS) prior to aerobic digestion in this study. The effects

of various operating conditions were investigated including electrolysis time, electric

power, current density, initial pH of sludge and sludge concentration. The study showed

that the sludge reduction increased with the electrolysis time, electric power or current

density, while decreased with the sludge concentration. Additionally, higher or lower pH

than 7.0 was propitious to remove organic matters. The electrochemical pre-treatment

removed volatile solids (VS) and volatile suspended solids (VSS) by 2.75% and 7.87%,

respectively, with a WAS concentration of 12.9 g/L, electrolysis time of 30 min, electric

power of 5 W and initial sludge pH of 10. In the subsequent aerobic digestion, the sludge

reductions for VS and VSS after solids retention time (SRT) of 17.5 days were 34.25% and

39.59%, respectively. However, a SRT of 23.5 days was necessary to achieve equivalent

reductions without electrochemical pre-treatment. Sludge analysis by Scanning Electron

Microscope (SEM) images and infrared (IR) spectra indicated that electrochemical pre-

treatment can rupture sludge cells, remove and solubilize intracellular substances, espe-

cially protein and polysaccharide, and consequently enhance the aerobic digestion.

ª 2010 Elsevier Ltd. All rights reserved.

1. Introduction Koscielniak, 1997; Bernard and Gray, 2000). However,

Waste Activated Sludge (WAS) generated during wastewater

treatment should be stabilized sufficiently to reduce its

organic content, pathogen contamination and odor problems

prior to ultimate disposal (Vlyssides and Karlis, 2004; Fytili and

Zabaniotou, 2009; Li et al., 2009). The most common methods

of sludge stabilization are biological processes of anaerobic

digestion and aerobic digestion. Compared with anaerobic

digestion, simplicity of process and lower capital costs are the

advantages of aerobic process. Aerobic digestion has been

a popular option for small or medium-sized wastewater

treatment plants because of these merits (Barbusinski and

7; fax: þ86 021 34203732. Zhu).

ier Ltd. All rights reserved

conventional aerobic digestion still requires large digestion

tanks due to its relatively long retention time (15e30 days) (Jin

et al., 2009).

During sludge digestion, the hydrolysis of large organic

molecules associated with microbial cells has been proven as

the rate-limiting step (Eastman and Ferguson, 1981; Shimizu

et al., 1993; Tiehm et al., 2001; Gronroos et al., 2005; Park and

Novak, 2007; Appels et al., 2008). Therefore, the pre-treat-

ment which disintegrates sludge flocs and disrupt microbial

cell walls of sludge was developed to improve subsequent

biological digestion. Several successful sludge disintegration

technologies include alkaline treatment (Lin et al., 1997, 1998;

.

.

wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 4 3 7 1e4 3 7 84372

Li et al., 2008), thermal treatment (Kim et al., 2003; Bougrier

et al., 2006; Salsabil et al., 2010), alkaline combined with

thermal hydrolysis (Neyens et al., 2003a; Vlyssides and Karlis,

2004), ultrasonic treatment (Wang et al., 1999; Neis et al., 2000;

Tiehm et al., 2001; Sangave et al., 2007; Yu et al., 2008; Jin et al.,

2009), ozone oxidation(Arodi et al., 2007; Dytczak and

Oleszkiewicz, 2008), hydrogen peroxide (Neyens et al., 2003b)

and biological hydrolysis with enzymes (Ucisik and Henze,

2008).

As a result of the more stringent environmental regula-

tions on the discharge of industrial and municipal waste-

water, electrochemical technology is considered a powerful

means of pollution control and has been widely used. It has

shown great versatility, high removal efficiency, lower

temperature requirement and environmental compatibility.

Themain regent, the electron, is clean (Rajeshwar et al., 1994).

In some situations, the technology may be the indispensable

step for the treatment of industrial effluents which contain

bio-refractory organic pollutants, such as landfill leachate

(Deng and Englehardt, 2007), phenol (Yavuz and Koparal,

2006), cyanides (Arellano and Martınez, 2007), cigarette

industry wastewater (Bejankiwar, 2002), textile (dye) waste-

water (Vlyssides et al., 2000; Korbahti, 2007), tannery waste-

water (Costa et al., 2008), etc.

Complete mineralization or partial degradation of organic

pollutants depends on the anode materials. It was reported

that the use of Ti/RuO2 anode produced a series of electro-

chemical steps which converted high biopolymer substances

to low-molecular-weight products. The low-molecular-weight

products then can be easily removed by the subsequent bio-

logical treatment (Torresa et al., 2003). It was indicated that

the combination of electrochemical and biological technology

might be a promising choice for the industrial wastewaters

that contain recalcitrant compounds.

Although the literature on electrochemical treatment of

activated sludge is rare, the capability of electrochemical

technique to decompose organic macromolecules to small

molecules observed from above studies may justify its appli-

cation in WAS treatment. In this study, electrochemical

method is first applied to pre-treat WAS, aiming to enhance

subsequent aerobic digestion. Firstly, the effects of electro-

chemical treatment on sludge reduction and solubilization

were evaluated and optimized under different electro-

chemical conditions. Secondly, the performances of aerobic

digestion of treated sludge and untreated sludge were

compared and assessed in terms of sludge reduction. The

feasibility of electrochemical pre-treatment on the enhance-

ment of WAS aerobic digestibility was finally discussed.

Table 1 e Characteristics of sludge samples.

Parameter Value

pH 6.69e7.07

Moisture content (%) 99.2e98.2

Conductivity (mScm�1) 794e1148

Chemical oxygen demand (COD) (mgL�1) 17,462e18,990

Soluble chemical oxygen demand (SCOD) (mgL�1) 36e52

Total solid (TS) (gL�1) 8.0e18.2

Volatile solid (VS) (gL�1) 5.5e12.7

Suspended solid (SS) (gL�1) 7.6e17.0

Volatile suspended solid (VSS) (gL�1) 5.4e12.6

Organic content (VSS/SS) (%) 0.68e0.74

2. Materials and methods

2.1. Sludge samples

In this study,WASwas obtained from the sludge returnwell of

the secondary clarifier of a municipal wastewater treatment

plant in Shanghai, China. The plant treats 50,000 m3d�1 of

wastewater with the anaerobic-anoxic-aerobic process. The

sludge samples were thickened to required solid concentra-

tions and stored at 4 � 1 �C prior to use. Maximum sludge

storage period was one week. Table 1 shows the characteris-

tics of sludge samples.

2.2. Electrochemical pre-treatment

All electrochemical experiments of waste activated sludge

were carried out in a 500 mL single-compartment glass cell.

Both the anode and the cathode were a pair of Ti/RuO2 mesh

plate electrodes of 7.0 � 10.0 cm2 size. The current was

supplied by a highly stable power unit (WYJ. 5 A 60V DC.

REGUL. ATED. POWER SUPPLY, Shanghai, China). Copper

wires were used for electrical circuit. During the experiments,

air was bubbled with an air pump (AIR PUMP, X-6500) to avoid

sludge settling and alleviate anode passivation.

Experiments were carried out at ambient temperature. All

samples were performed in triplicate and average, standard

deviation were calculated for each sample.

2.3. Aerobic digestion reactor

Aerobic digestion experiments were carried out in two plex-

iglass cylinders with an effective volume of 5 L each (Fig. 1).

One reactor was filled with control sample, and the other one

was filled with electrochemical pretreated sludge. After

adjusting the sludge pH to 7.0 approximately, inoculations

were performed with microbial consortia of 2% (V/V) of fresh

activated wastewater sludge at a solids concentration of

25 gL�1. The digesters were aerated by an air compressor (AIR

PUMP, X-6500) to maintain an uniform oxygen concentration

of 2 mgO2L�1 and good mixing between the electrochemically

treated sludge and the biomass. The digesters were operated

at room temperature (20e28 �C) for 28 days. Oxygen concen-

trationwasmonitored by anO2 probe at the top of the column.

Periodic samples (150mL each) were taken from the biological

reactor, filtered and analyzed for total solids (TS), volatile

solids (VS), suspended solids (SS) and volatile suspended solid

(VSS). During the period of incubation, the volume loss due to

evaporation was readjusted with distilled water.

2.4. Analytical methods

All analyses were evaluated using chemicals of analytical

grade. pH, TS, VS, SS, VSS, and SCOD were determined by the

StandardMethods (APHA et al., 1998). TP was determinedwith

the ammonium molybdate spectrophotometric method. TN

Fig. 1 e Experimental device for aerobic digestion.

wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 4 3 7 1e4 3 7 8 4373

was measured with the alkaline potassium persulphate

digestion-UV spectrophotometric method, and ammonia

nitrogen was determined by the Nessler’s reagent spectro-

photometricmethod. The sludge pHwasmeasured using a pH

meter (pHs-3C, Leici Co., Ltd., Shanghai, China). Conductivity

was determined by a conductivitymeter (DDSJ-308A, Leici Co.,

Ltd., Shanghai, China). The samples were centrifuged at 5000g

for 30min and then filtered through a 0.45 mmmembrane. The

filtrate was collected to measure SCOD, TP, TN and NH4þ-N.

2.5. Evaluation

The efficiencies of electrochemical pre-treatment and aerobic

digestion were evaluated by measuring the changes in terms

of SCOD, VS and VSS. VS or VSS reductions were calculated as

follow:

VS removal ¼ VS0 � VSVS0

� 100% (1)

VSS removal ¼ VSS0 � VSSVSS0

� 100% (2)

where VS0, VSS0 represented the VS, VSS concentration prior

to electrochemical pre-treatment or aerobic digestion.

Fig. 2 e Effect of the electrolysis time on the sludge organic

degradation (Note: error bars represent standard

deviation).

3. Results and discussion

3.1. Optimizing of electrochemical pre-treatmentconditions

The effects of electrochemical treatment on sludge disinte-

gration were studied to evaluate: the optimal pre-treatment

conditions in terms of organic matter solubilization and the

possible improvement of digestion.

3.1.1. Effect of electrolysis timeInvestigation of the effect of electrolysis time on the organic

degradation was carried out on sludge samples with

a concentration of 8.2 gL�1, at a constant electric power (5 W)

and pH 6.90. As shown in Fig. 2, the differences between VSS

and VS removals indicated the quantity of organic matters

solubilized to the liquid. It increasedwith the time, whichwas

consistent with the change of soluble chemical oxygen

demand. It was noted that the removal efficiencies of VS and

VSS increased rapidly within the time frame of less than

30 min, then increased at a slower pace than before. The

removal efficiencies of VS and VSS for the first 30 min were

2.4% and 4.9%, respectively, while they were only 3.8% and

8.3% after 240 min. Therefore, an electrolysis time of 30 min

was advisable for subsequent studies due to the consideration

of reactor volume and power cost.

3.1.2. Effect of electric powerElectric power reflects the energy input to the sludge treat-

ment system and its influence on the sludge disintegration

and reduction was investigated with a concentration of

12.7 gL�1, electrolysis time of 30 min, at an initial sludge pH of

6.70. The removal efficiencies of VS and VSS increased with

the increasing of electric power as shown in Fig. 3, and similar

trend was observed for SCOD. The results can be explained by

the composition of suspended solids: the mineral matters

constituted 20e30%, and the organic matters was 70e80%.

Only a small fraction of organic matters in the suspended

solids could be hydrolysable with an electric attack probably

due to refractory organic compounds. Higher power supplied

could lead to more organic matters decomposed and solubi-

lized. When electric power was greater than 5 W, the effect of

power on sludge reduction seems to level off. At the power

input of 5 W, the VS and VSS removal were 2.3% and 4.8%,

respectively, while they only increased to 3.5% and 7.5% at

power input of 14 W. Therefore, 5 W of electric power was

selected for the subsequent studies.

Fig. 3 e Effect of the electric power on the sludge treatment

(Note: error bars represent standard deviation).

Fig. 4 e Effect of the addition of Na2SO4 on the removal of

VS or VSS: (a) VS and VSS removal efficiencies (Note: error

bars represent standard deviation); (b) current density and

temperature after electrolysis.

wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 4 3 7 1e4 3 7 84374

3.1.3. Effect of current densityDifferent dosages of Na2SO4 were added to the sludge

(13.5 gL�1) to investigate the effect of current density on sludge

reduction. The experiment was carried out with 30 min elec-

trolysis time and a constant electric voltage of 12 V, at an

initial sludge pH of 6.70, and temperature of 19 �C. The results

were shown in Fig. 4. Both VS and VSS removal efficiencies

increased with the increasing of the current density as

indicted from Fig. 4. The removal efficiencies of VS and VSS

were sharply increased at low Na2SO4 concentration range of

0.004e0.04 molL�1, followed by a placid increase with

increasing the Na2SO4 concentration to 0.16 molL�1 where the

removal efficiencies were 4.75% and 16.1%, respectively. As

a result, the variation of Na2SO4 caused the increase of electric

power. Moreover, the temperature after electrolysis increased

from the beginning of 19 �C to 37 �C with the current density,

indicating electric energy converting to heat during the elec-

trolysis process. The temperature change is not obvious in the

pre-treatment without Na2SO4. The addition of Na2SO4 is to

investigate the impact of current density on electrochemical

pre-treatment with a constant electric voltage. It was not

applied to the subsequent optimizing experiments due to

temperature increase and additional chemical costs.

3.1.4. Effect of the initial sludge pHThe initial pH of sludge samplewas adjusted to 2, 3, 4, 5, 6, 7, 8,

9, 10, 11, 12 and 13 using 1 molL�1 of sodium hydroxide or

sulfuric acid solution, respectively. The operating conditions

were as following: sludge concentration 11.7 mgL�1, electric

power (4.9e5.2) W, electrolysis time 30 min and at ambient

temperature. The removal of the organic solid at each pH is

schematically shown in Fig. 5. As was evident, both of VS and

VSS removal efficiencies increased with the increasing of pH

under alkaline condition or the decreasing of pH in acid range.

When pH was 12.94 or 1.99, the higher removal efficiencies of

VS and VSS could be achieved, 5.08% and 17.81%, 5.16% and

8.13% respectively. While the initial sludge pH was neutral,

6.97, those of VS and VSS were the lowest and were 2.35% and

4.70%, respectively. That was to say, alkali and acid could

enhance the electrochemical pre-treatment of the sludge. In

alkaline treatment, hydroxy anions can destroy floc structures

and cell walls, resulting in natural shape losing of proteins,

saponification of lipid and hydrolysis of RNA. Neyens et al.

(2004) and Erdincler and Vesilind (2000) examined chemical

degradation and ionization of the hydroxyl groups

(eOH/eO�) could cause extensive swelling and subsequent

solubilization of gels in sludge, and after the destruction of

extracellular polymer substances, the cell walls being exposed

to a high pH could not withstand the appropriate turgor

pressure resulting in the disruption of cells and release of

intracellular substances. Gasco et al. (2007) also observed that

acid treatment caused themodifications in the organicmatter

composition of sewage sludge. Additionally, the increasing of

ion concentrations in the reactors increased due to the addi-

tion of NaOH or H2SO4 perhaps contributed to the sludge

reduction. In view of removal efficiency and chemicals cost,

the pH of 10 was chosen as the optimal pH, where the removal

efficiencies were 2.9% and 8.4%, respectively.

Fig. 5 e Effect of the initial sludge pH on the removal of VS

or VSS (Note: error bars represent standard deviation).

wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 4 3 7 1e4 3 7 8 4375

3.1.5. Effect of the initial sludge concentrationTo evaluate the impact of the initial sludge concentration on

the degradation of organic matters by electrochemical treat-

ment, experiments were conducted by varying initial solid

concentrationswith a sludge pH of 10, 30min electrolysis time

and a constant electric power of 5 W. Sludge samples with the

certain concentration could be obtained by being centrifuged

at 2000g for 15 min or being diluted with the supernatant.

Fig. 6 indicated the effect of initial sludge concentration on

electrochemical treatment. Both the removal efficiencies of

VS and VSS decreased with the sludge concentration

increased. The results suggested that at higher organic solid

concentrations, mass transfer limitation may be inhibit the

rate of electrochemical degradation.

Fig. 6 e Effect of initial sludge concentration on

electrochemical treatment (Note: error bars represent

standard deviation).

3.2. Performances assessment of aerobic digestion ofelectrolyzed sludge

The ability of electrochemical pre-treatment to solubilize or

remove particulate organic matter has been demonstrated by

the experiments in the first part of the paper, the comparison

and analysis of the treated sludge aerobic biodegradability

were required to optimize the coupling process of electro-

chemical pre-treatment and biodegradation.

3.2.1. Comparison of aerobic digestion performances betweenelectrochemical pre-treatment and controlThe electrochemical pre-treatment was carried out in the

following conditions: sludge concentration 12.9 gL�1, elec-

trolysis time 30 min, pH 10.0 and electric power 5 W. Fig. 7(a)

shows the changes in characteristics of sludge samples after

electrochemical pre-treatment. Since a small portion of the

organic solids was degraded or solubilized into the superna-

tant after electrochemical pre-treatment, the initial VS and

VSS concentrations of sludge samples after electrochemical

pre-treatment were slightly lower than the control samples

but different (P < 0.05), and their reduction percentages were

2.2% and 5.9%, respectively. In accordancewith it, COD, TP, TN

and NH4þ-N of the supernatant after electrolysis (shown in

Table 2) increased in some extent.

Fig. 7 e Comparison of electrochemical pretreated sludge

and control sludge (Note: error bars represent standard

deviation) (a) electrochemical pre-treatment (Note: the P

values of TSS, VS and VSS are 0.04, 0.03, 0.02, respectively);

(b) aerobic digestion.

Table 2 e Comparison of the water quality of supernatant(mgLL1).

SCOD TN TP NH4þ-N

Control 44 � 5 12.8 � 0.3 1.35 � 0.1 7.8 � 0.4

Electrochemical

pre-treatment

348 � 8 26.5 � 1.6 13.8 � 0.7 9.2 � 0.6

P 4.7E-07 1.1E-04 5.5E-06 0.033

Fig. 8 e SEM images of sludge cells: (a) control;

(b) electrochemical pretreated sludge.

wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 4 3 7 1e4 3 7 84376

Fig. 7(b) depicts the comparison of sludge aerobic digestion

performances between electrochemical pretreated sludge and

the control sample. Pretreated samples showed higher

removal efficiency from the very beginning of digestion.

During 1.65 d digestion, VS and VSS removed about 10.2% and

12.25%, respectively, for samples with electrochemical pre-

treatment, while VS and VSS of control samples reduced

about 6.13% and 8.38%, respectively. Pretreated sludge

reduction for VS and VSSwas 34.25% and 39.59%, respectively,

with electrochemical pre-treatment compared with 28.32%

and 33.79%, respectively, with control at the digestion time of

17.5 d. This is probably due to the release of soluble organic

carbon sources which were more biodegradable. In order to

achieve a VSS reduction greater than 38%, the requirement of

United States Environmental Protection Agency (U.S. EPA)

regulation, 23.5 d was theminimumdigestion time for control

sludge. Therefore, the electrochemical pre-treatment could

significantly enhance sludge biodegradability and aerobic

digestion efficiency.

3.2.2. Hypothesis on mechanism of improved aerobicdigestion by electrochemical pre-treatmentFig. 8 shows the scanning electron microscope (SEM) images

of control and electrochemical pretreated sludge cells. The

difference in cell appearance was obvious. The surface of

sludge cells (Fig. 8(a)) was relatively round and smooth, while

that of electrochemical pretreated sludge cell was deformed,

indicating that the sludge cell was broken by electrochemical

treatment and intracellular substances would be then solu-

bilized into the solution which could be readily utilized by

aerobic microorganisms.

As shown in Fig. 9, the Infrared (IR) spectra of the control

sludge sample reveal a number of absorption peaks, indi-

cating the complex nature of the sludge. The main absorption

band at 3800e2500 cm�1 was a symbol of eOH in the carboxyl

group (Tirkistani, 1998; Padmavathy et al., 2003; Choi and Yun,

2006). The absorption peak at 1384 cm�1 could be attributed to

the symmetrical stretching of the carboxylate anion (Choi and

Yun, 2006). The existence of phosphonate group was proved

from some absorption bands (P]O stretching at 1164 cm�1;

PeOH stretching at 941 cm�1; PeOeC stretching at 1047 cm�1)

(Pagnanelli et al., 2000). The IR spectra of the Sludge also dis-

played some characteristic absorption bands of amine group

(Choi and Yun, 2006): NeH bending band at 1662 cm�1; NeH

out of plane bending band near 700 cm�1; and CeN stretching

band at 1238 cm�1. The stretching band of NeH in the range of

3500e3300 cm�1 was completely shielded by the strong and

large band of carboxyl group in the range of 3800e2500 cm�1

Fig. 9 also shows the effect of electrochemical pre-treatment

on sludge. It was apparent that the characteristic absorption

bands above-mentioned became significantly weaker after

electrochemical pre-treatment, which was the indicative of

the degradation or solubilization of organic solids by the

electrochemical pre-treatment, especially carboxylate,

protein and polysaccharide.

3.3. Cost analysis

Electrochemical pre-treatment is feasible and can be cost

effectively based on the literature and this work. The total cost

of pre-treatment sludge technology would be less than that of

non-treated sludge as the energy consumption costs of the

two methods are comparable while the capital cost of the

digestion reactor with sludge pre-treatment is less than that

of non-treated sludge as the footprint of the former method is

smaller than the latter. 1) Regarding energy consumption,

Conventional Aerobic Digestion (CAD) requires 18e25 d of

sludge retention time, sometimes even 30 d, with the condi-

tions of 20 �C temperature, an oxygen concentration of no less

than 2 mg O2L�1 and 2% solid content. The total energy

consumption of CAD under such conditions is approximately

10e15 kW h m�3 (Environmental Protection Department,

2010). Based on this experiment results, the total energy

consumptions of pre-treatment technology would be

Fig. 9 e Infrared spectra of sludge samples after electrochemical pre-treatment.

wat e r r e s e a r c h 4 4 ( 2 0 1 0 ) 4 3 7 1e4 3 7 8 4377

approximately 12e16 kW h m�3, with electrolysis of approxi-

mately 5 kW h m�3 and aerobic digestion of 7e11 kW h m�3 2)

Most importantly, electrochemical pre-treatment can signifi-

cantly decrease aerobic digestion time. The digestion time

after electrochemical pre-treatment decreased from 23.5 d to

17.5 d as shown in the paper, in other words, electrochemical

pre-treatment could reduce roughly 26% of aerobic digestion

reactor volume, which is crucial for utilities with limited site

spaces, especially in Shanghai.

The purpose of this bench experiment is to reduce the

digestion time and reactor footprint. The detailed cost of the

technology will be evaluated in the subsequent pilot-scale

experiments.

4. Conclusions

The removals of VS and VSS and SCOD after electrochemical

pre-treatment increasedwith the increase of electrolysis time,

electric power or the addition of Na2SO4 chemicals. Addi-

tionally, the sludge reduction efficiencies decreased when

sludge concentration increased. The VS and VSS removal

efficiencies were 2.75% and 7.87%, respectively, for WAS with

a concentration of 12.9 gL�1, pH of 10, by 30 min electro-

chemical pre-treatment with an electric power of 5 W.

In the subsequent aerobic digestion, the sludge reductions

for VS and VSS were 34.25% and 39.59%, respectively, after an

aerobic digestion time of 17.5 d. However, an aerobic digestion

time of 23.5 d was necessary for the control samples to ach-

ieve equivalent reductions. The application of electrochemical

technique to WAS was proved to enhance the subsequent

aerobic digestion.

SEM images indicated that sludge cells were ruptured by

electrochemical pre-treatment, and intracellular substances

were solubilized into the solution which was readily utilized

by aerobic microorganisms. Additionally, compared with the

IR spectra of control sludge samples, electrochemical pre-

treatment could weaken the characteristic absorption bands

ofeOH in the carboxyl group, carboxylate anion, phosphonate

group and amine group. It revealed that organic matters, such

as protein and polysaccharide, solubilized into the

supernatant and enhanced its biodegradability. It is recom-

mended that additional work such as the biochemical analysis

should be done in the future studies.

Electrochemical pre-treatment could decrease the cost of

sludge aerobic digestion as a result of comparable energy

consumption and smaller aerobic digestion reactor volume

requirement.

Acknowledgements

This study has been supported by China Postdoctoral Science

Foundation (No.: 20090450698) and Shanghai Science and

Technology Commission (No.: 09dz1204104).

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