ww.sciencedirect.com

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 5 9 6 9e5 9 7 6

Available online at w

journal homepage: www.elsevier .com/locate/watres

Conditioning of wastewater sludge using freezing andthawing: Role of curing

Kai Hu a, Jun-Qiu Jiang a, Qing-Liang Zhao a,b,*, Duu-Jong Lee b,c,d, Kun Wang a, Wei Qiu a

a School of Municipal & Environmental Engineering, Harbin Institute of Technology, Harbin 150090, Chinab State Key Laboratory of Urban Water Resources and Environment (SKLUWRE), Harbin Institute of Technology, Harbin 150090, ChinacDepartment of Environmental Science and Engineering, Fudan University, Shanghai 200344, ChinadDepartment of Chemical Engineering, College of Engineering, National Taiwan University of Science and Technology, Taipei,

Taiwan 10617, China

a r t i c l e i n f o

Article history:

Received 14 April 2011

Received in revised form

18 August 2011

Accepted 29 August 2011

Available online 3 September 2011

Keywords:

Freeze/thaw treatment

Curing

Waste activated sludge

Mixed sludge

Solubilization

Dewaterability

* Corresponding author. School of Municipalþ86 451 86283017; fax: þ86 451 86282100.

E-mail address: qlzhao@hit.edu.cn (Q.-L.0043-1354/$ e see front matter ª 2011 Elsevdoi:10.1016/j.watres.2011.08.064

a b s t r a c t

Freeze/thaw (F/T) treatment is an efficient pre-treatment process for biological sludges.

When bulk sludge was frozen, tiny unfrozen regimes in the ice matrix were continuously

dehydrated by surrounding ice fronts, termed as the “curing stage”. This work demon-

strated that the F/T treatment could not only enhance sludge dewaterability, but also

solubilize organic matters from sludge matrix. Most enhancement of sludge dewaterability

was achieved during bulk freezing stage, with the waste activated sludge more readily

dewatered than the mixed sludges after treatment. Conversely, the freezing stage released

only limited quantities of organic matters to liquid. Conversely, the curing contributed

mostly on chemical oxygen demand (COD) solubilization and NH3eN release. The crys-

tallization of intra-aggregate moisture was claimed to damage cell membranes so to

release intracellular substances to surroundings. The F/T treatment with sufficient curing

is advised to effectively condition biological sludge as the feedstock of the following

anaerobic digestion process.

ª 2011 Elsevier Ltd. All rights reserved.

1. Introduction Studies on F/T for biological sludge and metal hydroxide

Organic matters hydrolysis presents the rate-limiting step in

sludge anaerobic digestion process (Elliott and Mahmood,

2007). Sludge pre-treatment techniques, including mechan-

ical (like sonication), chemical (such as alkali treatment),

thermal (heat treatment or freeze/thaw (F/T)) and biological

(enzymatic treatment), were studied in detail (Chu et al.,

2002a, 2002b; Whiteley and Lee, 2006). The F/T process pres-

ents a cost-effective sludge conditioning unit in case natural

freezing on sludge is feasible in field (Vesilind et al., 1991a;

Hedstrom and Hanaeus, 1999).

& Environmental Engine

Zhao).ier Ltd. All rights reserved

precipitates considered the associated changes in sludge

dewaterability (Martel and Diener, 1991; Parker et al., 1998a;

Wang et al., 2001; Kawasaki et al., 2004) and floc structure

(Vesilind et al., 1991a; Chang et al., 2004). Freezing tempera-

ture and time were revealed as two of the major factors that

influenced performance of F/T treatment. Wang et al. (2001)

demonstrated that improvement of sludge dewaterability

and degree of elution of intracellular water were more favor-

able at slow-frozen (�20 �C) than at fast-frozen (�80 �C) tests.Gao (2011) conducted bench scale experiments to examine the

effect of freezing temperature and freezeethaw cycles on the

ering, Harbin Institute of Technology, Harbin 150090, China. Tel.:

.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 5 9 6 9e5 9 7 65970

yielded sludge properties. During freezing, developing ice

front would partly engulf the floc and the force thereby built

up would compress the unfrozen part and pull apart the

network of the frozen part of sludge. By doing so, the sludge

was converted into a matrix of ice crystals and compacted

solid particles (Tao et al., 2006) with cell membrane integrity

damaged by the intracellular ice crystals (Thomashow, 1998).

The extracellular polymers (ECPs) were noted to release to the

sludge supernatant following F/T treatment (Hung et al., 1996;

Ormeci and Vesilind, 2001). Literature results demonstrated

that the enriched supernatant is favorable to enhance

anaerobic digestion of sludge (Montusiewicz et al., 2010).

When bulk sludge was completely frozen, there are tiny

unfrozen regimes in the ice matrix. Extended freezing, or

curing of the sludge, can significantly improve sludge dew-

aterability of drinking water residues (Vesilind and Martel,

1990; Parker et al., 1998b; Jean and Lee, 2000). In a city such

as Harbin, China, the average temperatures during November

to March are below freezing temperature (Wang et al., 2010),

making natural freezing a promising pre-treatment option to

sludge management. The sludge dumped into a freezing pool

during winter time will be kept frozen until it is thawed since

April and onward. Restated, during the whole year cycle with

complete freezing and complete thawing, most of the

freezing stage sludge will be in the curing state. Little atten-

tion was paid to unveil the effects of curing on the solubili-

zation of organic matters from biological sludges.

This study applied F/T treatment at �18 �C on wastewater

sludges and investigated the changes in physical and chem-

ical characteristics of sludge after F/T treatment. In particular,

the role of curing stage on sludge characteristics was clearly

demonstrated. Mechanisms corresponding to the noted

changes were discussed.

2. Materials and methods

2.1. Sludge samples

Sludge samples were collected from the primary sedimen-

tation tank (termed as primary sludge) and from the

secondary sedimentation tank (termed as the waste acti-

vated sludge (WAS)) in a municipal wastewater treatment

plant at Harbin City, China. All collected sludge samples

were first gravity thickened to around 97% w/w moisture

content. Then the mixed sludge samples were prepared by

mixing thickened primary sludges and thickened waste

Table 1 e Characterization of thickened WAS and mixed sludg

Mixed sludge Thickened WAS

TCOD/mgl�1 33,200 28,210

SCOD/mgl�1 920 948

pH 6.49 6.45

NH3eN/mgl�1 148 101

Alkalinity/mgl�1 720 580

a TCOD: total chemical oxygen demand; SCOD: soluble chemical oxygen d

volatile suspended solids.

activated sludge samples at 1:4 v/v to simulate the field

practice. The characteristics of sludge samples were shown

in Table 1.

2.2. F/T treatment

The thickened WAS and mixed sludge were placed in 550 ml

polyethyleneterephthalate bottles sealed with polyethylene

lids and frozen at �18 �C at different time periods. Following

freezing (and curing), the sludges were thawed for another 3 h

at 29 �C and at 47e56% relative humidity. Preliminary tests

revealed that complete freezing of sludge samples could be

reached in 3 h. Hence, the freezing tests at<3 hwere at freeing

stage; while those at >3 h were at curing stage.

2.3. Analytical methods

Total chemical oxygen demand (TCOD) and soluble chemical

oxygen demand (SCOD), total solids (TS), suspended solids

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

pH for the sludge samples before and after F/T treatment were

measured based on the Standard Analysis Methods (China

EPA, 2002). The sludge samples were centrifuged at 2770 � g

for 30 min prior to SCOD, alkalinity, NH3eN, SS and VSS

measurements. The VS and VSS contents were determined

after calcination at 600 �C for 1 h.

The COD solubilization was defined as the ratio of the

SCOD of treated sludge (SCOD)minus the initial SCOD (SCOD0)

divided by the initial particulate fraction of COD (CODp0) as

follows (Bougrier et al., 2008):

COD solubilizationð%Þ ¼ SCOD� SCOD0

TCOD0 � SCOD0¼ SCOD� SCOD0

CODp0

where TCOD0 is the initial sludge TCOD.

The particle size distribution was measured by dilution of

sludge supernatant using a Liquid Particle Counting System

(HIAC 9703, USA). The detected particles ranged from 2 to

300 mm. A drop of sludge sample was spread via pasteur

pipette onto a microscope slide, at which point the floc

structure was observed and photographed using an Olympus

BX051 at 200� magnification. 100 ml of sludge sample was

settled in a graduated cylinder and the settled volume of

sedimentation was recorded.

A vacuum filtration system equipped with Buchner funnel

was installed and adopted for a 100 ml sludge sample at

a pressure difference of 0.7 bar. The filtrate volume was

e.a

Mixed sludge Thickened WAS

TS/mgl�1 37,870 24,280

SS/mgl�1 35,870 22,400

VS/mgl�1 19,530 15,760

VSS/mgl�1 18,620 14,720

emand; TS; total solids; SS: suspended solids; VS: volatile solids; VSS:

0

10000

20000

30000

40000

50000

0 20 40 60 80

freezing time /h

TS

(V

S)

/mg•

L-1

TS of mixed sludge TS of WASVS of mixed sludge VS of WAS

Fig. 1 e TS and VS of mixed sludges and WAS versus

freezing time.

Table 2 e Settled sludge volumes after 24 h (initial sludgevolume: 100 ml).

Mixedsludge

Settledsludge

volume/ml

WAS Settledsludge

volume/ml

Raw 67 � 4 Raw 78 � 6

1 h freezing 59 � 4 1 h freezing 65 � 5

3 h freezing 55 � 5 3 h freezing 59 � 5

72 h freezing 46 � 3 72 h freezing 52 � 4

Table 3eCentrifugation of sludges (initial sludge volume:100 ml).

Freezingtime formixedsludge/h

Sedimentvolume/ml

Freezingtime forWAS/h

Sedimentvolume/ml

0 32 � 2 0 47 � 3

1 25 � 2 1 22 � 2

3 25 � 1 3 30 � 2

72 22 � 1 72 26 � 2

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 5 9 6 9e5 9 7 6 5971

recorded to determine the average specific resistance of filter

cake.

A digitally controlled centrifuge (TDL-40B, ANKE Shanghai,

China) with a rotational speed of 4000 rpm for 30 min was

used in the centrifugal settling tests. Four tubes of sludge

samples with an initial volume of 100 ml were centrifuged

with the sediment volumes recorded over time.

3. Results and discussion

3.1. Physical characteristics

The F/T treatment did not yield significant changes in the

sludge TS or VS (Fig. 1), a self-evident result since the freezing

and curing did not remove organic matters or induce evapo-

ration loss from sludge. This observation correlates with that

by El-Hadj et al. (2007).

The capillary suction time (CST) tests were conducted but

would not be discussed based on the comments by Ormeci

et al. (2001) that CST is not an appropriate indicator for

0

20

40

60

80

100

0 20 40 60 80 100settling time /h

sedi

men

tatio

n vo

lum

e /m

l

original sludge1h freezing3h freezing72h freezing

ba

Fig. 2 e Settling tests for original and treat

sludge dewaterability. Fig. 2a and b shows the sediment

volume versus settling time data for the original and the

treated sludges. The F/T treatment enhanced the settleability

of sludge samples as noted by the higher settling speed and

the less sediment volumes. It is also noticeable that within

1 h and 3 h freezing (and 3 h thawing), the improvement of

sludge settleability was noted marginal for both WAS and

mixed sludge. However, with a long curing time of about 69 h

(72 he3 h), the sludge settleability was significantly

improved. Curing has minimal effects on sediment volumes

(Table 2).

The F/T treatment enhances centrifugal settling of sludge,

correlating with the report by Vesilind et al. (1991b). Similar to

the gravity tests, the curing has minimal effects on sediment

volumes of the centrifugated sludge (Table 3). The 1e3 h

freezing of bulk sludge could not effectively improve filter-

ability of either mixed sludges or WAS. The 69-h curing had

0

20

40

60

80

100

0 20 40 60 80 100settling time /h

sedi

men

tatio

n vo

lum

e /m

l

original sludge

1h freezing

3h freezing

72h freezing

ed sludges. (a) Mixed sludge, (b) WAS.

Table 4 e Filtration tests for original and treated sludges(initial volume: 100 ml).

Freezingtime formixedsludge/h

Filtratevolume/ml

Freezingtime forWAS/h

Filtratevolume/ml

0 68 � 2 0 53 � 3

1 63 � 2 1 55 � 2

3 65 � 2 3 57 � 3

72 70 � 2 72 80 � 2

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 5 9 6 9e5 9 7 65972

negligible enhancement of mixed sludge filterability;

conversely, curing markedly reduced cake resistance for WAS

(Table 4). The result of curing on WAS was in agreement with

that by Vesilind and Martel (1990). Lee and Hsu (1994) also

noted that the F/T treated biological sludges could be almost

completely dewatered via gravitational filtration.

The F/T treatment did not alter particle size distributions

for mixed sludge or WAS at 1e3 h freezing. In particular, the

0

10

20

30

40

50

60

2~2.7

2.7~3.7

3.7~5.0

5.0~6.8

6.8~9.3

9.3~12

.6

12.6~

17.2

17.2~

23

perc

ent o

f pa

rtic

les

/%

a

0

10

20

30

40

50

60

70

80

90

2~2.7

2.7~3.7

3.7~5.0

5.0~6.8

6.8~9.3

9.3~12

.6

12.6~

17.2

17.2~

23

perc

ent o

f pa

rtic

les

/%

b

Fig. 3 e Particle size distribution for original and

69-h curing produced excess quantity of fine particles for

WAS, but had minimal effects on those for mixed sludge

(Fig. 3). This result correlates with Vesilind and Martel (1990)

which concluded that F/T treatment was most effective with

small particles.

3.2. Chemical characteristics

TheCODsolubilization reached0.6%formixedsludgeand1.6%

forWAS after 3 h freezingþ 3 h thawing F/T treatment (Fig. 4).

The presence of primary sludge increases resistance to solu-

bilization action by the F/T treatment of WAS. In the subse-

quent curing stage (3e72 h), the COD solubilization was

increasedwith curing time in a linearlymanner, reaching 7.5%

formixed sludge and 10.5% forWAS at the end of the 72-h test.

This level of solubilization is comparable to that from WAS

sample treated at 100 �C for 30 min (Bougrier et al., 2008) and

with 0.8 W/ml ultrasound for 5 min (Zhao et al., 2010).

The 1e3 h freezing þ 3 h thawing could release a limited

quantity of NH3eN from sludge (Fig. 5). Conversely, curing

effectively solubilizes NH3eN from sludge matrix into

.3

23.3~

31.7

31.7~

43.1

43.1~

58.6

58.6~

79.6

79.6~

108.2

108.2

~147.1

147.1

~200

original mixed sludge

1 h freezing

3 h freezing

72 h freezing

.3

23.3~

31.7

31.7~

43.1

43.1~

58.6

58.6~

79.6

79.6~

108.2

108.2

~147.1

147.1

~200

original WAS

1 h freezing

3 h freezing

72 h freezing

treated sludges. (a) Mixed sludge, (b) WAS.

0

2

4

6

8

10

12

0 20 40 60 80freezing time /h

CO

D s

olub

iliza

tion

/%

mixed sludge

WAS

Fig. 4 e COD solubilization with freezing time.

6.5

6.6

6.7

6.8

6.9

7

7.1

7.2

0 10 20 30 40 50 60 70 80

freezing time /h

pH

mixed sludge

WAS

Fig. 6 e Suspension pH with freezing time.

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 5 9 6 9e5 9 7 6 5973

supernatant. Using ultrasonic treatment a dramatic quantity

of NH3eN was released (Feng et al., 2009).

Fig. 6 showsthepHversus freezing timedata forboth treated

sludges. The suspension pH slightly decreased following F/T

treatment (from 6.89 to 6.57 for WAS and from 7.16 to 6.78 for

mixed sludge). The decrease in suspension pH is attributable to

the release of fatty acids from solid phase (Montusiewicz et al.,

2010), which was also noted by Stabnikova et al. (2008) for food

waste, by Ormeci and Vesilind (2001) for activated sludge, and

by Liu et al. (2009) onmarine intertidal sludge.

We conducted Fourier transform infrared spectroscopy

(FT-IR) tests for the original and treated sludge (Fig. 7). The

intensity of the 1546 cm�1 peak was decreased following F/T

treatment, corresponding to the solubilization of proteins into

supernatants.

3.3. Effects of curing on sludge conditioning

Curing is a storage process of frozen sludge under subfreezing

temperatures (Parker and Collins, 1997a). The unfrozen zones

presented in the frozen bulk sludge can be further dewatered

by the surrounding ice (Vesilind and Martel, 1990). As Jean

0

50

100

150

200

250

0 10 20 30 40 50 60 70 80freezing time /h

NH

3-N

/mg•

L-1

mixed sludge

WAS

Fig. 5 e NH3eN in suspension for treated sludge.

et al. (2000) mentioned, completeness of curing can be ratio-

nalized by the accomplishment of dehydration of moisture

that can be frozen. Jean et al. (2000) also claimed that themass

transfer rate of intra-aggregate water to diffuse to the growing

ice front determines the time needed for sludge curing.

Table 5 lists the percentage of changes of sludge properties

after F/T treatments for the present biological sludges. Most

enhancement of sludge dewaterability was achieved in the

freezing stage. The WAS was more readily conditioned by

1e3 h freezing þ 3 h thawing, as noted by the 73.1e81%

reduction of sediment volumes for WAS compared with the

57.1e70% for mixed sludge. Based on the conceptual models

by Vesilind and Martel (1990) and by Parker and Collins (1999),

the freezing process involved rejection and entrapment of

sludge flocs. Smaller solid particles associated with WAS

would be more readily moved by advancing ice front and be

dehydrated into solids pockets. Restated, the bulk freezing of

sludge is sufficient to transform puffy sludge structures into

compact aggregates to facilitate settleability and filterability.

According to the postulated mechanism for sludge freezing,

Vesilind and Martel (1990) pointed out that the freezing rate

was possibly the governing variable that determined the

dewaterability of freezeethaw sludge compared with curing

temperature and time, inasmuch as freezing temperature

affected the movement and aggregation of solids. If a proper

freezing temperature (freezing rate) was applied, the free

water surrounding the flocs and surface water surrounding

the particles would be frozen in sequence, causing the water

molecules being extracted from flocs interior to build the

crystalline structures, and forcing the particles migrated into

tightly compacted solids pockets. This conclusion on sludge

dewaterability was also proposed by Parker and Collins

(1997b).

Conversely, the freezing stage released limited quantities

of COD (15.5% for WAS and 8.1% for mixed sludge). Curing

contributed mostly on COD solubilization and NH3eN release

(Table 5). The interactions between unfrozen zones in bulk

sludge and the surrounding front not only dehydrate the

Fig. 7 e FT-IR spectra for original and treated sludges. (a)Mixed sludge, (b) WAS.

Table 5 e Relative contribution of freezing (1e3 h of freezing test) and curing (3e72 h of freezing test) for mixed sludge andWAS.

Sample Stage Settled sludge volume Centrifugal settling volume COD solubilization NH3eN release

Mixed sludge Freezing 57.1% 70% 8.1% 15.2%

Curing 42.9% 30% 91.9% 84.8%

WAS Freezing 73.1% 81% 15.5% 4.1%

Curing 26.9% 19% 84.5% 95.9%

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 5 9 6 9e5 9 7 65974

aggregates as commented by Jean et al. (2000), but also solu-

bilize the organic matters into supernatants. Freezing causes

cell disrupt through intracellular and extracellular ice crystals

formed during freezing (Thomashow, 1998; Ormeci and

Vesilind, 2001). As temperature decreases below 0 �C, ice

forms and accumulates in the intercellular spaces, which

results in the physical cell disruption (Thomashow, 1998).

However, most cellular damage results from the freeze-

induced dehydration. At temperatures below �10 �C (such as

�18 �C adopted in this study), with the consequent low water

potentials and severe dehydration, membrane damage can

occur in the form of “fracture-jump lesions” (Thomashow,

1998). In addition, the water in sludge cells expands during

freezing process, and cells vulnerable to the pressure of the

expanding ice may burst. It is also possible that the

compression and suction on cells exerted by the advancing ice

front may cause the cells to disrupt (Ormeci and Vesilind,

2001). Gao (2011) summarizes that intracellular ice formation

that usually occurs during rapid freezing may cause the

mechanical disruption of cellswhile slow freezing (such as the

one used in this study) often results in the release of more

outer-membrane materials. The release of ECPs and intracel-

lular materials to the surroundings contributed to the signif-

icant increase of SCOD and NH3eN concentrations in

freezeethaw sludge.

Ice formation is generally initiated in the intercellular

spaces, as opposed to intracellularly, as a result of high

freezing point and homogeneous ice-nucleation sites for the

former. So the surface water (difficult-to-freeze water) takes

a longer time to freeze (Thomashow, 1998; Vesilind and

Martel, 1990). The cell freezing process is governed by the

competition between the mass transfer (intracellular water

movement) and the heat transfer, which is distinguished by

a freezing rate of 3000 K/min (Silvares et al., 1975). The average

temperature-decreasing rate in this study was approx.

0.183 K/min, amuch lower valuewater transport would not be

dominating in the freezing process. Based on the calculation

by Jean et al. (2000), our curing time should be 3380 s, of the

same order of that noted in experiments.

Natural freezing and thawing can be a promising pre-

treatment stage of biological sludge to enhance dewater-

ability. In case the F/T treated sludge is to be used as feedstock

of the following anaerobic digestion process, sufficient curing

time is needed to allow development of intra-aggregate ice to

solubilize organic matters to supernatants.

4. Conclusions

The following conclusions are drawn based on the presented

experimental results:

(1) Freeze/thaw (F/T) treatment could enhance biological

sludgedewaterability. A 72-h treatment at�18 �Cdecreased

the sedimentation volumes by 31.2e31.3% formixed sludge

and by 33.3e44.7% for waste activated sludge, respectively.

(2) F/T treatment could facilitate mass transfer from the solid

phase into the aqueous phase. The maximum chemical

oxygen demand (COD) solubilization obtained in the study

were 7.5% for mixed sludge and 10.5% for waste activated

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 5 9 6 9e5 9 7 6 5975

sludge. And maximum increments in NH3eN concentra-

tion reached 45.3% for mixed sludge and 74.5% for waste

activated sludge.

(3) Most enhancement of sludge dewaterability was achieved

during bulk freezing stage, with the waste activated

sludges more readily dewatered than the mixed sludges

after F/T treatment.

(4) Unlike sludge dewaterability, the freezing stage released

only limited quantities of organic matters to liquid. Thus,

COD solubilization and NH3eN release reliedmostly on the

curing stage.

Acknowledgments

The authors gratefully acknowledge funding from Project

50821002 (National Creative Research Groups) supported by

National Nature Science Foundation of China, National Water

Pollution Control and Management Key Project (2009ZX07317-

008), partial supports by State Key Laboratory of Urban Water

Resource and Environment, Harbin Institute of Technology

(No. 2010DX17), and funding from Heilongjiang Province

Science Foundation for Youths (QC2009C113).

r e f e r e n c e s

Bougrier, C., Delgenes, J.P., Carrere, H., 2008. Effects of thermaltreatments on five different waste activated sludge samplessolubilisation, physical properties and anaerobic digestion.Chemical Engineering Journal 139 (2), 236e244.

Chang, M.R., Chiang, L.I., Lee, D.J., Liu, J.C., Wu, N.M., Chen, W.C.,Hsu, B.M., 2004. Conditioning of wastewater sludge fromscience-based industrial park using freezing and thawing.JournalofEnvironmentalEngineering-ASCE130 (12), 1552e1555.

China EPA, 2002. Water and Wastewater Monitoring andAnalyzing, fourth ed. Chinese Environmental Science Press,Beijing, China.

Chu, C.P., Lin, W.W., Lee, D.J., Chang, B.V., Liao, G.S., Peng, X.F.,2002a. Thermal treatment of activated sludge using liquidboiling. Journal of Environmental Engineering-ASCE 128 (11),1100e1103.

Chu, C.P., Lee, D.J., Chang, B.V., Liao, G.S., Tay, J.H., 2002b. Weakultrasonic pretreatment on anaerobic digestion of flocculatedwaste activated sludge. Water Research 36 (11), 2681e2688.

El-Hadj, T.B., Dosta, J., Marquez-Serrano, R., Mata-Alvarez, J., 2007.Effect of ultrasound pretreatment in mesophilic andthermophilic anaerobic digestion with emphasis onnaphthalene andpyrene removal.Water Research 41 (1), 87e94.

Elliott, A., Mahmood, T., 2007. Pretreatment technologies foradvancing anaerobic digestion of pulp and paper biotreatmentresidues. Water Research 41 (19), 4273e4286.

Feng, X., Lei, H.Y., Deng, J.C., Yu, Q., Li, H.L., 2009. Physical andchemical characteristics of waste activated sludge treatedultrasonically. Chemical Engineering and Processing 48 (1),187e194.

Gao, W., 2011. Freezing as a combined wastewater sludgepretreatment and conditioning method. Desalination 268(1e3), 170e173.

Hedstrom, A., Hanaeus, J., 1999. Natural freezing, drying, andcomposting for treatment of septic sludge. Journal of ColdRegions Engineering 13 (4), 167e179.

Hung, W.T., Chang, I.L., Lee, D.J., Hong, S.G., 1996. Sludgechemical composition changes under uni-directional freezing.Water Science and Technology 34 (3e4), 525e531.

Jean, D.S., Lee, D.J., 2000. Effects of electrolytes and curing onfreezing of ferric hydroxide sludge. Colloids and Surfaces A-Physicochemical and Engineering Aspects 162 (1e3), 285e288.

Jean, D.S., Chu, C.P., Lee, D.J., 2000. Effects of electrolyte andcuring on freeze/thaw treatment of sludge. Water Research 34(5), 1577e1583.

Kawasaki, K., Matsuda, A., Yamashita, H., 2004. The effect offreezing and thawing treatment on the solid liquid separationcharacteristics of bulking activated sludge. Kagaku KogakuRonbunshu 30 (5), 587e591.

Lee, D.J., Hsu, Y.H., 1994. Fast freezeethaw treatment on excessactivated sludges e floc structure and sludge dewaterability.Environmental Science and Technology 28 (8), 1444e1449.

Liu, H.Y., Wang, G.C., Zhu, D.L., Pan, G.H., 2009. Enrichment of thehydrogen-producing microbial community from marineintertidal sludge by different pretreatment methods.International Journal of Hydrogen Energy 34 (24), 9696e9701.

Martel, C.J., Diener, C.J., 1991. A pilot scale study of alum sludgedewaterability in a freezing bed. Journal of the AmericanWater Works Association 83 (12), 51e55.

Montusiewicz, A., Lebiocka, M., Rozej, A., Zacharska, E.,Pawłowski, L., 2010. Freezing/thawing effects on anaerobicdigestion of mixed sewage sludge. Bioresource Technology 101(10), 3466e3473.

Ormeci, B., Vesilind, P.A., 2001. Effect of dissolved organicmaterialand cations on freezeethawconditioning of activated andalumsludges. Water Research 35 (18), 4299e4306.

Parker, P.J., Collins, A.G., 1999. Dehydration of flocs by freezing.Environmental Science & Technology 33 (3), 482e488.

Parker, P.J., Collins, A.G., Dempsey, J.P., 1998a. Alum residual flocinteractions with an advancing ice/water interface. Journal ofEnvironmental Engineering 124 (3), 249e253.

Parker, P.J., Collins, A.G., Dempsey, J.P., 1998b. Effects of freezingrate, solids content, and curing time on freeze/thawconditioning of water treatment residues. EnvironmentalScience and Technology 32 (3), 383e387.

Parker, P.J., Collins, A.G., 1997a. Unidirectional freezing of waste-activated sludge: the presence of sodium chloride e

Comment. Environmental Science and Technology 31 (12),3740.

Parker, P.J., Collins, A.G., 1997b. Feasibility study on freeze/thawconditioning of pulp mill waste activated sludge. Journal ofCold Regions Engineering 11 (3), 245e250.

Stabnikova, O., Liu, X.Y., Wang, J.Y., 2008. Digestion of frozen/thawed food waste in the hybrid anaerobic solideliquidsystem. Waste Management 28 (9), 1654e1659.

Silvares, O.M., Cravalho, E.G., Toscano, W.M., Huggins, C.E., 1975.The thermodynamics of water transport from biological cellsduring freezing. Journal of Heat Transfer 97 (4), 582e590.

Tao, T., Peng, X.F., Lee, D.J., 2006. Interaction betweenwastewater-sludge floc and moving ice front. ChemicalEngineering Science 61 (16), 5369e5376.

Thomashow, M.F., 1998. Role of cold-responsive genes in plantfreezing tolerance. Plant Physiology 118 (1), 1e7.

Vesilind, P.A., Wallinmaa, S., Martel, C.J., 1991a. Freezeethawsludge conditioning and double-layer compression. CanadianJournal of Civil Engineering 18 (6), 1078e1083.

Vesilind, P.A., Martel, C.J., 1990. Freezing of water and wastewatersludges. Journal of Environmental Engineering 116 (5),854e862.

Vesilind, P.A., Hung, W.Y., Martel, C.J., 1991b. Agitation andfilterability of freeze/thawed sludge. Journal of Cold RegionsEngineering 5 (2), 77e82.

Wang, Q.H., Fujisaki, K., Ohsumi, Y., Ogawa, H.I., 2001.Enhancement of dewaterability of thickened waste activated

wat e r r e s e a r c h 4 5 ( 2 0 1 1 ) 5 9 6 9e5 9 7 65976

sludge by freezing and thawing treatment. Journal ofEnvironmental Science and Health Part A-toxic/hazardousSubstances & Environmental Engineering 36 (7), 1361e1371.

Wang, X.A., Zheng, M.Y., Zhang, W.Y., Zhang, S., Yang, T., 2010.Experimental study of a solar-assisted ground-coupled heatpump system with solar seasonal thermal storage in severecold areas. Energy and Buildings 42 (11), 2104e2110.

Whiteley, C.G., Lee, D.J., 2006. Enzyme technology and biologicalremediation: a review. Enzyme Microbial Technology 38 (3e4),291e316.

Zhao, Q.L., Hu, K., Miao, L.J., Wang, K., 2010. Effect of UltrasonicTreatment on Characteristics of Waste Activated SludgeEnvironmental Pollution and Public Health (EPPH 2010) withiniCBBE, June 21e23, 2010, Chengdu, China.