Radiation Physics and Chemistry 120 (2016) 49–55

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Radiation Physics and Chemistry

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Disintegration of excess sludge enhanced by a combined treatment ofgamma irradiation and modified coal fly ash

Yulin Xiang a,n, Lipeng Wang b, Yurong Jiao a

a College of Chemistry and Chemical Engineering, Yulin University, Yulin 719000, Shaanxi Province, Chinab Zhou Enlai School of Government, NanKai University, TianJin 300072, China

H I G H L I G H T S

� Effects of gamma irradiation and modified CFA on excess sludge were studied.

� The combined application could improve porosity of sludge cake.� The combined treatment is an effective method to disintegrate excess sludge.

a r t i c l e i n f o

Article history:Received 23 October 2015Received in revised form26 November 2015Accepted 30 November 2015Available online 2 December 2015

Keywords:Sludge disintegrationExcess sludgeGamma irradiationCoal fly ash modification

x.doi.org/10.1016/j.radphyschem.2015.11.0356X/& 2015 Published by Elsevier Ltd.

esponding author.ail address: yulinx@126.com (Y. Xiang).

a b s t r a c t

In order to improve the disintegration performance and accelerate the disintegration rate of excesssludge, the individual and combined influences of γ-ray irradiation and modified coal fly ash treatmenton the disintegration of excess sludge were investigated based on physicochemical properties of excesssludge. The changes in constituents of excess sludge were examined by means of UV/vis spectra and SEMimages. The results showed that the disintegration performance of excess sludge was effectively im-proved by gamma ray irradiation in the presence of modified coal fly ash. A new band from 250 nm to290 nm appeared on all irradiated sludge samples. The SEM images illustrated the cells surfaces of thesludge by the combined treatment were disfigured. The SCOD, soluble carbohydrate and protein fromsludge supernatant increased obviously with increasing modified CFA dosage from 0 to 0.2 g ml�1 anddose from 0 to 10 kGy. The sludge SRF and filter cake moisture declined significantly, and the filtrationspeed was faster. In conclusion, γ-ray irradiation-modified coal fly ash pretreatment is an effectivemethod to disintegrate excess sludge.

& 2015 Published by Elsevier Ltd.

1. Introduction

During the process of waste water treatment, a large quantityof excess sludge, which have been included in the ranks of ha-zardous substances, are produced (Low and Chase, 1999). Thehazardous substances in the excess sludge will bring to hu-mankind not only environmental pollution, but also unexpecteddisasters once the control system loses effectiveness (Wang et al.,2014). The management of excess sludge is a hardnut problem allthe time. At present, for the management of excess sludge, anae-robic and aerobic digestion processes have been widely applied tothe big and small sewage treatment plants, respectively. But inertbiological material which is hard to be degraded in excess sludgerestricts sludge digestibility severely.

In order to improve the sludge digestibility and accelerate thedisintegration rate of inert biological material, electron beam ir-radiation (Jean et al., 2015), gamma irradiation (Chu et al., 2011),mechanical disintegration (Monika and Justyna, 2014), ultrasonictechnology (Sahinkaya and Sevimli, 2013), and microwave irra-diation (Tyagi and Lo, 2013) have been explored. Recently, com-bination processes of these technologies have aroused a lot ofresearchers’ concern, such as microwave-hydrogen peroxide(Wang et al., 2015), ultrasonic-acid (Sahinkaya, 2015), physico-chemical pretreatment method (Tyagi and Lo, 2011) and so on.

For sludge and wastewater treatment, γ-ray and electron beamirradiation has been recognized as an effective technology becauseof destruction of organic pollutants and pathogen inactivation. Theprocessing time in case of electron beam irradiation is relativelyshort (Chu et al., 2011; Wang and Wang, 2007). It was reportedthat the technology had been used to disintegrate many bio-re-fractory materials in activated sludge (Kim et al., 2009) and

Y. Xiang et al. / Radiation Physics and Chemistry 120 (2016) 49–5550

disinfect wastewater for reuse (Tahri et al., 2010). Coal fly ash (CFA)generated from coal-fired power plants, is one such solid wastewith high contents of CaO, MgO, alumina-silicates and silicates.CFA can be used as a stabilizer for sludge improvement by killingpathogens and decreasing heavy metal availability, after CFA pre-treatment, the disintegration performance of sewage sludge wasobviously improved (Xu et al., 2012). Improving the disintegrationefficiency by using a γ-ray irradiation system and the modified CFAas a conditioner has not been reported.

In order to improve the disintegration performance of excesssludge, this study investigated the individual and combined in-fluences of γ-ray irradiation and modified CFA treatment pro-cesses. It aims to obtain more penetration into sludge disintegra-tion mechanism under γ-ray irradiation-modified CFA pretreat-ment in terms of SCOD, soluble protein, soluble carbohydrate,specific resistance to filtration (SRF) and the changes in con-stituents of excess sludge by means of chemical analysis, UV/visspectra and SEM images.

2. Materials and methods

2.1. Collection and preparation of materials

The excess sludge sample was collected from the secondarysedimentation tank of the Yuyang wastewater treatment plant inYulin, China. The pH of the sludge samples were in the range of7.7–9.6. Moisture contents of the initial sludge samples were about9871.5%. Before the disintegration, the pH of the samples wereadjusted to 11 using sodium hydroxide, and moisture contents ofthe samples were adjusted to 99.0% (wt/wt) by moving super-natant or diluting with ultrapure water. Total chemical oxygendemand (TCOD) concentrations of the samples at 99.0% wereabout 7997 mg l�1, while soluble chemical oxygen demand(SCOD) were about 67 mg l�1.

The CFA was collected from Big Baodang, Shaanxi, China andchemical constituents of the raw CFA are showed in Table 1. In thispaper, nitric acid was used as modification agent for CFA. Themodification conditions were as follows: nitric acid concentration,4 mol/L; ratio of CFA to acid of 1:4 g ml�1; soaking time, 4 h;stirring speed, 80 rpm. After filtration, the CFA was oven-dried at10075 °C for 4 h, then grinded and sieved (100 mesh). Themodified CFA was obtained.

2.2. Experimental setup and procedure

In order to investigate the effect of modified CFA and 60Co γ-rayirradiation on disintegration performance of excess sludge, threeexperimental programs were carried out as follows: modifiedCFA/60Co γ-ray irradiation, modified CFA alone and 60Co γ-ray ir-radiation alone. Above all, 300 ml of the preparative excess sludgewas placed in a 500 ml glass bottle. The experiment was initiatedby adding the preset amount of modified CFA (0 to 0.4 g ml�1) andstirred until well-mixed, then the glass bottle was sealed and ex-posed to doses of 0 to 20 kGy in a cobalt-60 irradiator (dose rate:70 Gy/min). The sample was turned 360 °ceaselessly during theirradiation process. Finally, the sample was cooled to room tem-perature (about 25 °C) in a water bath before analysis.

Table 1Chemical composition of the raw CFA.

Constituent C SiO2 Al2O3 Fe2O3

Content (%) 3.42 44.85 27.72 12.72

2.3. Analyses

The pretreated sludge was filtered with a buchner funnel(diameter: 9 cm) under 0.04 MPa vacuum pressure. The filter-ability is represented according to the specific resistance to fil-tration (SRF):

μϖ=

( )SRF

PA b21

2

where SRF is the specific resistance to filtration, mkg�1; μ is theviscosity of the filtrate, Nsm�2; ω is the weight of sludge cake perunit volume of filtrate, kgm�3; b represents the slope of filtratedischarge curve, (t/V vs. V), sm�6; A represents the filter area, m2;P represents the filtration pressure, Pa (Chen et al., 2010). In theprocess of filtration, produced sludge cake was dried at 100 °C todetermine the moisture of the sludge cake. The filtrate was mea-sured with a ultraviolet–visible spectrophotometer (WFZ UV-2100PC).

TCOD and SCOD were measured using the Standard Methods(APHA/AWWA/WEF, 2005). The soluble carbohydrate (SC) andprotein (SP) measurements were carried out by the phenolsul-phuric acid method and Lowry method (Dubois et al., 1956), re-spectively. In order to compare the excess sludge disintegrationefficiency of the different processes, the disintegration efficiency(DE) was calculated using Eq. (2) (Ganesh et al., 2013):

=−

− ( )DE

SCOD SCOD

TCOD SCOD 2after 0

0 0

where SCOD0 is the SCOD of the initial excess sludge, mg l�1;SCODafter is the SCOD of the excess sludge after treatment, mg l�1;and TCOD0 is the TCOD of the initial excess sludge, mg l�1.

For settleability of sludge, the sludge sample was led into a100 ml cylinder, and the varied supernatant volume was recordedin the range of 0–120 min at 10 min increments. The structurevariations of the sludge were examined by a scanning electronmicroscope (SEM, Quanta-600, FEI) before and after disintegration.Colony-Forming units (CFU), which is the activity in culturablebacteria, were calculated with beef extract-peptone agar plates for4872 h incubation at 36 °C. The experimental water were ultra-pure water. Each chemical was analytic grade. In order to mini-mize the systematic error, each experimental measurement wasreplicated 3 times and the average value was selected.

3. Results and discussion

3.1. UV–vis spectra evolution of sludge supernatant

After γ-ray irradiation and modified CFA pretreatment, the UV–vis spectra of excess sludge supernatant was measured, they areshown in Fig. 1. As can be seen, a new band from 250 nm to290 nm appeared on all irradiated samples. The result was similarto the previous study which the extracellular polymeric sub-stances from sewage sludge were disintegrated by gamma-ray ir-radiation (Xie et al., 2014). As is well known, nucleic acid andproteins are both important parts of microorganisms, and theabsorption peaks of pure nucleic acid and proteins are in the 260–280 nm wavelength range, which reflected sludge cells were

TiO2 CaO MgO K2O Na2O SO3

1.06 3.08 0.97 1.21 0.46 0.48

200 250 300 350 4000.0

0.5

1.0

1.5

2.0

2.5

doses20kGy15kGy10kGy 5kGy 0kGy

Abs

wavelength/nm200 250 300 350 400

0.0

0.5

1.0

1.5

2.0

2.5

3.0

doses20kGy15kGy10kGy 5kGy 0kGy

Abs

wavelength/nm

200 250 300 350 4000.0

0.5

1.0

1.5

2.0

2.5

3.0

doses20kGy15kGy10kGy 5kGy 0kGy

Abs

wavelength/nm200 250 300 350 400

0.0

0.5

1.0

1.5

2.0

2.5

3.0

doses20kGy15kGy10kGy 5kGy 0kGy

Abs

wavelength/nm

Fig. 1. UV–vis spectra of supernatant of sludge irradiated with different doses (modified CFA addition: a:0; b:0.05 g ml�1, c:0.2 g ml�1; d:0.35 g ml�1).

Y. Xiang et al. / Radiation Physics and Chemistry 120 (2016) 49–55 51

disintegrated, nucleic acid and proteins were released into sludgesolution. Thus UV–vis spectra of sludge supernatant inferred thatγ-ray irradiation could disintegrate the sludge flocs structure andrelease the substances such as nucleic acid, proteins into the su-pernatant, and more bound-water from sludge cells and flocsstructure were released. Therefore, the biodegradability and de-watering property of excess sludge were improved. In addition,along with nucleic acid and proteins, it might produce a few ofvolatile aromatic compounds in the treatment process, while Lureported that CFA owned preferential absorption to volatile aro-matic compounds, so volatile aromatic compounds can’t be re-leased into atmospheric environment (Lu et al., 2014). Fig. 1 alsoshowed that the absorption peaks increased with increasing dose,the increase rate was first rapid from 0 to 10 kGy, then the increaserate tended to become slow from 10 to 20 kGy. Under the sameirradiation condition, the absorption peak value first increased andthen decreased with modified CFA addition increasing (from 0 to0.35 g ml�1), when the modified CFA addition was 0.2 g ml�1, thepeak value achieved the maximum (Fig. 1). The absorbed dose andmodified CFA dosage were high, leading to the higher cost andlarger sludge volume. Therefore, the absorbed dose and modifiedCFA dosage should be stipulated to be 10 kGy and 0.2 g ml�1,respectively.

3.2. Excess sludge disintegration property under different treatmentprograms

To compare the effects of modified CFA and γ-ray irradiation onthe disintegration property of the excess sludge, three experi-ments were designed as follows: γ-ray doses were 0, 10 and10 kGy for modified CFA single, γ-ray irradiation single and

modified CFA/γ-ray irradiation treatment, respectively. Additionalquantities of modified CFA were 0.2, 0 and 0.2 g ml�1 for modifiedCFA single, γ-ray irradiation single and modified CFA/γ-ray irra-diation treatment, respectively.

As Fig. 2 shown, the values of SCOD, DE, SP and SC rise withextending time. At the same treatment time, the disintegrationefficiency of sludge by the combined treatment of gamma irra-diation and modified coal fly ash is highest, followed by singlegamma irradiation, and the disintegration efficiency by the singlemodified CFA pretreatment is lowest. When the aqueous solutionwas irradiated, intermediates such as ⋅OH, ⋅H, −eaq are generated.These free radicals have high reactivity, and they play a major rolein the sludge cell disintegration and bacteria inactivation (Chuet al., 2011). Under alkaline condition (sludge pH: 11), effects offree radicals treatment were more remarkable (Xie et al., 2014).The CFA was a good conditioner for sludge treatment. Chen et al.(2010) reported that the modified CFA by acid/base could exertgreatly conditioning effect on sludge disintegration due to markedimprovement of the specific surface area and adsorption proper-ties. In addition, although part of the energy of radiation was ab-sorbed by the modified CFA, the CFA with acid modifying agentcould be activated effectively by 60Co-γ rays (Wei et al., 2011), thenclearer effect can be obtained in sludge treatment process.Therefore, by means of marked adsorption properties of themodified CFA, the disintegration property of the excess sludge hadimproved noticeably under γ-ray irradiation (Chen et al., 2010).

3.3. Changes in filtration dewatering performance of excess sludge

The SRF of the fresh excess sludge was about1.94�1013 m kg�1, and dewatering performance was far from

0 5 10 15 20 25 300

500

1000

1500

2000

2500

3000

3500

4000

4500

single modified CFA single irradiationcombined process

SCOD

(mg/

L)

Time (min)0 5 10 15 20 25 30

0

10

20

30

40

50

single modified CFA single Irradiationcombined process

DE (

%)

Time (min)

0 5 10 15 20 25 300

500

1000

1500

single modified CFA single Irradiationcombined process

SP(m

g/L)

Time (min)0 5 10 15 20 25 30

0

50

100

150

200

250

300

350

400

single modified CFA single irradiationcombined process

SC(m

g/L)

Time (min)

Fig. 2. Effect of different treatment programs on the disintegration property of the excess sludge.

0 2 4 6 8 10 12 14 16 18 200

50

100

150

200

SRF/

(1011

m/k

g)

Dose(kGy)

a

0 2 4 6 8 10 12 14 16 18 2040

50

60

70

80

90

Slu

dge

cake

moi

stur

e(%

)

Dose(kGy)

b

Fig. 3. Effect of dose on sludge SRF (a) and sludge cake moisture (b).

Y. Xiang et al. / Radiation Physics and Chemistry 120 (2016) 49–5552

optimistic. To improve the sludge performance, the added amountof modified CFA was 0.2 g ml�1, a series of experiments was car-ried out under different doses. The influence of the dose on fil-tration dewatering performance was shown in Fig. 3a. As can beseen, the SRF greatly decreased as the dose increased. With a doseof 10 kGy the SRF was 3.56�1011 m kg�1, implying that the sludgedewatering performance was visibly improved. When the doseexceeded 10 kGy, the SRF decreased slightly with the increase ofdose. After sludge dewatering, the changes in the sludge cakemoisture with doses was displayed in Fig. 3b. The sludge cakemoisture reduced with the increase of the dose. With a dose of10 kGy the moisture decreased from 88.69% (untreated sludge) to

59.17% (pretreated sludge). The filtration speed of pretreatedsludge was more rapid than that of the untreated sludge. 15–20 min was needed to obtain 200 ml filtrate from 300 ml freshsludge, but with a dose of 10 kGy the duration was only 2 min. Themajority of free radicals with strong oxidizing capability weregenerated by gamma ray irradiation in sludge aqueous solution.These free radicals could destroy effectively the flocs structure ofsludge, and numerous bound-water were freed, dewateringproperty of sludge was promoted.

The influence of the modified CFA dosage on filtration dewa-tering performance was shown in Fig. 4a. The dose was fixed10 kGy. The SRF decreased as the modified CFA dosage increased.

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.350

50

100

150

200

SRF/

(1011

m/k

g)

Modified CFA dosage (g/ml)

a

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35

50

60

70

80

90

Slu

dge

cake

moi

stur

e(%

)

Modified CFA dosage(g/ml)

b

Fig. 4. Effect of modified CFA dosage on sludge SRF (a) and sludge cake moisture (b).

0 20 40 60 80 100 12005

101520253035404550

a

untreated sludge single modified CFA single irradiation combined process

Sup

erna

tant

am

ount

(ml)

Settling time (min)

Fig. 5. Settling curve (a: modified CFA dosage, 0.2 g ml�1; dose, 10 kGy) and sludge settling images (b: 1. untreated sludge; 2. sludge conditioned with modified CFA; 3.sludge conditioned with gamma irradiation; 4. sludge conditioned with combined process of modified CFA and gamma irradiation).

0 5 10 15 20 250

5

10

15

20

25

30

35

40

OUR

/OUR 0

(%)

Time (h)Fig. 6. Changes in sludge respiration with time (after treatment).

Y. Xiang et al. / Radiation Physics and Chemistry 120 (2016) 49–55 53

With a modified CFA dosage of 0.2 gml�1 the SRF was3.88�1011 m kg�1, showing that the dewatering performance ofexcess sludge was obviously improved. When the modified CFAdosage exceeded 0.2 g ml�1, the decrease of the SRF becameslowly with the increase of modified CFA dosage. After dewatering,

the changes in the sludge cake moisture with modified CFA do-sages was described in Fig. 4b. The sludge cake moisture decreasedwith the increase of the modified CFA dosage. With a modified CFAdosage of 0.2 g ml�1 the moisture decreased from 89.23% (un-treated sludge) to 61.09% (pretreated sludge). To obtain 200 mlfiltrate from 300 ml fresh sludge, with a modified CFA dosage of0.2 g ml�1 the duration was about 2.2 min. This is similar to thefindings of Karr and Keinath (1978). The untreated sludge includedplenty of fine particles (Wakeman, 2007), the fine particles couldfill into the pores of the sludge cake, thus the the sludge cakepermeability became poor. The modified CFA possesses firm par-ticles and irregular surface. The porosity of sludge cake was im-proved by constructing pores or channels between the sludgeparticles and modified CFA particles due to varied particle sizedistributions, so the filtrated resistance decreased.

3.4. Changes in settleability of excess sludge

A comparative analysis of the sludge settleability under differ-ent treatment conditions was done. Fig. 5a showed fluctuation ofthe supernatant amount during 120 min settling time. The set-tleability of untreated sludge was very poor, and settling phe-nomena was difficult for us to observe. Compared to the untreatedsample, a clear settling interface appeared at all pretreated sam-ples. The results indicated that the settling velocity with combinedtreatment of gamma irradiation and modified coal fly ash was the

Fig. 7. SEM images of excess sludge (a: before treatment, b: after treatment).

Y. Xiang et al. / Radiation Physics and Chemistry 120 (2016) 49–5554

highest, then that of the individual gamma irradiation, and finallythe velocity was the lowest for the individual modified coal fly ash.After 40 min, the supernatant amount was 27.4, 20.2 and 15.5 mlwith combined treatment, individual irradiation and individualmodified CFA, respectively. After 120 min, the supernatant amountreached the maximum which was 47.5, 28.8 and 22.1 ml, respec-tively. Fig. 5b showed settling images of the sludge of combinedtreatment, the sludge conditioned with individual irradiation andindividual modified CFA, and untreated sludge after 120 min set-tling, respectively. The settling performance of conditioned sludgewith irradiation and/or modified CFA were all improved. Thecombined process of gamma irradiation and modified coal fly ashis more efficient to improve sludge settleability than individualprocess of irradiation or modified coal fly ash.

3.5. Changes in activities of excess sludge and SEM analysis

Fresh sludge sample was 300 ml, and the added amount ofmodified CFA was 0.2 g ml�1. After exposure to doses of 10 kGy,the changes in activities of the pretreated excess sludge with timewas shown in Fig. 6. As can be seen, the microbial activity in thesludge gradually reduced. Microbial respiration was restrained byaround 61% at or near the end of irradiation and by 96% at 16-hafter irradiation. Culturable bacteria of the untreated sludge was1.16�107 CFU, while culturable bacteria of the pretreated sludgewas 4.9�104 CFU and remained constant. The result is similar tothe findings of Chu et al. (2011).

Fig. 7 showed that sludge flocs structures before and aftercombined treatment of modified CFA and gamma irradiation. TheSEM images illustrated that the cells appearances before and aftertreatment were distinctly different. The cells appearances of un-treated sludge were comparatively complete and smooth. Whilethe cells surfaces after pretreated sludge were disfigured, and fi-lamentous bacteria disappeared.

4. Conclusions

The individual and combined treatment effects of gamma ir-radiation and modified coal fly ash on the disintegration of excesssludge were studied. The experimental results showed disin-tegration efficiency under the combined action of gamma irra-diation and modified coal fly ash was higher than the individualaction. With a dose of 10 kGy and a modified CFA dosage of0.2 g ml�1, the sludge SRF declined from 1.94�1013 to 3.56–

3.88�1011 m kg�1, and the filter cake moisture cut nearly 30%.The filtration speed was faster. The disintegration mechanism ofthe excess sludge by gamma irradiation-modified coal fly ashtreatment is that the combined treatment could disintegratesludge flocs and cells and improve porosity of sludge cake, leadingto improvement of sludge characteristics. As a result, the combi-nation of gamma irradiation with modified coal fly ash promoteddisintegration of excess sludge. The combined treatment has beenconsidered to be as an effective method.

Acknowledgment

National Natural Science Foundation of china (21203163). TheNational Natural Science Gold project, Shaanxi provincial scienceand technology department foster industrialization projects(15JF035). The authors are grateful for the funding and supportprovided by the following projects: the Scientific Research StartingFoundation for high-level professionals in Yulin University ofChina (No. 12GK04).

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