Bioresource Technology 123 (2012) 189–194

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Bioresource Technology

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Optimized alkaline pretreatment of sludge before anaerobic digestion

Huan Li ⇑, Chenchen Li, Wenjie Liu, Shuxin ZouGraduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China

h i g h l i g h t s

" NaOH pretreatment was systematically analyzed for anaerobic digestion." The optimized condition of NaOH dose and pH control was concluded." The effects of high pH and high salinity on anaerobic digestion were investigated." The influence of alkaline pretreatment on sludge settleability was compared.

a r t i c l e i n f o

Article history:Received 28 May 2012Received in revised form 1 August 2012Accepted 3 August 2012Available online 10 August 2012

Keywords:Alkaline pretreatmentAnaerobic digestionSludge

0960-8524/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.biortech.2012.08.017

⇑ Corresponding author. Tel.: +86 755 26036105; faE-mail addresses: li.huan@sz.tsinghua.edu.cn, lihua

a b s t r a c t

NaOH was used to disintegrate a mixture composed mainly of primary sludge with biofilm sludge beforeanaerobic digestion in batch experiments. NaOH pretreatment dissolved some organic substances, andthe optimum dose was 0.1 mol/L. After the alkali-treated sludge was fed into the digesters, the higherpH delayed the start of digestion and reduced the biogas production during the initial stage, althoughthe system recovered after a lag phase when the dose was lower than 0.04 mol/L. Acid conditioningwas necessary, but the increased salinity also impacted on the digestion efficiency. For sludge pretreat-ment, the optimum NaOH dose was 0.1 mol/L, and the initial pH of the batch digesters needs to be con-trolled at less than eight. Under optimized conditions, the organic degradation rate was 38.3% and thebiogas yield was 0.65 L/g volatile suspended solid (VSS), whereas these values for the control were30.3% and 0.64 L/g VSS, respectively.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Waste activated sludge (WAS) is the main by-product of waste-water treatment plants (WWTPs). The disposal of sludge accountsfor up to 50% of the operating costs of a WWTP. To minimizeWWTP costs, anaerobic digestion is commonly used. This cantransform organic matter into biogas, thereby reducing the amountof final sludge solids that need to be disposed of while destroyingmost of the pathogens in the sludge and limiting odor problemsassociated with residual putrescible matter (Appels et al., 2008).However, an anaerobic digester requires a large volume due tothe long sludge retention time. Since the hydrolysis of sludge par-ticles is the rate-limiting step, pretreatment to disintegrate thesludge is used to accelerate sludge digestion or increase the degreeof degradation in a fixed digestion time. Methods of sludge disinte-gration include mechanical (Hwang et al., 1997; Nah et al., 2000),thermal (Bougrier et al., 2008; Appels et al., 2010; Nielsen et al.,2011), chemical (Lin et al., 1997; Navia et al., 2002; Devlin et al.,2011), ultrasonic (Neis et al., 2000; Hogan et al., 2004; Kim and

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x: +86 755 26036709.nsz@qq.com (H. Li).

Lee, 2012) and biological pretreatments. Some methods can becombined to disintegrate the sludge (Chiu et al., 1997; Vlyssidesand Karlis, 2004; Takashima and Tanaka, 2010; Vigueras-Carmonaet al., 2011; Saha et al., 2011). These pretreatments can disruptsludge flocs and cells, release inner organic matter, acceleratesludge hydrolysis and, consequently, improve the performance ofsubsequent anaerobic digestion (Weemaes and Verstraete, 1998;Kim et al., 2003).

Compared with other methods, alkaline pretreatment has sev-eral advantages, i.e. simple devices, easy to operate and high effi-ciency. Most of the investigations exhibited an increase inmethane production and decrease in volatile suspended solids(VSS), especially during low-dose alkaline treatment (Lin andChang, 1997; Lin and Wang, 2009; Navia et al., 2002; López Torresand Espinosa Lloréns, 2008). The preferred reagent, in most cases,was sodium hydroxide (NaOH), which was reported to yield great-er solubilization efficiency than calcium hydroxide (Ca(OH)2)(López Torres and Espinosa Lloréns, 2008). The dose of NaOH wascommonly controlled at a low level of 0.08–0.16 g/g total sludgesolids (TS) (Lin and Chang, 1997; Lin and Wang, 2009) or 0.25 g/gTS (Navia et al., 2002), although increasing the dose enhanced alka-line sludge disintegration (Li et al., 2008). If a high dose of NaOH

Table 1Characteristics of sludge samples.

Sludge A Sludge B

pH 7.0–7.5 7.2–7.5Water content (%) 98.0 98.3Total solid (TS, g/L) 20.00 16.50Volatile solid (VS, g/L) 10.80 7.06Suspended solid (SS, g/L) 16.70 15.80Volatile suspended solid (VSS, g/L) 8.72 6.64Total chemical oxygen demand (TCOD, mg/L) 13659 9600Soluble chemical oxygen demand (SCOD, mg/L) 3052 322

190 H. Li et al. / Bioresource Technology 123 (2012) 189–194

was used in alkaline sludge pretreatment, the residual NaOH in thepretreated sludge could destroy the bicarbonate buffer system inanaerobic digesters, and the excessive increase in pH could inhibitor even inactivate the anaerobic microorganisms. Therefore, it wassometimes necessary to neutralize the pretreated sludge with acid,but this would increase the cost of sludge pretreatment. On theother hand, in many cases of anaerobic digestion, it is necessaryto add appropriate amounts of alkali in the form of lime or sodiumor potassium hydroxide to the digesters, so that the pH can be keptin the neutral region (Mukherjee and Levine, 1992). Hence, it isimportant to optimize the alkaline dose to enhance the disintegra-tion effect, reduce alkali or acid consumption and increase the effi-ciency of anaerobic digestion.

After pretreatment with NaOH, the pH of the pretreated sludgewould decrease over time because of the consumption of NaOH.After the pretreated sludge was seeded, the pH of the blendedsludge would also change during the subsequent anaerobicdigestion. Therefore, the tolerance of anaerobic activated sludgeto alkaline conditions should be considered in terms of its impacton the subsequent anaerobic digestion. Only a few previous studiesinvestigated the variation of sludge pH during alkaline treatment(Xiao and Liu, 2006; Yang et al., 2007). When the initial pH ofthe pretreated sludge was lower than 10, the pH decreasedgradually to less than 8.5 after 24 h. Thus, the pretreated sludgecould be fed into the anaerobic digesters directly. When the initialpH of the pretreated sludge ranged from 12 to 13, the pH was stillclose to 12 after 24 h and acid neutralization was necessary beforethe pretreated sludge could be fed into the digesters (Yang et al.,2007).

In most related research, alkaline sludge treatment was com-bined with other treatment methods, e.g. thermal treatment orultrasonic treatment (Chiu et al., 1997; Vlyssides and Karlis,2004; Vigueras-Carmona et al., 2011; Chi et al., 2011). Data for asingle alkaline pretreatment preceding sludge anaerobic digestionwere very limited (Lin et al., 1997; Navia et al., 2002; López Torresand Espinosa Lloréns, 2008; Lin et al., 2009), especially in terms ofthe influence of initial pH and Na+ ion concentration on the sludgedigestion efficiency. To the best of the knowledge, this work fo-cused on alkaline sludge pretreatment before anaerobic digestion.The sludge used in the experiments was a mixture of primarysludge and biofilm sludge. The process of alkaline sludge disinte-gration was first examined using NaOH at different doses. The var-iation in sludge pH was investigated during alkaline pretreatmentand the subsequent anaerobic digestion in order to find the toler-ance of anaerobic activated sludge to alkaline pretreatment. Basedon these data, the parameters of alkaline pretreatment were opti-mized. Under the proposed conditions, the performance of the pre-treated sludge digestion was analyzed.

2. Methods

2.1. Sludge samples

The sewage sludge used in this study was collected from a localfull-scale municipal WWTP. In the plant, a biological aerated filterprocess was applied to clean the wastewater. The dischargedsludge, composed mainly of primary sludge with a small amountof biofilm sludge, was conditioned with polyacrylamide (PAM)and dewatered by centrifugation. In some of China’s cities, sludgedigestion is carried out in digesters remote from WWTPs and, thus,the sludge is dewatered in WWTPs and then diluted before diges-tion. Based on this situation, the dewatered sludge was collectedand stored at 4 �C in this work. Before alkaline treatment andanaerobic digestion, the dewatered sludge was diluted (sludge A,Table 1) using deionized water.

Digested sludge that was discharged from a laboratory-scalesemi-continuous anaerobic digester was used as the seed sludge(sludge B) in batch experiments of anaerobic digestion. The labora-tory-scale digester had an effective volume of 6 L and a sludgeretention time of 20 days. After 300 mL of digested sludge was dis-charged, 300 mL of sludge A was fed into the digester once a day.

2.2. Alkaline sludge treatment

Alkaline sludge treatments were conducted in 2.0-L batchmixed reactors. After NaOH was added, the sludge samples werestirred at 240 rpm for 30 min, which was the optimum durationtime with the highest efficiency (Lin et al., 1997; Navia et al.,2002; Cai et al., 2004; Li et al., 2008). In the first 30 min, the quan-tity solubilized was 60–71% of the total solubilized organic matterafter 24 h (Li et al., 2008). The dosage of NaOH ranged from 0.005to 0.5 mol/L. The solubilization of the organic substances in thesludge was measured using soluble chemical oxygen demand(SCOD). The degree of sludge disintegration was calculated as theratio of the increase in SCOD due to alkaline treatment to the max-imum possible SCOD increase:

DDCOD ¼ ðSCOD� SCOD0Þ=ðTCOD� SCOD0Þ ð1Þ

where SCOD0 is the SCOD of the untreated sludge samples andTCOD is the total chemical oxygen demand.

2.3. Variation in sludge pH

After the alkaline sludge (sludge A) treatment, the pH variationof the mixture of the treated sludge and the seed sludge (sludge B)was observed to investigate the tolerance of the anaerobic sludgeto alkaline treatment. The samples of sludge A were disintegratedwith 0.02, 0.04, 0.1 and 0.2 mol/L NaOH for 30 min, and then100 mL of treated sludge were mixed with 75 mL of seed sludge(sludge B) in a 250-mL jar. The initial pH values of the blendedsludge were 8.89, 9.31, 10.33 and 12.73, respectively. After themixtures were added to the jars, each jar was flushed with nitro-gen, in order to remove oxygen, and then sealed with a rubberplug. A pH probe was pushed through the rubber plug and sub-merged in the sludge. The jars were placed in a shaking water bath(SHA-C, ZhongDa Instrument, China) to keep the reaction temper-ature at 35 ± 2 �C. The pH changes of the blended sludge sampleswere recorded over 24 h and compared with those of the controlwithout alkaline pretreatment.

2.4. BMP tests

Biochemical methane potential (BMP) tests were carried out inthe above devices in order to further investigate the influence ofNaOH pretreatment on methane production. The samples of sludgeA were disintegrated with 0.005, 0.01, 0.02, 0.04, and 0.1 mol/LNaOH for 30 min, and then 100 mL of the treated sludge weremixed with 75 mL of the seed sludge (sludge B). The initial pH

H. Li et al. / Bioresource Technology 123 (2012) 189–194 191

values of the blended sludge samples were 7.67, 8.24, 8.67, 9.31and 10.50, respectively. The blended sludge samples were thenused for batch anaerobic digestion experiments. The biogas pro-duction was observed over 21 days by displacement of a saturatedsodium chloride solution (Salam et al., 2009).

More batch experiments were carried out in order to determinethe influence of acid conditioning of the pretreated sludge on thesubsequent anaerobic digestion. NaOH dosage was increased to0.5 mol/L. The pretreated sludge was first conditioned with HCland then used for BMP tests. The biogas production and the settlingability of the digested sludge were measured. The settling ability ofthe sludge was determined by measuring the volume of settledsludge after 30 min (SV30).

2.5. Analytical procedures

The water content, TS, volatile solids (VS), suspended solids (SS),VSS and chemical oxygen demand (COD) were measured accordingto standard methods (APHA, 2005). The pH of the sludge was mea-sured with a pH meter (EUTECH Cyberscan510, Singapore). Whenmeasuring sludge SCOD, the samples were centrifuged at 5000gfor 10 min, and then the supernatant was filtered through a mem-brane with a mesh size of 0.45 lm. The filtrate was used to deter-mine the SCOD. TCOD is the total COD of the sludge samples.

3. Results and discussion

3.1. The effects of alkaline pretreatment on sludge disintegration

The effects of NaOH pretreatment are presented in terms of thechange in sludge SCOD (Fig. 1). An increase in SCOD indicates thatmore organic substances were released. For the raw sludge (sludgeA), the SCOD increased substantially when the NaOH dose in-creased from 0.005 to 0.1 mol/L, whereas the increase in SCODwas very limited when the dose increased further from 0.1 to0.5 mol/L. Therefore, for sludge A, the optimum dose of NaOHshould be about 0.1 mol/L. At this dose, the DDCOD of sludge Awas 26.9%. The low value was attributed to the high initial SCOD(SCOD0) of sludge A, which was 22.3% of TCOD. Sludge A was com-posed of about 80% primary sludge and 20% biofilm sludge. The or-ganic substances in the primary sludge were easily dissolvedduring the diluting and stirring processes of the raw dewateredsludge, whereas the organic substances derived from the biofilmsludge were difficult to dissolve.

Different results were found for the alkaline treatment of the di-gested sludge (sludge B). The inflection point of the sludge SCOD

Fig. 1. Sludge SCOD after NaOH treatment at different doses.

change was at a dose of 0.2 mol/L, which was higher than thatfor sludge A. At this dose, the DDCOD of sludge A was only 14.1%.Sludge B was derived from the digestion of sludge A, and most ofthe degradable organic matter had been transferred into biogasand water. Either the residual organic matter was difficult to solu-bilize by NaOH treatment, or the particles containing the residualorganic matter were difficult to disrupt. Therefore, less disintegra-tion was obtained, and a higher NaOH dose was necessary to disin-tegrate the digested sludge. According to previous reports,disintegration effects have also been shown to be dependent onthe characteristics of the sludge. The optimum NaOH dosage forsludge disintegration was reported to be 0.04–0.1 mol/L or 0.08–0.16 g/g TS (Lin et al., 1997; Navia et al., 2002; Li et al., 2008),and the values of DDCOD reached 40–50% for WAS with high organ-ic content of 70–80%. Obviously, disintegration was more difficultto accomplish for sludge A or sludge B, which had a lower organiccontent, and the proper dose required to solubilize the sludge washigher.

The variation in SCOD verified the solubilization of organic mat-ter during NaOH treatment. Acceleration of the sludge hydrolysiswould improve the subsequent anaerobic digestion, but the in-crease in pH would inhibit anaerobic bacteria. After sludge A orsludge B was mixed with NaOH at different doses, values of the ini-tial pH and the pH after 30 min are shown in Fig. 2. An obvious in-crease in sludge pH was found with increasing NaOH dose. After30 min, the decrease in sludge pH was very limited, especially athigh doses. If the treated sludge was directly fed into digesters,the anaerobic digestion process would be influenced by the suddenchange in pH due to the residual NaOH. Hence, further informationwas needed about the tolerance of anaerobic sludge to alkalineconditions.

3.2. Tolerance of anaerobic sludge to alkaline conditions

After the treated sludge A was mixed with the seed sludge(sludge B), the pH variation of the blended sludge was recorded.The control was the blended sludge without NaOH pretreatment:its initial pH was 7.4 and the pH changed only slightly over 24 h.The initial pH of the blended sludge rose to about nine, and thesludge pH then decreased gradually to about eight during the sub-sequent 24 h of anaerobic digestion. The buffer system in theanaerobic digesters can weaken the effect of NaOH addition, and

Fig. 2. The pH at the beginning and after 30 min of NaOH sludge treatment ofdifferent doses.

Fig. 4. Cumulative biogas production from sludge batch anaerobic digestions withdifferent initial pH values (different NaOH doses).

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the organic acids produced by acid-forming bacteria would alsoneutralize some of the NaOH. When the initial pH rose to over10, the subsequent decrease in sludge pH was almost negligible,and the anaerobic digestion system did not recover in 24 h. In gen-eral, the appropriate pH is 6.8–7.2 for completely mixed digesters(Lay et al., 1997). Therefore, if the initial pH was higher than 10(NaOH dose was 0.1 mol/L or 0.2 g/g TS), the anaerobic digestionprocess may have been obliterated.

The effect of the initial pH on sludge anaerobic digestion wasfurther verified using BMP tests (Fig. 3). The blended sludge withan initial pH of 10.50 did not produce any biogas over 15 days (re-sults not shown in Fig. 3), which was in accordance with the pHvariation. When the blended sludge had an initial pH of 8.67 or9.33, the pH value dropped to about eight during the 24-h anaero-bic digestion according to our observations, but the effects of thetwo initial pH values on biogas production were very different.For the blended sludge with an initial pH of 9.33, the start of biogasproduction was delayed by 3 days compared to the control with aninitial pH of 7.02. When the initial pH of the blended sludge was8.24, the start of biogas production was delayed very little by alka-line pretreatment, but the rate of biogas production was indeedlower in the initial 3 days of the batch anaerobic digestion. Onlythe sample with an initial pH of 7.67 was not impacted by the in-creased pH. For the control, biogas production ended in 10 days,whereas the end time was prolonged for the other samples withhigher initial pH values. When the anaerobic bacteria survivedthe initial pH of 8.67 and 9.33, biogas production also recovered,but the maximum single-day production was still lower than thecontrol.

The cumulative biogas production was also compared amongsamples. The cumulative biogas production decreased withincreasing NaOH dose (Fig. 4). On one hand, pretreatment with alow dose of NaOH (e.g. 0.005 mol/L) did not dissolve the organicmatter sufficiently; on the other hand, pretreatment with a highdose reduced the activity of the anaerobic sludge. Although theanaerobic sludge could survive initial pH conditions of 8.2–9.3(Fig. 3), the efficiency of the anaerobic digestion still decreased.Therefore, the initial pH of the anaerobic digesters should be con-trolled at a pH lower than 8.0 after the alkali-treated sludge is fedinto the digesters. Since pretreatment with a low NaOH dose couldensure a low initial pH but did not improve the subsequent anaer-obic digestion, acid conditioning following pretreatment with ahigh dose was necessary.

After the 0.04 mol/L NaOH treatment, the sludge samples(sludge A) were conditioned with different doses of HCl and then

Fig. 3. Daily biogas production from sludge batch anaerobic digestions withdifferent initial pH values (2 g dry solid of sludge A was fed to each digester).

blended with the seed sludge (sludge B). Further BMP tests werecarried out in order to analyze the influence of HCl conditioningon anaerobic digestion. Biogas production is shown in Fig. 5. AfterNaOH pretreatment, the HCl conditioning produced an obviouseffect on biogas production during the subsequent anaerobicdigestion. When the initial pH of the blended sludge was adjustedto about 7.66, the efficiency of biogas production at the initial stagedid not decrease, and the cumulative biogas production increasedby 25% compared to the control. When the blended sludge was ad-justed to a pH of 8.47 or 8.86, biogas production was delayed by1 day. Moreover, cumulative biogas production showed almostno increment for an initial pH of 8.47, but a decrement was ob-served for an initial pH of 8.86. These results were similar to thosewithout HCl conditioning (Fig. 4). An initial pH of 8.5 did not weak-en the anaerobic digestion, and did not improve it either. Thatindicated the inhibition of the anaerobic sludge canceled out theeffect of alkaline sludge disintegration. Hence, the initial sludgepH should be controlled at a pH lower than 8.0 after NaOH pre-treatment and HCl conditioning.

Fig. 5. Biogas production of sludge batch anaerobic digestion after 0.04 mol/L NaOHtreatment and HCl conditioning.

Fig. 6. Biogas production of sludge batch anaerobic digestion after NaOH pretreat-ment at different doses (the initial pH was adjusted to 7–8).

H. Li et al. / Bioresource Technology 123 (2012) 189–194 193

3.3. The effect of alkaline dose on anaerobic digestion of the sludge

After NaOH pretreatment and HCl conditioning, the initial pH ofthe blended sludge was controlled at 7–8, and then the biogas pro-duction was recorded (Fig. 6). For digestions with a pretreatment of0.04 or 0.1 mol/L NaOH, biogas production was similar to the con-trol at the initial stage of anaerobic digestion, but an improvementwas seen after 5 days. At the initial stage, the soluble or easily dis-solved organic substances were consumed first. The substrate wassufficient for the anaerobic bacteria even without NaOH disintegra-tion. After that, the inner organic substances in the sludge particlesbecame the main substrate. NaOH pretreatment released these in-ner substances and accelerated the digestion. For the digestionwith 0.1 mol/L NaOH pretreatment, the organic degradation ratewas 38.3% and the biogas yield was 0.65 L/g VSS, whereas the val-ues for the control were 30.3% and 0.64 L/g VSS, respectively. Thebiogas yield was almost the same, and the increase in the totalamount was attributed to a higher degradation rate of the organicsubstances with the help of NaOH pretreatment.

During the digestion with pretreatment of 0.3 mol/L NaOH, thecumulative biogas production was higher than the control, but thedaily biogas production was lower at the initial stage. Moreover,the cumulative biogas production was still lower than that of thedigestion with 0.1 or 0.04 mol/L NaOH pretreatment. Pretreatmentwith 0.5 mol/L NaOH led to a stronger inhibition of the anaerobicbacteria due to the high concentration of Na+. In general, 3.5–5 g/L Na+ can moderately inhibit the activity of mesophilic methano-gens, and 8 g/L Na+ can lead to strong inhibition (McCarty, 1964).When the dose of NaOH was 0.04, 0.1, 0.3 and 0.5 mol/L, the in-crease in [Na+] was 0.9, 2.3, 6.9 and 11.5 g/L, respectively. Conse-quently, pretreatment with 0.3 or 0.5 mol/L NaOH followed byHCl conditioning formed a high salinity environment, which inhib-ited the activity of the anaerobic bacteria. The experimental resultsalso showed that anaerobic bacteria could adapt to a high salinityenvironment through long-term domestication. The lag phase ofbiogas production could also be shortened (Omil et al., 1995).The comprehensive effect of NaOH pretreatment on sludge anaer-obic digestion depended on improvement of sludge hydrolysis andavoidance of high salinity. Considering the disintegration effect onsludge A (Fig. 1), 0.1 mol/L was a sufficient NaOH pretreatment.

Amino-carbonyl reactions (browning reactions) are also recog-nized as one of the reasons for the deterioration of sludge hydroly-

sis-acidification efficiency (Gao et al., 2011). After thermal alkalinepretreatment at a pH of more than 11, the brown color of a sludgelysate was attributed to the products of the amino-carbonyl reac-tion, and some of the products were difficult to degrade. The colorevolution could be revealed by the UV–vis spectra (Xiang et al.,2011). However, similar spectra were not found for experimentsinvolving NaOH treatment without thermal conditioning. It wasdeduced that the hardly degradable products of the amino-car-bonyl reaction were not the reason for the decrement in biogasproduction.

After the batch digestions with NaOH pretreatment, the settlingability of the digested sludge was also measured. The values of SV30

were about 38–40%, which was lower than the 66% value of thecontrol and also lower than the value of the raw sludge (sludgeA). Hence, NaOH pretreatment at an appropriate dose could im-prove the comprehensive performance of sludge anaerobicdigestion.

4. Conclusions

The optimum NaOH dose for sludge disintegration was 0.1 mol/L. However, the residual NaOH delayed the start of anaerobicdigestion and reduced the amount of biogas produced at the initialstage. Acid conditioning should be used to maintain the initial pHof the digesters below eight. For sludge pretreatment, the optimumdose was also 0.1 mol/L. A higher dose would increase the salinityand inhibit the efficiency of sludge digestion. Under optimized con-ditions, the organic degradation rate was 38.3% and the biogasyield was 0.65 L/g VSS, and the settling ability of the digestedsludge was also improved.

Acknowledgements

This work was financially supported by The China Major Sci-ence and Technology Program for Water Pollution Control andTreatment (No. 2011ZX07317), The Natural Science Foundationof China (51008174), the fund of Key Laboratory for Solid WasteManagement and Environment Safety, Ministry of Education ofChina (SWMES201011) and the Shenzhen Science and TechnologyResearch and Development Fund (JC201005270309A).

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