Desalination 251 (2010) 58–63

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Improving anaerobic biodegradability of biological sludges by Fenton pre-treatment:Effects on single stage and two-stage anaerobic digestion

Gulbin Erden ⁎, A. FilibeliDokuz Eylül University, Department of Environmental Engineering, Tinaztepe Campus, 35160, Buca-Izmir, Turkey

⁎ Corresponding author.E-mail address: gulbin.erden@deu.edu.tr (G. Erden).

0011-9164/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.desal.2009.09.144

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 August 2009Received in revised form 28 September 2009Accepted 28 September 2009Available online 21 October 2009

Keywords:Anaerobic digestionBiological sludgeFenton processSludge minimization

In the present study, the effects of Fenton process on anaerobic sludge bioprocessing were investigated. Aratio of 0.067 g Fe(II) per gram H2O2, and 60 g H2O2/kg dried solids (DS) was applied to biological sludgesamples preceding anaerobic sludge digestion. Single stage anaerobic digestion under thermophilicconditions is compared with two-stage anaerobic digestion (mesophilic digestion prior to thermophilicdigestion). The comparison is in terms of solid reductions and specific methane productions. Fentonprocessed sludge gives higher solid reduction and higher methane production for each experiment. Thehighest reduction in sludge's solids was observed in a reactor operated under thermophilic conditions. Thesecond stage digestion under mesophilic conditions did not induce extra solid reduction. However, itfacilitated higher methane. Another observation is that, Fenton process led to decrease the biosolids'resistance to dewatering in terms of capillary suction time (CST), but had no effect on sludge dewatering onbelt-press application.

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© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Anaerobic digestion of waste activated sludge (WAS) is often slowdue to the rate limiting cell lysis step [17]. In order to improve hydrolysisand anaerobic digestion performance, floc disintegrationwas developedas the pre-treatment process of sludge to accelerate the anaerobicdigestion and to increase the degree of stabilization [4,32]. Sewagesludge disintegration can be defined as the destruction of sludge byexternal forces. The forces can be of physical, chemical or biologicalnature. A result of the disintegration process is numerous changes ofsludge properties [21]. The changesmay be summarized as disruption ofmicrobial cells in the sludge, thereby destroying the cell walls andreleasing the cell content; breakingupor disrupting the cellwalls, so thatsubstances protected by the former are released and dissolved; andopening up the cell walls of organisms, so that the substances containedin the cell are solubilized [30]. Increased stabilization degree of sludgewith disintegration process provides less sludge production,more stablesludge and more biogas production comparing the classical anaerobicdigestion [31]. Ultrasonic treatment [3,14,25,26,29,34,35], ozone oxida-tion [5,20,33], mechanical disintegration [16,23], alkaline treatment[6,18], thermal treatment [2] and biological hydrolysis with enzymes[1,15] were investigated for sludge disintegration purpose by severalresearchers inhalf-scale and lab-scale plants. Fentonprocess is oneof thecommonly used advanced oxidation techniques. Fenton's reagent is a

mixture of H2O2 and ferrous iron. The ferrous iron initiates and catalysesthe decomposition of H2O2, resulting in the generation of highly reactivehydroxyl (OH) radicals [12]. The OH radical is the main reactant in theprocess capable of decomposing a number of organic substances viaoxidation. The rate and extent of the Fenton reactions are dependent onsystem parameters including, iron and hydrogen peroxide concentra-tion, and solution pH. The application of the Fenton process fordisintegration of WAS may cause two phenomena: solubilization andmineralization of sludge solids. Part of activated sludge is mineralized tocarbon dioxide and water while part of sludge is solubilized tobiodegradable organics, which are easily accessible and can be digestedmuch faster in later biological process than sludge in a particular phase.Takumura et al. [28] applied the similar advanced oxidation method ofphoto-Fenton reaction toWAS in a batchphoto reactor for disintegrationpurpose; soluble chemical oxygen demand (SCOD) was achieved athighest level in the presence of 4 g H2O2/L, 40 mg Fe(II)/L, and 3000 mgMLSS/L, pH=3 for 6 h reaction time and effective disintegration wasobtained. At longer than 6 h retention time, COD was decreased andmineralization occurred. Nickel and Neis [24] applied Fenton process tothickened sludge and they noted optimum activity in the presence of25 g H2O2/kg DS, and 1.67 g Fe(II)/kg DS, pH=3 and at ambienttemperature andpressure. In these conditionsFentonprocess resulted ina considerable reduction of dried solid (DS) and volatile solid (VS)contents in the filter cake of approximately 20%, an improveddewaterability with a 30% reduction of the sludge volume, and a 30%increase of the cake DS-content when compared with the untreatedsludge sample. In another study, biological sludge is processed with0.07 g Fe(II) per gramof H2O2 and50 gH2O2/kgDS at pH=3andFenton

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processed sludge were digested in laboratory scale digester consisting1 L batch reactor atmesophilic and a 26.6% reduction in VSwas recorded[7]. In our previous study, 0.067 g Fe(II) per gramof H2O2 and60 gH2O2/kg DS was found as optimum dose using Box–Wilson ExperimentalDesign Method. Biological sludge was processed with 0.067 g Fe(II) pergram of H2O2 and 60 g H2O2/kg DS at pH=3, and 25.4% reduction in VSwas obtained in 8.5 L lab-scale digester operated with a 5 day sludgeretention time at mesophilic conditions. VS reduction in the reactor fedwith Fenton processed sludgewas 1.53 times greater than in the controlreactor at the end of the 20th operation day [11]. In this study, the sameratio of 0.067 g Fe(II) per gramH2O2, and60 gH2O2/kgDSwas applied tobiological sludge samples preceding anaerobic sludge digestion. Singlestage anaerobic digestion under thermophilic conditions was comparedwith two-stage anaerobic digestion (mesophilic digestion prior tothermophilic digestion). The comparison was in terms of solidreductions and specific methane productions.

2. Materials and methods

2.1. Sludge characterization

Waste activated sludge (WAS) was sampled from the municipalwastewater treatment plant in Izmir, which has extended aerationactivated sludge plantwith nutrient removal facilities. At the start-up ofthe reactors, inoculum sludge was taken from a full-scale upflowanaerobic sludge blanket (UASB) reactor treating beer industrywastewater. Dried solids (DS), volatile solids (VS), pH, electricalconductivity (EC), and CST are analyzed to determine the characteristicsof sludge. Results are shown in Table 1. All analyses are according to theprocedures given in the Standard Methods [27]. pH and electricalconductivity measurements were carried out with a 890 MD pH meterand a YSI Model 33 conductivity-meter, respectively.

2.2. Experimental procedure

2.2.1. Fenton processIn Fenton experiments, a ratio of 0.067 g Fe(II) per gram H2O2, and

60 g H2O2/kg dried solids (DS) was applied to a 1.5 L sludge sample. Thisdose combination is from an optimization study which is using Box–Wilson Statistical Design Method. Disintegration degree (DD) [22] wasusedas a systemresponse for evaluationofflocdisintegration. The resultswere given in our previous study [11]. The Fe(II)/H2O2 ratio used in thisstudy is in agreementwith [24]. Fentonprocess is carriedoutby adjustingthepH to 3withH2SO4. The second step is the addition of Fe(II) at certainconcentrations. After this,H2O2 is added to the sample. Themixed sampleis stirred at 100 rpm for 60 min. After reaction, the sample is neutralizedwith Ca(OH)2. In Fenton experiments, ferrous (FeSO4⋅7H2O) is used as

Table 1Properties of waste activated sludge and anaerobic inoculum sludge.

Parameters Activatedsludge

Anaerobicinoculum sludge

pH 7±0.2 8.22±0.1Electrical conductivity (EC, μS/cm) 7.21±1.42 3.12±0.3Redox potential (ORP, mV) 35±2 −185±3Dried solids (DS, %) 1.81±0.36 7.5±0.3Volatile solids (VS, %) 54.44±1.92 82.5±1.9Suspended solids (SS, mg/L−1) 13500±650 72800±3986Volatile suspended solids (VSS, mg/L−1) 9500±348 65250±4265Capillary suction time (CST, s) 142.5±16.48 400±15.9Crown press applicationFinal cake solids (%) 10.31 –

Particle size (µm)Surface weighted mean D [3,2] 18.444 221.876Volume weighted mean D [4,3] 77.212 1094.572d (0.1) 9.970 121.798d (0.5) 46.496 1204.415d (0.9) 160.299 1583.540

the source of Fe(II), ferrous (FeSO4⋅7H2O) is of analytical grade fromMerck. Hydrogen peroxide solution (37% (w/w)) in stable form, H2SO4

(98–99%) and NaOH are all of analytical grade from Merck.

2.2.2. Anaerobic digestion studiesFour lab-scale anaerobic reactors are used in sludgedigestion. Twoof

them are operated as control reactors without Fenton's application,where the others are fedwith Fenton processed sludge. In the first stageof anaerobic digestion studies, two lab-scale anaerobic reactorswith thevolume of 13.5 L were operated at 55±2 °C in thermophilic conditions.Control reactor is marked as TC, and the reactor fed with Fentonprocessed sludge is marked as TF. In the second stage of anaerobicdigestion, 8.5 L lab-scale anaerobic reactors operated at 37±2 °C inmesophilic conditionswere used. The control reactorwas coded as TMCand the reactor fed with Fenton processed sludge was coded as TMF. Inthe second stage, sludge taken to the TCwas fed to the reactor coded asTMC and sludge taken to the TFwas fed to the reactor coded as TMF. Thereactors were heated and the temperature was kept constantly by heattransfer oil jacket which is constructed from a stainless-steel. Thisreactor is operated with PLC for both stages. Mechanical mixers wereused in the reactors. In the start-up phase, inoculum sludge was fed toreactors. After that, 1/2 volume of the reactor content was withdrawnand the same volume of activated sludge was fed to the reactors. In ourprevious study [11], different sludge retention timeswere carriedout forthe same sludge samples in mesophilic conditions. 5 day sludgeretention times gave the best results in terms of stabilization degree.Hence, all reactors were operated as semi batch system with a 5 daysludge retention time in digestion studies. To see the temperature effecton anaerobic biodegradability of sludge, sludge retention time inreactors operated in thermophilic conditions was chosen as the samein reactors operated in mesophilic conditions. In semi batch systemswith 5 days of sludge retention time, 1/5 of the total volume of sludgein reactors was withdrawn to the reactors and the same volume ofsludge content was fed everyday during the operation period. Duringthe 30 day operation period, 2.7 L of sludge was withdrawn and thesame volume of sludge content was fed to the TC and TM reactors, then1.7volume of sludge from the TC and TM were withdrawn to the TMCand TMF respectively, and every day the same volume of sludgewas fedto the TMC and TMF.

2.3. Analytical methods

For system evaluations, pH and temperature were monitored dailywhile alkalinity, volatile fatty acid (VFA), and electrical conductivityvalues were measured three times in a week. For performanceevaluations, total dried solids (DS), volatile solids (VS), suspendedsolids (SS), volatile suspended solids (VSS), protein contents, particlesize distribution, daily total gas andmethane productions, and CSTweremeasured during the operation period. DS, VS, SS andVSS analyseswereregularly done according to the StandardMethods [27]. CST valueswereanalyzed with a Triton A-304 M CST-meter. Particle size distributionswere monitored by using a Malvern Mastersizer 2000QM analyzer. Thebelt-press simulator of crown press supplied from Phipps and Bird,Richmond, VA was used for evaluation of dewatering properties ofsludge. Sludge slurry (200 mL) was drained through a screen and thevolumecollected after 2 minwasmeasured. The solids remainingon thescreenwere then pressed and thefinal cake solids determined.Methaneproductions were determined by liquid displacement method. In thismethod gas passes through the distilledwater including 3% (w/v)NaOH[13]. Due to the lack of digital device to measure the amount of gasproduced, gas valves of the reactors were first closed for about 1h andthen opened. The liquid displacements were converted to the dailyproductions. Specific methane productions (SMP) were determined asmLCH4/gVS based on volatile solids and dailymethane productions. Gascomponents (CO, CO2, and H2S)were analyzed by a Drägermodel X-am7000 multi gas analyzer. Volatile fatty acid (VFA) measurements were

Fig. 1. Dried solid changes in the reactors as a function of operation time.Fig. 3. Suspended solid changes in reactors as a function of operation time.

Fig. 4. Volatile suspended solid changes in reactors as a function of operation time.

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done with an HPLC Agilent 1100 with C18 column. Extracellular poly-meric substances (EPS)were extracted from the samples using the heatextraction techniqueoriginated byGoodwin and Forster [9] and Frolundet al. [8]. Protein contents of EPS sampleswere analyzed byprotein assaykits (Procedure No. TP0300 Micro Lowry, Sigma).

3. Results and discussion

3.1. Anaerobic sludge digestion following floc disintegration by Fentonprocess

In digestion studies, pH and temperature parameters were moni-tored daily while alkalinity, redox potential, and volatile fatty acids(VFA) parameterswere analyzed three times in aweek for evaluation ofanaerobic digester performance. pH values varied from 6.80 to 8.80 inreactors. The temperature was kept at 37±2 °C in all mesophilicreactors. Redox potentials of reactor contents in a very negative range of−300 mV and −500 mV were observed. Total alkalinity values weremeasured regularly as a measure of the stability of the digesters. Analkalinity range of 1880–4343 mg CaCO3/L was measured during theoperation period. VFA contentwas also checked for reactor stability andVFA values did not exceed 1000–1500 mg/Lwhich is not recommendedfor anaerobic methanogens [19]. VFA values did not exceed thisrecommended range even during the first operation days.

DS changes in reactors as a function of operation time which isgiven in Fig. 1. During the few days in the first stage of anaerobicdigestion DS values decreased drastically, thermophilic conditionsplayed an important role in the decrease of DS-content. At the end ofthe 30th operation day, DS of sludge were decreased to 14.4%, 28.2%,22.7% and 27.1% with reference to the raw sludge for TC, TF, TMC, andTMF, respectively. In the first stage, DS reduction in reactor fed withFenton processed sludge was approximately two times higher thancontrol (reference) ones. The second stage of digestion did notprovide an extra reduction in DS for reactor coded as TF. In contrast,the second stage improved the DS reduction in control reactors.

Fig. 2. Volatile solid changes in reactors as a function of operation time.

VS measurement results are shown in Fig. 2. Better volatile solidsreductions were observed with Fenton processed sludge compared tocontrol ones in both first stage and second stage of anaerobicdigestion studies. At the end of the 30th operation day, VS of sludgewere decreased to 20.5%, 26.8%, 22.2% and 22.7% with reference to theraw sludge for TC, TF, TMC, and TMF, respectively. Fenton processimproved the VS destruction and the highest reductions in VS wereobtained in the reactor coded as TF. Better VS reductions wereobserved for TC and TF especially in the first fifteen days. Two-stagedigestion did not show higher effect on VS destruction than singlestage digestion. In our previous work, biological sludge (taken fromthe same treatment plant) was processed with Fenton's reagent withsame dose at pH=3 and Fenton processed sludge was anaerobicallydigested with a 5 day sludge retention time, at mesophilic conditionsand 21.5% of the VS reduction was obtained at the end of the 30 dayoperation period. Hence, thermophilic conditions are more effectivethan mesophilic conditions in VS reduction of sludge [11].

Fig. 5. Specific methane productions (SMP) during the operation period.

Fig. 6. Protein concentrations in reactors content during the operation.

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Figs. 3 and 4 demonstrate the changes of SS and VSS as a functionof operation time. The disintegration of the sludge cells was alsoreflected in decreasing SS contents of the sludge. SS and VSS decreasedquickly especially in the first week of operation period in reactorsoperated with thermophilic conditions. After fifteen days of operationmuch closed values of SS and VSS were observed for all reactors.

Higher reductions in SS were observed in reactors fed with Fentonprocessed sludge than in control reactors. Here, the efficiency of theFenton process in sludge solubilization has been confirmed by thepilot-scale digestion experiments. At the end of the 30 day operation,SS of sludge were decreased to 22.2%, 39.6%, 32.1% and 39.3% withreference to the raw sludge for TC, TF, TMC, and TMF, respectively.While the second stage of digestion caused an extra SS and VSS

Fig. 7. Particle size distributions in 1st, 5th, 15th,

reduction in control reactors, nearly same values of SS were observedin TF and TMF at the end of the 30 day operation.

The second stage did not significantly affect the SS and VSSreduction for reactors fed with Fenton processed sludge. VSS of sludgewere decreased to 31.1%, 46.3%, 43% and 40.8% according to the rawsludge for TC, TF, TMC, and TMF, respectively at the end of the 30 dayoperation. The minimum VSS of 5100 mg/L was achieved at the 30thday of operation, while the valuewas 32,670 mg/L at the end of the firstoperation day.

Fig. 5 shows the specific methane productions during the operationperiod. Specific methane productions were increasing with increasingoperation days for the first 20 days after the productions were almoststagnated. Reactors fed with Fenton processed sludge gave highermethane production rates compared to the control reactors. At the endof the 20 day period specific methane productions were calculated as400.3 mL methane/gVS, 547.3 mL methane/gVS, 544.6 mL methane/gVS and 561.1 mL methane/gVS for TC, TF, TMC, and TMF, respectively.Fenton pre-treatment provided 1.4 and 1.2 times higher methaneproductions in single stage of digestion and second stage of digestion,respectively, at the end of the 30 day operation period. Results showedthat two-stage digestion provides more methane production comparedto the single stage thermophilic digestion. Fenton process improved themethane productions in two-stage digestion too. Total methaneproductions in TF and TMF were 1.3 times higher than the totalmethane productions in control reactors of TC and TMC at the 30th dayof operation.

For reactors fed with Fenton processed sludge, H2S and CO valueswere higher than the control reactor, and Fenton process led to anincrease in H2S and CO levels in the reactors. For TF and TMF, H2Svalues higher than 100 ppm were recorded during the operation

and 30th operation days for reactor contents.

Table 2Particle size changes for the 1st, 5th, 15th and 30th operation days in reactor contents.

Sludge (ID) Sur. weightedmean D (3,2)

Vol. weightedmean D (4,3)

d (0.1) d (0.5) d (0.9)

TC — 1st day 78.683 394.766 31.105 313.088 907.525TF — 1st day 66.413 360.995 26.383 275.465 841.996TMC — 1st day 70.045 418.505 27.776 335.950 971.219TMF — 1st day 32.080 251.307 17.881 103.063 696.762TC — 5th day 24.971 207.544 16.437 71.972 627.874TF — 5th day 25.317 225.805 15.314 70.719 688.711TMC — 5th day 65.180 340.018 26.737 228.513 820.663TMF — 5th day 32.066 273.415 18.553 116.564 742.256TC — 15th day 19.465 71.255 12.397 47.392 119.066TF — 15th day 18.772 57.196 11.746 40.667 102.530TMC — 15th day 20.765 109.759 14.441 55.364 283.667TMF — 15th day 21.440 147.178 13.328 51.987 501.841TC — 30th day 20.025 67.968 14.465 49.461 120.852TF — 30th day 20.036 64.405 12.692 43.263 119.560TMC — 30th day 22.240 70.936 15.042 51.043 127.426TMF — 30th day 20.566 79.564 12.984 44.663 135.798

Table 3Final cake solids obtained from crown press application during the operation period.

ReactorID/days

Final cake solids, %

1 10 15 20 25 30

TC 13.09 13.2 10.8 None observed None observed None observedTF 13.7 13.3 11.6 None observed None observed None observedTMC 15.6 11.6 11.6 11.3 13 12.6TMF 15.5 12.1 10.9 10.9 12.1 12.5

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period. H2S contents did not exceed 1000 mg/L which is not recom-mended for anaerobic methanogens reactions [10].

Cell lysis transforming cell content into the medium is the first, andbreakdown of extra cellular polymeric substances (EPS) fraction in thesludge is the second stage of floc disintegration. The protein concentra-tions of reactor contents decreasedwith operation time in all reactors asshown in Fig. 6. At the end of the first operation day, lower proteinconcentrations were observed in the reactors operated with Fentonprocessed sludge than control ones. Besides, the second stage ofdigestion enhanced the degradation of protein contents of sludge.Particle size distributions in Fig. 7 indicate floc disintegration stemmingwith protein data. Particle size distribution of reactor contentsdecreased during the operation period. Particle size distribution wasat peak centered on around 500 to 900 µm for the first five operationdays. Two peaks were observed especially in the first operation days forall reactors. The second peak could be explained by a re-flocculationphenomenon [4]. This re-flocculationwasobtained especially in thefirstdays due to the release of intracellular or extracellular material and thesecond peak was decreased with operation time. Particle sizedistribution was at peak centered on around 52.481 µm for both TCand TMC at the end of the 30 days of operation. This valuewas recordedas 45.709 for both TF and TMF.

Table 2 shows the particle size changes for selected operation days.d (0.1), d (0.5), and d (0.9) are to say that 10%, 50%, and 90% of particles(in volume) having a diameter lower or equal to d (0.1), d (0.5), and d(0.9), respectively. Fenton process does not have a significant effect onparticle size, and similar results were obtained in the reactors coded asTC and TF in the first stage of digestion, the effect of Fenton process ondecrease of particle size can be clearly seen in the first operation days in

Fig. 8. CST variations in reactors as a function of operation time.

the second stage of digestion. Higher reductions were obtained inreactors fed with Fenton processed sludge compared to that in thecontrol reactors in the second stage of mesophilic digestion.

3.2. Evaluation of dewatering characteristics of digested sludge

CST is a quick and simple method to evaluate the filterability ofsludge. This method neglects the shear effect on sludge, and it cannotdetermine the dewaterability differences among dewatering processesbut gives an idea on the dewatering capacity of sludge. CST results givenin Fig. 8 are consistentwithprotein results. Degradation inprotein led tolessening in the biosolids' resistance to dewatering. In the first stagedigestion, CST of sludge decreased to 32.6% and 90.9% according to theraw sludge for TC and TF, respectively at the end of the 30 days ofoperation. Depending on the CST data, we can say that using Fentonprocess preceding anaerobic sludge digestion has a positive effect onsludge filterability. On the other hand, the second stage of digestion didnot improve the CST reduction in digested sludge. For evaluation ofdewatering characteristics of digested sludge a crown presswas used asa simulator for belt-press. The reactor contentswere regularlyprocessedthrough a crown press during the 30 days of operation period.

The results in Table 3, show no improvement in cake formation ofdigested sludge. While cake solids of raw sludge were 10.31%, cakesolids were not obtained after the15th operation day in TC and TF.Thermophilic conditions negatively affected the cake formation.

4. Conclusions

The effect offlocdisintegrationby Fentonprocess on anaerobic sludgedigestion performance was investigated. Results of this study showedthat anaerobic degradability of sludge can be enhanced using the Fentonprocess. The most effective sludge solubilization was observed in thereactor operated at thermophilic conditions and fed with Fentonprocessed sludge. 28.2% reduction in DS, 26.8% reduction in VS, 39.6%reduction in SS, and 46.3% reduction in VSSwere obtainedwith referenceto the raw sludge for this reactor. The second stage digestion at meso-philic conditions did not provide an extra improvement in soliddestruction. Significant reductions in EPS were obtained for digestedsludge especially pre-treated with Fenton process. Particle size distribu-tions indicated floc disintegration stemming with EPS data. Fenton pre-treatment increased the methane production rates for both single stagethermophilic digestion and two-stage digestion. Two-stage digestionprovides more methane production compared to the single stagethermophilic digestion. Total methane productions in TF and TMF were1.3 times higher than total methane productions in control reactors of TCand TMC at the 30th day of operation. Improved degradations in proteinwith the operation period led to decreases in biosolids' resistance todewatering. Reactors fed with Fenton processed sludge gave lower CSTresults in terms of better filterability. However, this process did notimprove sludge's cake formation on a crown press application.

Acknowledgements

The authors express sincere appreciation to The Scientific andTechnological Research Council of Turkey (TUBITAK) for supporting

63G. Erden, A. Filibeli / Desalination 251 (2010) 58–63

the study under award #105Y337: Sludge Disintegration UsingAdvanced Oxidation Processes. The authors also thank Izmir Waterand Sewerage Administration for providing sludge samples.

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