Infl uence of Pyrolytic Biochar on Settleability and Denitrifi cation of Activated Sludge Process

Xiao-feng SimaBing-bing LiHong Jiang

Biomass Clean Energy Laboratory,Department of Chem istry,University of Science and Technology of China,Hefei230026,China

Infl uence of Pyrolytic Biochar on Settleability and Denitrifi cation of Activated Sludge Process

Xiao-feng Sima,Bing-bing Li,Hong Jiang?

Biomass Clean Energy Laboratory,Department of Chem istry,University of Science and Technology of China,Hefei230026,China

Biochar is amassively produced by-p roduct of biomass pyrolysis to obtain renewab le energy and has not been fully used.Incom p lete separation of sludge and effl uent and insuffi cient denitrification of sewage are two ofmain factors that influence the effi ciency of activated sludge process.In this work,we proposed a new utilization of biochar and investigated the eff ect of biochar addition on the performance of settleability and denitrification of activated sludge.Results show that the addition of biochar can im prove the settleability of activated sludge by changing the physicochem ical characteristics of sludge(e.g.,flocculating ability,zeta-potential,hyd rophobicity,and extracellular polym eric substances constituents). Moreover,the dissolved organic carbon released from biochar obtained at lower pyrolysis temperature can im prove the nitrate removal effi ciency to a certain extent.

Biochar,Activated sludge,Settleability,Biological denitrification,Carbon source

I.INTRODUCTION

As a cost-eff ective method,activated sludge process has been w idely adopted inmost wastewater treatment p lants(WW TPs).However,the poor settling of activated sludge biom ass rem ains a comm on problem, which results in increased effl uent solid concentrations, decreased disinfection effi ciencies,and increased treatment costs[1].

Somematerials,such as diatom ite and powdered activated carbon,have been used as additives to im prove sludge settleability[2].However,the high cost of powdered activated carbon(1.65?9.90 dollar per kilogram [3,4])lim its its w ide application in activated sludge process.Biochar is the carbonaceous by-product of biomass pyrolysis for preparation of renewable liquid fuels[5,6],and its production cost is no more than one-tenth of the cost of powdered activated carbon[7, 8].Adding biochar into activated sludge processm ay change the sludge properties.For instance,O leszczuket al.[9]dosed biochar into activated sludge and found that biochar decreased the phytotoxicity of sludge and positively aff ected seed germ ination.In addition,the variations of sludge properties caused by biochar addition may facilitate the separation and post-treatment of excess sludge by im proving sludge settling property and increasing the heating value of dehydrated sludge[10,11].A lthough the investigations on the improvement ofsludge settling property by addition ofactivated carbon have been reported[2,12],the im provement of settleability of sludge by biochar addition has not been found.

No suffi cient carbon sources in urban sewages to act as electron donors for com p lete denitrification is another problem in WWTPs[13,14],and nitrogen in the effl uents is an important factor that causes eutrophication ofwater bodies[15]whichm ay threaten theaquatic ecosystem s and the living environment of hum an beings [16,17].Liquid carbon sources(e.g.,methanol,ethanol, and acetic acid)are w idely added into the activated sludge process as an external carbon source to promote denitrification,however,they are expensive and potentially hazardous chem icals[18,19].In addition to the stable carbon in the skeleton,a certain amount of dissolved organic matter is present in the biochar [20,21],whichmay be bioavailable and act as a carbon source for denitrification.Jam iesonet al.[22]characterized dissolved organic m atter in birch and map le biochars using optical analyses,and their findings suggest that pyrolytic temperature can influence the composition ofbiochar-derived dissolved organicmatterand further influence the bioavailability of the dissolved organicm atter.However,to our know ledge,m ost studies on biochar are in the field of soil remediation,few focus on the utilization of biochar-derived dissolved organic matter as a carbon source ofWW TPs.

In this work,biochar was used as as additive to improve the settleability of sludge and the denitrification in activated sludge process.Three kinds of biochars byfast pyrolysis of rice husk waste at different tem peratureswere prepared.The effects of biochar addition on the settleability and physicochem ical characteristics of sludge were evaluated.The eff ects of pyrolytic tem perature on dissolved organic carbon(DOC)released from biochar and denitrification of sludge were investigated; and the potential influencemechanism of biochar addition on settleability and denitrification were discussed.

II.M ATERIALS AND M ETHODS

A.M aterials

The rice husk was obtained from Anhui Y ineng Bioenergy Co.Ltd.,Hefei,China.The sam pleswere dried in an oven at 105?C for 12 h,and then pulverized by a rotary cutting m ill and screened to collect particles w ith sizes below 150μm(100m esh).Finally,they were stored in a glass desiccator for further use.A ll reagents used in this study are of analytical grade purity and purchased from Sinopharm Chem ical Reagent Co.,Ltd. Shanghai,China.

B.Preparation and characterization of biochar

The biochar was prepared by fast pyrolysis of rice husk at three diff erent tem peratures.Char-573,char-773,and char-973 denoted thebiochar produced at temperatures of 573,773,and 973 K,respectively.The pyrolysis experim ent was conducted in a vertical fixedbed reactor under a nitrogen atmosphere,which was described in our previouswork[23].

Elemental com positions(C,H,N,and O)of rice husk and biochars were determ ined using an elemental analyzer(vario EL III,Elementar,Germ any).Ram an experimentswere conducted w ith a LabRAM HR spectrometer using the green line of an argon laser (λ=514.5 nm)as excitation source.Dissolved organic carbon(DOC)of sam p les was determ ined on a TOC analyzer(TOC-VCPN,Shimadzu Co.)using a sequencing extractionmethod.Specifically,0.5 g of sam p lewas added to 30 m L of deionized water and shaken using the recip rocating shaker(180 r/m in)in the dark for 1 d.Them ixturewas then separated at 8000 r/m in on a centrifuge.The separated solid was added to 30 m L of deionized water for another shaking of 2 d,and the m ixture was separated again.The solid was added to 30 m L of deionized water for another shaking of 4 d and then separated.A ll the supernatant was collected and fi ltered through a 0.22μm membrane,and DOC was determ ined.A ll extraction experim ents were conducted in duplication.

C.Eff ects of biochar on sludge settleability

Ten of 500 m L graduated cylinders were used as laboratory-scale sequencing batch reactors(SBRs),and each reactor was inoculated w ith 500 m L of activated sludge taken from an aeration tank from the Zhuzhuanjing WW TP,Hefei,China.The seeding sludge had a sludge age of 10 d,a m ixed liquor suspended solids (MLSS)concentration of 8 g/L,and a sludge volume index(SVI)of 68.1 m L/g.A fter inoculation,the samp les(rice husk,char-573,and char-773)were added into the reactorsw ith sam p le/sludgem ass ratio of 20%, 10%,and 5%(w/w),respectively.The control experimentswere conducted under the same conditionsw ithout adding rice husk or chars into the reactor.

A fter that,the SBRs were m aintained at 28±1?C and operated in successiveeach cycle of12 h.One cycle consists of 5m in of influent addition,10 h of aeration, 100 m in of settling,and 15 m in of effl uent w ithd rawal. In each cycle,200 m L of effl uent was lightly sucked out by siphoning.The SBRs were then fed w ith synthetic wastewater,which had average COD,NH4+-N, and PO43?-P concentrations of 400,20,and 4 m g/L, respectively.SV30was measured during the settling stage of each cycle for indication.The physicochem ical propertiesof sludgewerem easured at the end of the experim ent,including M LSS,zeta potential(ζ-potential), flocculating ability,hydrophobicity,and the composition of extracellular polymeric substances(EPS).

D.Denitrifi cation batch experim ents

Batch m icrocosm experim ents were conducted to evaluate the capacity of biochar as a carbon source to enhance denitrification.The batch experiments were conducted in ten of 250 m L Erlenmeyer flasks.Then, 3 g of sam p les(rice husk,char-573,char-773,and char-973)and 150m L of syntheticwaste-water that contains 100mg/L NO3?-N and 10mg/L PO43?-P were added into the Erlenm eyer flasks.A fterwards,5m L of anaerobic granular sludge(the seed sludgewas taken from an anaerobic digester in a citrate-processing WW TP)was inoculated.The flaskswere then purged w ith high purity nitrogen for 20 m in to lower dissolved oxygen concentration.The flasks were sealed w ith glass stoppers and then incubated in a constant-temperature reciprocating shaker and agitated at 150 r/m in and 298±3 K. The control experim ent was conducted under the same conditions w ithout adding rice husk or chars into the flasks.

Two dup licated experimentswere conducted on each sam p le.The m ixture(30 m L)was taken from each flask on day 2,3,5,7,11,13,and 15,and the extracted volume was rep laced w ith 30 m L of fresh synthetic wastewater.The flasks were again purged w ith high purity grade of nitrogen for 20 m in,then sealed w ith glass stoppers,and incubated in the shaker.The m ixturewas centrifuged at 8000 r/m in for 20m in,and the supernatantwas collected and further fi ltered w ith a 0.22μm m embrane for NO3?-N,NO2?-N,and NH4+-N analyses.

E.M icrobial comm unity analysis

At the end ofbatch experim ents,the suspension samp les in control,char-573 group,and the inoculated sludgewere collected and centrifuged at 6000 r/m in for 10 m in.The supernatant was decanted and the pellet was stored at?20?C until DNA extraction.Upon thaw ing,the totalgenom ic DNA wasextracted from the sam ples using the Power Soil DNA Kit(M o Bio Laboratories,Carlsbad,CA,USA).Total DNA extraction as well as PCR-DGGE and 16S rRNA analysis were performed as described in supp lem entary m aterials). Nucleotide sequences were com pared to sequences in the NCBIGenBank database using the BLASTn search program.

F.Nitrogen release

Nitrogen release of sam p leswas determ ined using the sim ilar sequencing extraction method used in the determ ination of DOC.The concentrations of NO3?-N, NO2?-N,and NH4+-N in fi ltratewere also determ ined.

G.Analyticalm ethods

The concentrations of NO3?-N,NO2?-N,NH4+-N, and M LSSwere analyzed follow ing the standard m ethods for the exam ination ofwater and waste water[24]. Theζ-potential of sludge suspension was determ ined by aζ-potential analyzer(Zetasizer Nano ZS,Malvern Co.,UK).The flocculating ability was evaluated by the refloculation ability of sludge flocs after disruption[25].

The hydrophobicity of sludgewas estimated by measuring contact angle using axisymmetric drop shape analysis,which was described previously[26].The sludge suspension was deposited to form a smooth thin layer on 0.45μm acetate cellu lose m embranes,which werewashed tw icew ith double-distilled water and then p laced on a 1%agar plate.Prior to themeasurement, the membranes were mounted on glass slides and air dried for 10m in,and the advancing contact angleswere directly m easured using the sessile drop techniquew ith a drop of liquid water.The shape of a sessile distilled water droplet placed on the layer of biomasswas determ ined using a contact angle analyzer(JC2000A,Powereach Co.,Shanghai,China).A ll contact angle values were based on arithmeticmeans of at least 10 independentm easurem ents.

Heat extraction methods were used to extract EPS from sludge[27].The extracted solution was analyzed for total carbohydrates,protein,and hum ic substances. The total amount of EPS was expressed by the sum of carbohydrate,protein,and hum ic substances,which are the dom inant com ponents in EPS[28,29].Carbohydrates in EPSwere determ ined according to the phenolsulfuric acidmethod w ith glucoseasstandard[30].Proteinswere determ ined by the Folinmethod w ith bovine serum album in asstandard[31].Hum ic substanceswere determ ined by the im proved Folin m ethod[32].

H.Statistical analysis

A ll statistical analyses were performed using the IBM SPSS 17.0 software(SPSS Inc.,USA)for W indows.The Pearson product-m oment correlation coefficient(rp)was used for linear estimations.Correlations were considered statistically significant if the correlation coeffi cient wasm ore than the critical coeffi cient forP<0.05.

III.RESULTS AND D ISCUSSION

A.Characterization

The elem ental com positions of rice husk and biochars are listed in Table S1(supp lem entary materials).The C content in biochar slightly increases from 40.5%to 43.0%,whereas the H and O contents significantly decrease w ith the increase of pyrolytic tem perature.The ash,mainly consisting of inorganic salts,is diffi cult to be volatilized and retained in biochar,and its content increasesw ith the pyrolytic tem perature increase.

B.Eff ect of biochar addition on the settleability of activated sludge

1.Settleability of activated sludge

The SV30is an im portant index of the settleability of activated sludge.The results indicated that the settleability im proved w ith the addition of rice husk and biochars.To evaluate the im provem ent of settleability, the decreased percentage(DP)of SV30wasapp lied and calculated using Eq.(1):

where SV30controlis the SV30of the sludge in control reactor,and SV30xis the SV30of the sludge in other reactorsw ith the addition of biochars or rice husk.

The DPs of SV30w ith the addition of diff erent dosages of rice husk,char-573,and char-773 are shown in FIG.1.Overall,the addition of rice husk and chars can im prove the settleability of sludge(DPs of SV30ranged from 1.5%to 9.5%when rice husk and chars were added).A fter one cycle(12 h),DPs of SV30increased dramatically to the range of 5.8%?23.0%, which indicates that the sludge ism ore easily to be separated during this period.DPs decreased in the following two cycles and gradually increased again in the last cycles.Given that the optimum sludge retention tim e for good bioflocculation and low effl uent COD ranged from 2 d to 8 d[33],char-573 w ith 5%dosagemay bem ore suitable for practical usage in WW TP,in which the DP ism aintained at 16.2%?19.3%.

FIG.1 The decreased percentage(DP)of SV30in each reactor w ith diff erent dosage of char-573,char-773,and rice husk. (a)20%,(b)10%,(c)5%.

2.Physicochem ical characteristics of sludge

Thephysicochem icalcharacteristicsofsludgehavean im portant influence on sludge settleability[27,34].The im provem ent of the settleability of sludge by the addition of chars or rice husk may be closely related to the variationsof sludge properties.Therefore,some physicochem ical characteristics like contact angle,flocculating ability,and zeta potential of sludge in each reactor werem easured at the end of the experim ent and shown in FIG.2.The contact angleofsludge in the controlexperiment was84.1?,while it ranged from 71.8?to 81.3?in the reactors containing chars and rice husk.The decrease of contact angle indicated that the hydrophobicity ofsludge decreased after the addition of charsor rice husk.This result was probably due to the high porosity and high surface area of chars or rice husk,which enhanced the adsorption to water and hydrophilic sludge flocs[35].W ith the addition of chars or rice husk,the zeta potential of sludge decreased from?16.7 m V to the range between?20.0 and?21.5m V.Furthermore, flocculating ability was16.4%in the controlexperiment and increased to 25.2%or 52.9%after the addition of chars or rice husk,suggesting that the com pactability of sludge is significantly im proved[36].

It has been reported that EPS,mainly consisting of protein,hum ic substances,and carbohydrate,hasa stabilizing eff ect on the sludge floc[37,38].The concentrations of protein,hum ic substances,and carbohydrate in sludge w ith and w ithout chars/rice husk are shown in FIG.3.A fter the addition of chars or rice husk,the concentration of hum ic substances decreased from 11.56mg/g MLSS to 9.35?11.11 mg/g M LSS, and the concentration of carbohydrate decreased from 2.52 mg/g M LSS to 2.09?2.49mg/g M LSS,whereas the concentration of protein kept around 8.5 m g/g M LSS. The im proved flocculating ability of sludge by the addition of chars or rice husk may be attributed to the significant decrease in the concentration of hum ic substances[27,38].

FIG.2(a)Contact angle,(b)zeta potential,and(c)flocculating ability of sludge at the end of the experim ent.

3.Correlation analysis

The correlation between the settleability and the properties of sludgewas evaluated by Pearson productmoment correlation coeffi cient,and the resultsare summarized in Table I.The results indicated that the contact angle,flocculating ability,zeta potential,hum ic substances,and total EPS are significantly correlated w ith SVI(P<0.05).The contact angle,hum ic substances,and total EPS have a positive eff ect on SVI, whereas the flocculating abilitiesofsludgeand SVIhavea negative correlation.These results are in accordance w ith previous studies[36,39],which reveal that the physicochem ical characteristics of sludge are changed by biochar addition,and the settleability of activated sludge is im proved.

FIG.3 Dom inant com ponents of EPS in each reactor at the end of experim ent:p rotein,hum ic substances,and carbohyd rate.

TABLE I Person’s correlation coeffi cient(rp)andP-value between the characteristics and settleability of sludge.

C.Eff ect of biochar on denitrifi cation

1.DOC released from biochar

The DOC released from biocharmay beused asa potential carbon source form icrobes and is an im portant factor for enhanced denitrification[40].FIG.S1(in supp lementarymaterial)shows that theamount ofDOC released from biochar decreases significantly w ith the increasing pyrolytic tem perature,which suggests that few DOC of biochar obtained at high tem perature can be utilized by m icroorganisms in activated sludge.Nevertheless,biocharsw ith high DOC contentsarenot necessarily favorable for denitrification,because the organic com pounds in biochar usually varied w ith the pyrolytic temperature,and someof them arenocuous tom icroorganism s.For instance,pinewood-derived biochar water extracts contain carboxyl and hydroxyl hom ologous series,which have been found to have inhibitory effects on the grow th of aquatic photosynthetic m icroorganism[41].Therefore,to elucidate the role of biochar as a potential carbon source,denitrification performance should be evaluated.

FIG.4 The total rem oval am ount of NO3?-N in denitrification batch experiment.

2.Removal of nitrate

Denitrification by m icroorganism s is the m ain removal pathway of nitrate in sewage,during which a certain amount of carbon source is consumed.Rice husk, char-573,char-773,and char-973 were added into the synthetic waste water as a potential carbon source for denitrification.The total rem oval am ount of nitrate in the flasks during the 15 d run is presented in FIG.4. The removaleffi cienciesofnitratewereobviously different w ith dosing of diff erent solid m aterials.In control group,the removal amount of nitrate was 5.3 mg on second day and rose slow ly to 9.9 mg until the end of the experim ent.Due to the absence of external carbon source,the soluble organic carbon released from the hydrolysis processof sludge is responsible for the denitrification in the controlgroup[42].In the reactor w ith the addition of rice husk,the removalam ount ofnitratewas 3.1mg on second day,drastically rose to 15.7m g on 5th day,and continuously rose to 30.4mg during the later experiment time.For the group w ith char-573,the removalam ount ofnitratewasonly 4.4m g on second day, but quickly rose to 10.9 m g on third day and 18.0 mg on 5th day.A fterwards,it gradually rose to 28.5 mg during the later experiment time.For the group w ith char-773,the removal am ount of nitrate quickly rose to 6.5m g on second day and slow ly rose to 17.3m g at the end of the experiment.

FIG.5(a)Ram an spectra of biochars obtained at diff erent tem peratures.Ram an spectra of(b)char-773,and(c)char-973 w ith Gaussian peak fi t.

Considering that nitrate is diffi cult to be removed by adsorption,thus the significant increase of denitrification by dosing rice husk and biochars should be attributed to the com p lementary carbon source.However,the types of carbon source supp lied by diff erent solid materials are discrepant.Cellulose and sem icellulose are the dom inant carbon source in rice husk that are beneficial to the denitrification[18].In pyrolysis,cellulosic compounds are thermochem ically decomposed to produce volatiles,and some bioavailable organic com pounds are deposited or adsorbed on the carbon skeleton.The carbon source in biochars obtained at low tem perature are easily-decomposed organic compounds,whereasw ith the increasing pyrolytic temperature,fewer bioavailable organic com pounds rem ained. Thus,sludge w ith the addition of rice husk and char-573 exhibited good denitrification.To verify this hypothesis,we conducted a Raman analysis.The Raman spectra obviously indicate that the degree of aromatic condensation in the biochar increases w ith increasing pyrolytic tem perature(FIG.5(a)).D and G bands in Ram an spectra are evidence of polyaromatic hydrocarbons and graphitic m aterial,which are typically found at 1350 and 1530 cm?1,respectively[43].FIG.5(a) shows that the two peaks are absent in char-573,but appear in char-773 and char-973.At the sam e time, the results of peak fi t show that intensities ratio of D to G bandID/IGof char-773(1.89)is higher than that of char-973(1.24)(FIG.5(b)and(c)),which indicates that the degree of aromatic condensation in the biochar increasesw ith increasing pyrolysis tem perature[44].

3.Accumulation of nitrite and ammonium

Given the biochars often contain some contentsof N-containing species,N release experiments of biochars were performed.Amm onium was released from each sam p leand nitratewas released from ricehusk only,and no nitritewas detected in leachate(FIG.6(a)and(b)). The total amounts of nitrate and ammonium released from rice husk are 0.07 and 0.79m g/g,while those total amounts of ammonium released from char-573,char-773,and char-973 are 1.35,0.59,and 0.28mg/g,respectively.A lthough m ore amm onium is released by char-573 than other chars,its denitrification boost is still the best,which suggests that the effect of the N release from biochar on denitrification is negligible.

During denitrification process,the product nitrite may accumulate in the system.FIG.6(d)shows that nitrite accumulated significantly in fi rst 5 d of the experim ent.In the reactor containing rice husk,the nitrite concentration increased to 12.30 m g/L on second day,rose to 35.65 mg/L on third day,decreased on 5th day,and then disappeared during the rest of the experiment time.For char-573,the nitrite concentration was 8.91m g/L on second day,and rose quickly to 46.16mg/L on 5th day.A fterwards,an abrupt decrease of the nitrite concentration to 2.10 mg/L occurred on 7th day and the nitrite disappeared at the end of the experiment tim e.It was indicated that the com petition for lim ited carbon source existed in nitrate and nitrite respirations,and nitrite accumulation showed that the available carbon was insuffi cient during the denitrification process[45,46].As for reactorscontaining char-773 and char-973,the accumulation of nitrite is negligible because little carbon sourcewas provided.

Ammonium accumulation was observed in control and experim ent groups(FIG.6(c)).In control group, the amm onium concentration was 5.04 mg/L on second day,increased slow ly to 11.92 mg/L on 7th day, and remained between 11 to 13 mg/L until the end of the experim ents.This result further suggests the occurrence of sludge hydrolysis,during which the concentration of ammonium kept increasing[42,47].The change of ammonium concentration w ith time in reactorsw ith biochar showed a sim ilar trend.

4.M icrobial community changes

FIG.6 The amount of NH4+-N(a)and NO3?-N(b)released from rice husk and biochars by leaching experiments;the concentration of NH4+-N(c)and NO2?-N(d)of supernatants during denitrification batch experim ents.

The DGGE profi les show that them icrobial community com position in the sludge changed at theend of the batch experiments(FIG.S2 in supp lementary materials).Them ain bands in each sam p le are obviously different.Bands 1?3 are observed and extremely enriched in char-573 sam p le,whereas bands 4?6 are dom inant in the inoculated sludge sam p le,and bands 7 and 8 are enriched in control sam p le.DNA sequences were com pared w ith the NCBIGenBank database using the BLASTn search.Bands 1?3 have a high sim ilarity toParabacteroides chartaesp.,which is usually detected in bacterial community in anaerobic bioreactors[48]. The gene sequences from band 4?6 closely belong to the genusofCandidatus Cloacamonas acidam inovoranssp.,Treponema caldariasp.,andTherm osipho atlanticussp.,respectively,all of which commonly present in many anaerobic digesters[49?51].By contrast,band 7 affi liatesw ith the genusSulfuritalea hydrogenivoranssp.and band 8 affi liates w ith the genusSulfurisoma sedim inicolasp.,two facultative autotrophs[52,53], which indicates the m icrobial community in control group were gradually changed to facultative autotrophs because of the lack of carbon source.

D.App lication potential

From the above results,dosing biochar can significantly im prove the settleability of sludge by changing contact angle,zeta potential,and the flocculating ability of sludge.Therefore,the addition of biochar to improve the settleability of activated sludge can find practical usage in WW TP.However,although the biochar, particularly the biochar obtained at low pyrolytic temperature,was demonstrated to be usefu l to act as a carbon source for nitrate removal in denitrification phase, it should be taken into account that the release rate of DOC was very slow and the release period was m ore than 15 days.In m any types of waste water configuration(e.g.,A2O,SBR or their modified revision), activated sludge is internally partially recycled(spatially or tem porally)from denitrifying phase to anaerobic/aerobic phase.In such modes,the DOC gradually released from biocharmay be consumed in the aerobic phase,thus DOC released from biochar could not be fu lly used for denitrification.In addition,the biochar has been reported to increase p lant grow th and crop yields when used into soil[54],the excess activated sludge w ith the addition of biochar is m ore valuable for soil app lication.

IV.CONCLUSION

In this work,we introduce a new app lication of pyrolytic biochar in waste water treatment,the biochar which was added into the reactor can im prove the settling perform ance of sludge in the secondary sedim entation tank or in the setting process.Biochar obtained at low pyrolytic temperaturewas also demonstrated to be a potential carbon source for denitrification.

Supp lem entary m aterials:M icrobial community analysism ethod is shown in Text S1.Elementalcom positionsof rice husk and biochars(Table S1),the amount of DOC released from rice husk and biochars(FIG. S1),and DGGE analysis of samp les(FIG.S2)are also shown.

V.ACKNOW LEDGM ENTS

Thiswork was supported by the National Key Technology R&D Program of the M inistry of Science andTechnology(No.2012BAJ08B00),the Key Special Program on the S&T for the Pollution Control,and Treatm ent ofWater Bodies(No.2012ZX 07103-001),National 863 Program(No.2012AA 063608-01).

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ceived on December 20,2016;Accepted on April7,2017)

?Au thor to whom correspondence shou ld be add ressed.E-m ail: jhong@ustc.edu.cn,Tel.:+86-551-63607482,FAX:+86-551-63607482.