Process Biochemistry 46(2011)162–167
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Process
Biochemistry
j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /p r o c b i
孙羽幽o
Effect of biopolymer clusters on the fouling property of sludge from a membrane bioreactor (MBR)and its control by ozonation
Fei-yun Sun,Xiao-mao Wang,Xiao-yan Li ∗
Environmental Engineering Research Centre,Department of Civil Engineering,The University of Hong Kong,Pokfulam Road,Hong Kong,China
a r t i c l e i n f o Article history:
Received 26May 2010
Received in revised form 25July 2010Accepted 1August 2010
Keywords:
Biopolymer clusters (BPC)Membrane bioreactor (MBR)Membrane fouling Microfiltration Ozonation
Soluble microbial products (SMP)
a b s t r a c t
Organic substances in the liquid phase of the sludge in a membrane bioreactor (MBR)have a profound impact on membrane fouling.In this study,a single-fibre microfiltration apparatus was developed to investigate the fouling propensity of MBR sludge and the effectiveness of ozonation in membrane fouling mitigation.The results show that biopolymer clusters (BPC)in the MBR suspension had a significant influence on the fouling potential of the sludge.An increase in BPC concentration by 20%and 60%from around 3.5mg/l in the mixed sludge liquor drastically increased the fouling rate by 120%and 300%,respectively.Ozonation of the BPC solution greatly reduced the detrimental role of BPC in membrane fouling.An ozone dose of 0.03mg/mg TOC of BPC could reduce the mean BPC size from 38to 27␮m,which was further reduced to 12␮m at 0.3mg O 3/mg TOC of BPC.In addition to BPC destruction,ozonation apparently also modified the surface properties of BPC,resulting in an increase in the filterable fraction and a decrease in the liquid viscosity.Based on the experimental findings,an approach for MBR membrane fouling control is proposed that applies ozonation to the supernatant containing BPC in a side-stream application.
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1.Introduction
Membrane bioreactors (MBRs)are increasingly being used as an advanced technology for biological wastewater treatment and reuse.With the use of a membrane for sludge filtration,the MBR ensures complete solid–liquid separation [1,2].In MBRs,the sludge age and concentration can be effectively manipulated,affording this type of bioreactors several advantages over the conventional activated sludge (CAS)process [3].At the same time,however,because of the retention by the membrane,some of the soluble microbial products (SMP)and other colloidal substances are unable to escape from the system with the effluent [4,5].The organic interception by membrane filtration results in the formation and accumulation of organic foulants in the MBR sludge suspension,which in turn worsens the membrane-fouling problem.
The effect on membrane fouling of liquid-phase organic sub-stances in the MBR sludge mixture has long been recognised [6–10].Recent research reveals the presence of a group of large-sized organic solutes,termed biopolymer clusters (BPC),in MBR systems [11–13].BPC are neither biomass flocs nor SMP or extracellular polymeric substances (EPS).They can be larger than 10␮m in size,and are formed by the affinity clustering of SMP and loose EPS on
∗Corresponding author.Tel.:+852********;fax:+852********.E-mail address:xlia@hkucc.hku.hk (X.-y.Li).URL:web.hku.hk/xlia/(X.-y.Li).the membrane surface [14].It has been suggested that BPC may facilitate sludge deposition and the fouling layer formation on the membrane surface,and the detrimental role of BPC in membrane fouling has been demonstrated qualitatively during the operation of MBR systems [12,14].However,more systematic studies remain to be conducted to determine the correlation between the mem-brane fouling rate and the BPC content of the sludge mixture.In addition,the effect of changes in BPC properties on the fouling potential of the MBR sludge also merits investigation.
The reduction or modification of BPC in MBR sludge mixture is expected to be beneficial for the control of membrane fouling.Removal of BPC and their precursors,such as SMP and loose EPS,is an option,and indeed the use of adsorbents or coagulants in the MBR mixed liquor has been found effective in decelerating membrane fouling [15–19].However,continuous addition of these chemicals may either be harmful to the membrane due to physi-cal abrasion,as is the case for granular activated carbon,or affect the MBR treatment performance,as is the case with some coag-ulant metal ions (e.g.,Fe(II)and Fe(III)that are reportedly toxic to the nitrifying bacteria [20]).More recently,the ozonation of bulk sludge has been tested as a means of membrane fouling con-trol durin
g continuous MBR operation [21–24].The results show that at appropriate doses the membrane fouling rate can be effec-tively reduced,meanwhile,ozonation coupled with MBR appears to be an effective method for sludge reduction and toxic organic wastewater treatment [23,24].However,a possible overdose of ozone and its impact on the biomass activity is a concern with
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F.-y.Sun et al./Process Biochemistry46(2011)162–167
163
Fig.1.Schematic diagram of the single-fibre MF testing apparatus.
direct sludge ozonation.Moreover,the underlying mechanisms of sludge ozonation for membrane fouling mitigation are not well understood.
There is thus a need to determine the effect of BPC in MBR sludge mixture on membrane fouling,and to investigate the effective-ness of the ozonation of BPC in reducing the fouling propensity of sludge.In this study,a lab-scale MBR was operated to supply both biomass sludge and BPC dispersion.A newly designed single-fibre microfiltration(MF)system was fabricated for the membrane filtration-fouling tests on different sludge–BPC mixture samples under well-controlled hydrodynamic conditions.Ozonation was applied to the BPC solution only,rather than the entire sludge mix-ture,before mixing into the sludge suspension.The objectives of the experimental study were(1)to determine the fouling propensity of MBR sludge with different BPC contents,and(2)to investigate the effectiveness of the ozonation of BPC in minimising membrane fouling during sludgefiltration.The mechanism of sludge ozona-tion to mitigate fouling was also identified,and on this basis a more reliable ozonation approach for MBR fouling control is proposed.
2.Materials and methods
2.1.Filtration setup and operation
A single-fibrefiltration apparatus was fabricated for the sludgefiltration and fouling tests(Fig.1).The apparatus was made of a plexiglass tube1.5cm in internal diameter and50cm in height.A polyethylene(PE)hollow-fibre MF membrane(pore size=0.4␮m,diameter=0.14cm,working length=40cm,surface area=16cm2, Mitsubishi Rayon,Japan)was installed along the centreline of thefiltration tube.The sludge suspension in a feed tank was pumped through the MF test tube by a helical pump(SELTZ-L40II,Hydor,USA).A constant cross-flow rate of2l/min(0.19m/s) was applied by the recirculation of the sludge suspension for continuous mem-brane surface cleaning.The permeate was drawn out through the MF membrane by a suction pump(MasterFLEX,Cole-Parmer,USA)at a constantflux of37.5l/m2h. An electronic balance(Arrw60,OHAUS,USA)was used to record the permeate pro-duction during thefiltration-fouling tests.Unless sampled for analysis,the collected permeate was returned manually to the feed tank at regular intervals to maintain the same sludge concentration.A pressure sensor(PTX Ex-0129,Druck,USA)was installed before the suction pump to record the trans-membrane pressure(TMP) during sludgefiltration.Both the permeate production and TMP data were trans-ferred to a PC for continuous data recording(Fig.1).The membrane fouling rate
披荆斩棘的哥哥四公淘汰名单was measured by the increase in TMP with the amount of permeate produced(filtrate depth,L),or TMP/ L.After eachfiltration-fouling test,the membranefibre was taken off thefiltration tube and washed with100ml of DI water at40◦C to recover all of the sludge and foulants deposited on the membrane surface.The sludge and foulant dispersion were then settled for2h at4◦C and the supernatant was anal-ysed for total organic carbon(TOC)and chemical composition,including proteins (PN),polysaccharides(PS)and humic-like substances(HS).The sludge in the dis-persion was collected on afilter,dried for2h at105◦C and then weighed to obtain the suspended solids(SS)content.
2.2.MBR activated sludge and biopolymer clusters
The sample activated sludge(AS)and biopolymer substances for thefiltration tests were collected from a submerged MBR(SMBR).The laboratory SMBR had a working volume of5l and contained a submerged0.4␮m polyethylene MF mod-ule(surface area=0.2m2,Mitsubishi Rayon,Japan).The SMBR system had been in stable operation for more than four years before the present experiment[12,25]. The influent(feeding wastewater)to the SMBR was a mixture of a glucose-based synthetic wastewater prepared according to the basic recipe given in the Environ-mental Engineering Process Laboratory Manual of the AEESP[26]and domestic sewage collected from the Stanley Sewage Treatment Works in Hong Kong.The sewage fraction supplied around10%of the tot
al organic load in the influent.The wastewater influent had a chemical oxygen demand(COD)of around500mg/l and a COD:N:P ratio of100:9:3.NaHCO3was added to the influent at50mg/l or higher to maintain the pH of the MBR suspension between6.5and7.5.The biomass concen-tration,food-to-microorganism(F/M)ratio,solid retention time(SRT)and hydraulic retention time(HRT)of the SMBR system were10g/l,0.2g COD/g SS d,25d and8h, respectively.
The AS mixture collected from the SMBR was settled for1h,and the settled sludge was then diluted with a0.05%NaCl solution to a mixed liquor suspended solids(MLSS)concentration of3g/l.Large organic substances,or biopolymer clus-ters,were obtained from the cake sludge(CS)deposited on the surface of the membrane in the SMBR.When the membrane was seriously fouled,the CS layer was scraped off the membrane using a spatula.The CS was then re-suspended and dis-persed by stirring it in a0.05%NaCl solution.The CS suspension was then separated by sedimentation at4◦C for12h and the supernatant was collected.The organic substances in the CS supernatant were regarded as biopolymer clusters[12,14].The CS supernatant,or BPC solution,was analysed for TOC and PN,PS and HS content.
The BPC solution was added into the AS suspension(3g/l)at different doses. Each sludge suspension was then tested for its fouling propensity using the single-fibre MFfiltration apparatus.In t
天涯海角歌词his way,the effect of the BPC content in the sludge mixture on the fouling potential of the sludge during membranefiltration was determined.
2.3.Ozonation of the BPC solution
Ozonation was also applied to the BPC solution with an intention to modify the BPC properties before their addition to the sludge.Ozonation was performed quantitatively by adding ozone-containing water into the BPC solution.Ozone was generated in the gaseous phase by an ozone generator(5000BF,Enaly,China)that was supplied with pure oxygen.To dissolve the ozone in water and prepare an ozone solution,500ml ultra-pure water(Milli-Q-Advantage,Water Purification,Millipore, USA)was bubbled with the ozone gas at4◦C for10min or longer.The ozone concen-tration achieved in the ozone solution was about8mg/l.A pre-determined amount of the ozone solution was then added to30ml of the BPC solution.The mixed solution was placed in the dark and stirred for5min at60rpm to ensure complete ozonation. Similar to the previous sludgefiltration tests,the ozonated BPC solution was added into the AS suspension at different doses,and the sludge mixtures were then tested for their fouling potential using the single-fibrefiltration apparatus.
164  F.-y.Sun et al./Process Biochemistry46(2011)162–167
2.4.BPC characterisation
In the characterisation of the organic substances in the BPC solution,the fraction that could not pass through a0.4␮m membranefilter(polycarbonate,Osmonics, USA)was defined as non-filterable BPC.The proportion of non-filterable BPC to the total organic content in the BPC solution was termed as the BPC cut-off ratio[11].The BPC solution before and after ozonation and itsfiltrate were analysed to determine the TOC concentration and the PN,PS and HS content.
The BPC size distribution was determined using a laser diffraction particle anal-yser(LS13320,Beckman Coulter,USA).Before the particle sizing and counting,the BPC in the solution were stained with NanoOrange(Molecular Probes,Eugene,USA), which is afluorescent probe that targets proteins in organic polymers.Five millilitres of NanoOrange dye solution was added to30ml of BPC solution for afinal dye con-centration of20mg/l,and the mixture was kept in the dark for30min.After staining, the transparent BPC became detectable by a laser particle analyser[14].Moreover, both before and after ozonation,the BPC wasfiltered on a membranefilter and examined directly under a confocal laser scanning microscope(CLSM)(LSM Pas-cal,Zeiss,Thornwood,USA),following the procedures described previously[5,27]. For the CLSM observations,BPC(actually non-filterable BPC)and other solids col-lected on a0.4␮m black polycarbo
nate membrane(25mm,Osmonics,USA)were stained using a combination of two probes:SYTO9to target the bacterial cells and ConA-TRITC to target the polysaccharides with d-glucose or d-mannose[12].
2.5.Analytical methods
The TOC was measured by a TOC analyser(IL550TOC-TN Analysers,Lachat,USA) using the high-temperature combustion method.The protein and humic concentra-tions were determined via an UV/vis spectrophotometer(Lambda25,Perkin Elmer, USA)following the modified Lowry method using albumin bovine(Sigma,Germany) and humic acid(Fluka,Italy),respectively,as the standards[28].The polysaccharide content was measured according to the phenol method using glucose as the stan-dard[29].The MLSS concentration of the sludge was measured in accordance with the standard methods[30].The concentration of dissolved ozone in the ozonated water was determined based on the UV absorbance as measured by an UV/VIS spec-trophotometer(Lambda25,Perkin Elmer,USA)following the conventional indigo method[31,32].The liquid viscosity was measured by a vibration viscometer(SV-10, A&D,Japan).
3.Results and discussion
3.1.Significance of BPC in membrane fouling
The membrane fouling rate for the sludge samples was well indi-cated by the increase in TMP during thefiltration process(Fig.2). For thefiltration-fouling tests with the single-fibre MF apparatus, the sludge suspension collected from the SMBR was kept at a SS concentration of3g/l.At a constantfiltrationflux,the membrane fouling rate shown by the TMP increase was reflective of the foul-ing propensity of the sludge samples.As all of the conditions were identical except for the amount of BPC added to the sludge mix-ture,the comparative results directly demonstrate the effect of BPC on membrane fouling,and clearly show that an increase in BPC concentration in the MBR sludge mixture led to a significant accel-eration in membrane fouling during the sludgefiltration.However, it should be pointed out that afixed signal-fibre MF membrane was used in the present study to determine the fouling rate dur-ing sludgefiltration.The actual MBR situation is more complication with aeration and membranefibre movement.The membranefibre movement caused by aeration turbulence is expected to reduce the membrane fouling rate;however,the contact between membrane fibres in a membrane module would reduce thefluid shear over the membrane surface,worsening the fouling situation.
The control sludge(without extra BPC)had a background liquid-phase organic concentration C0of aro
und3.5mg/l.A small BPC addition of0.2C0resulted in a notable increase in the membrane fouling rate(Fig.2a),and a further addition of BPC beyond0.6C0 increased the fouling rate dramatically.The membrane fouling rate during sludgefiltration increased almost linearly with the BPC content in the sludge suspension(Fig.3).The results of the well-controlled sludgefiltration experiments thus prove that BPC are a crucial foulant in MBR systems.BPC are a group of organic solutes formed by the affinity clustering of soluble and colloidal
substances Fig.2.Membrane fouling rate indicated by TMP increase during thefiltration of the sludge(SS concentration=3g/l)with different BPC dose ratios:(a)BPC without ozonation;(b)BPC after ozonation at0.18mg O3/mg TOC.C0:background organic (TOC)concentration in the sludge suspension;C BPC:TOC of the BPC added.
on the membrane surface during MBR operation[11].It is believed that large-sized BPC in the MBR sludge mixture function as a“glue”that facilitates sludge attachment and the formation of a fouling layer on the membrane surface[12–14].
3.2.Reduction of the membrane fouling rate by BPC ozonation
Ozonation was applied to the BPC solution before its addition into the sludge suspension.No residual ozone was found in the ozonated BPC solutions.At the ozone dose employed,which was less than1mg O3/mg TOC of BPC,the amount of BPC removed
was
Fig.3.Membrane fouling rate of the sludge during MFfiltration as a function of the BPC content:comparison of the fouling effect between raw BPC and ozonated BPC.
F.-y.Sun et al./Process Biochemistry46(2011)162–167
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Fig.4.Changes in the sludge and BPC deposition rates in the fouling layer on the single-fibre membrane during sludgefiltration as a function of the ozone dose applied to the BPC solution.The BPC dose ratio C BPC/C0=1.6.
minimal,as shown below(Fig.5in Section3.3)by the insignificant TOC reduction,but BPC destruction by ozonation was expected.The fouling test results demonstrate that ozonation can greatly reduce the detrimental effect of BPC on membrane fouling during sludge filtration.Upon ozonation of the BPC solution,the membrane foul-ing rates of the sludge–BPC mixtures decreased significantly compared with the identical test cases without ozonation(Fig.2b). The average fouling mitigation efficiency by BPC ozonation was over70%(Fig.3).Thus,the ozonation of BPC may be an effective fouling control measure in SMBR systems.郝云否认家暴>杨紫个人资料简介
After eachfiltration test,the fouling sludge layer on the single-fibre membrane was collected and analysed for the solid and BPC content to determine the average deposition rates of the solid mat-ter and BPC on the membrane surface during sludgefiltration.The
享乐团results show that the deposition rates of both the solids and
BPC
Fig.5.Comparison of the organic content and chemical composition of BPC before and after ozonation at different ozone
doses.Fig.6.Change in(a)the size distribution and(b)the mean size of BPC after ozonation at different ozone doses.
decreased as the ozone dose applied to the BPC solution increased (Fig.4).It should be noted that the biomass solids were the pre-dominant foulant material(over95%)in the fouling(cake)layer on the membrane surface.The BPC content in the sludge cake layer averaged around14.2mg TOC/g SS.The proportional deposition of BPC and suspended solids suggests that BPC function as the“glue”in cake layer formation.In comparison,BPC after ozonation appar-ently lost their“gluing”capability to a great extent.However,the effectiveness of ozonation in reducing foulant attachment on the membrane surface did not continue to increase with an increasing amount of ozone.This implies that the improvement of the sludge filterability by BPC ozonation has a limit.Fortunately,a small ozone dose is effective in reducing the fouling potential of sludge.
3.3.Destruction of BPC by ozonation
As stated previously,ozonation did not result in significant BPC oxidation or organic mineralization.The TOC concentration remained largely unchanged in the BPC solutions after ozonation at different doses(Fig.5).Moreover,according to the chemical analy-sis,ozonation did not le
ad to a clear trend of change in the chemical composition of BPC in terms of the polysaccharide,protein and humic content(Fig.5).Apparently,oxidation of the organic poly-mers by ozone at the doses applied did not reach the level of their component units,that is,simple sugars for polysaccharides,amino acids for proteins and aliphatic or phenolic acids for humic sub-stances.The main change brought about by ozonation appeared to be the breaking up of large BPC.This is demonstrated by the signifi-cant reduction in BPC size after ozonation.According to the particle size analysis(Fig.6),a small ozone dosage of0.03mg/mg TOC of BPC decreased the volume-based mean size of BPC from38to27␮m.
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Fig.7.CLSM observation of BPC before and after ozonation:(a 1)and (a 2)before ozonation;(b 1)and (
b 2)ozone dose ratio =0.03mg O 3/mg TOC;(c 1)and (c 2)ozone dose ratio =0.18mg O 3/mg TOC (red:polysaccharides in the BPC;green:bacterial cells).(For interpretation of references to color in this figure legend,the reader is referred to the web version of this article.)
As the ozone dose increased to 0.30mg/mg TOC of BPC,the mean BPC size decreased to about 12␮m.However,further increases in the ozone dose resulted in little decrease in BPC size.This indicates that larger BPC are more vulnerable than small BPC to break-up by ozonation.
The breaking up of large BPC by ozonation was further confirmed by CLSM examination (Fig.7).BPC could be well observed by stain-ing their polysaccharide components with fluorescent ConA-TRITC.The CLSM images show that large BPC,many of which were larger than 50␮m,disintegrated into smaller BPC after ozonation at small ozone doses of 0.03–0.18mg/mg TOC of BPC.
In addition to BPC break-up,ozonation at a low dose also altered the chemical properties of the BPC.Ozone is a selective oxidant that reacts faster with some chemicals or functional groups than with others [33].Polymeric substances and their clusters contain several sites that are reactive to ozone.For example,the glyco-sidic bonds inherent in chains of polysaccharides can be easily cleave
d by ozone attack,resulting in their breakdown into short-chain polysaccharides or oligosaccharides [34,35].Some reactive sites that are located at the branches of polymeric substances are readily cut by ozonation from the main chains [36].This leads not only to the fragmentation of BPC,but also the modification of their surface properties.
The viscosity of the BPC solution decreased as a result of ozona-tion (Fig.8).A high viscosity generally suggests a high fouling potential of the feed liquid [37,38].The significantly higher viscos-ity of the BPC solution than water is probably due to the abundance of large BPC and their interaction.The decrease in viscosity of the BPC solution after ozonation thus not only reflects the breaking up of the BPC,but also implies the modification of their surface properties.
3.4.Effect of BPC destruction on membrane fouling control
Due to the size reduction and possible modification of the sur-face properties of BPC after ozonation,the cut-off ratio of the BPC by filtration decreased (Fig.8).In other words,the portion of fil-terable BPC increased considerably after ozonation,although the total amount of BPC hardly changed.The cut-off ratio provides an indication of the fouling propensity of BPC dispersion [11],as the
fouling resistance greatly depends on the amount of foulant depo-sition.A reduction in size leads to a decrease in BPC retention due to the steric effect [39],whereas surface property modification affects the gelling propensity of the polymeric substances [40].
An SMBR is an almost completely enclosed system that does not allow the overflow of loose sludge
flocs or organic foulants from the system.As a result,fouling materials,including SMP,loose EPS and colloidal organics,accumulate in the bioreactors.Sludge filtration through a large membrane surface provides a unique condition for BPC formation from polymeric organic substances [11].BPC within the sludge cake deposited on the membrane greatly increase the filtration resistance of the cake layer.The detachment of BPC from the membrane by aeration turbulence brings BPC back into the sludge suspension,which in turn worsens the fouling potential of the sludge [12,13].It is therefore desirable to remove or destruct BPC regularly in an MBR system.
Previous studies showed that direct sludge ozonation could practically reduce the membrane fouling rate in SMBR [19].Huang and Wu [22]demonstrated that in continuous SMBR operation,ozonation of the bulk sludge at 0.25mg O 3/g SS effectively con-trolled membrane fouling.The experimental findings of this study indicate that the underlying mechanism of ozonation for fouling minimisation reported in previous studies is probably the
effec-
Fig.8.Changes in the viscosity and cut-off ratio of the BPC solution after ozonation.