Separation and Purification Technology 132 (2014) 10–15

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Separation and Purification Technology

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The effect of fluidized media characteristics on membrane foulingand energy consumption in anaerobic fluidized membrane bioreactors

http://dx.doi.org/10.1016/j.seppur.2014.04.0491383-5866/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +82 32 860 7502.E-mail address: jeonghwankim@inha.ac.kr (J. Kim).

Muhammad Aslam a, Perry L. McCarty a,b, Jaeho Bae a, Jeonghwan Kim a,⇑a Department of Environmental Engineering, Inha University, Incheon, Republic of Koreab Department of Civil and Environmental Engineering, Stanford University, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 3 March 2014Received in revised form 29 April 2014Accepted 30 April 2014Available online 9 May 2014

Keywords:Anaerobic fluidized membrane bioreactorFluidized mediaMembrane foulingEnergy consumption

The effects of fluidized media characteristics on membrane fouling and energy consumption in theanaerobic fluidized membrane bioreactor (AFMBR) were evaluated. Both adsorbing granular activatedcarbon (GAC) and non-adsorbing silica and polyethylene terephthalate (PET) beads were evaluated. Forfresh GAC, the energy requirement for fluidization increased with size, but smaller sizes led to greaterreduction in membrane fouling. But when the GAC adsorption capacity was first exhausted, the reversewas noted, the larger particles reduced membrane fouling best. Non-adsorbing silica particles and PETbeads demonstrated similar results to pre-adsorbed GAC, lower fouling was accomplished by the largermedia that had a higher energy requirement for fluidization. The media packing ratio was also ofimportance, the higher the packing ratio up to 50%, the greater the reduction in fouling. Fouling reductionwas also somewhat better at a given energy expenditure with lower specific gravity PET beads than withdenser and smaller pre-adsorbed GAC particles.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Anaerobic treatment is a potential cost-effective technology fordomestic wastewater treatment as it is less productive of wastebiosolids and generates renewable bioenergy in the form of meth-ane [1–3]. However, anaerobic treatment is not expected to meetstringent effluent quality standards unless the slow-growing anaer-obic microorganisms can be maintained within the reactor [2,4–6].For this reason there has been growing interests in anaerobic mem-brane bioreactors (AnMBR), which combine anaerobic treatmentwith membrane filtration [1,7–9]. However, membrane fouling isa concern as it reduces membrane life-time and increases capitaland operational costs. The addition of media such as powdered acti-vated carbon (PAC) has been found to reduce membrane fouling inmembrane bioreactors (MBR) [10,11]. But, when used in gas sparg-ing or cross-flow membrane bioreactors, the energy consumptionrate to reduce fouling can still be quite high.

The anaerobic fluidized membrane bioreactor (AFMBR) is analternative AnMBR, which combines an anaerobic fluidized bedreactor (AFBR) with submerged membrane filtration [9]. Here,

granular activated carbon (GAC) is being used as the fluidizedmedia, and not only supports the active biofilm, but also controlsmembrane fouling through its scouring action along the mem-branes. Unlike the common gas-sparging fouling mitigationmethod or cross-flow filtration mode, membrane scouring byfluidized GAC particles under bulk recirculation alone throughthe reactor results in a relatively low energy requirement [9,12].

While GAC fluidization has proven to effect low-energy foulingcontrol, other materials might be more cost effective. The objectiveof this study was to evaluate competing traits of different typesand sizes of fluidized media for effectiveness in reducing mem-brane fouling and for energy requirement for fluidization. Twoimportant, but perhaps competing, criteria for this evaluationare, (1) the fluidized media must play a key role in providing phys-icochemical functionalities to reduce membrane fouling, and (2)fluidization of the media should require little energy. It is likelythat better reduction in membrane fouling through the scrubbingaction on membrane surfaces may require more energy. Media costis another trait of importance. Additionally GAC is a well-knownand effective adsorbent, which may add to its ability to reducefouling, as well as to improve its effectiveness for increasingtreatment stability and efficiency. Thus, while there are severaldifferent criteria for judging fluidized media effectiveness, thisstudy was focused on how media characteristics affect foulingmitigation and energy expenditure.

M. Aslam et al. / Separation and Purification Technology 132 (2014) 10–15 11

2. Experimental

2.1. Anaerobic fluidized bed membrane reactor

The acrylic AFMBR used in this study is illustrated in Fig. 1. Itwas 350 mm high, 25 mm wide and 160 mm long with effectivewater volume of 1.0 L and with a settler at the top. It contained adouble-sided flat-sheet polyvinylchloride (PVC) hydrophobic MFmembrane with nominal pore size of 0.4 lm (Pure Envi Tech Inc.,Korea). PVC flat-sheet membranes have been applied widely inMBRs due to their low costs, high mechanical strength, andwell-defined water channels between membranes relative to hol-low-fibers. The flat-sheet membrane was 150 mm wide and250 mm high with a total effective surface area of 0.04 m2. Liquiddepth in the lab-scale AFMBR reactor was 490 mm. A diffuser platethat contained 12 5-mm holes was installed at the bottom of thereactor to support the fluidized media and to allow entrance ofthe recirculating water to fluidize the media. The feed solutionfor this AFMBR was the effluent from a 3.93 L anaerobic fluidizedbed reactor (AFBR) with GAC as the fluidized media as describedelsewhere [9]. The feed to the AFBR had a total COD (TCOD) of513 mg/L, and contained 250 mg COD/L each of sodium acetateand sodium propionate, and another 10 mg COD/L of a mixtureof yeast extract and oleic acid. Additionally it contained 0.19 g/LNH4Cl and 10 mL/L of filtered anaerobic digester supernatant(COD of about 5 mg/L) from a local wastewater facility as sourcesof nitrogen and micronutrients.

The AFMBR was charged with AFBR effluent for each study andwas operated at ambient temperature. The top open sections of theflat-sheet membrane were connected to a peristaltic pump(Masterflex, Model No. 7520-57, USA). Permeate was generallyextracted with the peristaltic pump at a constant flux of 50 L/m2/h and recycled back into the AFMBR to maintain reactor waterlevel. Sufficient upward flow of fluid was maintained by fluidrecirculation with a magnetic pump using a flow-rate controller

Fig. 1. Experimental set-up for the batch-fed a

(Blue-white, F-450) to allow the fluidized media to rise and coverthe whole surface of the membrane. As a fouling indicator, thesuction pressure required to maintain the permeate flux was mon-itored over time with a vacuum pressure meter (EW-68604-00,Cole Parmer) connected in the permeate line.

2.2. Fluidized media and adsorption test

The same GAC (MRX-M, Calgon Carbon Corp., Pittsburgh, USA) asused for the AFBR was used in the AFMBR to observe the impact onmembrane fouling and energy requirements. The effect of the GACpacking ratio was studied using 10%, 30% or 50% of the total effectiveheight of the main membrane reactor. Recirculation flow rate wasadjusted to expand the GAC so that it completely covered theeffective surface areas of the flat-sheet membranes. To investigatethe effect of GAC sizes on membrane fouling, GAC particles werefractionated by sieve separation into sizes of >2 mm, 0.85–2 mm,0.5–0.85 mm, 0.18–0.5 mm, and <0.18 mm. Silica particles wereused as one alternative fluidized media and had a similar averagerange in size but higher specific gravity than the GAC. Anotheralternative media studied was polyethylene terephthalate (PET)beads. Properties of each fluidized media are summarized in Table 1.

Pre-adsorbed as well as fresh GAC media were evaluated. Pre-adsorbed GAC was prepared by filling it into a small-scale column30 mm in diameter and 470 mm high. AFBR effluent was thenpassed through the column continuously at a flow rate of 0.15 L/hand the effluent from the column was analyzed for COD changeswith time until the effluent concentration equaled the influentconcentration. The GAC was then considered to be pre-adsorbed.

2.3. Analytical methods

Measurements for total and soluble chemical oxygen demand(TCOD and SCOD), and total and volatile suspended solids (TSSand VSS) were performed according to procedures in Standard

naerobic fluidized membrane bioreactor.

Table 1Properties of each fluidized media tested with the AFMBR system.

GAC Silica PET beads

Specific gravity 2.3 2.6–2.8 1.3Size (mm) 1–2 1–2 3 ± 0.5Image

12 M. Aslam et al. / Separation and Purification Technology 132 (2014) 10–15

Methods [13]. For SCOD analysis, samples were first filteredthrough 1.2 lm GF/C filters. TSS and VSS were determined byfiltering samples through 1.2 lm GF/C filters followed by dryingfor 2 h in an oven at 100 �C. The sample was then kept in a desic-cator for 30 min before TSS analysis, and then placed in an oven at550 �C for 30 min followed by another 30 min in desiccation priorto weighing to measure the VSS concentration.

2.4. Energy requirements for fluidization

The theoretical energy requirement for fluidization wasestimated for each fluidized media using the power requirementequation [9]:

P ¼ QcE1000

ð1Þ

where P is the power requirement (kW), Q is the recirculation flowrate used to fluidize the media (m3/s), c is 9800 N/m3, and E repre-sents the hydraulic pressure head for fluidization (m). The energyused per unit volume of permeate was found by dividing the aboveby the permeate flow rate of 0.002 m3/h, which is equivalent to theapplied set-point membrane flux of 50 L/m2/h.

3. Results and discussion

In order to determine the energy requirement for fluidization ofdifferent size fresh GAC particles, the packing ratio was set at 50%.Fig. 2 illustrates that fresh GAC particle size affects the energyrequirement for fluidization significantly. The higher energyrequired for the larger particles was mainly because of theincreased recirculation flow rate they required for complete fluid-ization, rather than by the hydraulic head loss, which was similarfor all GAC sizes. For example, recirculation flow rates required

Fig. 2. Effect of GAC size on energy requirement for fluidization in the AFMBR(packing ratio: 50%).

for fluidizing GAC with sizes of 0.85–2 mm and 0.5–0.85 mm were6 and 3 L/min, respectively, while the head losses for these GACsizes were 0.018 and 0.014 m. Recirculation flow rates for eachGAC size range are listed in Table 2. From Fig. 2, the energy require-ment for GAC fluidization appears to grow exponentially withparticle size, illustrating the negative impact larger particles canhave on energy needs.

3.1. Relationship between particle size and packing ratio on energyrequirements

The effect of packing ratio on energy requirement for fluidiza-tion was then evaluated using different sizes of fresh GAC, withresults illustrated in Fig. 3. Overall energy requirement for fluidiza-tion increased significantly, both with particle size and withpacking ratio. As packing ratio increased, the mass of GAC forfluidization increased, which in turn increased the hydraulic pres-sure head. While this may suggest that a lower packing ratio isbest, the lower packing ratio for a given particle size resulted ina higher fouling rate. For example, for GAC particles ranging in sizefrom 0.5 to 0.85 mm, the suction pressure increased to 0.18 barafter 4 h filtration with a 50% packing ratio, but increased to amuch greater 0.4 bar with a 10% packing ratio. However, increasingthe packing ratio to 70% did not provide a better fouling reductionthan a 50% packing ratio for all GAC sizes tested (data not shown).Thus, in the packing ratio range of 10–50%, there is a trade offbetween better fouling reduction at higher ratios but lower energycosts at lower ratios. Such trade offs have cost implications thatdeserve further evaluation for AFMBR system design.

3.2. Relationship between GAC sizes and membrane fouling

Fig. 4 presents transient behavior of suction pressure with timeusing fresh GAC particles of different sizes at the same permeateflux and packing ratio, in comparison with suction pressureincrease without GAC fluidization. Hydraulic flow without GACfluidization was not effective in reducing membrane foulingalthough the recirculation flow rate used was higher (15 L/min)than that required for GAC fluidization. The smaller fresh GAC sizesled to greater reduction in membrane fouling. The suction pressurefor the smallest GAC tested (<0.18 mm), almost equaled the nearzero increase with DI water. It would be attractive if the smallestGAC material would provide both the best fouling control (Fig. 4)and the lowest energy requirement (Fig. 2). However, it is likelythat with fresh GAC, fouling control was by sorption of foulants,which would have been much greater with the small particles,rather than by scouring of the membranes. For example, Hu andStuckey [14] found that even small powdered activated carbon(PAC) performed better than GAC for fouling control in a biogassparged AnMBR, which they attributed to its better adsorption

Fig. 5. Effect of media size (GAC and silica particles) on adsorption breakthrough.

Fig. 6. Effect of pre-adsorbed GAC size on membrane fouling (set-point permeateflux: 50 L/m2/h, packing ratio: 30%, Qr: 15 L/min without GAC, 0.8 L/min ford < 0.18 mm, 2.0 L/min for 0.18 < d < 0.5 mm, 11.5 L/min for d > 2 mm where Qr isrecirculation flow rate.

Table 2Energy requirements for fluidization of different GAC and silica sizes.

GAC/silica(d < 0.18 mm)

GAC/silica(0.18 < d < 0.5 mm)

GAC/silica(0.5 < d < 0.85 mm)

GAC/silica(0.85 < d < 2 mm)

GAC(d > 2 mm)

Permeate flux (L/m2/h) 50/50 50/50 50/50 50/50 50Permeate flow rate (L/h) 2/2 2/2 2/2 2/2 2/2Packing ratio (%) 50/50 50/50 50/50 50/50 50Recirculation flow rate (L/min) 0.5/1.5 1.5/3.5 3.0/7.5 6.0/11 9.0Head loss (m) 0.01/0.047 0.013/0.060 0.014/0.067 0.018/0.076 0.019Energy for fluidization (kW h/m3) 0.0004/0.006 0.0016/0.017 0.003/0.03 0.009/0.07 0.014

Fig. 4. Effect of fresh GAC size on membrane fouling (set-point permeate flux:50 L/m2/h, packing ratio: 50%).

Fig. 3. Combined effect of packing ratio and size of GAC particles on energyrequirement for fluidization.

M. Aslam et al. / Separation and Purification Technology 132 (2014) 10–15 13

effect, although both physical scouring and adsorption were likelyto have played important roles in the fouling reduction found [13].

In order to examine this finding further, the adsorption effect ofGAC was constrained by using pre-adsorbed GAC with a similarrange in sizes. Here, the pre-adsorbed GAC was first prepared inparallel using four fixed bed adsorption columns, three of whichcontained different GAC sizes of >2 mm, 0.18–0.5 mm, and<0.18 mm, and the forth a non-adsorbing silica. Here as expected,the smaller GAC sizes illustrated a higher adsorption capacity, asillustrated in Fig. 5 by the time taken for the effluent COD to equalthe influent COD.

After the pre-adsorbed GAC was prepared, it was evaluated in afouling test. Due to the limited amount of pre-adsorbed GAC pro-duced, the packing ratio was reduced to 30%. The recirculation flowrates required for fluidization of pre-adsorbed GAC were 0.8, 2.0and 11.5 L/min for GAC sizes <0.18 mm, 0.18–0.5 mm and>2 mm, respectively. Recirculation flow rate without pre-adsorbedGAC was set as 15 L/min.

Fig. 6 indicates that with pre-adsorbed GAC, the best foulingcontrol was obtained with the larger rather than the smallerGAC, thus confirming that the opposite result found with freshGAC was because sorption was the main factor then effectingfoulant reduction. This confirms that GAC reduces fouling bothby sorption of foulants and by membrane scouring, the relativebenefit of each depending upon the length of time of GAC usage.With longer periods of usage, the GAC adsorption capacity is likelyto be spent, and then the larger GAC particles would be best forfoulant reduction, but this benefit comes with the cost of a higherenergy requirement.

In order to better understand the relationship between energyrequirements and membrane fouling, other materials that had lit-tle sorption capacity were evaluated. First, silica particles having

Fig. 7. Effect of silica size on membrane fouling (set-point permeate flux:50 L/m2/h, packing ratio: 50%).

Fig. 8. Relationship between suction pressure increase with time for 3 mm PETbeads and 0.5–0.85 mm silica particles in comparison with no fluidized media. Flowrate of 15 L/min, permeate flux of 50 L/m2/h, and packing ratio of 10%).

Fig. 9. Relationship between TMP change rate and energy for fluidization fordifferent non-adsorbing fluidized media and packing ratios (set-point permeateflux: 50 L/m2/h).

14 M. Aslam et al. / Separation and Purification Technology 132 (2014) 10–15

the same size range of 0.18–0.5 mm as with pre-adsorbed GACparticles were selected for study. The silica particles required muchmore energy for fluidization than GAC particles of similar size(Table 2), primarily because of their higher specific gravity (2.8vs. 2.2 for GAC). This difference is reflected in a higher fluidizationhead loss for the same volume and size of material, and also ahigher required upflow recirculation rate, both of which add tothe energy requirement. Fig. 7 depicts the change in suctionpressure with time for silica particles. As with pre-adsorbed GAC,membrane fouling was less with the larger silica particles, whichrequired more energy for fluidization. Since silica particles do nothave an adsorption capacity (Fig. 5), mitigation of membranefouling results primarily by the scouring action of silica particlesmoving over membrane surfaces.

In order to further investigate the relationship between differ-ent non-adsorbing fluidized media and membrane fouling for sizeswhere energy requirements for fluidization were similar, PETbeads and silica particles were compared (Fig. 8). For this test, a10% packing ratio was used for both silica and PET beads. For thePET beads characterized as 3 mm in uniform size and specificgravity of 1.4, a 15 L/min recirculation flow rate was required forcomplete fluidization at 10% packing ratio. For similar energy con-sumption under these operating conditions, smaller silica particlesranging from 0.5 to 0.85 mm were required as their specific gravityof 2.6 was much higher. At 15 L/min as recycle rate with a 10%packing ratio, the head loss for fluidization of silica particles wassomewhat higher than PET beads (0.011 vs. 0.009 m). The energyrequirements for fluidization at a 50 L/m2/h flux and the aboveother conditions were 0.013 kW h/m3 for the silica particles and0.010 kW h/m3 for the PET beads. Fig. 8 illustrates that fouling mit-igation under similar energy utilization conditions with the PETmedia was more pronounced than with the heavier but smallersilica particles. Based upon this limited comparison of twonon-adsorbing materials, it appears that larger media with lowerspecific gravity are more capable of fouling reduction with a lowerenergy requirement.

The relationship between fouling rate and energy expenditurefor the non-adsorbing fluidized media tested in this study, whichincluded silica particles, pre-adsorbed GAC particles, and PETbeads, is illustrated in Fig. 9. Fouling rate is reported here as therate or transmembrane pressure (TMP) change determined bydividing the final suction pressure by 4 h. Fig. 9 data illustrates thatfor a given medium at a given packing ratio, a lower fluidizationenergy requirement (that is smaller particles) is associated withan increase in the fouling rate. Also, the fouling rate for a given

medium is significantly affected by its packing ratio. For example,with silica particles at a given energy requirement, the TMP changewith time was significantly lower with a higher packing ratio. Butwhen comparing different media at the same packing ratio andwith a similar energy requirement, the larger PET particles reducedthe fouling rate somewhat more than did the smaller silica parti-cles. While these data are useful for indicating factors affectingthe fouling rate, more studies are needed with materials of differ-ent sizes, packing ratios, and permeate fluxes to better generalizethese comparisons and draw firmer conclusions about whichmedia might actually provide the better reduction in fouling withthe lowest energy requirement.

4. Conclusions

GAC can reduce membrane fouling both by adsorption of fou-lants and by the scouring action of GAC along membrane surfaces.Sorption is the dominant process with fresh GAC. Here smaller par-ticles, which have a higher sorption capacity and require lessenergy for fluidization, are preferred. However, after sorptioncapacity is diminished, then membrane scouring dominates. Here,fouling reduction was found to be a function of energy expendi-ture. For example, fouling reduction was best obtained with largerGAC particles that required more energy for fluidization. Increasingthe packing ratio of GAC particles from 10% to 50% increased the

M. Aslam et al. / Separation and Purification Technology 132 (2014) 10–15 15

energy required for fluidization and also lowered the fouling rate.Other non-adsorbing materials such as silica and PET beadsbehaved similarly to pre-adsorbed GAC, that is larger sizesprovided greater fouling reduction, but required more energy forfluidization. Larger lower-specific gravity materials such as PETappeared to perform somewhat better than denser materials suchas silica. However, further studies are needed to better generalizethe various factors affecting the relationship between energyexpenditure and fouling reduction.

Acknowledgements

This research was supported by the Korea Ministry of Environ-ment as a ‘‘Global Top Project’’ (Projec No.: GT-12-B-01-012-0),and by an INHA University research grant.

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