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INFLUENCE OF OPERATING CONDITIONS ON FOULING BEHAVIOR IN WASTEWATER MEMBRANE BIOREACTOR PROCESSES

John-Paul Nywening1, Hongde Zhou1 and Hadi Husain2

1School of Engineering, University of Guelph, Guelph, ON, Canada, N1G 2W1

2Zenon Environmental Inc., 845 Harrington Court, Burlington, ON, Canada L7N 3P3 ABSTRACT Membrane fouling was studied using three pilot-scale submerged membrane bioreactors operated at a series of permeation and aeration conditions to treat municipal wastewater. The transmembrane pressure increases were used to calculate the fouling ratios to compare the relative fouling rates. The results showed that the trends of fouling resistances differed greatly, depending on the permeate flux and mixed liquor characteristics. A stable fouling resistance can result when the filtration is operated at sustainable permeate flux conditions. At the unsustainable permeate flux conditions, the fouling resistance increased exponentially as the filtration progresses. In all the cases, the fouling ratios increased with permeate flux and decreased with aeration intensity. Furthermore, the effects of aeration intensity on fouling ratio is independent of the permeate flux and vice versa. Finally, the variation of fouling ratios at different operating conditions strongly depends on the sludge characteristics of mixed liquor and it appears that more than one parameter of mixed liquor is needed to define their relationship. KEYWORDS Aeration, filterability, fouling, membrane bioreactor, wastewater treatment. INTRODUCTION Membrane bioreactors (MBRs), combining conventional wastewater activated sludge processes with membrane filtration, yield excellent solid-liquid separation efficiency and high quality effluent. Additional advantages include a small footprint, robust resistance to influent variations, reduced sludge production and modular design. However, their capital and operating costs have limited widespread application in wastewater treatment and reuse even though these costs have drastically declined in recent years due to the process improvement and technological advance in membrane production (Belfort, et al. (1994); Cui and Taha (2003)). Between 1994 and 2000, treatment costs declined by 80% while energy requirements dropped by 85% (Husain, 2005). Further advances to reduce process costs are yet desirable. Sparging aeration for effective fouling control accounts for a large portion of the operating energy costs for submerged MBRs. Fouling, which occurs through the deposition and adsorption of dissolved and suspended solids, increases the hydraulic resistance, thereby, limiting the permeate flux across the membrane surface and necessitating the frequent membrane module cleaning and replacement. It is generally accepted that fouling occurs in the form of adsorption, pore deposition, cake formation, concentration polarization and/or biofouling. Their relative magnitudes depend on membrane material and configuration, operating conditions and mixed

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Copyright 2006 Water Environment Foundation. All Rights Reserved©

liquor characteristics. Particles may be transported to the membrane surface by the permeate flux. Depending on the particle-particle interactions, the particle-membrane interactions and the hydrodynamic condition on the membrane surfaces, accumulated particles can be transported back into bulk liquid via Brownian diffusion, inertial lift and shear induced dispersion (Wiesner, et al. (2005)). Design modifications and operation improvements have led to substantial increases in process efficiency by improving the dispersion of accumulated solids. In early applications, sidestream recirculation was employed to maintain high crossflow velocities to control cake build-up. This method, however, is uneconomical in large scale applications due to process complexities, high energy cost associated with pumping and increased fouling from the breakup of large floc (Shimizu et al. (1996)). With the introduction of submerged operation in the mid 1990’s, the membrane modules were immersed directly in the bioreactor while the effluent was withdrawn by vacuum. Vigorous sparing aeration coupled with the lower fluxes resulting from this operating mode greatly reduces fouling. Nevertheless, the energy required to provide sufficient sparging air usually exceeds that to run a conventional activated sludge plant. Numerous researches have shown that the membrane fouling decreases with the airflow rate (Chang and Judd (2002); Engelhardt et al. (1998)). A long-term reduction in aeration can also lead to a rapid accumulation of fouling material on the membrane surface (Chang et al. (2002)). However, it has also been shown that for submerged MBRs there is an optimum aeration rate which maximizes fouling suppression (Bouhabila et al. (1998); Le-Clech et al. (2003)). Chua et al. (2002) suggested that beyond this critical value, the fouling may actually increase because the shear force breaks up larger particles and the biomass film. Further, there may be an optimal aeration duration and sequence. Guinzbourg (2003) found that reducing the aeration on-time to off-time ratio by a factor of 6 did not produce a significant change to the TMP across the membrane surface. Further studies showed that the rates of membrane fouling are specific to each mixed liquor. The extracellular polymer substances (EPS) have been considered among the most important components affecting the membrane fouling but different extraction and analytical techniques have produced conflicting results (Martin (2005)). Differing results have also been reported for the effects of MLSS concentration (Yamamoto et al. (1989)) but generally it is agreed that it poses little direct influence (Rosenberger et al. (2002)). Rather it is the particle size distribution and the quantity of colloidal particles (Fan (2005); Wisniewski and Grasmick (1998)) that appear to contribute significantly to cake formation. Additionally, particle surface charge and pH have been found to affect the filterability of mixed liquor. The objectives of this study are to assess the effects of key operating conditions on membrane fouling including permeate flux, aeration intensity and aeration sequence, and to examine the roles of sludge characteristics at different operating conditions.

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MATERIALS/METHODS Pilot Plant Setup Figure 1 shows the schematic for three parallel ZW-500 MBR pilot plants (A, B, C) used in this study. Raw wastewater from the City of Guelph Wastewater Treatment Plant was filtered through a 0.75mm drum screen to remove large particulate matter prior to being fed to the 9250L aerobic tank. A centrifuge pump was used to transfer the mixed liquor at five times the permeate flow rate from the aerobic tank to the bottom of a 950L membrane tank while the membrane tank overflow was returned by gravity. Permeate was withdrawn by vacuum at a constant flow rate from the outside to the inside of three submerged ZW-500C membrane modules. A 100L tank retained permeate for backwashing and excessive mixed liquor was wasted from the membrane tank using a sludge wasting pump. Three dosing pumps were used to pump different chemical reagents for membrane cleaning and mixed liquor conditioning. Air was supplied to the aerobic tank from the City of Guelph Wastewater Treatment Plant. A dedicated centrifugal blower was used to supply sparging air to the membrane tank via two coarse air diffusers located at the bottom of the membrane modules. A PLC was used to control the pilot operation and continuously record MLSS concentration, permeate flow rate, temperature, transmembrane pressure, tank volumes, etc. Tables 1 and 2 summarize the main equipment specifications and operating conditions, respectively. Figure 1 - Setup of ZW-500 pilot plant.

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Table 1 - Main specifications of MBR pilot plant. Parameter Unit Value Screen opening mm 0.75 Aerobic tank m 2.4L x 2.9W x 2.5H Aerobic tank effective volume m3 12.3 Membrane tank m 0.85L x 0.47W x 2.4H Membrane tank effective volume m3 0.95 Membrane type ZW-500C-SMC Fibre length m 1.6 Fibre outer diameter mm 1.9 Fibre internal diameter mm 0.9 Nominal membrane pore size µm 0.04 Number of membrane modules per plant 3 Total membrane area per pilot m3 66.9 - 69.7 Maximum permeation transmembrane pressure Pa 0.83 x 105 Number of pilot plants 3 Table 2 - Main pilot operating parameters under normal conditions. Parameter Unit Value HRT h ~ 6 SRT days 18 MLSS g/L 8-12 F/M ratio ~ 0.2 Cyclic aeration s-on/s-off 10/10 Permeation/Relaxation min 10/1 Permeate flux (J) L/m2/h 24-44 Mixed liquor recirculation rate (5J) L/m2/h 120-220 Experimental Procedure To examine the effects of operating conditions on membrane fouling, a series of pilot tests were performed in which one process variable was manipulated at a time. To completely remove reversible fouling materials from the membrane surface each test was commenced with a 45 second backwash during which a continuous air sparging rate of 61.2 m3/h was maintained followed by an additional 15 seconds of backwash during which the aeration intensity and sequence were set to the desired test conditions to allow the hydrodynamics within the membrane tank to stabilize before permeation. Immediately after the backwash, permeation was initiated and proceeded at the selected permeate flux and aeration intensity until a predetermined transmembrane pressure, typically no less than 51.7 kPa, was reached to examine the dynamics of solids deposition. Finally, the membrane modules were backwashed for one minute at a flow rate of 39.1 L/m2/h to remove the reversible fouling materials. The stepwise test procedure is summarized in Table 3. Table 4 summarizes the detailed experimental arrangements for each series of pilot tests.

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In addition to observing the progression of fouling, the critical and sustainable fluxes were determined in a stepwise flux method similar to that suggested by Ognier, Wisniewski and Grasmick (2002) as illustrated in Fig. 2. Prior to measurement, the membranes were cleaned by air scouring while the permeation was ceased for 30 minutes. Each permeation step lasted 10 minutes and the incremental flux was 3.4 l/m2/h. The transmembrane pressure (TMP) at the end of each step was then plotted as a function of the permeate flux. The critical flux was obtained at the point where the TMP deviated from the straight line extended from the origin. The sustainable flux was defined as the maximum flux below which the TMP increased linearly with permeate flux, even though a small, stable amount of fouling occurred and the straight line did not intersect the origin. Above the sustainable flux, TMP increases exponentially with permeate flux. Table 3 - Filterability test procedure.

Aeration Step Procedure Flux

L/m2/h Intensity m3/h

Sequence s-on/s-off

Duration s

1 Backpulse 39.1 61.2 Continuous 45

2 Backpulse 39.1 40.8 - 61.2

(10/10) (10/20) (10/30) (10/40)

15

3 Permeation 39.1 – 58.6 40.8 - 61.2

(10/10) (10/20) (10/30) (10/40)

Min: -51.7 kPa

4 Backpulse 39.1 61.2 Continuous 60 Table 4 - Summary of experimental arrangement.

Test Pilot Flux L/m2/h

Intensity m3/h

Aeration Sequence s-On/s-Off

I A, B, C

39.1 45.6 52.1 58.6

40.8 47.6 54.4 61.2

10/10

II A 32.6 39.1 45.6

40.8 10/10

III B 32.6 39.1 45.6

43.4 10/10

IV C 32.6 39.1 45.6

40.8 51.0 61.2

10/10

V A, B, C 45.6 40.8 51.0 10/10

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61.2 A 39.1 40.8 B 39.1 43.4 VI C 39.1 61.2

10/10 10/20 10/30 10/40

Figure 2 - Critical and sustainable flux definition.

0

10

20

30

40

50

0 10 20 30 40Flux (L/m2/h)

TMP

(kPa

)

Sustainable Flux

Critical Flux

Wastewater and Mixed Liquor Characterization Daily composite samples were taken from the screened raw wastewater while grab samples were collected from the mixed liquor in the aerobic and membrane tanks and the permeate. The collected mixed liquor samples were used to measure time to filter (TTF) and diluted solid volume index (DSVI) on site. A portion of mixed liquor samples was filtered immediately to minimize biodegradation. Permeate, raw water, mixed liquor and filtrate samples were stored in an ice-packed cooler and transported to the Environmental Engineering Laboratory at the University of Guelph. Total suspended solids (TSS), mixed liquor suspended solids (MLSS), mixed liquor volatile suspended solids (MLVSS), total dissolved solids (TDS), total solids (TS), TTF, total nitrogen (TN), total phosphorus (TP), ammonium nitrogen (NH4-N) and chemical oxygen demand (COD) were determined for the wastewater and filtrate samples according to Standard Methods (APHA, AWWA and WEF (1995)). Because MLSS concentrations were much higher than in conventional activated sludge processes, the samples for the DSVI measurement were dilute with permeate to a ratio of 3:1 so that the concentration was approximately 3 g/L. Total organic carbon (TOC) was analyzed using a TOC analyzer (TOC-VCSH, Shimadzu, Japan). The difference between the filtrate passing through a 1.5 μm filtration paper (934-AH, Whatman, USA) and the permeate collected from the membrane modules is referred to as colloidal TOC. Table 5 summarizes the main analytic results for the screened raw water and the mixed liquor from the aerobic. Table 6 lists the suspended solids concentrations in the aerobic and membrane tanks.

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Table 5 - Main characteristics of raw wastewater and mixed liquor filtrate.

Screened Raw Water Mixed Liquor Filtrate in Aerobic Tank Parameter Units

Mean ± St.Dev. No. of Samples Mean ± St.Dev. No. of

Samples COD mg/L 569.1 ± 384.0 19 19.6 ± 9.8 55 COD (Soluble) mg/L 150.0 ± 89.7 18 - - NH4-N mg/L 38.9 ± 4.8 11 6.2 ± 3.4 55 NH4-N (Soluble) mg/L 33.3 ± 5.1 11 - - TN mg/L 65.7 ± 32.3 19 25.5 ± 17.8 32 TN (Soluble) mg/L 34.1 ± 15.6 18 - - TP mg/L 7.0 ± 2.0 19 1.7 ± 0.7 54 TP (Soluble) mg/L 3.8 ± 1.4 19 - - pH 7.6 ± 0.2 19 7.6 ± 0.1 55 TSS mg/L 241.6 ± 58.6 14 - - TS g/L 1.4 ± 0.2 14 - - Turbidity - - 0.4 ± 0.5 55 Table 6 - Suspended solid concentrations in the aerobic and membrane tanks.

Aerobic Tank Membrane Tank Parameter Units

Mean ± St.Dev. No. of Samples Mean ± St.Dev. No. of

Samples MLSS g/L 9.2 ± 1.5 59 10.4 ± 1.8 68 MLVSS g/L 7.3 ± 1.2 53 8.4 ± 1.6 53 Data Analysis and Interpretation The total filtration resistance (Rt) was calculated from the flux (J) and transmembrane pressure as:

JTMPRt μ

= (1)

The fouling resistance (Rf) was then obtained by subtracting the clean water resistance (Rw) from the total filtration resistance:

wtf RRR −= (2) To account for variations in MLSS concentration and differences in permeate flux, the fouling resistance was plotted as a function of mass of MLSS intercepted per unit area of membrane surface (Am) rather than time (t).

m

tpt A

MLSSVM

⋅= )( (3)

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where Vp(t) is the total volume permeated at time t. Below the maximum sustainable flux at a specified aeration intensity, stable operation can be obtained. The resultant steady state filtration resistance decreases with aeration intensity and increases with permeate flux. Thus, the fouling ratio (F) has been proposed to compare the influence of these operating conditions on membrane fouling:

)(

)(

reff

if

RR

F = (4)

where Rf(i) is the steady state fouling resistance and Rf(ref) is the reference fouling resistance at the same mass of MLSS intercepted per unit area of membrane surface. A lower fouling ratio indicates a lower rate of fouling. Figures 3a and 3b illustrate the typical fouling resistances and the fouling ratios at different aeration intensities using the fouling resistances at an aeration intensity of 61.2 m3/h as reference values (Qg(ref0). Significantly, it was shown that the fouling ratios are almost constant throughout each of filtration tests, suggesting that this single value can be used as an indicator to compare the influence of different aeration intensities and permeate fluxes on the rate of fouling. Note that the initial deviation from constant F may be the result of adsorption and pore blocking at the beginning of filtration. In contrast, Fig. 3c shows that, when permeation was conducted above the maximum sustainable flux, the fouling resistance continued to increase with the mass of MLSS intercepted. Consequently, filtration was terminated once the maximum operating resistance is reached. It was observed that the mass of MLSS intercepted at the point of filtration termination increases with aeration intensity and decreases with permeate flux. Thus, the fouling ratio (F) has been proposed as:

)(

)(

it

reft

MM

F = (5)

where Mt(i) is the mass of MLSS intercepted by the membrane and Mt(ref) is the reference mass of MLSS intercepted by the membrane at the same filtration resistance. Again, the calculated fouling ratios were constant throughout each filtration tests except in the initial stage due to different fouling mechanisms (See Fig. 3d).

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Figure 3 - Determination of fouling ratios at sustainable conditions: (a) and (b) (Test I: Pilot B); and unsustainable conditions: (c) and (d) (Test V: Pilot A).

0.0

0.2

0.4

0.6

0.8

1.0

0 150 300 450Volume * Concentration / Area

(g/m2)

Rf (

1012

/m)

J = 39.1 L/m2/h

0

1

2

3

4

0 150 300 450Volume * Concentration / Area

(g/m2)

Rf(i

)/Rf(r

ef)

Qg(ref) = 61.2 m3/hJ = 39.1 L/m2/h

a) b)

— B: 40.8 m3/h — B: 47.6 m3/h — B: 54.4 m3/h — B: 61.2 m3/h

0.0

1.0

2.0

3.0

0 100 200 300 400 500Volume * Concentration / Area

(g/m2)

Rf (

1012

/m)

J = 45.6 L/m2/h

0.0

1.0

2.0

3.0

0.0 1.0 2.0 3.0Mt(ref)/Mt(i)

Rf (

1012

/m)Qg(ref) = 61.2 m3/h

J = 45.6 L/m2/h

c) d)

— A: 40.8 m3/h — A: 51.0 m3/h — A: 61.2 m3/h RESULTS AND DISCUSSION Sustainable Operation Fig. 4a shows the typical effects of permeate flux on the fouling ratios at different aeration intensities. Note that the fouling ratios for Fig. 4a were calculated based on the reference fouling resistances measured at the permeate flux of 39.1 L/m2/h. In general, the fouling ratios increased with permeate flux. This is consistent with many previous observations that the stable fouling resistances increase linearly with permeate flux (Lin, Rao and Shirazi (2005); Ould-Dris et al. (2000)). More significantly, all the fouling ratio curves overlap each other, suggesting that the influence of permeate flux on the fouling ratio is independent of the aeration intensity. An exception is the fouling curve at the aeration intensity of 61.2 m3/h. This may be explained by

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the fact that at this aeration intensity, the permeate flux is near the critical flux. Below the critical flux the influence of permeate flux on the fouling resistance is zero. Similarly, Fig. 4b shows the typical trend of fouling ratios as a function of aeration intensity. The fouling ratios decreased with aeration intensity because more fouling material was sheared away from the membrane surface at higher aeration intensities. Again, all the fouling ratio curves were nearly identical at the different permeate fluxes, except for one obtained at the permeate flux of 39.1 L/m2/h. This is because the permeate flux was approaching the critical flux. It has been reported that as the permeate flux is reduced and approaches the critical flux, surface deposition declines significantly and the contribution of pore deposition and adsorption to the total fouling resistance becomes more substantial (Bouhabila, Aim and Buisson (1998); Chang et al. (2002)). These fouling mechanisms are less sensitive to shear introduced by sparging aeration (Bouhabila, Aim and Buisson (1998)) resulting in a smaller influence on the fouling ratio. Likewise, the fouling materials that contribute to pore blocking and adsorption are smaller and less sensitive to variation in permeate flux than the larger MLSS particles that contribute to cake formation. As a result they continue to be deposited when flux is reduced. Figure 4 - Effects of a) permeate flux and b) aeration intensity on fouling ratio for sustainable conditions (Test I: Pilot C).

0.0

1.0

2.0

3.0

4.0

5.0

35 40 45 50 55

Permeate Flux (L/m2/h)a)

Foul

ing

Rat

io

40.8 m3/h47.6 m3/h54.4 m3/h61.2 m3/h

J(ref) = 39.1 L/m2/h

0.0

0.2

0.4

0.6

0.8

1.0

1.2

40 45 50 55 60 65

Aeration Intensity (m3/h)b)

Foul

ing

Rat

io

39.1 L/m2/h45.6 L/m2/h52.1 L/m2/h

Qg(ref) = 40.8 m3/h

Unsustainable Operation Figures 5a and b show the typical fouling ratio trends measured at unsustainable test conditions as a function of permeate flux and aeration intensity, respectively. Again, almost identical curves were observed at the different aeration intensities or permeate fluxes. This indicates that the effects of permeate flux and aeration intensity are independent one another. However, the fouling ratios did not vary linearly with permeate flux, reflecting the occurrence of cake compaction at unsustainable filtration condition.

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Figure 5 -Influence of a) permeate flux and b) aeration intensity on the fouling ratio at unsustainable conditions (Test IV: Pilot C).

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

30 35 40 45 50

Permeate Flux (L/m2/h)a)

Foul

ing

Rat

io

40.8 m3/h51.0 m3/h61.2 m3/h

J(ref) = 39.1 L/m2/h

0.8

1.0

1.2

1.4

40 45 50 55 60 65


Foul

ing

Rat

io

32.6 L/m2/h

39.1 L/m2/h

45.6 L/m2/h

Qg(ref) = 61.2 m3/h

Effects of Mixed Liquor Characteristics on Fouling Ratios Fig. 6 compares the fouling ratios at sustainable conditions for the different mixed liquors generated in the three pilot plants. As shown, the fouling ratio trends for permeate flux and aeration intensity varied greatly for the different mixed liquors tested. The largest variation with permeate flux was observed for the mixed liquor in Pilot A, followed by Pilots B and C. In contrast, the fouling ratios for the mixed liquor in Pilot B were much more sensitive to aeration intensity than those for Pilots A and C. This is reasonable as different mixed liquor characteristics affect the transport of fouling materials to the membrane and the removal of accumulated solids from the surface. It should be noted that at the lowest permeate fluxes and the highest aeration intensities examined during sustainable operation, the influence on fouling ratios declined (see Figures 6a and b). This is again due to the fact that the filtration was operated near the critical flux and aeration intensities.

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Figure 6 – Comparison of fouling ratios between different mixed liquors at sustainable conditions as a function of: a) permeate flux and b) aeration intensity (Test I).

0.0

5.0

10.0

15.0

20.0

35 40 45 50 55 60Permeate Flux (L/m2/h)

a)

Foul

ing

Rat

io

Pilot APilot BPilot C

J(ref) = 39.1 L/m2/hQg = 61.2 m3/h

0.0

0.5

1.0

1.5

2.0

2.5

40 45 50 55 60 65


Foul

ing

Rat

io


Qg(ref) = 61.2 m3/hJ = 39.1 L/m2/h

Figures. 7a and b plot the fouling ratios for different mixed liquors at unsustainable operating conditions as functions of permeate flux and aeration intensity, respectively. In general, the permeate flux had a non-linear impact on fouling ratio. In contrast, the effects of aeration intensity on fouling ratio were almost linear. In both cases, however, the magnitude of these impacts was specific to each of mixed liquors. Figure 7 – Comparison of fouling ratios between different mixed liquors at unsustainable conditions as a function of: a) permeate flux (Tests II, III and IV) and b) aeration intensity (Test V).

0.0

0.5

1.0

1.5

2.0

30 35 40 45 50Permeate Flux (L/m2/h)

a)

Foul

ing

Rat

io


J(ref) = 39.1 L/m2/hQg = 40.8 m3/h

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

40 45 50 55 60 65Aeration Intensity (m3/h)

b)

Foul

ing

Rat

io


Qg(ref) = 61.2 m3/hJ = 45.6 L/m2/h

Attempts were made to relate to the fouling ratio to different characteristics of mixed liquors (see Table 7). It has been suggested that the mixed liquor with the lowest concentration of colloidal solids would often have the best filterability. Thus, for Test I the filterability of Pilot C mixed liquor was the best while Pilot A mixed liquor was the worst. This agrees with the critical and sustainable flux measurements as well as the fouling resistances measured experimentally. It appears that at the sustainable conditions, the effect of permeate flux on the fouling ratio also declines as the mixed liquor quality improves (see Fig. 6a). However, in Fig. 6b there does not

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appear to be a clear relation between the effects of aeration intensity and mixed liquor filterability. At unsustainable flux conditions, no relationship can be established between the fouling ratio and the characteristics of mixed liquors. Table 7 – Summary of main mixed liquor characteristics.

Pilot MLSS MLVSS Colloidal Solids TTF DSVI4 Critical

Flux** Sustainable

Flux** Test g/L g/L mg/L s ml/L L/m2/h L/m2/h

A 10.1 - 16.0 244 480 28.8 52.6 B 10.9 - 5.0 187 480 33.9 59.4

I

C 11.1 - 4.7 180 450 33.9 64.8 II A 11.9 9.3 17.0 497 475 28* 42* III B 11.5 9.0 15.6 330 475 15* 28* IV C 9.0 7.9 23.9 562 500 23* 30*

A 11.9 9.3 17.0 497 475 28* 42* B 11.5 9.0 15.6 330 475 15* 28*

V

C 9.0 7.9 23.9 562 500 23* 30* A 11.9 9.3 17.0 497 475 28* 42* B 11.5 9.0 15.6 330 475 15* 28*

VI

C 8.3 6.7 20.8 343 500 23* 28* *Critical and sustainable values determined a day earlier or later. **Determined at an aeration intensity of 61.2m3/h and an aeration sequence of 10s-on/10s-off. Effects of Aeration Sequence on Fouling Ratios Cyclic aeration is intermittent air sparging and has been demonstrated to be an effective operating strategy for reducing the net air flow rate while maintaining fouling control. Thus, the effects of the cyclic aeration sequence on the fouling ratio were examined (see Fig. 8). Although different aeration intensities were used, the effects are specific to the mixed liquor: pilots A and B had similar aeration intensities but very different responses while pilot B and C, though very different in aeration intensity, had more similar responses. In terms of the colloidal particle concentration and the critical and sustainable fluxes, Pilot A had better filterability than Pilots B and C. However, additional tests should be completed to verify whether or not the effect of aeration sequence on fouling ratios increases with mixed liquor filterability.

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Figure 8 – Effect of cyclic aeration on fouling ratio at unsustainable conditions (Test VI).

0.0

2.0

4.0

6.0

8.0

0.4 0.5 0.6 0.7 0.8 0.9

Fraction of Permeation Time without Aeration

Foul

ing

Rat

io

Pilot A (40.8 m3/h)

Pilot B (43.4 m3/h)

Pilot C (61.2 m3/h)

Aeration(ref) = 10 sec on/10 sec off

J = 39.1 L/m2/h

Aeration Efficiency Optimization Aeration efficiency can be determined by calculating the volume of permeate produced per unit of air used:

gg

m

g

m

PIJA

QJA

Efficiency ==

where J is permeate flux (L/m2/h), Am is the membrane area (m2) and Qg is the air flow rate (m3/h) calculated as the aeration intensity (Ig) multiplied by the fraction of permeation time with aeration (Pg) (e.g. at an air sparging sequence of 10s-on/10s-off Pa equals 0.5). A contour plot of aeration efficiency calculated at an aeration sequence of 10s-on/10s-off is illustrated in Fig. 9. Although the values are only dependent on membrane area, the operational boundary conditions will be specific to the mixed liquor quality. Fig. 10 was constructed by plotting the steady-state TMP contours as a function of permeate flux and aeration intensity based on the results from Figures 4a and b. Operating conditions that produce a TMP exceeding 51.7 kPa are considered unsustainable while fluxes on the left side of the plot are below the critical flux and, thus, are independent of aeration intensity. Combining Figures 9 and 10, Fig. 11 shows that, for this mixed liquor, the highest aeration efficiency occurs at the lowest aeration intensity.

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Figure 9 - Contour plot of aeration efficiency determined for an aeration frequency of 10s-on/10s-off.

Figure 10 - Contour plot of steady-state TMP based on sustainable conditions specified for Test I: Pilot C.

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Figure 11 - Aeration efficiency determination (Test I: Pilot C).

CONCLUSIONS The effects of key operating parameters on membrane fouling in terms of fouling ratio were examined for different mixed liquors in submerged MBR wastewater pilot plants. The following conclusions were drawn: 1. The trends of fouling resistances differed greatly, depending on the permeate flux and sludge

characteristics of mixed liquor. A stable fouling resistance can result when the filtration is operated at the sustainable permeate flux conditions. At the unsustainable permeate flux conditions, the fouling resistance increased exponentially as the filtration progresses.

2. The fouling ratio defined by Eqs. 3 and 4 for sustainable flux and unsustainable conditions, respectively, can be easily used to quantify the relative impacts of key operating parameters on membrane fouling.

3. The effects of aeration intensity on fouling ratio is independent of the permeate flux and vice versa.

4. The variation of fouling ratio at different operating conditions also strongly depends on the characteristics of mixed liquor. Furthermore, it appears that more than one characteristic is needed in order to define their relationship.

5. The aeration efficiency is based MBR design but the peak efficiency at sustainable operating conditions will be specific to the mixed liquor quality

ACKNOWLEDGEMENTS The authors acknowledge the financial support from Earth and Environmental Technologies-Ontario Centres of Excellence, Zenon Environmental Inc. and the Natural Sciences and Engineering Research Council of Canada (NSERC). Many staff from Zenon Environmental Inc.

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and City of Guelph Wastewater Treatment Plant were involved in the wastewater sampling and pilot plant operation. REFERENCES

APHA, AWWA, WEF (1995) Standard methods for the examination of water and wastewater. American Public Health Association, Washington, D. C., various paging.

Belfort, G., Davis, R. H., Zydney, A. L. (1994) Behavior of suspensions and macromolecular solutions in crossflow microfiltration. J. Membr. Sci. 96(1-2), 1-58.

Bouhabila, E. H., Aim, R. B., Buisson, H. (1998) Microfiltration of activated sludge using submerged membrane with air bubbling (application to wastewater treatment). Desalination 118(1-3), 315-322.

Chang, I. and Judd, S. J. (2002) Air sparging of a submerged MBR for municipal wastewater treatment. Process Biochemistry 37(8), 915-920.

Chang, I., Clech, P. L., Jefferson, B., Judd, S. (2002) Membrane fouling in membrane bioreactors for wastewater treatment. J. Environ. Eng. 128(11), 1018-1029.

Chua, H. C., Arnot, T. C., Howell, J. A. (2002) Controlling fouling in membrane bioreactors operated with a variable throughput. Desalination 149(1-3), 225-229.

Cui, Z. and Taha, T. (2003) Enhancement of ultrafiltration using gas sparging: A comparison of different membrane modules. Journal of Chemical Technology and Biotechnology 78(2-3), 249-253.

Engelhardt, N., Firk, W., Warnken, W. (1998) Integration of membrane filtration into the activated sludge process in municipal wastewater treatment. Water Science and Technology 38(4-5 pt 4), 429-436.

Fan, F. (2005) Fouling Mechanisms and Control Strategies for Improving Membrane Bioreactor Processes. Thesis 1(1), 1-183.

Guinzbourg, B. F. (2003) Identification of Pore Blocking and Cake Filtration Mechanisms in a Submerged MBR Pilot Plant. , 1-67.

Le-Clech, P., Jefferson, B., Judd, S. J. (2003) Impact of aeration, solids concentration and membrane characteristics on the hydraulic performance of a membrane bioreactor. J. Membr. Sci. 218(1-2), 117-129.

Lin, C., Rao, P., Shirazi, S. (2005) Effect of operating parameters on permeate flux decline caused by cake formation - A model study. Desalination 171(1), 95-105.

Martin, W. (2005) Wastewater Sludge Characteristics and Implications on Membrane Fouling. , 1-146.

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Ognier, S., Wisniewski, C., Grasmick, A. (2002) Characterisation and modelling of fouling in membrane bioreactors. Desalination 146(1-3), 141-147.

Ould-Dris, A., Jaffrin, M. Y., Si-Hassen, D., Neggaz, Y. (2000) Effect of cake thickness and particle polydispersity on prediction of permeate flux in microfiltration of particulate suspensions by a hydrodynamic diffusion model. J. Membr. Sci. 164(1-2), 211-227.

Rosenberger, S., Kruger, U., Witzig, R., Manz, W., Szewzyk, U., Kraume, M. (2002) Performance of a bioreactor with submerged membranes for aerobic treatment of municipal waste water. Water Res. 36(2), 413-420.

Shimizu, Y., Uryu, K., Okuno, Y., Watanabe, A. (1996) Cross-flow microfiltration of activated sludge using submerged membrane with air bubbling. Journal of Fermentation and Bioengineering 81(1), 55-60.

Wiesner, M. R., Tarabara, V., Cortalezzi, M. (2005) Processes of particle deposition in membrane operation and fabrication. Water Science and Technology 51(6-7), 345-348.

Wisniewski, C. and Grasmick, A. (1998) Floc size distribution in a membrane bioreactor and consequences for membrane fouling. Colloids Surf. Physicochem. Eng. Aspects 138(2-3), 403-411.

Yamamoto, K., Hiasa, M., Mahmood, T., Matsuo, T. (1989) Direct solid-liquid separation using hollow fiber membrane in an activated sludge aeration tank. Water Science and Technology 21(4-5 pt 1), 43-54.

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