Microstructured spacers for submerged membrane filtration systems

Journal of Membrane Science 446 (2013) 189–200

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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci

Microstructured spacers for submerged membrane filtration systems Clemens Fritzmann, Matthias Hausmann, Martin Wiese, Matthias Wessling n, Thomas Melin Aachener Verfahrenstechnik—Chemical Process Engineering, RWTH Aachen University, Turmstraße 46, D-52064 Aachen, Germany

art ic l e i nf o

a b s t r a c t

Article history: Received 15 March 2013 Received in revised form 18 June 2013 Accepted 23 June 2013 Available online 2 July 2013

We introduce a new type of membrane spacer design, based on the geometry of static mixers, for application in submerged membrane filtration. In particular we demonstrate the potential of the spacers to reduce air sparging and energy consumption. New operational modes are proposed and discussed critically. Critical flux measurements are performed with and without spacer at various process parameters such as spacer/channel height, air sparging rate and bubble size. The most relevant energy reducing parameters of submerged systems are determined with the Design of Experiments (DoE) methodology. Using the new spacers, the critical flux can be increased by 100%. Furthermore, fouling monitored by the trans-membrane pressure change can be significantly reduced by a factor of 7.5 when combined with elevated cross-flow velocities. & 2013 Elsevier B.V. All rights reserved.

Keywords: Submerged membrane system Spacer Fouling control Hydrodynamics Energy consumption

1. Introduction Submerged membrane systems are nowadays established technologies to efficiently treat domestic and industrial wastewaters. As common for all membrane technologies, membrane fouling occurring during filtration constitutes a major issue limiting the membrane performance. To achieve sustainable flux, different operation strategies frequently linked to high operational costs have to be undertaken. The major energy input in the case of submerged membrane bioreactors (MBR) is due to aeration. In many membrane applications, spacers are successfully implemented into membrane modules to reduce fouling, increase mass transfer and thereby improve the filtration performance. It is normally assumed that commercial net type spacers suffer from fouling when a certain turbidity is surpassed. However we suggest that well engineered spacer geometries might even be able to cope with turbid solutions. The application of the microstructured spacer has proven to be efficient in terms of mass transfer enhancement [1]. If designed without dead volumes in the flow channel fewer “traps” for particulate fouling and fouling agglomerates may exist and as a consequence the risk of channel blockage is reduced. New microstructured membrane spacer types hence appear to be viable even in systems with high solid loads. For fouling mitigation in submerged membrane systems with high fouling potential often air scouring of the membranes is performed [2–4]. A major

n

Corresponding author. Tel.: +49 241 80 954 88. E-mail address: [email protected] (M. Wessling).

0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.06.033

bottleneck of the submerged systems is the high energy consumption associated with air scouring, typically responsible for 50–80% of the overall operating costs [5]. This paper evaluates the application of structured spacers to submerged membrane systems and the interaction of spacer and air scouring with regard to the process performance aiming at the reduction of air sparging, energy consumption and lower overall process costs. The influence of main design and operational parameters such as channel/spacer height, air sparging rate and bubble size on the filtration performance with and without the use of structured spacers is evaluated in single parameter experiments using a model fouling suspension. In addition, the interactions between the main operational parameters are studied by means of the Design of Experiments (DoE) methodology and the most promising measures to reduce energy consumption are quantified. 2. Background 2.1. Air scouring in submerged flat-sheet membrane systems State-of-the-art submerged flat sheet systems comprise several vertically aligned flat sheet membrane panels, as schematically depicted in Fig. 1. The typical distance between panels is 6–10 mm. At each panel, permeate is withdrawn from the top and collected in the permeate manifold. The system of parallel panels is implemented in an outer partially open housing and the whole set-up is immersed in the feed mixture, e.g. in the activated sludge tank of a MBR plant.

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Fig. 1. Typical set-up of a flat sheet submerged membrane system after Melin and Rautenbach [6].

Air is introduced from below for sludge aeration and more importantly for scouring of the membrane via a diffuser pipework to ensure good distribution of the air bubbles. The rising bubbles induce an upward directed cross flow between the panels, while outside the outer housing of the submerged system suspension flows downwards in the so-called down-comers. The bubbles that rise between the panels induce secondary flows and wakes that result in the destabilization of the concentration boundary layer. In addition the gas bubbles partly displace the concentration boundary layer since they penetrate into the boundary layer [2]. For larger bubbles with bubble size 460% of the channel width, a falling film downwards at the membrane surface can be observed resulting in a region of increased shear forces [7]. A change in local pressure is observed when a bubble passes along the membrane, which contributes to the mass transfer enhancement and influences the cake structure [2], which was shown by Laborie et al. [8] and Cabassud et al. [9]. The increase in superficial cross flow velocity as a result of the rising bubbles is of minor importance for fouling reduction at typical aeration rates found in submerged membrane systems [2]. Ndinisa et al. [10] as well as Sur and Cui [11] analyzed the influence of the aeration related operational parameters on process performance in submerged flat sheet membrane systems. They describe that with increasing air flow rate the effect of aeration increases, i.e. higher fluxes and/or lower fouling rates are observed until a plateau is reached at which further increase in aeration has no additional effect on the filtration performance. Larger air bubbles have been reported to be more effective for mass transfer enhancement, but again a threshold value has been determined at which a further increase in size has no additional effect on mass transfer rates [12]. There exists a strong relationship between channel width and cleaning efficiency of the bubbles [10].

In practical operation, especially in full-scale plants and in spite of continuous air scouring, operational problems or shortcomings have repeatedly been reported that result in a decrease in flux over time, either due to irreversible fouling [13,14] or clogging of the flow channels [15]. Judd [16] attributes the clogging of the channels to inhomogeneities of air distribution and states that better distribution of air will be advantageous with regard to both fouling and clogging control. Cui et al. [2] in the same context ask for module design that promotes homogeneous flow distribution and maximizes the bubble efficiency. Uneven bubble distribution encountered in submerged membrane systems can stem from an insufficient or uneven air distribution of feed flow upwards in the membrane channels due to interactions with the down-comer. Further, within a single channel, bubbles tend to move to the center of the channel [10], resulting in imbalanced mechanical cleaning of the membrane and regions with low shear and stagnant flow, which are prone to clogging and fouling [17,18]. Willems et al. [19] investigated the distribution of bubbles in spacer filled channels of the net type structure and found that more even distributions are feasible which strongly depend on the liquid velocity. As a step towards better fouling control via distribution of air bubbles Ndinisa et al. [10] investigated the influence of straight vertical baffles between flat sheet membranes. They found that slower fouling rates and a higher critical flux were observed with baffles implemented in the feed channel. They contributed the positive effect of the baffles to a better distribution as well as an advantageous form of the bubbles, since real slug flow [2] was observed. Further, the baffles prevented the bubbles from moving towards the center of the panel. Here, we propose the application of new structured membrane spacers to submerged systems in combination with air sparging. Against intuition – spacers will cause clogging in suspension flow – we hypothesize that structured spacer may well be suited for application in submerged systems and filtration of high solid loads. The absence of continuous fouling regions, fewer regions of stagnant flow as well as the absence of filaments perpendicular to the flow are expected to result in low risk of channel clogging. The design of the spacer seems further suited to be used in the larger channels typically encountered in submerged systems (6–10 mm), since an increase in size of the spacer does not increase dead volume in the channel to the same extent as larger net spacers. To the best of the authors knowledge, the combination of spacer structures with flat sheet membranes in air sparged membrane systems has not been investigated before.

3. Materials and methods 3.1. Spacer The geometry of the new spacer comprises two layers of double helix form filaments exclusively aligned in the mean direction of flow in the feed channel. The double helix form filament being the core element of the helically microstructured spacers (MSS) is shown in Fig. 2a. It is characterized by the three geometric properties – L, which is the length of one spiral coil (3601), the height of the filament D and filament width d. Several of the filaments aligned in parallel build one layer of the spacer geometry as indicated in Fig. 2b. Filaments within one layer have similar twist orientation (either left- (L) or right(R) twisted) opposite to the filaments in the second layer. This leads to an overall geometry where the two layers partly fit into each other leading to a mechanically stable layer structure as shown in Fig. 2c. In addition to the geometry parameters for the single twisted filaments L, D and d, the overall spacer geometry is characterized by means of the spacer height h, as well as the distance b between two adjacent filaments in one single layer (Fig. 3). Within this

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Fig. 2. (a) Double-helix form twisted filament, (b) single layer of twisted elements, and (c) two-layer spacer.

Fig. 3. Filament (left) and spacer (right) cross section; the shaded area displays one repeating spacer element.

work, for all spacer geometries considered, filaments in one layer are in direct contact with the neighboring filaments. b is therefore equal to the filament height D. Further, the two layers touch, which defines geometry parameter h, for a fixed filament height and thickness. The geometry therefore displays only two geometrical degrees of freedom if dimensionless geometrical parameters are used, namely L/D and d/D. For hydraulic characterization of the spacer either the hydraulic diameter dh or the channel height h is used. The helically structured spacers display a constant free cross sectional area along the channel axis and are calculated according to dh ¼

4A U

ð1Þ

with A as the free cross sectional area and U being the wetted perimeter. For one repeating element of width b ¼D this leads to dh ¼

4Dhε 2πd þ 4ðDdÞ þ 2D

ð2Þ

The channel porosity ε is defined as ε ¼ 1

As Ach

ð3Þ

with the spacer cross section As and the cross section of the empty channel Ach. Using these geometric parameters ε can be expressed as

Prior to the evaluation of the spacer induced hydrodynamical improvements of submerged MBR-applications, first, the motivation for development of the MSS and its particular design is outlined. The spacer design aims at four distinct improvements:

   

intensified mass transport; low feed side pressure losses; reduction of fouling propensity and fouling extent; and minimum risk of channel blockage.

3.1.1. Intensified mass transport Flow within the helix form filament structure is expected to result in swirling flows with the fluid repeatedly forced towards the membrane. Both flow features, i.e. forced flow to the membrane as well as swirling flow, have been described to enhance mass transport [1,20]. As opposed to the net spacer, swirling flow is not a result of more or less coincidental secondary flow characteristics, but is a direct result of the spacer geometry and can be controlled by adjusting the geometry parameters of the MSS. The two-layer design was chosen to avoid the rejected components that are transported from the upper to the lower membrane without mixing with the bulk fluid. When the fluids in both spacer layers meet in the middle of the channel, one hopes to facilitate rapid mixing.

2

ε ¼ 1

0:5πd þ 2ðDdÞd Dh

which in combination with Eq. (2) gives dh.

ð4Þ

3.1.2. Reduction of feed channel pressure loss In the helically structured spacers a constant free cross sectional area exists in the flow channel and discontinuities in the

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flow field are avoided. The phenomena of such repeating discontinuities, illustrated in Fig. 4, can be recognized as cork-screw or zigzag flow. It is associated with high form drag [20] and kinetic losses [21]. The absence of filaments perpendicular or diagonal to the mean direction of flow as in the case of the helically structured spacers promises to reduce form drag and kinetic losses, especially if a sufficiently thin spacer filament d is applied. 3.1.3. Fouling influence With regard to fouling, the net type spacer is characterized by a line contact between each filament and the membrane surface. This line contact results in larger areas of low shear forces [23] on the membrane in the vicinity of the spacer, which has been shown to result in initial deposition of foulants [24]. Unlike the net spacer, the MSS displays point like contact between spacer and membrane, which should result in smaller regions of low membrane shear stress and reduced dead volumes, further reducing the risk of fouling in the feed channel. The spacer geometry uses the structured two-layer design described in Section 3.1, defined in Figs. 2 and 3, but the geometry is adjusted for the larger channel width. The geometrical dimensions of the spacers are given in Table 1. The spacers used are geometrically similar, have a porosity ε of 0.80 and the spacer height is varied from 4 to 10 mm. Only one filament height to helix length ratio (L/D ¼8) is considered. The larger L/D ratio was chosen since it is associated with a relatively low pressure drop and the high pitch of the spacer filaments is expected to result in higher bubble rise velocities, which will increase shear rates. The spacer covers the entire active membrane area. The spacer structure was produced via the Objet rapid prototyping process [25]. 3.2. Experimental setup All experiments were performed with a semi-automated submerged MBR system equipped with a tailor-made membrane test cell. The system set-up is shown schematically in Fig. 5. Feed is supplied from the 180 l vessel B1 to the test cell with one single membrane panel implemented and flow channels on both sides of the panel. The front of the membrane test cell, shown in Fig. 5 (right), is made from PMMA to allow visual observation of the two-phase feed flow. The feed channel height can be adjusted from 4 to 6 and 10 mm to be able to implement different spacer heights. Air is distributed at a distance of 60 mm below the

Fig. 4. Zigzag flow in net spacer geometries adopted from da Costa et al. [22].

Table 1 Geometrical dimension of the spacers applied in the filtration experiments. Spacer height h [mm]

Filament height D [mm]

d/D

L/D

4 6 10

2.75 4.1 7

0.15 0.15 0.15

8 8 8

membrane panel from 14 nozzles along each flow channel. The nozzle diameter can be changed among 0.5, 1.0 and 2.0 mm. The permeation rate is adjusted by means of a peristaltic pump connected to the permeate outlet on top of the panel. Feed and permeate pressure are consistently monitored for measurement of the trans-membrane pressure (TMP); metering precision for the TMP is 10 mbar. The system was operated in full recirculation mode to ensure constant concentrations in all experiments. Instead of the typical loop reactor design, where cross flow is induced by the rising bubbles, a forced cross flow is used in the experiments. This is a more general approach and results are thus also representative for external side stream MBR systems. Current side stream MBRs, which are mostly tubular systems, are operated without air, but at high cross flow velocities of 1–6 m/s to prevent fouling. 3.3. Membrane The membrane used in the experiments was a non-commercially available membrane from AGFA Gevaert. The so called integrated permeate channel (IPC) membrane is a fully back-flushable flat sheet membrane due to an inner textile that prevents ballooning of the membrane during back-flush [26]. The active size of the membrane on each side of the panel was 169 mm width  260 mm height. The width of the flow channels is similar to the width of the active membrane. No details on the membrane used in this work may be given due to contractual constraints. 3.4. Fouling suspensions To ensure stable and reproducible conditions, the first set of experiments was performed with a silica–water model suspension. The silica suspension has been prepared from silica-sol Bindzil 9950 by Eka Akzo Nobel and deionised water (o1 mS/cm) with a total silica concentration of 0.6 vol% if not otherwise stated. The average particle size determined by a Coulter LS is 90 nm. The silica suspension has been chosen since it displays shear thinning behavior similar to mixed liquor suspended solids (MLSS), as shown in Fig. 6. 3.5. Experimental methodology Three sets of experiments are performed with increasing experimental complexity: (a) critical flux measurements, (b) single parameter experiments to investigate the influence of each of the operational parameters on cleaning efficiency and (c) full factorial design to assess the interaction effect of the operational parameters. 3.5.1. Specification of process parameters Aeration rate in this paper is specified based on the width of the membrane panel and is denoted as specific aeration demand based on panel width (SADW). SADW is a well suited characteristic number that can be easily re-calculated to other common definitions of aeration rate such as membrane specific aeration demand SADm or plant foot print specific aeration. For definition of benchmark results of the filtration experiments with model suspension standard conditions have been defined as listed in Table 2, which are applied throughout the experiments of this paper if not otherwise stated. An aeration rate SADW of 0.75 N m3/(m h) is a typical value of practical MBR operation [16] and since it is aimed at a reduction of air scouring, the SADW of 0.75 N m3/(m h) marks the upper bound for aeration in this study. Calculation of the cross flow velocity (CFV) is based on the empty channel throughout this work without taking into account

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Fig. 5. Experimental set-up.

3.5.2. Critical flux measurements Critical flux experiments using model suspension served the purpose to identify and choose suitable filtration conditions in the single parameter experiments. In addition, the critical flux experiments give a first impression on the general impact of the application of the structured spacers. The critical flux experiments are performed using the improved flux steps method developed by van der Marel et al. [27] with alternating filtration and relaxation times of 15 and 2 min respectively. Flux is increased by 2 l/m2 h between filtration intervals. The critical flux in this study is defined as the flux at which the TMP-gradient dTMP/dt exceeds 100 Pa/min [27]. The method by van der Marel et al. enables to detect irreversible fouling. In preliminary experiments it was shown that for the model suspension used in the experiment no irreversible fouling occurs. Fig. 6. Shear dependent viscosity of the model suspension. MLSS represents digestates of different solid content. Data points are measurements for silica suspension.

Table 2 Definition of standard testing conditions. Operating parameter Aeration rate Flux Liquid cross flow velocity Nozzle size Channel height

0.75Nm3/(m h) 35.0 l/m2 h 0.05 m/s 1.0 mm 6.0 mm

the volumetric gas flow rate. Using a rather low cross flow velocity of 0.05 m/s as standard conditions enables to differentiate between the effects of rising bubbles and additional background liquid flow. For the parameters nozzle size and channel height the medium values are used as standard conditions. Finally, the defined standard flux of 35 [l/(m² h)] followed from the critical flux experiments. Thirty-five LMH is above critical flux under standard conditions as will be seen in the results section on the critical flux experiments and thus the TMP increase can be monitored for evaluation of the cleaning efficiency in the single parameter experiments.

3.5.3. Single parameter experiments In the single parameter experiments, the general influence of spacer implementation in the feed channel on the fouling rate is evaluated varying the spacer height. Further, the effect of aeration rate, cross flow velocity, nozzle size and back-flushing is analyzed. The results are then compared with the filtration performances without application of spacers. Fouling rates are assessed by means of the TMP increase over time. Backflushing frequency and intensity have been arbitrarily chosen, but are typical operational values of MBR praxis [16]. Judd proposes to use a back-flush strength of 1–3 times the filtration flux. In this work back-flush intensity has not been varied and was set equal to the filtration flux. The back-flushing interval consists of 270 s filtration time and 30 s back-flushing. For better comparison of the results from filtration experiments with and without backflushing identical net fluxes are applied as suggested by Wu et al. [28] if not otherwise stated. This gives a filtration and backflush flux of 43.75 LMH compared to the 35 LMH flux in the experiments without backflushing. 3.5.4. Design of experiments Design of experiments or factorial design (DOE) is a standard method that is widely used in many fields of (membrane) process design [29,30]. For details on the DOE methodology readers are referred to standard text books such as Montgomery [31]. The DOE methodology allows the comparison of the influence of individual

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operating parameters and in addition enables to identify and quantify interactions of these factors. In the factorial Design Experiments of this paper “2n” experiments are performed, i.e. all factors investigated are evaluated at an upper (+) and a lower (  ) bound. Effects and interdependencies of the chosen parameters of Table 3 are evaluated using the Yates scheme [32]. Four factors are analyzed regarding their effect on TMP rise; these are cross flow velocity, aeration rate, use of intermittent aeration, and the implementation of a spacer geometry in the feed channel. Intermittent aeration has been chosen as an additional parameter to the ones investigated in the single parameter experiments since it has been proposed as a measure to reduce energy consumption in submerged systems [33] and thus competes with the use of spacers to enhance bubble efficiency. The intermittent aeration interval of 10 s aeration and 20 s pausing has been arbitrarily chosen but seems suitable according to the results by Guibert et al. [33]. The intermittent aeration experiments have been performed at similar net air flux as the continuous aeration experiments. Feasible upper and lower bounds for cross flow velocity and aeration rate have been determined in the single parameter experiments. The factors of the DOE and their upper and lower bounds are given in Table 3. The evaluation of the factorial design experiments is based on the TMP rise. However, in some of the experiments a steady TMP was observed after an initial rise of the filtration resistance as will be seen in the results section. Therefore, TMP rise is defined in the DOE evaluation criteria either by the average gradient until TMP reached 390 mbar (pressure based termination criteria) according to Δp 390 mbar ¼ Δt tðp ¼ 390 mbarÞ

ð5Þ

or using the pressure level reached after 2400 s filtration time (time based termination criteria): Δp pðt ¼ 2400 sÞ ¼ Δt 2400 s

ð6Þ

The arbitrary choice of time- and pressure-based termination criteria limits quantitative evaluation and comparison of the effects on fouling of the individual operating factors. However, the defined TMP rise is well suited for quantitative assessment since the order of significance of the individual effects will not change with the choice of the termination criteria.

4. Results 4.1. Critical flux experiments with model suspension The critical flux (CF) experiments with and without application of structured spacers are presented in Fig. 7. The CF experiments have been performed under standard conditions defined by Table 2, i.e. in the 6 mm channel. The critical flux was 18 LMH without application of the spacer, while it increased to 34–36 LMH Table 3 DOE parameter bounds and definitions. Parameter Spacer (h ¼ 6 mm) Cross flow velocity [m/s] Aeration rate [N m3/m h] Intermittent aerationa

A B C D a

Lower bound (  )

Upper bound (+)

Not implemented 0.015 0.25 Off

Implemented 0.1 0.75 On

Aeration interval: 10 s aeration, 20 s pausing.

with the spacer implemented in the feed channel. The spacer thus caused an increase of the critical flux by nearly a factor of two under the standard conditions. As a result of the critical flux experiments the standard flux used in the single parameter experiments has been set to 35 LMH, which is approximately the critical flux under the conditions chosen in the CF experiments, i.e. with strong aeration.

4.1.1. Single parameter experiments 4.1.1.1. Influence of channel and spacer height. The effect of the channel and spacer height on TMP increase during filtration is illustrated in Fig. 8. Channel and spacer heights of 4, 6 and 10 mm have been implemented and the above mentioned standard conditions have been applied. Without application of spacers an increase in bubble efficiency can be observed when channel height is decreased from 10 to 6 mm. While TMP reaches 0.4 bar after 484 s with the 10 mm gap width, it takes 578 s for the 6 mm channel. The lower average TMP gradient confirms the results of Ndinisa et al. [10] who used channel widths of 7 and 14 mm and found lower TMP increase for the smaller gap width. However, in the experiments of the study at hand, no additional benefit in cleaning efficiency can be observed with a further decrease in channel height from 6 to 4 mm and for both channel widths TMP reached 0.4 bar after 578 s. The increase in bubble efficiency with decreasing gap width from 10 to 6 mm can be explained by the bubble size and form observed in the feed channel. Using the standard nozzle size of 1 mm a large proportion of the bubbles is significantly smaller than 10 mm as can be seen in Fig. 11, which shows a photo of the bubbles rising in the feed channel. Due to their limited size only few of the bubbles become slugs, such slugs being described as the most efficient form in terms of cleaning effectiveness [10]. With smaller gap width a larger proportion of the bubbles is about the same size as the channel, which results in a thin falling film between the bubble and the membrane with elevated shear stress and high mass transport [34]. With spacers implemented in the feed channel an increase in cleaning efficiency is observed with a change in spacer height from 10 to 6 mm as well. In fact, for the 6 mm spacer no TMP increase can be seen after the initial filtration phase, thus filtration flux is below the critical flux validating the results of the critical flux experiments. As opposed to the case without spacer, a smaller gap of 4 mm results in a lower filtration performance compared to the 6 mm spacer. It is expected that the lower cleaning efficiency follows from the smaller channels in the spacer structure. Bubbles cannot rise unhindered, which reduces rise velocity and efficiency of the bubbles. The actual velocity of the bubbles could however not be measured due to the spacer partially covering the view onto the rising bubbles. It must be concluded that spacer choice needs careful optimization and adjustment of bubble size. Interesting to note when comparing the filtration with and without spacer is that even with the least effective spacer height of 10 mm, the time until a TMP of 0.4 bar has been reached was substantially extended, even compared to the best performing 4 and 6 mm channel heights in the filtration runs without spacer. The effect of the spacer can be attributed to the increased shear induced by the altered hydrodynamics as well as its influence on form and path line of the bubbles. As can be seen from Fig. 9 (left side), which shows an area with smaller bubbles rising in the spacer structure, even smaller bubbles take on slugs form. Further, the smaller bubbles are repeatedly transported to the membrane surface increasing the bubble efficiency. Two of the formed slugs are indicated with arrows.

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Fig. 7. Critical flux measurements with alternating filtration and relaxation times of 15 and 2 min.

Fig. 8. Influence of channel height on filtration.

Fig. 9. Left: bubble flow inside the spacer structure 10 mm spacer; right: bubble flow in the 6 mm spacer (1 mm nozzle size).

In addition to the slug form bubbles, also larger bubbles can be observed in the feed channel. In the right-hand picture of Fig. 9, showing the channel with the 6 mm spacer implemented, lighter

areas can be identified, which are larger bubbles that rise in the spacer structure and along the membrane surface. Thus a mixture of bubble forms is observed, smaller slugs and undefined larger

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bubbles. From visual observation it can be concluded that with decreasing spacer height less slugs form in the spacer structure but to fully assess the influence of the spacer structure and height on bubble size and form further investigation is needed. Visual observation further hints that bubbles are more equally distributed over the membrane, which might be an additional reason for lower TMP increase over time. 4.1.1.2. Influence of spacers on aeration efficiency. Fig. 10 shows the influence of aeration rate on fouling under otherwise standard conditions with and without spacer implemented. Aeration is varied between 0.25 and 0.75 N m3/(m h). In the filtration experiments without spacer, the fouling rate slows down with an increase in gas flow rate as observed by Ndinisa et al. [10]. However, at elevated flow rates only a minor effect on fouling reduction was observed. While the time until a TMP of 0.4 bar is reached is extended by 12% when aeration is increased from 0.25 to 0.5 Nm3/(m h), a further increase to 0.75 N m3/(m h) elongates filtration time only by an additional 4%. With application of the spacer the standard flux of 35 LMH is below critical flux at the highest gas flow rate of SADW ¼0.75 N m3/ (m h) as has been shown in the CF experiments and no TMP rise is

seen. At a SADW of 0.5 N m3/(m h) a low but noticeable TMP increase is observed, while at the lowest gas flow rate, a stronger increase in TMP is seen. At SAD¼ 0.25 N m3/(m h) fouling formation, i.e. the time until a TMP of 0.4 bar is reached, slows down by a factor of 3.5, compared to the case with maximum aeration and no spacer, further illustrating the potential of the spacer to reduce aeration rate and energy consumption without limiting process performance. 4.1.1.3. Influence of nozzle size. The effect of nozzle size on fouling behavior was investigated with nozzle sizes of 0.5, 1 and 2 mm under standard conditions. Pictures of the spacer free feed channel for the different nozzle sizes are shown in Fig. 11, while results from filtration runs with and without spacers are depicted in Fig. 12. As can be seen in the experiments without spacer implemented, increasing the nozzle diameters generates less but bigger bubbles, while at the same time fouling is reduced. The small difference in filtration performance for the 1.0 and 2.0 mm nozzles indicates that bubble size is already close to the optimal point at which no further increase in bubble efficiency will be seen as predicted by Cheng and Li [35]. As opposed to the channels without spacer, with spacers, the nozzle size has a significant

Fig. 10. Influence of aeration rate on TMP increase.

Fig. 11. Bubble pattern in non-spacer filled channel with 0.5 mm (left), 1 mm (center) and 2 mm nozzles (right).

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Fig. 12. Influence of nozzle size on TMP increase.

influence. The increase in nozzle size from 0.5 to 1.0 to 2.0 mm results in an increase in filtration performance, with flux below critical flux for the 1 mm nozzle, i.e. no further increase in TMP is observed after the first initial TMP rise. Constant TMP can be observed for an extended time of up to 2500 s as well. Since for the 1.0 and 2.0 mm nozzle flux was below critical flux no difference in filtration performance was detected. 4.1.1.4. Influence of liquid cross flow velocity. Although in submerged system liquid background flow and aeration are coupled, in this work both influences are evaluated independently to be able to extrapolate results to systems with forced flow. Fig. 13 quantifies the fouling affinity observed during the filtration tests under standard testing conditions as defined in Table 2 with no spacer implemented in the feed channel. The influence of liquid cross flow velocities was evaluated between 0.015 m/s and 0.3 m/s. Up to a velocity of 0.1 m/s an increase in cross flow velocity results in a lower TMP gradient. The time until the TMP reaches 0.4 bar is extended by 52% if CFV is raised from 0.05 to 0.1 m/s. Hence, the background liquid flow has a substantial impact on fouling behavior. Interesting to note in Fig. 13 is the lower performance at 0.3 m/s liquid cross flow, which can be explained by visual observations of the bubble movement in the channel. Up to CFVs of 0.1 m/s the bubbles oscillate horizontally, while at 0.3 m/s their affinity to oscillate decreased noticeably and bubbles raised up on steady vertical lines. The oscillation and the associated random movement of the bubbles result in more effective scouring on the membrane surface. Higher liquid cross-flow velocities cause higher wall shear stress but this is counterproductive if bubble-oscillation is inhibited. In the filtration tests with spacers a TMP of 0.4 bar was not reached after 2500 s under standard conditions, and with the exception of the lowest cross flow velocity of 0.015 m/s a steady TMP could be observed after an initial rise of the filtration resistance. With increasing CFV, the TMP plateau can be found at a lower value indicating that less areas of the membrane are covered with foulants and that the increase in velocity results in an additional cleaning effect. Movements of the bubbles and bubble path lines strongly depend on the background liquid flow rate. Typical bubble pathways are shown in Fig. 15. While at the lowest cross flow velocity (0.015 cm/s) bubbles tend to move diagonal to the mean direction of flow and inside the diagonal channels defined by the spacer geometry, at elevated cross flow (0.3 m/s) bubbles circle around the spacer filaments in vertical direction and are repeatedly

transported into the inner spacer structure. At 0.05–0.1 m/s a mixture of these two flow regimes can be observed. To further evaluate the influence of the background liquid flow, a second test series with SADW ¼ 0.1 N m3/(m h) has been performed with spacer implemented for cross flow velocities of 0.01– 0.25 m/s; the results are presented in Fig. 14. The TMP gradient at 0.01, 0.05 and 0.1 m/s is on average 13–19% higher than in the previous filtration runs without spacer and high aeration. As opposed to the filtration without spacer a further increase in velocity gives additional benefit regarding cleaning efficiency. At elevated cross flow of 0.25 m/s, the TMP reaches a steady level, which was not found in the filtration runs without spacer even with 750% higher gas flow. 4.1.1.5. Use of back-flushing. Finally, the combination of spacer and backflushing is examined. Fig. 16 illustrates the results from filtration runs with and without spacer implemented in the feed channel. As stated above similar net flux is applied, i.e. filtration with backflushing is performed with an actual filtration flux of 43 LMH. As a result of increased flux rate, rapid fouling is seen at the start of the experiments. In the case of filtration without spacer, backflushing is not capable to ensure stable operation and after three backflushing intervals, TMP already reaches 0.7 bar. With spacers implemented in the feed channel, the increase in filtration resistance is less severe during the filtration interval and as a consequence TMP oscillates around a stable value. It must be noted that we did not aim to optimize the backflushing intensity and interval length in this study. However it should be well possible by choice of a suitable interval length and backflushing intensity that a stable TMP can also be reached without spacer. It becomes obvious from the filtration tests performed, that there is an added value in combining spacer application with backflushing. 4.1.2. Design of experiments The additional benefit from factorial design experiments is that DOE allows direct comparison of the effects of the operating and design parameters and it enables to identify interdependencies between variables. The resulting effects and interdependencies of the factors evaluated in the factorial design experiments are listed in Table 4. “+” and “ ” denote if the parameters investigated were fixed at their upper or lower limit. Trial ( ) defines the hypothetical operating and design point when all factors are fixed at a medium setting. TMP rise at this operating point is 6.033 mbar/s. A negative sign in the sixth column of Table 4 indicates slow-down of the TMP rise

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Fig. 13. Influence of liquid velocity on TMP rise.

Fig. 14. Influence of liquid cross flow velocity at minimum aeration SADW ¼0.1 N m3/(m h).

Fig. 15. Influence of background liquid cross flow velocity on bubble pathline. Dashed lines indicate non-visible bubble movement in the inner structure of the spacer.

Fig. 16. Efficiency of backflushing with and without application of spacers.

compared to the medium operating point when a factor is fixed at its upper bound, while a positive effect increases fouling tendency. The combinatorial effect of factors, listed in the last column of Table 4, is calculated by addition of the single effects and of the effect interdependencies. It is given in percentual reduction of the TMP rise compared to case ( ). Factorial design can be further used to evaluate whether an effect is significant, i.e. if the measured effect goes beyond the experimental deviation. Therefore, a test of significance using a short-cut method was performed. All effects and interdependencies have been determined to be significant. From the results of the factorial design experiments it can be concluded that the most effective way to reduce fouling in the evaluated range of operating and design criteria is the use of spacers, followed by increasing the cross flow velocity, followed by increasing the aeration rate, followed by the use of intermittent aeration. The application of spacers reduces TMP rise from the above stated average value of 6.033 mbar/s by 3.982 mbar/s or 66% when all other parameters are kept at their defined medium level. The combinatorial effect of the second and third most effective factors, higher cross flow and higher aeration (bc) set to their upper bounds, is significantly less pronounced than the effect of the application of spacers, namely savings of “only” 2.762 mbar/s instead of 3.982 mbar/s. Expressed in percentual change TMP rise (Δp/Δt) is reduced by 45% instead of 66%. Close to optimum conditions are obtained by combination of spacers with high cross flow velocity (ab), which results in 92.2% decrease of the TMP rise. Only slight improvement is seen by

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further reduce energy consumption in the processing of fluids with high solid loads.

Table 4 Factor effects and interdependencies. Trial Aa Ba Ca Da Effects and interCombinatorial effect of factors dependencies [mbar/s] and interdependencies [%] (-) a b ab c ac bc abc d ad bd abd cd acd bcd abcd

– + – + – + + – – + – + – + – +

– – + + – – + – – – + + – – + +

– – – – + + + – – – – – + + + +

– – – – – – – + + + + + + + + +

6.033 –3.982 –1.730 0.150 –1.512 0.178 0.480 0.787 –0.616 0.187 0.212 0.150 0.023 0.223 –0.115 –0.131

– -66.0 -28.7 -92.2 -25.1 -88.1 -45.8 -93.4 -10.2 -73.1 -35.4 -93.4 -34.9 -91.2 -54.1 -94.5

a A: spacer application, B: cross flow velocity, C: aeration rate, D: intermittent aeration.

Nomenclature Ach As b d dh D h L Δp Δt ϵ SAD SADW SADm

additional combination with effects c and/or d (abc, abd, abcd), although the stand alone effect of high aeration (c) is nearly as significant as the effect of high liquid cross flow velocity (b). Intermittent aeration (d) has a much lower effect compared to factors A–C, but it is nevertheless a valuable option to limit fouling since only limited module and process design changes are required to modify existing technology to include intermittent aeration. With the exception of second and third order dependencies bcd and abcd, all interdependencies, especially the first order dependencies between the single factors promote faster TMP rise, i.e. the effect of the factors partly cancel out if combined.

5. Conclusions Microstructured spacers have been applied to submerged membrane systems with air scouring. In critical flux measurements using model fouling suspension an increase in critical flux by 100% was observed with spacer implemented in the feed channel. Single parameter experiments varying the most important operating and design parameters validated the increase in process performance. It was further shown that an optimum spacer height exists for which process performance is maximized. In comparison, it was found that process performance increases with decreasing channel height without application of spacers. Air flow rate could be significantly reduced by application of the structured spacers. It was shown that by combination of spacers with operation at higher cross flow velocities aeration rate could be reduced by a factor of 7.5 without loss in process performance. The beneficial effect of the spacer is assumed to result from the altered hydrodynamics with increased membrane shear forces as well as intensified bubble cleaning efficiency. Slug flow could be observed even for smaller bubbles and bubbles are repeatedly forced towards the membrane surface. In addition a more equal distribution of bubbles is found. Using the Design of Experiments methodology it was derived that the most effective way to reduce fouling is the use of spacers, followed by increasing the cross flow velocity, followed by increasing the aeration rate. Combination of spacer and high cross flow velocity has the most significant effect on fouling rate. The strong influence of the background cross flow velocity in combination with spacers indicates that the application of the spacers in side stream membrane systems is a promising option to

cross section of the empty channel (m²) cross section of the spacer (m²) distance of two spacer filaments in one layer (m²) spacer filament width (m) hydraulic diameter (m) spacer filament height (m) spacer height/channel height (m) length of one spacer 3601 coil (m) pressure difference (bar) time difference (s) porosity (%) specific aeration demand (N m³/m h) specific aeration demand based on panel width (N m³/m h) membrane specific aeration demand (N m³/m h)

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