Fouling scenarios in hollow fiber membranes during mini-plant filtration tests and correlation to microalgae-loaded feed characteristics

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Fouling scenarios in hollow fiber membranes during mini-plant filtration tests and correlation to microalgae-loaded feed characteristics Sucipta Laksono a, Ibrahim M.A. ElSherbiny a, *, Stefan A. Huber b, Stefan Panglisch a, * a b

Mechanical Process Engineering and Water Technology, University of Duisburg-Essen, 47057 Duisburg, Germany DOC-Labor Dr. Huber, Eisenbahnstraße 6, 76229 Karlsruhe, Germany

A R T I C L E I N F O

A B S T R A C T

Keywords: Ultrafiltration membranes Mini-plant operation Algal organic matter Pore blocking fouling mechanisms Fouling reversibility

Fouling mechanisms inside polyethersulfone hollow fiber membranes during mini-plant filtration tests using microalgae-loaded feed water were modeled. Multiple filtration cycles tests were conducted in dead-end mode at conditions analogous to full-scale operation conditions using four microalgae species, individually employed in intact and lysed conditions, without prior separation of algae cells, cell debris and algal organic matter (AOM). Fouling mechanisms contributions, fouling rates and fouling resistances per one cycle were statistically quan­ tified employing classical pore blocking filtration and resistance-in-series models via special algorithm. The results were reliably correlated to main microalgae feeds characteristics (i.e., AOM composition, average size distribution, conformation), retention of AOM substances, besides morphology and nature of the formed fouling layers. Standard blocking and intermediate blocking mechanisms resulted rather in irreversible fouling. Nature of intermediate fouling mechanism was interestingly altered based on microalgae characteristics, and conse­ quently, influencing fouling reversibility and overall membrane performance. Helical microalgae cells tended to block membrane pores causing strong fouling, while rectangular and cylindrical microalgae cells showed a combined fouling phenomenon (competitive blocking of membrane pores and fouling layer voids). Moreover, cake filtration mechanism rather contributed into better fouling reversibility; however, for microalgae feeds having high hydrophobic organic carbon and biopolymers contents, strong irreversible fouling was revealed, irrespective of cake filtration mechanism contribution.

1. Introduction Pressure-driven membrane processes, ultrafiltration (UF) and microfiltration (MF), are among efficient separation technologies for purification of algae-containing water [1–3]. Nevertheless, intensive efforts are being devoted to find out main critical parameters influencing UF/MF membranes performance during full-scale operation, as well as reliable interpretations of the governing fouling mechanisms. Recent studies have emphasized that deposition of retained algal organic matter (AOM), either on membranes surface or inside the pores, is responsible for membrane performance decay rather than microalgae cells and debris [4,5]. AOM comprises biopolymers (hydrophilic high molecular weight polysaccharides, cationic proteins and amino-sugars, >20,000 Da), humic substances (≤1000 Da) and building blocks (natural break­ down products of humics, 300–450 Da), besides low molecular weight neutrals and acids (both are <350 Da) [6,7]. AOM is also often classified regarding secretion or excretion into extracellular organic matter

(EOM), produced during algae metabolism, and intracellular organic matter (IOM), released only after cell breakage [5,8]. Both substances are mainly composed of proteins and polysaccharides at different pro­ portions [9]. Moreover, a fraction of AOM representing sticky poly­ saccharides and glycoproteins is known as transparent exopolymer particles (TEPs) [10], which have been reported to be responsible for organic fouling in UF/MF membranes [6,11] and biofouling in reverse osmosis membranes [12,13]. Numerous studies have investigated individual vs. combined fouling contributions by main AOM fractions, EOM and IOM. For instance, Liu et al. related the significant flux decline of flat polyvinylidene difluoride (PVDF) membranes at constant pressure filtration using blue-green microalgae to the formation of a compressed cake layer containing algae species and cell debris rather than irreversible internal fouling due to EOM and IOM [5]. Nevertheless, the formed cake layer improved IOM retention and, consequently, mitigate pore blocking fouling during combined filtration experiments [5,9]. Moreover, Huang et al. have

* Corresponding authors. E-mail addresses: [email protected] (I.M.A. ElSherbiny), [email protected] (S. Panglisch). https://doi.org/10.1016/j.cej.2020.127723 Received 4 September 2020; Received in revised form 23 October 2020; Accepted 10 November 2020 Available online 28 November 2020 1385-8947/© 2020 Elsevier B.V. All rights reserved.

Please cite this article as: Sucipta Laksono, Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2020.127723

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GmbH, Germany) with total membrane active surface area of 0.051 m2 were employed. Every module contained ten capillaries; each had length of 30 cm, diameter of 0.9 mm and nominal barrier pore diameter of ~0.02 µm. As receiving, membrane modules were soaked in NaOH at pH 12 for 1 day, followed by a chemical cleaning (see Supplementary Data, Section S1.1) to remove preserving chemicals. Thereafter, cleaned modules were stored in 0.05% Na2S2O5 ready for use.

studied organic fouling of hollow fiber PVDF membranes by blue-green microalgae; EOM was observed to induce more irreversible fouling than IOM [8]. Furthermore, fouling of cellulose membrane filters was correlated to biopolymer and TEP concentrations in diatom microalgae feeds [4]; however, no correlation was found with algal cell density or chlorophyll-a concentration. Similar findings were earlier reported [6], where fouling of flat sheet polyethersulfone (PES) by blue-green and diatom microalgae was studied at constant filtration rate. In another work, fouling of polyvinyl chloride hollow fiber by algae-rich freshwater was related to different molecular weight distributions of EOM [14]. Nevertheless, the reliability of several studies in literature, in this context, might be seen limited since EOM as well as IOM coexist natu­ rally in algae-containing water at real conditions. Hence, filtration of algae feeds without prior separation is considered more reasonable for respective investigations. Moreover, no detailed studies have been so far reported concerning the impact of individual chemical substances in AOM on the combined fouling of hollow fiber membranes at operating conditions close to full-scale application. In parallel, most of literature on membrane fouling by microalgae was conducted using flat sheet membranes [1,2]. Few studies investi­ gated the fouling behavior of PVDF hollow fiber membranes by marine green microalgae at constant flux operation [15,16], while others filtered blue-green microalgae at constant pressure operation [3,8]. One recent work has studied the performance of PES hollow fiber membranes in multiple filtration cycles (up to 9 cycles) using diatom microalgae at constant filtration rate of 80 L/m2 h (equivalent to 26 L/m2 per cycle) [4]. However, such adopted filtration conditions are inconsistent with the frequently applied operating conditions at full-scale application [17]. Organic fouling of UF/MF membranes by algae cells, cell debris, and AOM is generally analyzed in literature using three models. Pore blocking filtration model was exclusively employed for flat sheet membranes [9,18], whereas fouling of hollow fiber membranes was often analyzed using Darcy law and Modified Fouling Index models [4,6,19]. Yet, pore blocking model has not been employed to model fouling mechanisms occurred inside hollow fiber membranes during multiple cycles filtration tests. In the current work, the performance of hollow fiber PES modules in mini-plant experiments, performed at operating conditions close to industrial-scale application, was studied and related to the main microalgae and feed characteristics, besides membrane retention. In membrane-based algae removal applications, very different alterations of permeability are often observed in hollow fiber membrane systems, not only from cycle to cycle but also within a single cycle. For instance, the fouling rate at the beginning of a certain filtration cycle may differ than during the rest of filtration. Therefore, a critical objective was to identify different fouling mechanisms that occur throughout single filtration cycle, and consequently, to determine reliably the fouling backwashability or irreversibility. This consideration is different from most of the established fouling models or indices for hollow fiber membranes that are limited to the estimation of fouling reversibility in multiple filtration cycles tests. Four microalgae species were individu­ ally employed in two extreme cell conditions, intact (i.e., growth / algae bloom phase) and lysed (i.e., death and ultimate cell breakage phase), without prior separation of EOM and IOM. Pore blocking filtration model and resistance-in-series model were specifically employed to model alterations in fouling scenarios in hollow fiber membranes within a single cycle and over multiple cycles. Thereafter, fouling reversibility was determined and attempted to be correlated with fouling mecha­ nisms contributions and respective rates per one filtration cycle.

2.2. Microalgae Four microalgae types were individually used in intact and lysed cell conditions. Fig. 1 presents images (using confocal laser scanning mi­ croscopy (CLSM), Leica TCS SP8, Leica, Germany) for algae species employed in this study: Chlorella sorokiniana (CS; single spherical cells, Chlorophyta phylum) as green microalgae, Arthrospira platensis (AP; filamentous helical cells, Cyanobacteria phylum) as blue-green micro­ algae, besides Thalassiosira rotula (TH; flat rectangular cells, Ochrophyta phylum) and Chaetoceros calcitrans (CC; cylindrical cells, Ochrophyta phylum) as brown diatom microalgae. Algae stock solutions were pur­ chased from Bluebiotech GmbH, Germany. Green and blue-green microalgae were cultivated in BG-11 medium, while brown diatom microalgae were cultivated in F/2 medium; description of inhabitation media is provided in Section S2. Stock solutions were alternatively exposed to light and dark at a ratio of 16:8 h and aerated to maintain oxygen concentration and buffering process. 2.3. Preparation and characterization of algae feed solutions Algae stock solutions were characterized regarding chlorophyll-a concentration (Algae lab analyzer, bbe Moeldaenke, Germany; mea­ surement error is 1 µg/L), and particle size distribution (Multisizer 4, Coulter Counter, Beckman, USA). Intact algae feed was prepared using microalgae cells as received. Certain volume was added into 70-liter tank containing RO-quality water (conductivity < 50 μS/cm, organic carbon content < 0.2 ppm) to prepare algae feed with a total particle volume of ~15 million µm3/ml. For preparation of lysed algae feeds, identical volumes were employed. Prior to dilution in RO-quality water, intact cells were disrupted using ultrasound (UIP 250, Hielscher, Ger­ many) at frequency of 24 kHz and power of 250 Watt for 2 h; temper­ ature was set at 17 ◦ C ± 2 to prevent protein degradation. Feed solutions were kept under stirring to prevent sedimentation of algae cells and other suspended particles. In case of intact feeds, vigorous stirring was avoided to prevent algae cells wall breakage. Because of the degradation effect of chlorophyll-a during storage, samples were collected from feed tanks on the day of experiment to be rechecked using Algae lab analyzer and Coulter Counter. Complete analysis of chromatographic dissolved organic carbon (DOC) was performed using Liquid Chromatography combined with Organic Carbon Detection (LC-OCD, DOC-Labor Dr. Huber, Karlsruhe, Germany). Prior to measurement, samples were filtered through 0.45 µm membrane filters. Detailed description for measurement basis and procedures were reported [7]. 2.4. Mini-plant filtration experiments 2.4.1. Membrane testing unit Fully automated mini-plant testing unit (Convergence B.V., Netherlands), equipped with Coriolis flowmeters, was employed. A schematic illustration for mini-plant testing system is shown in Fig. 2(a). Membrane modules were vertically installed, and feed solution was pumped from the bottom side. Filtration experiments were conducted in inside-out dead-end mode at room temperature (21 ◦ C ± 2).

2. Materials and methods

2.4.2. Algae filtration experiments A diagram describing hollow fiber membranes testing and cleaning procedures is presented in Fig. 2(b). Prior to filtration experiments, hollow fiber membranes were compacted via filtration of RO-quality

2.1. Membrane PES hollow fiber membrane modules (Multibore®, supplied by inge 2

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Fig. 1. CLSM images for green, blue-green and diatom microalgae in intact and lysed conditions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

water at permeate flux of 500 L/m2⋅h for 4 h. Pure water permeability was then measured, being usually in range of 650–700 L/m2⋅h⋅bar. Miniplant filtration experiments using intact and lysed algae feeds were performed at operating conditions close to industrial-scale application. Mini-plants enable studying membranes behavior at full-scale operating conditions without the need for complicated and expensive pilot-scale constructions [17,20]. Multiple filtration cycles tests were carried out at constant flux mode, permeate flux of 100 L/m2⋅h. Every filtration cycle lasted for 45 min (equivalent to specific total permeate volume of 65 L/m2) and was followed by mechanical backwashing at flux of 230 L/ m2⋅h using RO-quality water for 55 s (30 s to bottom side and 25 s to top side). Filtration experiments were automatically aborted either when 16 filtration cycles were attained (equivalent to total filtered volume of 1,040 L/m2), or membrane permeability decreased to 100 L/m2⋅h⋅bar. Afterward, typical chemical cleaning was performed. A description for chemical cleaning procedures and an assessment of the cleaning effi­ ciency are supplied in Section S1.

software. Furthermore, nature and condition of deposited microalgae species were analyzed using CLSM equipped with 63x water immersion objective. Fouled membranes were stained for 10 min in a mixture of 100 µL of 1 mg/mL Concanavalin A tetramethylrhodamine (ConA) and 1,5 µL of 1.67 mM SYTO 9 (Molecular Probes, Eugene, US) in the dark. Fluorescent red color by ConA (excitation at 561 nm and emission at 565–610 nm) indicates carbohydrate binding proteins, while fluorescent green color by SYTO 9 (excitation at 488 nm and emission at 495–560 nm) refers to DNA and RNA [21].

2.5. Morphological characterization of the fouling layers

where CP is solute concentration in permeate, and CF is solute concen­ tration in feed solution.

2.6. Membranes retention Samples were collected from algae feed solutions and permeates after 30 min of first filtration cycle and analyzed using LC-OCD to determine the retention of different AOM substances, Eq. (1). ) ( CP (1) R = 1− CF

Morphology of the formed fouling layers was characterized employing corresponding flat sheet PES membranes (inge GmbH, Ger­ many) with active surface area of 13.8 cm2 and nominal barrier pore diameter of ~0.02 µm. Algae feed amount equivalent to specific total permeate volume of 65 L/m2 was filtered at constant flux of 100 L/m2⋅h using Millipore holder (Millipore Corporation, USA). Cross-section morphology of fouled flat sheet membranes was analyzed using Quanta 400 FEI Scanning Electron Microscope (SEM, Thermo-Fisher Scientific, USA); fouling layer thickness was estimated using ImageJ

2.7. Analysis of fouling inside hollow fiber membranes in mini-plant tests Fouling behavior by different microalgae was primarily investigated by measuring membrane permeability decay over multiple cycles. Membrane permeability, W, is defined by Eq. (2). W=

3

J ΔP

(2)

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each filtration cycle was modelled independently, i.e., irreversible fouling remaining from previous cycle was not considered. Therefore, this model was combined with resistance-in-series model to consider irreversible fouling existing inside the membrane over multiple cycles. Analysis of fouling mechanisms was performed using special algorithm and employing MATLAB R2019a software. A description for the adapted methodology is provided in Section S4. Percentage contribution (%) for each fouling mechanism was calculated using Eq. (6).

where J is water flux, ΔP is transmembrane pressure. Normalized permeability, W’ , was calculated as in Eq. (3). W’ =

WVsp W0

(3)

where, WVsp is membrane permeability at certain filtered specific volume (Vsp , Eq. (4)), W0 , is permeability of clean membrane. The membrane performance decay during mini-plant filtration tests was monitored via plotting W’ vs. Vsp. Vsp =

J t

%SB, %IB, %CF =

(4)

(6)

where, VSB/IB/CF is specific filtered volume range prevailed by blocking mechanism SB, IB or CF, respectively.

The presented data are the average of 5 data sets, which were the best representative data sets (i.e., with low redundancy) selected from several mini-plant filtration experiments for each microalgae type.

2.7.1.2. Determination of specific fouling rate. Specific fouling rate for ( ) each blocking mechanism dW was quantified over multiple dt

2.7.1. Modeling of fouling mechanisms using pore blocking filtration model

SB,IB,CF

filtration cycles from the slope of the relation between the change in membrane permeability (ΔW) and filtration time (t), as in Eq. (7). ⃒ [ ] ⃒ ⃒Wt − W0 ⃒ dW ⃒ = ⃒⃒ (7) ⃒ dt t

2.7.1.1. Chronological analysis of fouling mechanisms inside hollow fiber membranes. Classical pore blocking filtration model at constant flux filtration [22] are expressed by unified differential Eq. (5). dp = k’ pn dv

VSB/IB/CF × 100% Vsp

(5)

2.7.2. Determination of fouling resistances during mini-plant filtration experiments The alteration in membranes fouling resistances over multiple filtration cycles was estimated employing resistance-in-series model that is based on Darcy’s law (Eq. (8)).

where dp represents the change in pressure, dv represents the change in specific filtered volume, k’ is constant, and n is known as blocking index indicating the fouling mechanism type, cake filtration (CF, n = 0), in­ termediate pore blocking (IB, n = 1), standard pore blocking (SB, n = 1.5), complete pore blocking (n = 2); characteristic equations are pro­ vided in Table S3. Notwithstanding, this model was basically developed for flat sheet membranes [9], it has been extended here to model fouling mechanisms during multiple cycles tests, for the first time. The fouling in

J=

ΔP

μR T

where RT is total membrane resistance (i.e., intrinsic membrane resis

Fig. 2. (a) Schematic illustration for mini-plant testing system; (b) diagram describing hollow fiber membrane testing and cleaning procedures. 4

­

(8)

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tance in case of pure water filtration, Rm ), μ is feed solution viscosity (for simplicity, pure water viscosity was used). RT comprises three specific resistances, Rm , hydraulic reversible fouling, Rrev , and hydraulic irre­ versible fouling,Rirr (Eq. (9)). More information concerning application of Rirr to estimate chemical cleaning efficiency is provided in Section S1. J=

ΔP

information on changes of algae feed influence on hollow fiber membranes fouling behavior when they are increasingly fouled, i.e., over multiple filtration cycles. • Investigation of fouling mechanisms percentage contributions per one cycle, and respective rates, can potentially reveal possible cor­ relations between individual fouling mechanisms and alteration of fouling reversibility inside hollow fiber membranes.

(9)

μ(Rm + Rrev + Rirr )

3. Results and discussion

The quantification of membrane fouling resistances is illustrated in Fig. 3. Reversible fouling resistance per one cycle (Rrev(n) ) was obtained using Eq. (10), employing RT and fouling resistance remaining after mechanical backwashing (RBW ):

3.1. Characterization of algae feed solutions Algae feed solutions were characterized, and results for chlorophyll-a measurement, particle size distribution, and LC-OCD analysis are introduced in Table 1. Typical chlorophyll-a concentration was generally reported to be in range of 5–80 and 1–10 µg/L in case of eutrophic freshwater and seawater, respectively [23,24]. Here, chlorophyll-a con­ centration in green and blue-green microalgae feeds was in range of 170–225 and 22–32 µg/L for intact and lysed feeds, respectively. Lower chlorophyll-a concentration measured for lysed algae feeds is mainly attributed to the cell disruption process (cf. Section 2.3). In contrast, chlorophyll-a concentration in diatom microalgae feeds varied according to the algae type. Chlorophyll-a concentration for CC feeds was in range of 100–130 and 45–47 µg/L for intact and lysed feeds, respectively, where TH feeds exhibited chlorophyll-a concentration in range of 57–75 and 20–21 µg/L, respectively. It is worth mentioning that all algae feed solutions, in particular diatom microalgae, were prepared in RO-quality water for the sake of eliminating the influences by highly concentrated salt background water on the feed characteristics and membrane fouling behavior. Since cake filtration was predicted to be the main fouling reason, total particle volume was preferentially set constant for all algae types, and prior to lysing process, rather than chlorophyll-a concentration (cf. Table 1). Intact algae feeds were affirmed to exhibit correspondent total particle volume values in range of 14–18.2 × 106 µm3/mL, while lysed algae feeds had essentially lower values in range of 1.1–2.9 × 106 µm3/ mL. Nevertheless, intact algae feeds showed different total particle number distributions that might be correlated to different algae cells

(10)

Rrev(n) = RT(n) − RBW(n)

Irreversible fouling resistance per one cycle (Rirr(n) ) was determined by Eq. (11) using the remaining fouling resistances after mechanical backwashing within two consecutive cycles. Rirr(n) = RBW(n) − RBW(n−

(11)

1)

Cumulative irreversible fouling (Rirr(T) ) was calculated using Eq. (12). (12)

Rirr(T) = RBW(n) − Rm

Percentage contribution of Rrev(n) relative to total membrane fouling per one cycle (%Rrev(n) ) was estimated via Eq. (13). %Rrev(n) =

Rrev(n) RT(n) − RBW(n−

= 1)

RT(n) − RBW(n) RT(n) − RBW(n− 1)

(13)

The presented membrane fouling graphs and results are the average of 5 data sets for mini-plant filtration experiments for each microalgae type. Consolidation of the outputs of pore blocking and resistance-in-series models can contribute into better understanding of fouling scenarios inside hollow fiber membranes: • Since fixed amount of algae feeds (prepared at comparable total particle volume) was filtered per one cycle, monitoring fouling re­ sistances (Rrev(n) , Rirr(n) ) and their percentage contributions can give

Fig. 3. Schematic illustration for determination of membrane fouling resistances for every filtration cycle as described in Section 2.7.2; RT represents total membrane resistance; Rm represents intrinsic membrane resistance in case of pure water filtration; Rrev represents hydraulic reversible fouling; Rirr represents hydraulic irre­ versible fouling. 5

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applied stress) in changing the nature of AOM substances and on increasing the total number of respective particles. Low molecular weight acids, LMWA (aliphatic acids existing in algae inhabitation media or in cell debris), were revealed to be among the minor DOC constituents, ≤ 11% in all algae feeds. Another organic fraction is hy­ drophobic organic carbon, (HOC, sparingly soluble humins and anthropogenic compounds [7]). Alike CDOC components, lysed AP showed 5 times higher HOC portion than in case of intact AP, whereas the opposite trend was interestingly noticed in CC algae feeds such that HOC content in intact CC was 2 times higher than lysed CC. Moreover, HOC portions in CS feeds seemed to be below the device detection limit.

conformation (cf. Fig. 1) and size (cf. Table 1). Much lower total particle number values were inversely measured for lysed algae feeds. Further­ more, particle size distribution curves are provided as Supplementary Data, Figure S5. Intact CC, TH and AP feeds exhibited monodisperse particle size distribution with maximum peaks at 5–10, 8–15 and 20–25 µm, respectively. Whereas intact CS feeds showed a bimodal distribution with maximum peak at 2–5 µm, besides wide peak at 5–15 µm that might be correlated to aggregated algae cells due to AOM slimy substances existing on the cells walls (i.e., EOM, as revealed from CLSM analysis, Section 3.3). Particle size distribution analysis for lysed algae feeds did not show recognizable peaks within device detection range that reflects, along with chlorophyll-a and total particle volume analyses, ultimate algae cells breakdown during the lysing process. Moreover, LC-OCD was employed to analyze chromatographic DOC (CDOC) components in algae-loaded feeds (cf. Table 1). Green and bluegreen microalgae feeds showed higher DOC contents (1228 and 921 ppb-C for intact CS and AP, respectively) than diatom microalgae feeds (582 and 398 ppb-C for intact TH and CC, respectively). Besides, lysed algae feeds were found to have higher DOC contents than intact algae feeds. For instance, lysed AP feeds had DOC of 2298 ppb-C compared to intact AP with 921 ppb-C. Notwithstanding such clear trend, one tech­ nical issue should be considered while analyzing LC-OCD data that large intact algae cells were separated prior to measurement (cf. Section 2.3), which might influence the observed variations in organics contents be­ tween intact and lysed feeds that was not the case during mini-plant filtration tests. Therefore, the variations of organic components be­ tween intact vs. lysed feeds that are >50% are only considered while discussing the membrane performance results. In parallel, building blocks (BB), biopolymers (BP) and amphiphilic low molecular weight neutrals (LMWN) were the major DOC constitu­ ents in all algae feeds; green and blue-green microalgae feeds exhibited generally higher contents than diatom microalgae feeds. Besides, CS feeds showed the highest BB portions (>60%), whereas other algae feed solutions exhibited BB fractions ≤ 55%. Additionally, BB content in lysed AP (759 ppb-C) was 2 times the portion in intact AP (370 ppb-C); LMWN portion showed also similar trends. LMWN are degradation products of BP. Furthermore, in case of TH, AP, and CS algae feeds, BP content had an increase of 82–110% for lysed feeds than intact feeds, respectively. This might be related to the impact of lysis process (i.e.,

3.2. Performance of hollow fiber membranes in mini-plant filtration tests 3.2.1. Filtration of green and blue-green microalgae feeds Performance curves for hollow fiber membranes during multiple filtration cycles tests, normalized permeability vs. specific filtered vol­ ume; are presented in Fig. 4. In case of CS algae, significant performance decay of 60% within the first filtration cycle accompanied by an overall severe membrane fouling were observed during filtration of lysed algae feeds (cf. Fig. 4a). Subsequently, filtration experiment was automati­ cally aborted after 4 cycles only (equivalent to total specific filtered volume of 260 L/m2) due to reaching the maximum allowed pressure, which was the most drastic overall membrane fouling in this study. In contrast, moderate membrane fouling (normalized permeability decline of 37% at first cycle) was revealed during filtration of intact CS that was better removed by mechanical backwashing; consequently, filtration experiments could last for 16 filtration cycles (equivalent to ~1040 L/ m2). Additionally, retention (ℝ) for individual AOM substances during first filtration cycle was investigated; see Fig. 4c–d. Similar ℝDOC (16%) was interestingly measured for lysed and intact CS feeds. Nevertheless, higher ℝBP was detected in case of lysed CS (52%) than intact feeds (45%). Considering higher BP content in lysed CS feeds than intact feed (cf. Section 3.1 and Table 1), this may interpret steeper normalized permeability decline caused by lysed CS such that BP substances are proposed to result in a rather surface fouling. Interestingly, certain retention was measured for LMWA and LMWN (ℝLMWA = 56% and LMWN = 29%) during filtration of intact CS, despite of their very low molecular weight (and, consequently, small molecular size), which may

Table 1 Main characteristics and chemical compositions of intact and lysed microalgae-loaded feeds. Algae feed solution

Chlorophyll-a (µg/L)

Chlorella Sorokiniana (CS)

intact

197–225

lysed

30–32

Arthrospira Platensis (AP)

intact

170–182

lysed

22–25

intact

57–75

lysed

20–21

intact

100–130

lysed

45–47

Thalassiosira Rotula (TH) Chaetoceros Calcitrans (CC)

Total particle volumea (µm3/mL) ⋅106 – Total particle numbera (particle/mL) ⋅104

14.0 ± 1.4 48.7 ± 7.9 2.9 ± 0.2 11.9 ± 0.3 15.7 ± 0.9 13.0 ± 1.6 2.0 ± 0.4 2.4 ± 0.3 14.5 ± 1.3 11.5 ± 2.3 1.7 ± 0.4 2.9 ± 0.5 18.2 ± 2.4 28.2 ± 1.6 1.1 ± 0.4 1.6 ± 0.4

a

Algae size (µm)b

LC-OCD analysis (ppb-C)c DOCd

HOCe

2–5, 5–15 <2

1228

–j

1464

–j

20–25

921

<2

2298

8–15

582

<2

675

5–10

398

<2

405

72 8% 386 17% 70 12% 66 10% 87 22% 38 9%

Chromatographic DOC BPf

BBg

LMWNh

LMWAi

125 10% 267 18% 184 21% 368 19% 64 13% 117 19% 34 11% 51 14%

783 64% 880 60% 370 41% 759 37% 280 55% 300 49% 163 52% 197 54%

180 15% 241 17% 303 34% 671 35% 121 24% 154 25% 87 28% 109 30%

140 11% 75 5% 64 5% 162 9% 47 9% 37 6% 27 9% 11 3%

Measurements were repeated 5 times and standard deviation was calculated. Limitation of measurement aperture for Coulter Counter is 2 µm. c Statistical analysis performed by DOC-Labor Dr. Huber for assessment of reproducibility of measured DOC values using comparable sample water quality showed less than 5% margin of error for all fractions besides HOC (Hydrophobic organic carbon) with a margin of error of about 16% (Huber 2020). d Dissolved organic carbon, e Hydrophobic organic carbon, f Biopolymer, g Building blocks, h Low molecular weight neutrals, I Low molecular weight acids j HOC portion was below the device detection limit b

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Fig. 4. Performance curves for PES hollow fiber membranes in mini-plant filtration experiments using green and blue-green microalgae feeds at constant filtration rate of 100 L/h m2: (a) CS, (b) AP, and retention at first filtration cycle for individual AOM chemical substances: (c) intact CS, (d) lysed CS, (e) intact AP, (f) lysed AP. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

indicate their adhesive capacity either to retained algae cells or mem­ brane matrix. Nevertheless, lower retention was measured in case of lysed CS, ℝLMWA = 11% and ℝLMWN = 14%. Lysed AP feeds were found to cause the most severe normalized permeability decline per one filtration cycle, 76% at 1st cycle, (cf. Fig. 4b). This should be generally correlated to high CDOC (including BP) and HOC contents; however, overall membrane fouling was less intense than in case of lysed CS; filtration could last for 9 cycles (equivalent to ~585 L/m2), i.e., higher fouling reversibility. Moreover, in case of intact AP feeds, less rigorous performance decay of 58% within

first cycle was noticed that was better mechanically backwashable such that mini-plant experiments could last for 16 cycles. Membrane reten­ tion performance is also demonstrated in Fig. 4e–f. Considering higher DOC, BP, HOC fractions in lysed AP feeds than intact feeds (cf. Section 3.1 and Table 1), higher ℝDOC, ℝBP and ℝHOC measured during filtration of lysed AP feeds (23, 57, 66%, respectively) than intact AP (13, 33, 63%, respectively) can interpret the rigorous membrane fouling in case of lysed AP than intact AP, mostly via surface fouling by high molecular weight and adhesive organic substances. Moreover, low ℝBB and ℝLMWN were measured, while ℝLMWA was in range of 32–38 %. 7

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3.2.2. Filtration of diatom microalgae feeds Performance curves for hollow fiber membranes during mini-plant filtration of TH and CC algae feeds are illustrated in Fig. 5a and b, respectively. At comparable total particle volume, diatom microalgae feeds caused generally less overall membrane fouling compared to green and blue-green microalgae feeds. This is most likely due to the approx. 40–80% lower DOC content in diatom microalgae feeds. Mini-plant tests using intact and lysed TH feeds could last for 16 cycles that indicates mild and mechanically backwashable membrane fouling. Intact TH feeds caused a performance decay of 25–39% in the first 3 cycles, af­ terwards nearly constant performance decline of ~40% was observed

for 4th–16th cycles. Accordingly, fouling by intact TH feeds was revealed to be the lowest in this study. Lysed TH feeds showed more normalized permeability decline per filtration cycle (33–62%). This may be related to higher BP portion in lysed TH than intact feeds (cf. Section 3.1 and Table 1). Moreover, membrane retention within 1st cycle is introduced in Fig. 5c–d. Less membrane fouling and better back­ washability than in case of green and blue-green microalgae filtration could be interpreted by low DOC contents in TH feeds. In addition, the lysis process could produce cell debris and higher number of small AOM particles. Those particles could be big enough to be retained by the membrane, but they could also act potentially as adhesive collector for

Fig. 5. Performance curves for PES hollow fiber membranes in mini-plant filtration experiments using diatom microalgae feeds at constant filtration rate of 100 L/ h m2: (a) TH, (b) CC, and retention at first filtration cycle for individual AOM chemical substances: (c) intact TH, (d) lysed TH, (e) intact CC, (f) lysed CC. 8

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particles, either in suspension (i.e., feed solution) or as layer on the membrane surface, that might otherwise pass through the membrane. This might explain higher retention values measured for organic frac­ tions (ℝBP, ℝHOC and ℝDOC) in case of lysed microalgae feeds compared to intact feeds.

Nevertheless, an irregular fouling behavior was observed during filtration of CC algae feeds (cf. Fig. 5b); intact CC caused severer per­ formance decay per filtration cycle than lysed CC (38% vs. 21% at 1st filtration cycle, respectively), which was not mechanically back­ washable and led unexpectedly to automatic aborting of mini-plant tests

Fig. 6. Morphological characterization for corresponding fouled flat sheet PES membranes by one filtration cycle (45 min, 65 L/m2) using microalgae-loaded feeds. 9

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using intact CC feeds after 9 filtration cycles only. Considering compa­ rable DOC contents (cf. Table 1), such unforeseen higher fouling of intact CC feeds may be related to higher HOC content compared to lysed CC (130%). Membrane retention performance is shown in Fig. 5e–f. Lower ℝHOC (25%) and ℝDOC (26%) were determined in case of intact CC than lysed CC (68% and 34%, respectively); however, considering HOC content in the feed, more adhesive organic substances are expected to accumulate on membrane surface by intact CC than lysed CC. Higher LMWA and lower ℝLMWN were also measured for intact CC than lysed CC. Additionally, similar ℝBP was found in intact and lysed CC, while ℝBB values were the highest in this study.

Rirr(n) (Fig. 7b), indicating that, most likely, comparable irreversible fouling fractions remained inside hollow fiber membranes every cycle (i. e., adhering either on / in membrane pores or in the swelling layer). Moreover, pore blocking model revealed that intermediate blocking (IB) and cake filtration (CF) were the governing fouling mechanisms in all cycles (Fig. 7b); however, their percentage contributions (%IB and %CF) were oppositely changing such that %CF increased gradually until it dominated at later cycles (cf. Table S7). This may interpret the improvement of fouling layer backwashability over multiple cycles. Nevertheless, as illustrated in Fig. 7a and Table S7, intermediate ( ) ( ) dW blocking rate dW was higher than cake filtration rate in all dt dt

3.3. Morphological characterization of deposited fouling layers

cycles, irrespective %IB and %CF, which may be correlated to the increasing trend for cumulative irreversible fouling, Rirr(T), cf. Fig. 7a. In case of lysed CS, Rrev(n), Rirr(n) and Rirr(T) showed steeper increase than in case of intact CS until attaining the maximum at the 4th cycle (Fig. 8a), after which filtration was aborted. Besides, Rirr(n) at the 1st cycle was the highest in this study (Table S6). Irreversible fouling contribution (%Rirr(n)) exhibited almost 100% increase compared to intact CS (cf. Fig. 8b and Table S6). This severe fouling may be corre­ lated to standard blocking mechanism (SB), as revealed by pore blocking model. SB (via cell debris and adhesive low molecular weight sub­ stances, cf. Section 3.2.1) prevailed the fouling mechanisms in all cycles ( ) (cf. %SB, Fig. 8b), besides, dW was found to be the strongest fouling dt

IB

SEM micrographs for corresponding fouled flat sheet membranes by intact algae feeds revealed that most of retained algae cells were deposited on membranes surfaces in intact condition (Fig. 6a–d); typical conformation of intact algae cells (as depicted in Fig. 1) were recognized in the fouling layers. SEM micrographs for fouled membranes by lysed algae feeds (Fig. 6e–h) demonstrated that fouling layers composed of cell debris and formless substances. Furthermore, fouling layers thick­ nesses were analyzed (see Table 2). Intact algae feeds were emphasized to form thicker fouling layers than in case of lysed feeds (30–330%) that is correlated to algae cells conformation and particle size distribution analyses (cf. Section 3.1 and Figure S5). Thicker fouling layers, formed by intact feeds, are also imagined being less dense compared to thinner fouling layers by lysed feeds, which can explain severer fouling caused by lysed feeds (except CC). Moreover, intact green and blue-green microalgae feeds showed interestingly thicker fouling layers than intact diatom microalgae feeds that is consistent with algae particle size measurements (cf. Section 3.1 and Table 1). Condition of algae species in fouling layers was also analyzed using CLSM. Images for fouling layers formed by intact feeds were prevailed by green color emphasizing that the deposited algae cells were in intact condition (Fig. 6i–l); besides the confined red color surrounding green algae cells is referring to metabolism substances, EOM [25]. CLSM im­ ages for fouling layers formed by lysed feeds were dominated by red color indicating AOM and cell debris (Fig. 6m–p).

SB

rate, Fig. 8a and Table S7. In parallel, analysis of membrane fouling by intact AP emphasized that fouling reversibility was higher compared to intact CS, especially at early cycles (Fig. 9a and Table S6). Rrev(n) and %Rrev(n) showed stable linear trend and dominated in all cycles, indicating that analogous proportions of backwashable fouling deposited on the membrane every cycle (Figure S8a). Nevertheless, Rirr(T) exhibited linear trend with a limited increase, whereas Rirr(n) showed interestingly a decreasing trend (Fig. 9a and Table S6). High fouling reversibility during filtration of intact AP was accompanied by higher %CF (and accordingly lower %IB) compared to intact CS, Figure S8a and Table S7, due to bigger AP algae cells and considerable high molecular weight fractions. Nevertheless, ( ) ( ) dW dW and at early cycles were among the highest in this study dt dt IB

Mechanical reversible and irreversible fouling for hollow fiber membranes during multiple cycles filtration using intact CS feeds were analyzed (Fig. 7 and Table S6). Both reversible and irreversible fouling per one cycle, Rrev(n) and Rirr(n), exhibited linear increase over multiple cycles (Fig. 7a); however, Rrev(n), and thus %Rrev(n) (Fig. 7b), showed more increase and prevailed the membrane fouling in all cycles. On the other hand, the increase of Rirr(n) was rather limited, and consequently % Table 2 Fouling layers thickness values estimated using SEM analysis for corresponding fouled flat sheet membranes by one filtration cycle (45 min, 65 L/m2) using algae-contaminated feeds. Chlorella Sorokiniana (CS) Arthrospira Platensis (AP) Thalassiosira Rotula (TH) Chaetoceros Calcitrans (CC)

IB

Average fouling layer thickness* (µm) intact lysed intact lysed intact lysed intact lysed

0.60 ± 0.14 ± 1.28 ± 0.44 ± 0.46 ± 0.24 ± 0.55 ± 0.43 ±

CF

(Fig. 9a and Table S7), which may indicate the adhesion capacity of HOC and retained LMWA, as well as explain strong performance decay per cycle observed in Fig. 4b. Alike intact AP, lysed AP showed higher fouling reversibility than lysed CS, Rrev(n) exhibited stable linear trend over multiple cycles (Fig. 9b and Table S6). %Rrev(n) predominated membrane fouling in all cycles and showed a slight increase (Figure S8b and Table S6). Rirr(T) exhibited steeper increase over multiple cycles compared to intact AP. However, Rirr(n) showed a decreasing trend (Fig. 9b and Table S6), which may refer to low portions of unbackwashable fouling deposited inside hollow fiber membrane at later cycles, besides one might corre­ late it to a kind of combined fouling phenomenon, where fouling occurred in both barrier layer and fouling layer. Last assumption is supported by predomination of IB, over CF, in most cycles (Figure S8b ( ) and Table S7), while SB was not applied. dW was found to be the dt

3.4. Analysis of membrane fouling resistances and modeling of fouling mechanisms inside hollow fiber membranes

Microalgae-loaded feed

CF

highest in this work (Table S7) that was responsible for the drastic performance decay and filtration shutdown after 9 cycles. In case of intact TH, fouling reversibility showed constant trend over multiple cycles, Rrev(n) values were among the lowest in this study (Fig. 10a and Table S6). Nevertheless, %Rrev(n) prevailed all cycles and reached 99% (Figure S9a). Furthermore, Rirr(T) exhibited a slight increasing trend, but its values were also the lowest in this study (Fig. 10a and Table S6). Notwithstanding, Rirr(n) showed a clear decreasing trend, IB dominated over CF in all cycles with almost stable fouling rates that were among the lowest in this work (Figure S9a and

0.11 0.04 0.17 0.08 0.21 0.05 0.18 0.12

* Measurement of fouling layer thickness was repeated 5 times at different points on the fouled membranes surface, average values and standard deviation were calculated.

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Fig. 7. (a) Fouling mechanisms rates and membrane fouling resistances, as well as (b) their percentage contributions inside hollow fiber membranes during multiple filtration cycles using intact CS feeds.

Table S7), which may interestingly refer to an alteration in IB nature over multiple cycles as later discussed in Section 3.5. In case of lysed TH, Rrev(n) dominated and increased linearly over multiple cycles (Fig. 10b and Table S6), while %Rrev(n) exhibited lower values compared to intact TH and decreased for later cycles (Figure S9b). Contrary to intact TH, Rirr(n) showed a linear increasing trend over multiple cycles, besides IB dominated over CF in most of

cycles, while %IB was lower than in case of intact TH (Figure S9b and ( ) had constantly higher values compared Table S7). Nevertheless, dW dt IB

to intact TH in all cycles; higher values were determined for later cycles (Fig. 10b and Table S7), which may interpret the increasing trend of Rirr (n) and Rirr(T) over multiple cycles. Analysis of membrane fouling by intact CC revealed a steep linear 11

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Fig. 8. (a) Fouling mechanisms rates and membrane fouling resistances, as well as (b) their percentage contributions inside hollow fiber membranes during multiple filtration cycles using lysed CS feeds.

)

(

increase for both reversible and irreversible fouling resistances over multiple cycles (Fig. 11a); however, Rrev(n) and %Rrev(n) prevailed all cycles, but %Rrev(n) decreased in favor of %Rirr(n) for later cycles (Fig. 11b and Table S6). Moreover, Rirr(T) value at the 9th cycle was found to be higher than Rirr(T) values at the 16th cycle by other intact algae types. The increase in fouling irreversibility over multiple cycles was accompanied by predomination of IB over CF in all cycles (Fig. 11b)

and an increase of

dW dt

IB

at later cycles (Fig. 11b and Table S7), most

likely due to deposition of adhesive organic substances on the fouled ( ) membrane (cf. Section 3.2.2), while dW showed a decreasing trend. dt CF

In case of lysed CC, the fouling reversibility had linear increasing trend and %Rrev(n) was dominating in all cycles (Fig. 12); however, Rrev 12

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values were lower than in case of other lysed algae types (Table S6). Irreversible fouling showed also linear increasing trends; the increase in Rirr(T) values was higher compared to Rirr(n). Additionally, Rirr(n) values at early cycles were interestingly noticed to be lower compared to other lysed algae types (Table S6). Low irreversible fouling resistances were ( ) ( ) dW also accompanied by dW and values among the lowest in dt dt

elaborated for better understanding of fouling scenarios inside hollow fiber membranes during multiple filtration cycles. Generally, green and blue-green algae-containing feeds exhibited higher CDOC (including, BP, BB and LMWN), HOC (not in case of CS) and chlorophyll-a contents than diatom algae-containing feeds, at the same total algae particle volume. This could explain the overall severer membranes performance decay and higher blocking mechanism fouling rates caused by CS and AP feeds than in case of TH and CC feeds. Nevertheless, the trends for fouling reversibility per cycle and reversibility of the total fouling layer varied substantially depending on algae type and cell condition. This implies that nature of blocking fouling mechanisms and/or their impact on membranes performance may be altered during multiple filtration cycles, because of either different algae feeds characteristics (e.g., conformation, size, AOM matrix), or different interaction of algae sub­ stances with membrane material and/or already attached substances.

(n)

IB

CF

this study (Fig. 12a and Table S7); despite the fact that IB prevailed over CF in all cycles (Fig. 12b). 3.5. Elaboration of fouling scenarios inside hollow fiber membranes during mini-plant filtration experiments The knowledge gained concerning algae feeds characteristics, membranes performance, and membranesfouling propensity is

Fig. 9. Fouling mechanisms rates and membrane fouling resistances inside hollow fiber membranes during multiple filtration cycles using intact AP (a) and lysed AP (b). 13

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Fig. 10. Fouling mechanisms rates and membrane fouling resistances inside hollow fiber membranes during mini-plant multiple filtration cycles using intact TH (a) and lysed TH (b).

Moreover, lysed algae feeds were generally characterized by higher DOC contents, significantly lower chlorophyll-a and smaller particle size dis­ tribution, but with higher fouling potential than intact algae feeds, except for CC. Combination of pore blocking model and resistance-in-series model enabled reliable analysis of fouling mechanisms inside hollow fiber membranes over multiple filtration cycles. Generally, SB and IB were found to induce strong performance decay and high irreversible fouling, whereas CF was revealed to rather contribute into more fouling reversibility, depending on AOM composition. Most of mini-plant ex­ periments in this study comprised two fouling mechanisms, IB exhibited higher fouling rates than CF in all filtration cycles; however, overall fouling reversibility was essentially determined by the contributions thereof as well as fouling rates values. In such cases, one interesting approach to control algae fouling is to integrate flocculation as a

pretreatment to induce agglomeration in algae feed water prior to membrane filtration; as a result, %CF can be increased, and conse­ quently, fouling layer backwashability could be promoted. Constant fouling rates, as in case of intact algae feeds (except CC), were empha­ sized to be accompanied by good fouling reversibility, and stable membrane performance. On the other hand, in few fouling tests (i.e., using lysed CS), the three fouling mechanisms were found to occur, SB caused the worst fouling followed by IB then CF, besides the fouling irreversibility was the highest. In conclusion, the capability of esti­ mating the alteration in fouling resistances and contribution of fouling mechanisms over multiple cycles confer credibility to the current anal­ ysis approach. In details, rigorous permeability decline and less fouling reversibility by lysed CS, compared to intact CS, are related to higher BP ratio, Be­ sides, one might claim that low ℝLMWN and ℝLMWA, along with cell debris 14

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(as depicted by CLSM images), may rather contribute to internal fouling that interprets elevated Rirr(n) at early cycles and prevailing of SB over IB and CF in all cycles. Whereas, in case of intact CS, increasing %CF could guarantee more stable performance and better fouling backwashability. Contrary to other algae types, severe permeability decline per one cycle caused by lysed AP feed is correlated to high DOC (particularly BP and HOC). Furthermore, strong permeability decline per one cycle in

case of intact AP, despite of relatively moderate DOC ratio, is better related to the biggest algae cells size distribution that is also inferred by formation of the thickest fouling layers. But, in both cases, fouling backwashability was better compared to fouling caused by CS. Never­ theless, in case of lysed AP, high fouling reversibility could not over­ come the highest average IB and CF fouling rates, especially at early cycles. Consequently, in such cases, shortening filtration cycle period

Fig. 11. (a) Fouling mechanisms rates and membrane fouling resistances, as well as (b) their percentage contributions inside hollow fiber membranes during multiple filtration cycles using intact CC feeds. 15

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could improve potentially membranes performance, extend operation periods, and decrease chemical cleaning frequency. In case of TH feeds, notwithstanding mild membrane fouling and high fouling reversibility, %IB prevailed %CF in majority of filtration cycles; %IB was indeed among the highest in this work. However, based on the experience with green and blue-green microalgae feeds, consid­ erable %IB was always accompanied by moderate-to-low fouling reversibility. Such different impact of IB refers to an alteration in nature

of IB mechanism. To understand this phenomenon, hollow fiber mem­ branes performance at the first 10 L/m2 of the 1st cycle was analyzed (Figure S10). Lower normalized permeability decline rates were noticed for diatom microalgae feeds compared to green and blue-green micro­ algae feeds. Based on Huang, Young and Jacangelo [26], in IB mecha­ nism, deposited foulants particles on the membrane surface may compete with membrane pores for hosting newly approaching particles. Accordingly, lower fouling rate at initial filtration periods may indicate

Fig. 12. (a) Fouling mechanisms rates and membrane fouling resistances, as well as (b) their percentage contributions inside hollow fiber membranes during multiple filtration cycles using lysed CC feeds. 16

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irreversible fouling was found, despite of prevailing of cake filtration mechanism.

that IB was, most likely, occurring in the fouling layer as well, i.e., rectangular-shaped TH algae favored fouling layer formation. Subse­ quently, combined fouling phenomena are rather expected for next filtration cycles because of the random nature of fouling and hydraulic cleaning. This interesting scenario could also interpret high fouling layer reversibility during filtration of TH feeds. Moreover, in case of intact CC, %IB prevailed %CF in all cycles, be­ sides hydraulic irreversible fouling was the highest for an intact algae feed. High HOC fraction in the feed along with retained low molecular weight substances may strengthen the adhesion capacity of small-sized cylindrical CC algae cells inducing more %IB on the fouled surface rather than membrane pores that is consistent with low performance decay rate in Figure S10d and irregular increasing trend of IB fouling rate. Consequently, fouling inside hollow fiber membranes by feeds containing diatom microalgae are emphasized to occur via rather combined fouling scenarios, whose nature and impact on membranes performance are highly influenced by not only feed solution chemistry, but also microalgae characteristics (e.g., cell conformation and size), as well as the employed filtration and cleaning conditions.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Authors acknowledge the support by Dr. Johannes Koch at Imaging Centre, University of Duisburg-Essen, for CLSM analysis. Authors are also grateful to Christian Staaks from inge GmbH, Germany for fruitful collaboration and providing multibore® membranes. The first author acknowledges PhD scholarship offered by German Academic Exchange Service (DAAD). Appendix A. Supplementary data

4. Summary and conclusions

Supplementary data to this article can be found online at https://doi. org/10.1016/j.cej.2020.127723.

Fouling scenarios inside PES hollow fiber membranes during miniplant multiple filtration tests using algae-loaded water, in intact and lysed conditions without prior separation of algae cells, cell debris and AOM, were reliably investigated at operating conditions analogous to full-scale application. Employing special algorithm, it was possible, for the first time, to quantify statistically alterations in fouling mechanisms contributions and respective rates per one cycle as well as fouling reversibility percentage during multiple filtration cycles. The fouling propensity of hollow fiber membranes was properly correlated to feed and microalgae characteristics as well as retention trends for various AOM substances; the following conclusions were revealed.

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• Biopolymers, building blocks, and hydrophobic organic carbon had the strongest impact on membrane performance. • Good biopolymer retention was often accompanied by steep per­ formance decay per cycle, particularly in case of green and bluegreen microalgae. • In certain cases, good retention of low molecular weight AOM sub­ stances was correlated to combined fouling phenomenon, i.e., competitive blocking of membrane pores and fouling layer voids. • Lysing process altered substantially all algae feeds characteristics and resulted mostly in severe membrane fouling. • Microalgae cell conformation and size were found to influence the nature of intermediate blocking mechanism; helical microalgae cells tended to rather block the membrane pores resulting in a strong fouling, while rectangular and cylindrical microalgae cells showed a combined fouling phenomenon that resulted in mild membrane fouling. Furthermore, conclusions regarding the relation between fouling mechanism contributions and fouling reversibility: • Better fouling reversibility and stable performance were found in case of constant fouling rates per multiple filtration cycles, while varied fouling rates indicated less reversible combined fouling mechanisms. • Standard blocking mechanism caused the highest irreversible fouling • Intermediate blocking fouling rate was always higher than cake filtration fouling rate. The higher the intermediate blocking fouling rate the higher extent of irreversible fouling. • Higher cake filtration contribution in total membrane fouling resulted in better fouling reversibility; however, in cases, where hydrophobic organic carbon and biopolymers were high, strong 17

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