H2O2 for mitigating the fouling of a ceramic MF membrane caused by soluble algal organic matter

H2O2 for mitigating the fouling of a ceramic MF membrane caused by soluble algal organic matter

Journal of Membrane Science 493 (2015) 683–689 Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: www.elsevier...

539KB Sizes 0 Downloads 8 Views

Journal of Membrane Science 493 (2015) 683–689

Contents lists available at ScienceDirect

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

Effect of feedwater pre-treatment using UV/H2O2 for mitigating the fouling of a ceramic MF membrane caused by soluble algal organic matter Xiaolei Zhang, Linhua Fan n, Felicity A. Roddick School of Civil, Environmental and Chemical Engineering, RMIT University, GPO Box 2476, Melbourne, VIC 3001, Australia

art ic l e i nf o

a b s t r a c t

Article history: Received 21 April 2015 Received in revised form 2 July 2015 Accepted 12 July 2015 Available online 20 July 2015

Soluble algal organic matter (AOM) resulting from cyanobacterial blooms can cause severe fouling for water treatment membranes as it contains a large proportion of high molecular weight (MW) organics such as biopolymers and humic-like substances. UV/H2O2 advanced oxidation of feedwater containing AOM derived from Microcystis aeruginosa was investigated as a means of mitigating the fouling of a ceramic microfiltration (MF) membrane and degrading the algal toxin microcystin-LR. The result was compared with the pre-treatment using coagulation with ACH. UV/H2O2 treatment and coagulation achieved a marked and comparable reduction in total fouling resistance, however coagulation led to considerably lower irreversible fouling. The significant reduction in membrane fouling by the pretreatments was attributed to the effective breakdown/removal of the very high MW biopolymers ( Z20,000 Da) and high MW organic substances (  10,000 Da) in the AOM. UV/H2O2 treatment resulted in the generation of lower MW substances due to partial oxidation of the large molecules, and consequently greater irreversible membrane fouling compared with coagulation. Microcystin-LR was completely inactivated by UV/H2O2, whereas coagulation was ineffective for removing it. & 2015 Elsevier B.V. All rights reserved.

Keywords: Algal organic matter Ceramic membrane Fouling mitigation Microfiltration Water treatment

1. Introduction Ceramic membranes are being used increasingly in water and wastewater treatment due to their inherent advantages such as good selectivity, robustness and high water productivity, and their increasing cost competitiveness [1]. However, membrane fouling caused by naturally occurring organic matter in the feedwater remains a major limiting factor for most industrial membrane water treatment processes [2], as the fouling can lead to reduced productivity, reduced permeate quality, increased energy consumption and maintenance cost [3]. Worsening eutrophication problems in many aquatic systems can cause blooms of harmful algae and cyanobacteria, resulting in large amounts of soluble algal organic matter (AOM) entering water treatment processes and causing problems in water quality and treatment efficiency [4]. AOM has been demonstrated to cause severe fouling for low-pressure polymeric [5–8] and ceramic membranes [9,10] as it contains a high proportion of high molecular weight (MW) biopolymers such as polysaccharides and proteinaceous substances which have been identified as the most n

Corresponding author. Fax: þ 613 9639 0138. E-mail address: [email protected] (L. Fan).

http://dx.doi.org/10.1016/j.memsci.2015.07.024 0376-7388/& 2015 Elsevier B.V. All rights reserved.

problematic foulants for the membranes. Moreover, the blooms can lead to the release of harmful algal metabolites including toxins into the drinking water, posing a threat to human health [11]. Feedwater pre-treatment is a common practise for reducing membrane organic fouling as it can remove the organic components with high fouling potential or transform them into the organics with low fouling tendency prior to membrane filtration [12]. It has been demonstrated that treatment of the feedwater containing the AOM released from a common cyanobacterial bloom species Microcystis aeruginosa with conventional coagulants can effectively reduce the organic components causing reversible and irreversible fouling of a ceramic MF membrane [13]. Advanced oxidation processes (AOP) such as UV/H2O2 have been utilised to degrade organic compounds such as geosmin, methylisoborneol (MIB) and toxins derived from algal blooms in drinking water treatment [14]. AOP, which generate highly oxidising hydroxyl radicals (OH) that can break down large organic compounds into smaller molecules and eventually mineralise them [15], may have the potential for decreasing the concentration of membrane foulants and hence improving membrane performance [16]. It is of particular interest to integrate AOP treatment with ceramic membranes as the AOP can also be used to control product water quality, disinfect the water, and clean these membrane systems

X. Zhang et al. / Journal of Membrane Science 493 (2015) 683–689

which can withstand the harsh oxidising environment and the presence of residual oxidants. However, very limited work has been done to investigate the effect of UV/H2O2 process as a feedwater pre-treatment for improving the performance of low pressure membranes including ceramic water treatment membranes [17]. The aim of the present study was to investigate the effect of UV/H2O2 on the mitigation of the fouling of a ceramic MF membrane (0.1 mm, alumina) caused by the AOM released from M. aeruginosa at lab scale. For the UV/H2O2 pre-treatment tests, the AOM solution was subjected to two UV doses (fluences of 16 J cm  2 and 32 J cm  2, respectively) with initial H2O2 concentrations of 0.25 mM and 0.5 mM. The impact of the UV/H2O2 treatment on fouling mitigation was evaluated by membrane performance (i.e., flux decline in constant-pressure multi-cycle filtration, fouling resistance) and characterisation of the organic matter at the different stages of the treatments, and compared with coagulation with ACH. The effect of UV/H2O2 and coagulation on algal toxin removal was also investigated by determining the fate of the microcystin-LR spiked into the feedwater.

Relief valve Retentate valve Permeate valve P2

Heat exchanger

BF3

Membrane

684

Feed tank

Cooling water in

P1

Pump

BF3: Back-flush device P1, P2: Pressure gauge

Fig. 1. A schematic diagram of the ceramic MF membrane rig [19].

2.4. Multi-cycle MF tests 2. Experimental 2.1. Cultivation of algae and AOM extraction M. aeruginosa (CS 566/01-A01) was purchased from CSIRO Microalgae Research Centre (Tasmania, Australia). The algal cultures were grown in 5 L Schott bottles at 22 °C using MLA medium [18] under humidified aeration. A 16/8 h light/dark cycle was used to simulate natural light conditions. Algal cultures were harvested at the 35th day of growth (stationary phase). Centrifugation (3270g for 30 min) of the algal cell suspensions and the subsequent filtration of the supernatant (using 1 mm membranes) were conducted to extract the soluble AOM. 2.2. Feedwater preparation To mimic the presence of AOM in drinking water, the extracted AOM was diluted to a DOC concentration of 4.3 70.2 mg/L with tap water (1.470.1 mg DOC/L) to make the feedwater for the tests. The pH of the AOM solutions before and after the treatment was regulated at approximately 7 using 1 M HCl or 1 M NaOH. The impact of the feedwater pre-treatments on the algal toxin removal was investigated by dosing 15 mg L  1 microcystin-LR (Z95%, Sapphire Bioscience) into the prepared feedwater prior to the treatments. 2.3. Coagulation and UV/H2O2 treatment Coagulation was conducted at room temperature (20 72 °C) using a laboratory jar tester unit (Phipps and Bird, PB-700) with rapid mixing for 1 min at 200 rpm, followed by slow mixing for 20 min at 30 rpm. The optimum dosage determined in a previous study [13] of 5 mg Al3 þ L  1 ACH (aluminium chlorohydrate, Megapac 23, 40% w/w) was utilised. After the jar test, the supernatant of the coagulated water was immediately filtered (5 mm, Advantec) to remove flocs. UV/H2O2 treatment was carried out using an annular reactor with a centrally mounted UV lamp. The average irradiated area was 464 cm2, and the path length was 1.94 cm. A UVC lamp (39 W, Australian Ultra Violet Services, G36T15NU) was used to provide UVC irradiation (254 nm). The average fluence rate of the lamp was determined as 8.91 mW cm-2. UV doses of 16 J cm  2 (UV16) and 32 J cm  2 (UV32) and initial H2O2 concentrations of 0.25 mM and 0.5 mM were used for the feedwater pre-treatment.

MF tests were carried out with a single-channel tubular ceramic MF membrane (nominal pore size 0.1 mm, 0.005 m2, alumina, Pall). A schematic diagram of the ceramic filtration rig (XLAB5, Pall) is shown in Fig. 1. The rig is equipped with a progressing cavity pump (PCM, France), a heat exchanger, and a back-flush device (BF3). The transmembrane pressure (TMP) was measured as the average of inlet and outlet pressures, which were obtained from the pressure gauges P1 and P2. The multi-cycle filtration runs were conducted in inside-out and dead-end mode at a constant TMP of 7072 kPa. Approximately 450 mL of feedwater was filtered for each cycle. In this study, backpulsing was used for membrane cleaning. Backpulsing is an in situ method for cleaning the membrane by applying a pressure (normally 2 times higher than the forward pressure) on the permeate side. When the pressure is applied, the permeate liquid is forced back through the membrane to the feed side. As a result, the cake layer attached on the membrane is lifted and detached from the membrane surface. Backpulsing is commonly used for backwashing ceramic membranes, since they are able to withstand higher pressures. It was reported that a rapid (duration 2 s) high pressure backpulse (170 kPa) with compressed air could effectively restore the membrane flux during the MF of the secondary effluent for the same filtration system [19]. Thus, the same backpulse method was used at the end of each filtration cycle to remove reversible foulants from the membrane. Each test was run with 5 filtration cycles. All filtration tests were run in duplicate and the fouling results are reported as average values. The fouling resistance (R) value for each filtration trial was calculated by Eqs. (1) and (2) using the flux (J) values determined before and after conducting backpulsing. The Rtotal refers to the total fouling resistance after MF of the AOM solutions, the Rreversible/Rirreversible is associated with the reversible/irreversible fouling resistance and Rmembrane is the resistance of a clean membrane.

J=

ΔP μRtotal

Rtotal = R membrane + R reversible + Rirreversilbe

(1)

(2)

After each test, the fouled membrane was cleaned by soaking it in NaOCl solution (approximately 1000 ppm available chlorine) at 70 °C for 45 min as suggested by the manufacturer. The chemical

X. Zhang et al. / Journal of Membrane Science 493 (2015) 683–689

2.5. Unified membrane fouling index (UMFI) The unified membrane fouling index (UMFI) developed by Huang et al. [20] was used to assess membrane performance for multi-cycle MF under constant pressure. The detailed procedure, and the equation derivations and calculations can be found elsewhere [21]. The model for UMFI is expressed by Eq. (1), where the UMFI can be calculated using linear regression when the reciprocal of the normalised flux ( J0 /J ) increases linearly with the specific permeate volume (V).

J

= 1 + UMFI × V

1.6

Un-treated AOM 0.25 mM H 2O 2 UV16

1.4

0.50 mM H 2O 2 UV16 0.25 mM H 2O 2 UV32

1.2

0.50 mM H 2O 2 UV32 Coagulation ACH

0.8 0.6 0.4 0.2 0.0 0

100

200

300

400

500

-2

Specific volume (Lm ) 16

(3)

However, the reciprocal of the normalised flux ( J0 /J ) may be a non-linear function of V, where the membrane fouling is not linearly dependant on the specific volume. In this case, UMFI can be calculated using a 2-point method instead of fitting all the filtration data to the equation (i.e., the first and the last data points can be used to determine the index). In this study, the initial and final points of the multi-cycle MF results were used to calculate the UMFI.

Backpulsing

1.0

Un-treated AOM 0.25 mM H 2O 2 UV16

14

0.50 mM H 2O 2 UV16 12 10

J0 /J

J0

1.8

Normalized flux (J/J0)

cleaning procedure fully restored the pure water flux of membrane. The cleaned membrane was then used in further experiments. Prior to each test, Milli-Q water was filtered through the membrane at 70 kPa for 10 min to remove membrane cleaning agent. After that, the initial water flux was determined by filtering the Milli-Q water at 70 kPa for 10 min.

685

y=1+0.0170x y=1+0.0180x y=1+0.0050x

0.25 mM H 2O 2 UV32

y=1+0.0021x

0.50 mM H 2O 2 UV32

y=1+0.0013x

ACH

y=1+0.0015x

8 6 4

2.6. Analytical methods 2

DOC and UV absorbance at 254 nm (UVA254) were determined using a Sievers 820 TOC analyser and a UV/vis spectrophotometer (UV2, Unicam), respectively. pH was measured with a Hach Sension 156 pH meter. The apparent molecular weight distribution of the AOM was determined by size exclusion chromatography using liquid chromatography with DOC and UV detection (LC–OCD– UVD) at the Water Research Centre of the University of New South Wales, Sydney, Australia. The LC–OCD system (LC–OCD Model 8, DOC-Labour Dr. Huber, Germany) utilised a SEC column (Toyopearl TSK HW-50 S, diameter 2 cm, length 25 cm) and the chromatograms were processed using the Labview based programme Fiffikus (DOC-Labor Dr. Huber, Germany). The details of this technique are described by Huber et al. [22]. The microcystin-LR concentration of water samples was measured using Abraxis Microcystin Strip Test PN 520020 (0–5 ppb detection limit) and PN 520022 (0–10 ppb detection limit) obtained from Abraxis LLC (Warminster, PA, USA). The Abraxis Microcystin Strip Test is based on a rapid immunochromatographic method, which recognises microcystins and nodularins and their congeners by specific antibodies [23]. The reliability of Abraxis test strips for the determination of microcystin-LR in a wide range of water matrices was validated using high performance liquid chromatography by Roddick et al. [24]. All sample analyses were duplicated or triplicated as appropriate, and average values are reported.

3. Results 3.1. Impact of UV/H2O2 and coagulation treatment of feedwater on MF performance Multi-cycle MF tests were conducted in order to obtain the UMFI for the assessment of membrane fouling and its mitigation

0 0

100

200

300

400

500

-2

Specific volume (Lm )

Fig. 2. Multi-cycle MF tests on the un-treated, UV/H2O2 treated and coagulated AOM solutions (a) normalised flux, (b) UMFI (calculated using the data points of the first (v¼ 0, J0/J¼ 1) and last filtration cycle).

under various feedwater pre-treatment conditions. The normalised flux for the un-treated, UV/H2O2 treated and coagulated AOM solutions is presented in Fig. 2a. The UMFI values are shown as the slopes of all straight lines plotted using the two-data point method with the flux data (Fig. 2b). MF of the un-treated AOM solution led to a severe flux decline for all 5 filtration cycles, with UMFI of 0.0170 m2 L  1 obtained. UV/H2O2 treatment with the UV dose of 16 J cm  2 (UV16) and initial H2O2 concentration of 0.25 mM gave only a slight improvement in flux compared with the un-treated AOM solution for the first 2 filtration cycles. After that, their flux was almost the same. Increasing the initial H2O2 concentration to 0.50 mM led to an improved flux over the five filtration cycles. The UMFI of the UV/H2O2 treated solution was markedly lower at the initial H2O2 concentration of 0.5 mM compared with that at 0.25 mM (0.0050 cf. 0.0180 m2 L  1). Oxidative treatment with a higher UV dose (32 J cm  2) (UV32) also led to a greater flux improvement for the both initial H2O2 concentrations. Coagulation with ACH (5 mg Al3 þ L  1) gave comparable flux improvement to the UV 32/0.50 mM H2O2 treatment. The UMFI values for the AOM solutions treated by the two methods were also similar (i.e., 0.0015 cf. 0.0013 m2 L  1). However, considerably higher reversible fouling resistance and lower irreversible fouling resistance for the ACH-treated water as shown for each filtration cycle compared with the UV 32/0.50 mM H2O2-treated water, implying the different fouling mitigation effects of the two treatments (see Fig. S1 in Supplementary Materials). Characterisation

686

X. Zhang et al. / Journal of Membrane Science 493 (2015) 683–689

6

100

Coagulation ACH UV16/0.25 mM H2O2

5

70

UV32/0.25 mM H2O2 UV16/0.50 mM H2O2

4

60

UV32/0.50 mM H2O2

DOC removal (%)

80

LMW acid and HS Building blocks

OCD response

90

50 40 30

AOM feed Coagulated AOM feed UV/H 2O2 treated AOM feed

Humic like HMWS

3

Biopolymers

2

1

20 0 20

10

40

0

70

UV32/0.25 mM H2O2

DOC rejection (%)

100

UV16/0.50 mM H2O2 UV32/0.50 mM H2O2

50 40 30

AOM feed Coagulated AOM feed UV/H2O2 treated AOM feed

0.3

UVD response

80

un-treated AOM Coagulation UV16/0.25 mM H2O2

60

80

0.4

100 90

60

Retention time (min)

0.2

0.1

20 10 0 Fig. 3. Comparison of UV/H2O2 and coagulation feed pre-treatment (a) DOC removal and (b) DOC rejection by the ceramic membrane.

of the changes in organic matter before and after the pre-treatments was therefore carried out in order to obtain a better understanding of the fouling mitigation mechanisms involved. 3.2. Characterising the impact of UV/H2O2 and coagulation treatment of feedwater on membrane fouling 3.2.1. DOC The DOC removal by the UV/H2O2 treatment and coagulation, and rejection by the membrane were examined (Fig. 3). There was greater DOC reduction for the higher UV dose at the same H2O2 dosage (20% cf. 36% for 0.25 mM and 40% cf. 50% for UV16 and UV32, respectively). A markedly higher DOC removal was obtained by coagulation (72%). The un-treated AOM solution resulted in greater DOC rejection (52%) by the membrane (Fig. 3b) compared with the pre-treated AOM solutions (10–12%) except for the UV16/ 0.25 mM H2O2 treatment (42%), suggesting that the pre-treatments led to the removal or breakdown of the AOM components which were responsible for the severe fouling of the ceramic membrane. 3.2.2. Size exclusion chromatography The impact of the coagulation and UV/H2O2 treatment on AOM was considered to be significantly different. UV/H2O2 could completely mineralise the small organic molecules and/or partially oxidise large molecules (such as bioploymers) and consequently break them down into smaller molecules. For the coagulation, the

0.0 20

40

60

80

100

Retention time (min) Fig. 4. Comparison of LC chromatograms for the un-treated AOM, coagulated AOM and UV/H2O2 treated AOM (a) OCD response, (b) UVD response. (HMW ¼high molecular weight substance, HS ¼humic substance).

organic matter would be removed by binding with insoluble flocs and then precipitation. In order to examine the effects of coagulation and UV/H2O2 treatment on AOM, the apparent molecular weight distributions of the un-treated, coagulated and UV32/ 0.5 mM H2O2 treated (AOM solutions were determined by LC– OCD–UVD (Fig. 4). The un-treated AOM solution contained significant amounts of very high (Z 20,000 Da) and high MW substances (  10,000 Da), medium-MW components (i.e., humic-like substances, 1000 Da and building blocks, 350–500 Da), and lowMW substances ( o350 Da). The peaks for biopolymers, humics and building blocks were reduced significantly after coagulation, whereas the peak for high MW substances (smaller biopolymers) disappeared. The results suggested that coagulation was very effective for removing AOM compounds over a wide MW range. For the UV/H2O2 treated AOM, the very high MW biopolymers were degraded almost completely, whereas less reduction in high MW substances, humic-like and building block compounds was obtained compared with coagulation. A significant increase in low MW acids and humic substances (HS) was shown, which was attributed to the production of smaller molecules from the breakdown of larger molecules during the oxidative treatment. These results demonstrated that UV/H2O2 treatment could effectively decrease the concentration of the high fouling potential biopolymers.

X. Zhang et al. / Journal of Membrane Science 493 (2015) 683–689

compounds in AOM were more susceptible to the UV/H2O2 treatment than coagulation. To gain further information about the impact of the treated organic matter on the membrane filtration, the molecular weight distributions of the un-treated, coagulated and UV/H2O2 treated AOM before and after MF were compared (Fig. 5). After MF of the un-treated AOM solution, the very high MW substances (biopolymers) were retained almost completely by the membrane (Fig. 5a), whereas the high MW substances, humic-like and building block-like compounds were moderately retained. MF of the coagulated AOM solution resulted in small reductions in the biopolymers and low MW substances (Fig. 5b), whereas significantly greater reductions in the remaining high MW substances, and low MW acid and HS were shown for the UV/H2O2 treated sample (Fig. 5c).

6

AOM feed AOM permeate

4

3

2

1

0 20

40

60

80

100

Retention time (min) 6

OCD response

5

Coagulation feed Coagulation permeate

4

3

2

1

0 20

40

60

80

100

Retention time (min)

6

5

UV/H2O2 feed

OCD response

UV/H2O2 permeate

3.3. Fate of algal toxin during UV/H2O2-MF and coagulation-MF processes The concentration of microcystin-LR in the un-treated and pretreated AOM solutions, and MF permeate, was measured to investigate the fate of the algal toxin during the various treatments (Fig.6). The microcystin-LR concentration in the MF permeate for the un-treated AOM solution was about 33% lower than that in the feedwater, indicating some rejection of the toxin compounds by the membrane. A control filtration test on the solution with tap water and microcystin showed that no microcystin was rejected by the MF membrane. This suggested that microcystin molecules could pass through the MF membrane completely in the absence of the AOM, as the molecular size of microcystin-LR (MW 995 Da) was much smaller than the pore size of the MF membrane. The rejection of microcystin molecules by the membrane in the presence of AOM was most likely due to the pore restriction by and/or the entrapment of the microcystin molecules in the AOM foulant layer formed on the membrane surface. It was also possible that some microcystin molecules attached to the large AOM molecules as a result of molecular interaction. In order to clarify the cause of the retention, the concentration of microcystin in the permeate from the first 5 min of MF was monitored. No microcystin rejection was observed during this period and the rejection started to increase with filtration time (see Fig. S2 in Supplementary materials). This suggested that the rejection of microcystin molecules was not due to molecular

4

25

Un-treated AOM ACH treated AOM Tap water ACH treated tap water

3

20 2

1

0 20

40

60

80

100

Retention time (min) Fig. 5. Comparison of LC–OCD chromatograms for the (a) un-treated, (b) coagulated and (c) UV/H2O2 treated AOM before and after MF.

For the LC–UVD chromatograms, the only UV-absorbing compounds in the AOM were humic-like, building blocks and LMW acids and HS. The overall UVD response for coagulated AOM was higher than for the UV/H2O2 treated AOM, despite the OCD response for coagulated AOM being much lower than for the UV/H2O2 treated AOM. This indicated that the high UV-absorbing

-1 MC-LR (ug L )

OCD response

5

687

15

10

5

0

Feed

Permeate

Fig. 6. Comparison of the microcystin concentration in the un-treated and coagulated feed water/tap water before and after MF: (a) AOMþ microcystin and (b) tap waterþ microcystin.

688

X. Zhang et al. / Journal of Membrane Science 493 (2015) 683–689

interaction, but to the AOM compounds on membrane surface which served as a barrier to their passage through the membrane. The concentration of microcystin spiked into both tap water and AOM solution remained unchanged after coagulation, indicating that coagulation was ineffective for removing the algal toxin. The result was consistent with other reports [25,26] where conventional coagulation failed to remove microcystin. No decrease in microcystin concentration was shown after the MF of the coagulated AOM or tap water containing the toxin. This was expected as coagulation resulted in the removal of a great amount of large AOM molecules from the feedwater, and hence reduced the possibility of formation of a dense AOM foulant layer on the membrane surface to prevent microcystin molecules passing through. The effectiveness of UV/H2O2 for degrading microcystin was determined using UV/0.25 mM H2O2 and UV/0.50 mM H2O2 treatment on the AOM solutions made by spiking microcystin-LR (15 mg L  1) into tap water and tap water containing AOM. No microcystin was detected in either of the solutions after 1 min of irradiation (UV dose 0.5 J cm  2), demonstrating that UV/H2O2 treatment was very effective for degrading the algal toxin. The effect of UV alone (without the addition of H2O2) on microcystin removal was also examined (see Fig. S3 in Supplementary materials). The results showed that the microcystin could be inactivated by direct UVC (254 nm) irradiation, but a higher UV dose (2.5 J cm  2) was required. This UV dose is much higher than that commonly used in large scale disinfection practice (0.02–0.2 J cm  2) [27], which may suggest that the algal toxin would not be completely inactivated under the operating conditions generally used for disinfection.

4. Discussion In general, there are two fouling reduction mechanisms associated with feedwater pre-treatment: reduction of organic loading and structural changes in the organic matter [12]. In this study, coagulation with ACH gave a markedly higher reduction in DOC than the UV32/0.5 mM H2O2 treatment, yet they achieved comparable flux improvement efficiency. This may indicate both of these two mechanisms played important roles in fouling mitigation when UV/H2O2 feed pre-treatment was used. The significant reduction in flux declines after coagulation and UV/H2O2 treatment was attributed to the effective reduction in very high MW biopolymers and high MW substances as shown by the LC–OCD results. Compared with coagulation, the UV/H2O2 treated feedwater contained less biopolymers, but additional lower MW substances. The increased concentration of lower MW compounds resulting from the cleavage of the higher MW compounds by the oxidative treatment led to greater irreversible fouling of the membrane, as indicated by their greater rejection by the membrane. The smaller compounds could have entered the inner pore structure of the membrane, resulting in hydraulically irreversible fouling due to adsorption and/or entrapment. The positive effect of the UV/H2O2 treatment in mitigating membrane fouling caused by natural organic matter has also been reported in a study by Malek et al. [17,28], who found UV-based oxidation feed pre-treatment could significantly mitigate the organic fouling of a polymeric MF membrane. They also found that the irreversible fouling was increased due to the small molecules generated from the irradiation process. Microcystin-LR was retained to a considerable extent by the ceramic MF membrane due to the presence of AOM in the feedwater. This may imply that water treatment plants need to implement proper measures to manage the membrane reject/retentate streams during cyanobacterial blooms, as the reject may

contain a significant concentration of algal toxin. The effective inactivation of microcystin by direct UV irradiation provided that the dose is sufficient suggests that the membrane retentate could be treated by a UV disinfection system to eliminate the toxin and hence reduce the associated risk. Feedwater coagulation was an effective approach to maintain permeate flux and mitigate irreversible membrane fouling. However, it was ineffective for removing the algal toxin, which may require a pre- or post-treatment such as UV irradiation.

5. Conclusions The impact of UV/H2O2 treatment of feedwater for mitigating the fouling of a ceramic membrane caused by soluble algal organic matter was investigated and compared with coagulation. The effect of UV/H2O2 for degrading microcystin-LR was also examined to justify the applicability of UV/H2O2 treatment for water quality control during cyanobacterial blooms. The UV/H2O2 process could achieve a marked improvement in flux performance, which was comparable to the coagulation treatment using ACH. However, coagulation performed better than UV/H2O2 for mitigation of irreversible fouling as the UV/H2O2 led to the generation of additional smaller molecules, resulting in their greater access to the membrane internal pore structures and hence greater irreversible fouling. However, the UV/H2O2 process was very effective for breaking down the microcystin, whereas coagulation was ineffective for removing it. This study demonstrated that, unlike coagulation, UV/H2O2 treatment of the feedwaer could simultaneously enhance MF performance and control product water quality during cyanobacterial blooms in water catchments. Furthermore, the UV/H2O2 treatment does not produce any sludge compared with coagulation, which would significantly save the costs on sludge management. However, AOP are generally considered to be more expensive than coagulation processes due to their higher capital costs and energy consumption [29], more detailed cost analyses for full-scale applications should therefore be conducted to justify their utilisation. Further work may also be required for mitigating the internal membrane fouling caused by the AOP-treated water, with a view to further enhancing the membrane performance.

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2015.07. 024.

References [1] B. Hofs, J. Ogier, D. Vries, E.F. Beerendonk, E.R. Cornelissen, Comparison of ceramic and polymeric membrane permeability and fouling using surface water, Sep. Purif. Technol. 79 (2011) 365–374. [2] P. Bacchin, P. Aimar, R.W. Field, Critical and sustainable fluxes: theory, experiments and applications, J. Membr. Sci. 281 (2006) 42–69. [3] C.R. Bartels, M. Wilf, K. Andes, J. Iong, Design considerations for wastewater treatment by reverse osmosis, Water Sci. Technol. 51 (2005) 473. [4] S. Babel, S. Takizawa, Microfiltration membrane fouling and cake behavior during algal filtration, Desalination 261 (2010) 46–51. [5] F. Qu, H. Liang, Z. Wang, H. Wang, H. Yu, G. Li, Ultrafiltration membrane fouling by extracellular organic matters (EOM) of Microcystis aeruginosa in stationary phase: influences of interfacial characteristics of foulants and fouling mechanisms, Water Res. 46 (2012) 1490–1500. [6] Y. Goh, J. Harris, F. Roddick, Impact of Microcystis aeruginosa on membrane fouling in a biologically treated effluent, Water Sci. Technol. 63 (2011) 2853–2859. [7] Y. Goh, J. Harris, F. Roddick, Reducing the effect of cyanobacteria in the microfiltration of secondary effluent, Water Sci. Technol. 62 (2010) 1682.

X. Zhang et al. / Journal of Membrane Science 493 (2015) 683–689

[8] N. Lee, G. Amy, J.-P. Croué, Low-pressure membrane (MF/UF) fouling associated with allochthonous versus autochthonous natural organic matter, Water Res. 40 (2006) 2357–2368. [9] X. Zhang, L. Fan, F.A. Roddick, Influence of the characteristics of soluble algal organic matter released from Microcystis aeruginosa on the fouling of a ceramic microfiltration membrane, J. Membr. Sci. 425–426 (2013) 23–29. [10] X. Zhang, L. Fan, F.A. Roddick, Understanding the fouling of a ceramic microfiltration membrane caused by algal organic matter released from Microcystis aeruginosa, J. Membr. Sci. 447 (2013) 362–368. [11] Y. Wu, P.G. Kerr, Z. Hu, L. Yang, Removal of cyanobacterial bloom from a biopond–wetland system and the associated response of zoobenthic diversity, Bioresour. Technol. 101 (2010) 3903–3908. [12] H. Huang, K. Schwab, J.G. Jacangelo, Pretreatment for low pressure membranes in water treatment: a review, Environ. Sci. Technol. 43 (2009) 3011–3019. [13] X. Zhang, L. Fan, F.A. Roddick, Feedwater coagulation to mitigate the fouling of a ceramic MF membrane caused by soluble algal organic matter, Sep. Purif. Technol. 133 (2014) 221–226. [14] H. Ou, N. Gao, Y. Deng, H. Wang, H. Zhang, Inactivation and degradation of Microcystis aeruginosa by UV-C irradiation, Chemosphere 85 (2011) 1192–1198. [15] K. Liu, F.A. Roddick, L. Fan, Impact of salinity and pH on the UVC/H2O2 treatment of reverse osmosis concentrate produced from municipal wastewater reclamation, Water Res. 46 (2012) 3229–3239. [16] W. Song, V. Ravindran, B.E. Koel, M. Pirbazari, Nanofiltration of natural organic matter with H2O2/UV pretreatment: fouling mitigation and membrane surface characterization, J. Membr. Sci. 241 (2004) 143–160. [17] F. Malek, J.L. Harris, F.A. Roddick, Interrelationship of photooxidation and microfiltration in drinking water treatment, J. Membr. Sci. 281 (2006) 541–547. [18] C. Bolch, S. Blackburn, Isolation and purification of Australian isolates of the toxic cyanobacterium Microcystis aeruginosa Kütz, J. Appl. Phycol. 8 (1996) 5–13.

689

[19] S.T. Nguyen, Mitigation of Membrane Fouling in Microfiltration & Ultrafiltration of Activated Sludge Effluent for Water Reuse (Ph.D. thesis), School of Civil, Environmental & Chemical Engineering, RMIT University, 2010. [20] H. Huang, T.A. Young, J.G. Jacangelo, Unified membrane fouling index for low pressure membrane filtration of natural waters: principles and methodology, Environ. Sci. Technol. 42 (2007) 714–720. [21] A.H. Nguyen, J.E. Tobiason, K.J. Howe, Fouling indices for low pressure hollow fiber membrane performance assessment, Water Res. 45 (2011) 2627–2637. [22] S.A. Huber, A. Balz, M. Abert, W. Pronk, Characterisation of aquatic humic and non-humic matter with size-exclusion chromatography–organic carbon detection–organic nitrogen detection (LC–OCD–OND), Water Res. 45 (2011) 879–885. [23] J. Metcalf, G. Codd, Analysis of cyanobacterial toxins by immunological methods, Chem. Res. Toxicol. 16 (2003) 103–112. [24] F. Roddick, A. Meizler, T. Nguyen, L. Fan, Detection of microcystin-LR in lagoontreated water by Abraxis strip test, in: Proceedings of Ozwater, 2011. [25] K. Himberg, A.M. Keijola, L. Hiisvirta, H. Pyysalo, K. Sivonen, The effect of water treatment processes on the removal of hepatotoxins from Microcystis and Oscillatoria cyanobacteria: a laboratory study, Water Res. 23 (1989) 979–984. [26] B.-L. Yuan, J.-H. Qu, M.-L. Fu, Removal of cyanobacterial microcystin-LR by ferrate oxidation–coagulation, Toxicon 40 (2002) 1129–1134. [27] USEPA, Ultraviolet disinfection guidance manual for the final long term 2 enhanced surface water treatment rule, US Environmental Protection Agency, Washington, DC, 2006. [28] F. Malek, J. Harris, F. Roddick, Photooxidative pretreatment to improve sustainable operation of the microfiltration of drinking water, Dev. Chem. Eng. Miner. Process. 14 (2006) 219–226. [29] O. Autin, C. Romelot, L. Rust, J. Hart, P. Jarvis, J. MacAdam, S.A. Parsons, B. Jefferson, Evaluation of a UV-light emitting diodes unit for the removal of micropollutants in water for low energy advanced oxidation processes, Chemosphere 92 (2013) 745–751.