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drinking water, nitrate and perchlorate removal, ion exchange membrane bioreactor, ..... lead to a less favourable hydrodynamic situation in the biocompartment; ...
Biofouling diagnosis and effect of membrane cleaning in an ion exchange membrane bioreactor A. R. Ricardo, R. Valério, M.A.M. Reis, J.G. Crespo, S. Velizarov REQUIMTE / CQFB, Department of Chemistry, FCT, Universidade Nova de Lisboa, P-2829-516 Caparica, Portugal Tel: +351 21 294 83 00, Fax: +351 21 294 85 50, email: [email protected]

Abstract This study is focused on the effect of the membrane module configuration on the performance of an Ion Exchange Membrane Bioreactor (IEMB) for treating drinking water contaminated with the inorganic anionic pollutants nitrate and perchlorate. This hybrid process combines the transport of these two target anions from contaminated water through an anion exchange membrane with their biological reduction in a separate compartment. The performance of a plate-and-frame module configuration, consisting of a series of anion exchange membranes was investigated. It was found that water streams contaminated with ClO 4 and NO 3 were effectively treated, keeping their concentrations below respective recommended levels for drinking water supplies. The IEMB operation with this module allowed for addressing common problems encountered in practice such as membrane fouling effects. It was found out that the biofilm formed at the membrane surface contacting the water compartment can be minimized by controlling ethanol addition to the biocompartment. Alkaline treatment of membrane with 1 % NaOH generally restored the original membrane properties. A simulated membrane ageing protocol due to cleaning proved that the alkaline treatment did not significantly affected the properties of an anion-exchange membrane Ralex AMH-PES for at least 10 repeated washing cycles. Keywords drinking water, nitrate and perchlorate removal, ion exchange membrane bioreactor, biofouling, membrane cleaning

INTRODUCTION The Ion Exchange Membrane Bioreactor (IEMB) is a process based on Donnan dialysis that combines the transport of charged anionic micropollutants (e.g., nitrate, perchlorate and bromate) through an appropriate anion exchange membrane with their biological reduction to harmless products [Velizarov et al, 2000/2001]. The membrane physically separates the treated water from the biomedium, thus avoiding secondary water contamination by microbial cells, nutrients and metabolic by-products. Moreover, due to the biological reduction of the pollutants no concentrated toxic brine is produced. Chloride added to the biocompartment is used as a major driving counter-ion to the counter transport of target anionic pollutants from the water to the biocompartment. Due to Donnan exclusion of the coions (cations in this case), since the total electrolyte concentration is higher in the biocompartment, an electrical potential difference is established between the two solutions, thus causing transport of nitrate and perchlorate from the water to the biocompartment against their own concentration gradients. Once in the biocompartment, they are reduced to innocuous species (nitrogen gas and chloride, respectively) by an anoxic mixed microbial culture. The process efficiency for the simultaneous removal of nitrate and perchlorate from contaminated water streams has already been proved [Matos et al, 2006]. However, these studies were performed in a laboratory-scale module, in which a single membrane sample was separating two identical flow channels, a configuration that did not require inclusion of spacers. Therefore, considering a possible large-scale application, the performance of a plate-and-frame module configuration with a number of anion-exchange membranes and spacers was investigated. This design allows also to better address situations expected to occur in real operation facilities such as membrane fouling. Ion-exchange membrane fouling is a potential drawback since it can reduce the flux, increase membrane resistance and energy consumption [Wang et al, 2011]. In drinking water treatment facilities, the principal fouling agent is natural organic matter (NOM) composed of various organic compounds including extracellular polymeric substances (EPS) [Porcelli and Judd, 2010]. Anion-exchange membranes are particularly susceptible to fouling since in natural waters the colloids present are usually negatively charged [Wang et al, 2011], thus facilitating electrostatic interactions with the positively charged membrane fixed functional groups. Alkaline treatment using NaOH is the most common one for cleaning membranes fouled with NOM since it can remove organic foulants by hydrolysis and solubilisation mechanisms [Liu et al, 2006]. However, NaOH has been also found to affect membrane chemical structure and properties [Zhu and Nyström,1998; Guo et al, 2009]. Therefore, membrane ageing, i.e. membrane degradation in time, is

an important issue when dealing with chemical cleaning, since the membrane lifespan can be significantly reduced. Since the major operating costs in ion-exchange membrane assisted processes are usually related to membrane replacement [Dammak et al, 2009], it is essential to maintain the original membrane properties as long as possible. This work aims to demonstrate the feasibility of using a plate-and-frame membrane module for an IEMB case study: treatment of tap water contaminated with nitrate and perchlorate and to investigate the effect of membrane fouling, especially that occurring on the membrane surface contacting the water compartment, on the process performance. Alkaline treatment was applied for membrane cleaning and its effect on physical parameters such as membrane density, water content and thickness as well as on the anion-exchange capacity was investigated.

MATERIALS AND METHODS Experimental set-up IEMB studies. The polluted water was prepared with tap water from the Lisbon public network supplemented with 60 ppm of nitrate and 100 ppb of perchlorate. The experiments were performed in a modified electrodialysis module, EDR-Z-Mini purchased from Mega (Czech Republic) containing 12 anion-exchange membranes of the Ralex AMH-PES type (Mega, Czech Republic) (see Figure 1). In this modified electrodialysis unit, only anion-exchange membranes were used and the height of the channels through which biomedium was circulating was increased to 4 mm (instead of the commonly used 0.8 mm). This increase was done to allow for biofilm development in these channels, which were linked to a stirred anoxic vessel (thus forming a biocompartment). To the stirred anoxic vessel, fresh biomedium (K 2 HPO 4 1 g/L; KH 2 PO 4 0.6 g/L; NH 4 Cl 0.233 g/L; MgSO 4 0.1 g/L; NaCl 5.86 g/L) was continuously added. An enriched mixed microbial culture, originally taken from a wastewater facility was used to reduce nitrate and perchlorate in the biocompartment. Ethanol was used as the carbon source and electron donor and its concentration in the biofeed was changed, depending on the objective of the experiment. The hydraulic retention time in the biocompartment was maintained at 5.84 days and the recirculation through the membrane module at a flow rate of 1620 ml/min. The other module channels (diluate channels in the original electrodialysis module) were linked to the treated water compartment. The solution in the water compartment was recirculated at a flow rate of 700 ml/min and polluted water was continuously fed and withdrawn from this compartment at a flow 2 rate of 3.6 ml/min to guarantee a water treatment rate of 0.204 L/(m .h). In the two terminal (electrode) compartments, deionised water was recirculated at a flow rate of 500 ml/min. Donnan dialysis studies. The Donnan dialysis experiments were performed in the modified electrodialysis module with the aim to follow possible membrane fouling on the membrane surface contacting the treated water in the absence of ethanol and microbial culture in the biocompartment. This experiment was conducted under the same operating conditions as for the IEMB experiments except for adding inoculum and ethanol to the biocompartment.

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Figure 1: Schematic representation of the nitrate, perchlorate and chloride transport in an IEMB plate-and-frame module. For simplicity reasons, only six anion-exchange membranes are shown

Ethanol permeation studies. These studies were performed in the modified electrodialysis module using 1 g/L aqueous solution of ethanol in the feed solution (dilute channel) and deionized water in the receiving solution (concentrate channel). In both compartments, the solutions (1L) were recirculated at 700 ml/min. Samples from both compartments were taken for ethanol determination and the ethanol diffusion coefficient was estimated according to the procedure followed in [Fonseca et al, 2000].

Membrane cleaning Cleaning of membranes after IEMB operation was performed in three steps. Firstly, the membrane module was unset and the fouling material was physically removed by scrapping it from the membrane surface with a spatula (Figure 2A) and rinsing the membrane with deionised water. Then, the membranes and the spacers were stacked again and a chemical cleaning with 1% NaOH aqueous solution was performed (Figure 2B), as indicated by the manufacter (Mega, Czech Republic). The alkaline solution was recirculated in both water and biocompartment channels for 30 min at 700 ml/min. Finally, the module channels were rinsed with deionised water until a neutral pH.

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After chemical cleaning Figure 2: Photos of a membrane before and after cleaning. A: Biofilm scrapping from the membrane surface contacting the biocompartment; B: Membrane before and after cleaning with 1 % NaOH

Accelerated ageing A new membrane sample was submitted to consecutive chemical cleaning procedures. The membrane was submerged in a flask containing 100 ml of a 1% aqueous solution of NaOH for 30 minutes at 30ºC under orbital agitation of 200 rpm. Then, the membrane was washed with deionised water until a pH around 7 was obtained. After each cleaning sequence, a small piece of this membrane was cut to investigate the mechanical and chemical characteristics of the membrane. Between each cleaning procedure, the membrane was stored in 0.1% NaCl overnight.

Methods of membrane characterization Membrane apparent density was determined by measuring thickness and volume of small square 2 pieces (4 cm ) of membrane. Membrane thickness was measured with a micrometer and the density was determined by calculating the ratio of membrane wet weight in Cl form to its corresponding geometric volume. The water uptake (u) was determined as the fraction:

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in which the W h and W d represent correspondingly the wet and dry weight of the membrane sample. For ion-exchange capacity determination, the membrane was equilibrated for 16 h in 1M aqueous solution of HCl and rinsed with deionised water in order to remove the free chloride anions. The samples were then equilibrated in 20 ml of 0.5 M aqueous solution of Na 2 SO 4 for converting the 2membrane from a Cl form to a SO 4 form, according to the Mohr method [Li et al, 2006]. The liberated Cl were quantified by ion-exchange chromatography and the ion-exchange capacity determined by dividing the number of moles of Cl released by the membrane dry weight. All tests were performed in triplicate, except for the membrane thickness measurements which were repeated 6 times.

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The concentrations of NO 3 , ClO 4 and Cl were analyzed using a “Dionex” ion exchange chromatography system. The NO 3 and Cl analyses system consisted of an ED50 electrochemical detector and Ionpac AG9 Guard and Analytical AS9 (4mm) columns and an Anion Suppressor-Ultra (4mm) at 30ºC. In the case of ClO 4 , the columns used were AG16 (guard) and AS16 (analytical) column, respectively. The limit of ClO 4 detection was 1 ppb when injecting a sample of 1 ml. Ethanol was measured by HPLC with a Biorad Aminex HPX-87H column using 0.01N H 2 SO 4 aqueous solution at 0.5 ml/min as the mobile phase.

RESULTS AND DISCUSSION IEMB performance The treated water nitrate and perchlorate levels were found to be below the recommended ones for both ions in drinking water supplies during the whole duration of the experiment (Figure 3A). This performance was observed even when the microbial culture was not reducing efficiently the two pollutants in the biocompartment (Figure 3B). As it can be seen, during the first days of operation, nitrate and perchlorate accumulated in the biocompartment to values higher than those in the treated water. However, since the ion transfer is governed by Donnan dialysis principles, the addition of chloride to the biocompartment guaranteed efficient transport of nitrate and perchlorate against their individual concentration gradients. It is important to note also that the IEMB operated with the plateand-frame module showed very similar results if compared to those obtained in a previously used single membrane module [Matos et al, 2006], thus confirming the good scale-up potential of the IEMB process. In the beginning of the plate-and-frame IEMB module operation, the carbon source (ethanol) concentration in the biocompartment was kept limiting in order to avoid ethanol diffusion through the membrane into the water compartment. After 6 days, the ethanol addition was increased 8 times, from 0.56 g/L to 4.6 g/L, to avoid biological limitation. This increase led to ethanol permeation through the membrane and corresponding secondary contamination of the treated water by this nutrient. The determined ethanol diffusion coefficient for the membrane Ralex AMH-PES used was equal to -7 2 2.80x10 cm /s. This value is still considerably lower than the diffusivity of ethanol in water -5 2 (D (ethanol/water) = 1.28x10 cm /s at 25ºC). Previous studies performed with a membrane with a two -7 2 times lower permeability to ethanol (D (ethanol/Excellion membrane) = 1.42x10 cm /s), proved that ethanol transport to the treated water can be avoided at ethanol concentrations below about 200 mg/L in the biocompartment [Matos et al, 2008]. Therefore, to avoid simultaneously microbial growth limitation and possible diffusive transport of ethanol across the membrane, the ethanol feeding strategy must be carefully controlled by a gradual increase of ethanol concentration during the IEMB process start-up

phase. After this period, the biofilm formed on the membrane surface contacting the biocompartment starts acting as an additional reactive barrier for ethanol penetration into the treated water and, therefore, the ethanol level in the bulk of the biocompartment becomes less critical.

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Figure 3: Time course of concentrations of nitrate, perchlorate and ethanol in the treated water (A) and in the biocompartment (B) of the plate-and-frame IEMB module

In the plate-and-frame IEMB module configuration, the height of the biocompartment channels was set to 4 mm, instead of the typically used channel heights of 0.8 mm to 1 mm in order to guarantee that possible excessive biofilm formation would not cause channel clogging. Furthermore, the spacers used had higher and more open mesh dimensions of 5/2.9 mm in a diamond shape design, in order to decrease the possible area for biofilm attachment on the spacers. It was observed that the formation of the biofilm on the membrane surface contacting the biocompartment has a positive effect on process performance by avoiding ethanol permeation into the treated water since, due to its biological activity, the ethanol concentration at the membrane surface is much lower than the ethanol concentration in the bulk solution. Using bigger channels is expected to lead to a less favourable hydrodynamic situation in the biocompartment; however, due to the relatively high concentration of the major driving counter-ion (chloride) in the biocompartment, the contribution of the hydrodynamic parameters to the resistance to transport of counter-ions into this compartment becomes much less significant [Velizarov et al, 2000/2001].

Membrane fouling in the water compartment For many drinking water sources, the presence of natural organic matter (NOM) cannot be avoided, thus providing a source of possible membrane fouling. However, the appearance of an additional and relatively easily utilizable carbon source like ethanol, can favour more rapid microbial growth leading to formation of a biofilm on the membrane surface contacting the treated water compartment channels. Therefore, as expected, during an IEMB operation without controlling the ethanol addition to the biocompartment, a fouling layer was detected on the membrane surface contacting the water compartment. This resulted in a rapid clogging of the water channels and decrease in the water recirculation flow rate (Figure 4, closed circles). The membranes were therefore subjected to alkaline cleaning in order to remove the foulants attached and ethanol was gradually increased. This was able to restore the initial operating conditions and the water flow rate in the recirculation-loop of the water compartment showed values closed to a control case of no ethanol addition (see Figure 4). Since the Ralex anion-exchange membrane used is nonporous, the fouling agents attached only on the membrane surface and the fouling could be related to the decrease in the flow rate of the recirculation-loop of the water compartment.

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Figure 4: Decrease in water flow rate in the water compartment recirculation loop of the IEMB module at different ethanol feeding strategies

The gradual increase of ethanol during the first days of operation was performed by starting with 0.7 g/L of ethanol and increase it concentration by 15% every 6-7 days. During this start-up period, ethanol was able to support the biofilm development at the membrane surface contacting the biocompartment. After 45 days, the ethanol concentration in the feed to the biocompartment was fixed at 1.85 g/L and biofilm formation was stabilized since no increase in the pressure drop of the biocompartment recirculation loop was observed.

Membrane properties after accelerated membrane ageing In the present study, biofouling was effectively eliminated by treatment with 1% NaOH according to the membrane manufacturer´s indications. It is well known that NaOH encourages dissolution of organic matter and promotes cleavage of polysaccharides and proteins [Porcelli and Judd, 2010]. However, chemical cleaning might also damage the membrane polymeric material [Al-Amoudi and Lovitt, 2007]. Therefore, the selection of an adequate cleaning protocol should be done not only on the basis of its cleaning efficiency but also considering its effect on the membrane properties. Successive chemical cleaning procedures may provoke ion exchange membrane ageing, thus affecting significantly the ion transport [Dammak et al, 2009]. After NaOH treatment, the flux across a polysulfone ultrafiltration membrane was restored; however, it was found out that the zeta potential of the membrane changed [Zhu and Nyström, 1998]. Similar behaviour was observed by [Guo at al, 2009] who assumed that the reason for flux recovery values above 100% had been due to membrane surface modification caused by NaOH cleaning. Therefore, the impact of successive chemical cleaning events, in order to simulate membrane ageing on the physicochemical properties of the anion exchange membrane used in this study, was evaluated. Accelerated membrane ageing is a procedure that simulates successive cleaning events in order to evaluate possible long-term effects. This procedure has been successfully applied for different types of membranes [Zondervan et al, 2007; Dammak et al, 2009; Antony et al, 2010]. After each chemical treatment, the membrane density, thickness, water content and ion-exchange capacity were determined (Figure 5). Although standard deviations of the experimental data are relatively high, it appears that the membrane density slightly decreased for the 10 washing cycles performed; while the wet membrane thickness increased after chemical cleaning reaching a plateau of about 630 µm after 5 cleaning procedures. In any case, this value is only 5% higher compared to the thickness of a new membrane. An increase in membrane water content was only observed after the first chemical cleaning and then remained unchanged for the next nine cleaning procedures. It was found that these physical alterations did not lead to a significant change in membrane functionality, since the ion exchange capacity was maintained for the 10 washing cycles. It seems that NaOH only slightly affected the membrane polymeric structure. The heterogeneous anion-exchange membrane studied has a polyester fibers matrix and uses polyethylene as the binder (http://www.mega.cz). The results obtained are in agreement with those reported for artificial ageing of a cation exchange membrane exposed to oxidant agents [Dammak et al, 2009]. According to these authors, ionexchange capacity was not affected but membrane thickness and water content increased during the contact time with the oxidant agent. Therefore, it was suggested that the effect was limited to scission of some polymeric chains constituting the membrane polymeric material.

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Figure 5: Evolution of values of characteristic parameters of a Ralex AMH-PES membrane subjected to repeated cleaning procedures by 1% NaOH. A: membrane density, B: membrane thickness; C: membrane water content and D: membrane anion-exchange capacity

CONCLUSIONS The applicability of a plate-and-frame IEMB module configuration for the treatment of drinking water contaminated with nitrate and perchlorate was evaluated. • The IEMB process was successfully operated maintaining the nitrate and perchlorate concentrations in the treated water below their recommended levels for drinking water supplies; • Permeation of ethanol across the membrane to the treated water was avoided with a start-up procedure involving a gradual increase of ethanol feeding to the IEMB biocompartment; • Biofouling was successfully removed by alkaline treatment with 1 % NaOH; • The membrane anion-exchange functionality was not altered for 10 repeated cleaning procedures with 1 % NaOH; however, slight changes in other membrane properties, namely an increase in membrane thickness and water content were documented, thus suggesting alterations in the membrane polymeric material. It appears worth to further investigate these membranes using ATR-FTIR in order to gain more insight into the nature of the polymeric material modification.

ACKNOWLEDGMENTS Ana Rita Ricardo acknowledges FCT, Lisbon, Portugal for the PhD scholarship SFRH/BD/25275/2005.

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