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Application Of Forward Osmosis On Microalgae Harvesting

Congress: 2015
Author(s): XUE JIN (GLASGOW, UK), Mathieu Larronde-Larretche
UNIVERSITY OF GLASGOW1

Keyword(s): Sub-theme 6: Links with the energy, food and environmental sectors,
Abstract Introduction

Microalgae applications have recently gained increasing attention in wastewater treatment, biofuel production and carbon dioxide biofixation [1]. However, efficiency of biomass harvesting and concentration techniques remains economically insufficient [2]. In the meantime, forward osmosis (FO) has shown promising results as cost-effective filtration method, and has just started to be investigated for the harvesting of microalgal biomass [3, 4]. As many aspects of FO for algae harvesting remains unknown, the objectives of this study were to (1) systematically investigate the effect of draw solution (DS) chemistry on flux behavior and algal harvesting efficiency, (2) assess the impact of membrane type, orientation and feed spacer, and (3) gain a deep understanding of the mechanisms governing FO fouling during algae harvesting.

Methods/Materials

Harvesting experiments were conducted with a feed solution containing 0.2 g/l of Scenedesmus obliquus biomass, in a custom fabricated FO membrane cell providing an effective membrane area of 200 cm². Different draw solutions (commercial sea salts ; NaCl ; MgCl2 ; CaCl2) were investigated with two commercial FO membranes: cellulose triacetate (CTA) and thin film composite (TFC). Experiments conducted were aimed to achieve a concentration factor of 4 by removing 75 % of water from the algae suspension. Two successive cleaning steps, deionizd water flushing and osmotic backwashing, were then conducted in order to better understand the fouling mechanisms involved. Permeating water flux, reverse solute diffusion and variation of biomass concentration were measured during each experiment. Extracellular proteins and carbohydrates, well known to enhance membrane fouling, were analysed from samples taken during each experiment. Membrane samples were also analysed by scanning electron microscopy (SEM).

Results and Discussion

First, CTA and TFC membranes were investigated with both orientations for the harvesting of Scenedesmus obliquus with sea salts as DS. For both membranes, the active layer facing feed solution orientation (AL-FS) exhibited lower flux than the active layer facing the draw solution (AL-DS) orientation, due to well-known internal concentration polarization (ICP) effects. With AL-FS orientation, the TFC membrane achieved a higher initial water flux than CTA membrane due to its higher water permeability, but a greater flux decline because of its lower salt rejection. In the AL-DS orientation, the final water flux significantly declined by 59.5% for TFC membrane and 45.1% for CTA membrane, due to the combination of (1) internal adsorption of algal biomass inside the porous support layer and (2) pore clogging enhanced concentrative ICP which is due to reduced mass transfer coefficient of support layer [5]. In contrast, the AL-FS orientation exhibited a superior fouling resistance, with a final water flux loss below 15% for both membranes, caused by the deposition of algal biomass onto the active layer surface. Regarding these findings, the CTA membrane in the AL-FS orientation was selected for additional study on the effect of DS chemistry. Different DS electrolytes were investigates for Scenedesmus obliquus harvesting. Algae biomass did not cause much membrane fouling for NaCl and MgCl2. However, when Ca2+ ions were present in the draw solution, flux declined to a greater extent. At the end of experiments, the overall extent of flux loss followed the order of CaCl2 >> sea salts (containing 0.82 g/L of Ca2+) > NaCl ≈ MgCl2. This indicates that calcium ions cause severe membrane fouling during algae harvesting. The efficiency of biomass concentration was also affected due to algae deposition onto membrane and/or feed spacer. When CaCl2 was used as DS, the greatest loss in both water flux (63.2%) and algal biomass (44.8%) is explained by the diffusion of Ca2+ ions from draw solution into feed solution. Ca2+ ions bind preferentially to oxygen atoms of carboxylate groups in a highly organized manner and form bridges between adjacent algal cells as well as their extracellular polysaccharides (EPSs) and soluble microbial products (SMPs) leading to an egg-box-shaped gel network [6]. Large microalgae flocs were formed in the feed tank, increasing (1) the rate of algae deposition onto both membrane and feed spacer and (2) the compressibility of the fouling layers. The loss of biomass observed for all experiments suggests that (1) most of the algae deposition takes place onto the mesh spacer in the feed channel rather than onto membrane and (2) the biomass deposited on feed spacer may not augment the hydraulic resistance significantly. To investigate impact of feed spacer on harvesting efficiency, FO experiments with sea salt as DS were performed without spacer in the feed channel. Without feed spacer, 97% of the algal biomass was harvested in the feed tank, significantly higher than the 74% achieved with a feed spacer. These findings revealed a negative effect of using feed spacer on algae harvesting efficacy due to the easy accumulation of algal cells inside spacer. However, the beneficial effect of feed spacer has been proven to improve mass transfer in crossflow membrane filtration systems [7]. Hence, feed spacer needs to be further optimized in terms of material and geometry to reduce the risk of microalgae cell accumulation into the spacer and enhance mass transfer over the membrane surface in the feed channel.

Conclusion

This study demonstrated the significant impact of membrane orientation and draw solution chemistry on fouling behaviour and concentration efficiency during the harvesting of Scenedesmus obliquus biomass with forward osmosis. The presence of calcium ions in the draw solution is pointed out as a key parameter, enhancing membrane fouling through binding phenomenon. Feed spacer has been highlighted to greatly reduce algae harvesting and needs to be improved to achieve a greater biomass harvesting. The findings of this study provides significant insights into the FO process design in terms of draw solution selection, membrane module design and fouling control. 1. Razzak, S.A., et al., Integrated CO2 capture, wastewater treatment and biofuel production by microalgae culturing—A review. Renewable and Sustainable Energy Reviews, 2013. 27(0): p. 622-653. 2. Greenwell, H.C., et al., Placing microalgae on the biofuels priority list: a review of the technological challenges. Journal of the Royal Society Interface, 2010. 7(46): p. 703-726. 3. Zou, S., et al., The role of physical and chemical parameters on forward osmosis membrane fouling during algae separation. Journal of Membrane Science, 2011. 366(1–2): p. 356-362. 4. Cath, T.Y., A.E. Childress, and M. Elimelech, Forward osmosis: Principles, applications, and recent developments. Journal of Membrane Science, 2006. 281(1–2): p. 70-87. 5. Tang, C.Y., et al., Coupled effects of internal concentration polarization and fouling on flux behavior of forward osmosis membranes during humic acid filtration. Journal of Membrane Science, 2010. 354(1–2): p. 123-133. 6. Gao, D.-W., et al., Membrane fouling in an anaerobic membrane bioreactor: Differences in relative abundance of bacterial species in the membrane foulant layer and in suspension. Journal of Membrane Science, 2010. 364(1–2): p. 331-338. 7. Zou, S., et al., Direct microscopic observation of forward osmosis membrane fouling by microalgae: Critical flux and the role of operational conditions. Journal of Membrane Science, 2013. 436(0): p. 174-185.

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