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Fouling Structure In Fo And Its Effect On Osmotic Backwash Efficiency

Congress: 2015
Author(s): Sorcha Daly (Edinburgh, UK), Andrea Semiao
The University of Edinburgh : Institute for Infrastructure and the Environment1

Keyword(s): Sub-theme 13: Non-conventional sources of water,
AbstractIntroduction:

Potable water availability is an increasing issue owing to the world's clean water scarcity. Water strained areas possess 41% of the population, (Service 2006). Seawater and saline aquifers make up 97.5% of all water on Earth, causing an increase in the use of reverse osmosis (RO) desalination for potable water production, (Shannon, Bohn et al. 2008). However, the required hydraulic pressures needed for RO water permeation result in high energy demands. Furthermore, membrane fouling in RO reduces water production and also increases energy demand (Ang, Yip et al. 2011, Liu, Rainwater et al. 2011, Bar-Zeev and Elimelech 2014).

Forward osmosis (FO) has been proposed as a pre-treatment process to recover clean water from wastewater, simultaneously diluting seawater entering the RO step. This lowers the osmotic pressure of the seawater potentially reducing RO energy requirements and fouling. In FO, two liquids of different osmotic pressures are separated by a water-permeable and salt-rejecting membrane. The osmotic pressure difference is the driving force for water permeation from the feed side (wastewater) to the draw side (seawater), (Lee, Boo et al. 2010), eliminating the need for hydraulic pressure.

Like RO however, FO is prone to fouling by contaminants in wastewater and seawater (Lee, Boo et al. 2010, Liu and Mi 2012, Valladares Linares, Li et al. 2013) reducing the process efficiency. Osmotic backwashing can remove the fouling layer, where the permeate flux is reversed from the draw side to the feed side ether by switching the circulating solutions or by injecting a pulse of high salt concentration to the feed side, (Ramon, Nguyen et al. 2013). This minimizes discharge of cleaning chemicals to the environment and potentially allows for full recovery of the initial flux.

Studies show that osmotic backwashing restores the initial flux to varying extents due to concentration polarisation and fouling layer characteristics (Arkhangelsky, Wicaksana et al. 2012, Valladares Linares, Li et al. 2013). These results invite studying conditions affecting osmotic backwashing of fouled FO membranes such as hydrodynamics and feed and draw solution characteristics. Understanding these could advance the optimisation of osmotic backwashing of the FO membranes, which will encourage the use of this process prior to RO desalination.

The aim of this study is to investigate the various factors affecting fouling reversibility in FO to design an optimum cleaning strategy to restore membrane performance. Osmotic backwashing and salt cleaning efficiency will be studied using NaCl (RO brine) and pure water (RO permeate) for different fouling layer characteristics of alginate, humic acid and a mix of these layers. This could determine how efficient osmotic backwashing is depending on fouling layer characteristics and why. 

Methods/Materials:

Membranes: The membrane used in this study will be a Hydration Technology Innovations FO membrane with an area of 0.0048m2. Three custom built membrane cells will be used in parallel. FS and DS solutions: The DS will be prepared with NaCl to mimic seawater. The FS will be prepared with varying compositions and quantities of alginate and humic acid to achieve fouling layers with varying structures. Alginate is used to represent polysaccharides that are present in wastewater. Humic acid is used to represent natural organic matter also present in wastewater.

Fouling and cleaning: Fouling will be carried out for 4 days with the active layer facing the FS. Cleaning by osmotic backwashing and salt cleaning will be carried out my changing the concentration of the feed solution. To implement osmotic backwashing and salt cleaning the feed channel will be injected with a pulse of a high concentration NaCl solution. This should reverse the direction of the flow potentially removing the fouling layer. To compare the effects of salt cleaning with osmotic backwashing another cleaning experiment will be performed after fouling under the same conditions, this time the DS will be replaced with DI to observe osmotic backwashing efficiency without the effects of salt cleaning.

Fouling characterisation: To identify the cleaning performance, the fluxes of the fouled membrane before and after cleaning will be measured using a DI water FS and a NaCl DS solution.

Conclusion:

It is expected that varying the fouling layer structure will affect the cleaning efficiency. Ang, W. S., et al. (2011). "Chemical cleaning of RO membranes fouled by wastewater effluent: Achieving higher efficiency with dual-step cleaning." Journal of Membrane Science 382(1–2): 100- 106. Arkhangelsky, E., et al. (2012). "Effects of scaling and cleaning on the performance of forward osmosis hollow fiber membranes." Journal of Membrane Science 415–416(0): 101-108. Bar-Zeev, E. and M. Elimelech (2014). "Reverse Osmosis Biofilm Dispersal by Osmotic Back-Flushing: Cleaning via Substratum Perforation." Environmental Science & Technology Letters 1(2): 162-166. Lee, S., et al. (2010). "Comparison of fouling behavior in forward osmosis (FO) and reverse osmosis (RO)." Journal of Membrane Science 365(1–2): 34-39. Liu, C., et al. (2011). "Energy analysis and efficiency assessment of reverse osmosis desalination process." Desalination 276(1–3): 352-358. Liu, Y. and B. Mi (2012). "Combined fouling of forward osmosis membranes: Synergistic foulant interaction and direct observation of fouling layer formation." Journal of Membrane Science 407– 408(0): 136-144. Ramon, G. Z., et al. (2013). "Osmosis-assisted cleaning of organic-fouled seawater RO membranes." Chemical Engineering Journal 218(0): 173-182. Service, R. F. (2006). "Desalination Freshens Up." Science 313(5790): 1088-1090. Shannon, M. A., et al. (2008). "Science and technology for water purification in the coming decades."Nature 452(7185): 301-310. Valladares Linares, R., et al. (2013). "Water harvesting from municipal wastewater via osmotic gradient: An evaluation of process performance." Journal of Membrane Science 447(0): 50-56. Valladares Linares, R., et al. (2013). "Cleaning protocol for a FO membrane fouled in wastewater reuse." Desalination and Water Treatment 51(25-27): 4821-4824.

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