1. Title & Journal

 

Combined organic-inorganic fouling of forward osmosis hollow fiber membranes

 

2. Background of author

 

Elizabeth Arkhangelsky (a, b), Filicia Wicaksana (b), Chuyang Tang (a, b),

Abdulrahman A. Al-Rabiah (c), Saeed M. Al-Zahrani (c), Rong Wang (a, b)*

 

a School of Civil and Environmental Engineering, Nanyang Technological University, Innovation Center, Block 2, Unit 237,

18 Nanyang Drive, 50 Nanyang Avenue, 639798 Singapore, Singapore

b Singapore Membrane Technology Center, Nanyang Technological University, 639798 Singapore, Singapore

c Chemical Engineering Department, King Saud University, Riyadh 11421, Saudi Arabia

 

3. Summary

 

 

3.1 논문의 목적

 Organic fouling 이나 inorganic fouling 중 한 가지에 집중하거나, flat membrane에 초점이 맞춰져 있는 기존의 논문들과 달리, 이 논문은 hollow fiber membrane을 이용한 실험결과를 통하여 FO membrane의 combined organic-inorganic fouling 을 조사하는 데 있다.

 

3.2 Materials and methods

 

 1) Feed solution ? 4.5L

   - The feed solution for baseline test: 10mM NaCl with pH 6.25

   - The feed solution for fouling test: a mixture of alginate, BSA, and silica in the presence of absence of Ca²⁺ ions

  2) Draw solution

   - 5M NaCl for throughout the experiments

  3) Velocity

   - cross-flow velocity 6, 12, 24 cm/s

   - The permeate flux is determined by measuring the weight changes of the feed solution with a digital mass balance

  4) Cleaning procedure (5 min)

   - 1^st step: Flush the active layer surface of the membrane with MilliQ water at 30cm/s cross flow velocity

   - 2^nd step: Rinse the membrane with 500ml of fresh ultrapure water for 2 min

 5) Four phases for the experiments

(1) Verification of fundamental frequencies of crystal under dry air conditions

(2) Establishing a stable baseline by running the background solution for 10-15min

(3) The adsorption phase using BSA or alginate solution

(4) Cleaning phase

 

 

 

 

 

 

 

 

 

 

 

3.3 Results and discussion

  

  1) Effect of Ca²⁺ ions: individual and combined fouling

    - The flux decline was purely caused by fouling and was not due to concentration polarization effect (1a).

   - The interaction between foulants may contribute to a more severe membrane fouling (1a).

   - The flux decline with BSA and silica colloids demonstrated negligible (1b).

   - The filtration of alginate resulted in more water flux drop (16%) than others (1b).

   - Calcium ions are able to bind to the carboxylic functional groups of alginate and form a highly organized gel layer with a structure resembling an egg-box (1b).

   - Reduced back diffusion of salts and increased the cake-enhanced osmotic pressure cause the flux decline. The interaction between foulants may contribute to a more severe membrane fouling (1b).

   - BSA+ alginate was comparable to the flux decline caused by three-component system (27 vs. 30%) suggesting that the presence of colloidal silica did not contribute significantly to the flux decline (1c).

 

  

  2) Effect of osmotic driving force, shear and hydrodynamic regime

  - Higher draw solution concentration generated greater osmotic driving force that resulted in higher water flux across the membrane (2a).

 - The increase in permeation flux was responsible for a more severe fouling indicated by a more rapid flux decline at higher draw solution concentrations (2a).

 - By comparing results from baseline and fouling experiments, it appeared that membrane fouling was the main factor that contributed to the decline in flux (2a).

 - Membrane fouling propensity increased with the decrease in cross-flow velocity (2b).

 - High shear could alleviate membrane fouling by preventing foulants from depositing on the membrane surface (2b).

 - The increase of cross-flow velocity after 180 min of FO process could not restore the water flux to its initial value (3).

 - However, further flux decline could be prevented since the water fluxes during 180-360 min of operation remained relatively stable (3).

 

   3) Fouling reversibility

- Surface flushing with water was more effective than increasing the hydrodynamic shear during filtration (4).

- The effectiveness of cleaning process will depend on the severity of fouling that is also governed by various factors.

- A more severe fouling may requires a more intense cleaning or vice versa.

- Further optimization of cleaning process is necessary to reduce the duration and the amount of water spent for membrane cleaning.

   4) Fouling mechanism

- In the presence of salts, the flux decline of alginate-silica system was similar to the one with alginate alone.

- A strong interaction between silica and BSA was observed.

- The injection of sodium and calcium salts intensified the adsorption of BSA onto silica surface, and a more rapid frequency reduction occurred.

- The zeta potential measurements revealed that both alginate and BSA had negative net surface charges in water at pH 6.25 (table2).

- BSA was able to bind to both alginate and silica in binary mixture.

- The binding of protein and polysaccharide to silica appeared to be strong in salt solution.

- The injection of salt to the solution had decreased the Debye length that allowed the foulants to get closer to each other without being repelled.

- BSA-alginate-silica interactions appeared to be very weak in pure water. Alginate could entrap the protein, thus limiting further binding to silica.

 

3.4 Conclusions

 

 - The severity of FO fouling was controlled by combined effects of feed composition, membrane surface chemistry, foulant properties and characteristics, permeate flux, and hydrodynamic shear.

 - Surface flushing could fully restore the water flux to the level of pristine membrane, independent of the degree of fouling.

 

4. Originality & Creativity

 이 논문은 기존 논문과 달리 정삼투 멤브레인에서의 combined organic-inorganic fouling에 대하여 실험적으로 조사하였다.

 

5. Reviewer contact: 심예영 (yyshim@gist.ac.kr)