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
3.1 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
- 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)
- 1st step: Flush the active layer surface of the membrane with MilliQ water at 30cm/s cross flow velocity
- 2nd 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 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
- 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.
- 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 (email@example.com)