Title: The development of membrane fouling in full-scale RO processes
Journal: Journal of Membrane Science
Authors: Kai Loon Chen, Lianfa Song, Say Leong Ong and Wun Jern Ng
Corresponding author: Lianfa Song
Institute: 
Department of Civil Engineering, Center for Water Research, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore

The original and creativity of paper: 
This paper developed a predictive model for membrane fouling in full-scale RO process. Also, the authors introduced fouling potential and a collective parameter to characterize fouling property of feed water. 

Summary: 
In this study, numerical simulation have been performed to examine membrane fouling behaviors occurred in a full-scale RO process under various conditions.  The aims of these simulations were to study the formation of membrane fouling in full-scale RO process and also investigate the effects of membrane fouling on the performance of full-scale system. All simulations conducted in this paper were performed using these values were showed in Table 1.  

For the model development, an increase in membrane resistance was used as an indicator of membrane fouling that directly measureable with a simple filtration experiment. Be applying the concept of fouling potential, feed water characteristic could be directly related to fouling intensity on a RO membrane. Moreover, in order to provide more realistic description of full-scale RO process, local variations in flow properties and parameter were taken into consideration in this model. 

Table 1 RO parameter values for computer simulation
Parameters    Values
 

Simulation and discussion
1.    Fouling development in the membrane channel
Fig. 1 shows that, when fouling has not occurred yet (at the beginning stage), the local membrane resistance is equal to the clean membrane resistance along the entire channel. Over 15 days of operation, local membrane resistance increased for the first 4 m of membrane channel, the highest increase occurring at the feed end of the membrane channel. The local resistance of the remaining 2 m of membrane channel remains constant. This result indicates that membrane fouling starts from the feed end of the membrane channel.

 
Fig. 1. Membrane resistance along membrane channel with increasing operation time (in days). 

Moreover, the profiles of permeate flux at different operating time versus distance from the feed end of membrane channel are illustrated in Fig. 2. At the beginning of filtration operation, permeate is mainly produced within the first 4 m of membrane channel whereas there is no permeate flux being produced in the last 2 m of the membrane channel. The highest permeate flux occurs at the entrance of membrane channel and the flux decreases rapidly to zero at the 4 m location. 
 
Fig. 2. Permeate flux along membrane channel with increasing operation time (in days). 

2.     Effect of fouling on average flux
The average permeate flux in a 6 m long membrane channel has been simulated over a period of 180 days. The results show that the average permeate flux remains constant during the first 60 days of operation and then it begins to decline after 60 days. The results indicate that the average permeate flux in a full-scale RO process can remain constant for sometimes (60 days, in this case) even when membrane fouling occurs immediately at the beginning of filtration operation. 
 
Fig. 3. Change in average permeate flux with time with a feed that has a fouling potential of 3.5×109 Pa s/m2. 

3.    Effect of fouling on salt concentration
The result in Fig. 4 shows that salt concentration increases significantly from 10,000 to approximately 80,000 mg/l within the first 3 m of the channel and remains constant over the remaining portion when the membrane is clean (without fouling). The increase in salt concentration becomes slower along the channel and it takes a longer distance to reach the equilibrium salt concentration due to membrane fouling develops. Interestingly, the salt concentration profiles of Day 120 and Day 180 do not increase much along the membrane channel. Because, the overall permeate produced by the membrane channel is relatively small at this stage of the operation. Moreover, the salt concentration profile of the process is operated for 60 days or longer unable to reach its equilibrium concentration within the channel length.  

 
Fig. 4. Salt concentration along membrane channel with increasing operation time (in days). 
4.    Feed water fouling potential and fouling development
Fig. 5 shows that the average permeate fluxes start from the identical initial value for feed water with different fouling potentials. This is because the initial fluxes are determined by the clean membrane resistance and have not been affected by membrane fouling. After filtration, the fluxes of the feed water with higher fouling potentials decline earlier than those with lower fouling potentials. 
 
Fig. 5. Change in average permeate flux with time with various feed water k-values: (1) 1.5×109 Pa s/m2, (2) 3.5×109 Pa s/m2, (3) 7.0×109 Pa s/m2, (4) 1.1×1010 Pa s/m2, and (5) 1.5×1010 Pa s/m2. 
5.    Channel length and fouling development
Fig. 6 shows that the average permeate fluxes of 1 and 3 m channels decline right from the start of filtration operation. For RO process with a small surface area, membrane fouling has an immediate impact on average flux. On the other hand, there are initial periods of constant average fluxes for longer membrane channels (6 and 9 m).
 
Fig. 6. Change in average permeate flux with time and with various channel lengths. 

6.    Clean membrane resistance and fouling development
The behaviors of membrane fouling on two systems (system [1] and system [2]) are different. An initial period of constant average permeate flux is observed for the system with a clean membrane resistance of 1.8×1011 Pa s/m [1]. On the other hand, the average permeate flux of the system with a clean resistance of 8.0×1011 Pa s/m [2] declines gradually with time right from the start of filtration operation. These simulations demonstrate that membrane fouling can induce immediate flux decline in RO processes employing less permeable membranes. 
 
Fig. 7. Change in average permeate flux with time and with various membrane intrinsic resistances: (1) 1.8×1011 Pa s/m, and (2) 8.0×1011 Pa s/m. 



Application & further study: This predictive model can be applied in full-scale RO process for more effective fouling control and more widespread application. 


By Monruedee Moonkhum
Email: moon@gist.ac.kr