Title: Determination of critical flux by mass balance technique combined with direct observation image analysis
Journal: Journal of Membrane Science
Authors: Y.P. Zhangb, A.W.K. Lawab, A.G. Fanea
Corresponding author: Y.P. Zhang

Institute: 
a Singapore Membrane Technology Centre (SMTC)
b School of Civil and Environmental Engineering, Nanyang Technological University, Singapore 637723, Singapore

The original and creativity of paper: This paper introduces an improved method to determine the critical flux, during crossflow microfiltration, by combining mass balance and image analysis. The proposed method gave a reasonable value of deposition fraction.

Summary: 

Image capturing
Images were obtained from the Direct Observation Through Membrane (DOTM) technique. Figure 1 shows the schematic diagram of the DOTM system. 


Fig. 1. Experimental set-up including the DOTM facilities 

Then, the obtained images were converted to a threshold value. Figure 2 shows the image conversion processes. 

Image conversion processes
1. Images were captured through a microscope by a digital CCD camera.
2. They were further analyzed using image analysis software (Image-pro plus, Media Cybernetics).
3. All the colored images from the experiments were first converted into gray-scale images having intensity values of 0?255 (0 for black and 255 for white).
4. These gray-scale images were then converted to binary images by thresholding, i.e., partitioning the images into only black and white pixels at an arbitrary threshold value using the image analysis software.
5. In order to determine a threshold value, it was necessary to compare the converted binary image to original.
6. The converted black and white images were used to determine the fractional coverage by particle at a given time through individual pixel enumeration

Fig. 2. Determination of fractional surface coverage. (a) raw image obtained from direct microscopic observation; (b) conversion into gray-scale; (c) applied open filter to remove noises; (d) the filtered image was converted into a black and white image by choosing an intermediate gray-scale threshold.

Finally, the fouling layer coverage in membrane could be converted to the mass of the foulant using membrane fouling coverage and particle deposited mass balance equation. 

Membrane fouling coverage

Coverage=(A(coveraged))/(A(observation))

where A (coveraged) is the total particle covered area on membrane surface. A (observation) is observation area.

Particle deposited mass balance

dCoverage/dt=3/2 Cb/ρd θ(J-Jc)

where Cb is particle concentration in the bulk solution, J is operation flux, Jc is critical flux, and θ is fractional deposition.

Application & further study: The methodology is can be applied to extract information from fouled membrane image which obtained from other techniques. 

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