Thesis - faculty.ait.ac.th - Asian Institute of Technology

As there was no significant improvement when the HRT was increased from 16 to 24
h in terms of COD removal, further investigations were done at these two HRT, with 16 h
HRT followed by 24 h HRT.
4.3.3 Membrane Fouling and Membrane Resistance
The membrane fouling is the result of accumulation of rejected particles on the top of
the membrane (external fouling), or deposition and adsorption of small particles or
macromolecules at the pores or within the internal pore structure (internal fouling) of the
membrane (Guell, et al., 1999). The processes that contribute to the fouling are varied.
They include adhesion of the colloidal matters and macromolecules on the external and
internal surface, growth and adhesion of biofilms on the membrane surface, precipitation
of solved matters, aging of the membrane, etc (Gunder, 2001). Because of the complex and
diverse relationships, it is not possible to localize and define fouling clearly. The adverse
effects of the membrane fouling is the reduction of the permeate flux.
In the present study, a constant flux was maintained in the membrane bioreactors.
The resistance of the membrane influences the permeate flux. To maintain a constant flux,
the flow rate was increased correspondingly by adjusting the suction pump. The rapid
membrane fouling is indicated by a sudden increase in the transmembrane pressure. As a
high transmembrane pressure is a result of the membrane fouling process, it was used as a
parameter indicating requirement of cleaning. The membrane in the membrane reactors
were cleaned when the transmembrane pressure difference increased significantly. The
membranes were cleaned before it reached the maximum pressure to prevent damage to the
membrane operation. The transmembrane pressure difference of the YMBR and BMBR
systems is given in Figure 4.20. The detailed results are given in Table E-3 and E-4 of
Appendix E. Though the two reactors, with bacterial and yeast culture did not show much
difference in the performance, the yeast reactor showed an added advantage of lower
membrane fouling and thus, longer membrane life.
The cleaning was done by first flushing the membrane with tap water to remove the
cake layer from the membrane surface. Later, a 3% sodium hydroxide solution was filtered
through the membrane and then, washed with tap water. Finally, 1% solution of nitric acid
was filtered through the membrane followed by tap water. This cycle was repeated until
the membrane resistance was almost equal to the initial membrane resistance.
The frequency of cleaning was greater in bacterial membrane bioreactor than the
yeast membrane bioreactor. The frequency of membrane fouling is presented in the Table
4.10 for both the systems. The membrane resistance after cleaning is presented in Table
4.11. The detailed calculation is summarized in Table D-3 and D-4 and Figure D-1 and D-
2 of Appendix D. The bacterial system was first cleaned after 63 days of operation while
the yeast based system was cleaned after 80 days. It could be said that the membrane with
yeast reactor could be operated 27% more than the bacterial system. Further, in a total of
181 days of operation of the MBR systems, the BMBR was cleaned five times compared to
three times in the YMBR system. The operating time of the yeast membrane was about 1.3
times longer than the bacteria membrane.
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