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

The probable reason for frequent fouling in bacterial system than yeast system could
be EPS formation. The EPS formation in reactor was greater in bacterial system than in the
yeast system. The mechanism of biofilm development in the YMBR is different from that
of the BMBR. In the YMBR, the yeasts attach itself physically to the membrane surface
during filtration instead of getting trapped in a matrix as the bacterial cell. The yeast cells
usually attach together by means of physical interwinding of mycelia or pseudomycelia
(Nishihara ESRC Ltd., 2001).
Another probable reason for frequent fouling in the bacterial based membrane
bioreactor could be the size and nature of bacterial cells in comparison with the yeast cells.
The bacterial cells have a size of 0.5 to 1.0 µm diameter for the spherical shaped, and 0.5
to 1.0 µm wide and 1.5 to 3.0 µm long for the cylindrical (rods) shaped bacteria whereas
the size of yeast is around 5 to 30 µm length and 1 to 5µm width. The large yeast cells are
said to form a dynamic membrane on the top of the original membrane that is capable of
entrapping some of the protein aggregates. This may enhance the recovery of the viscous
aggregates and thus slowing down the fouling layer on the surface of the primary
membrane. Thus, the yeast interactions slow down the pore blocking by capturing a
significant fraction of protein aggregates (Guell, et al., 1999). In addition to this low
operating pH, poor adhesion capacity and low viscosity could be other reasons for low
fouling frequency in the yeast based MBR systems (Dan, 2002). Thus, yeast sludge can
reduce membrane fouling rate more significantly than the bacterial sludge. Therefore, it
could be suggested that the use of yeast system in the membrane bioreactor could be
beneficial as it has the potential to reduce the operating and maintenance costs of the
treatment system.
4.4 Application of Yeast and Bacteria Based Membrane Bioreactors in Ammonia
Stripped Leachate Treatment
Leachate with high load of refractory compounds, low value of BOD/COD ratio,
heavy metals and high concentration of nitrogen compounds, especially ammoniacal
exhibit difficulty in treatment (Dichtl, et al., 1997). Biological treatment becomes difficult
when the regarded leachate is inhibitive, toxic and older-less biodegradable (Geenens, et
al., 1999). Due to the presence of high ammonium content in the leachate, it could be
suggested that removal of ammonium is required prior to biological membrane treatment.
4.4.1 Ammonia Stripping Studies
The toxicity of ammonia-bearing waste to bacteria, algae, zooplankton and fish is a
universal phenomenon. Ammonia has been shown to be toxic in oxidation ponds where
high free ammonia and pH inhibit photosynthesis (Abeliovich and Azov, 1976). The
activated sludge process has also been shown to fail due to the ammonia toxicity and
phosphorous limitation (Keenan, et al., 1984). In addition, Cheung, et al. (1993) through
algal toxicity suggested that ammonia concentration as ammoniacal nitrogen is a major
factor governing the toxicity of landfill leachate. Along with this, it was difficult to
overcome the ammonia toxicity and to treat the leachate containing a low COD/N ratio
with biological process (Keenan, et al., 1984; Robinson and Maris, 1985; Cheung, et al.,
1997). Therefore, there is a need to reduce the concentration of ammonia in the leachate
below inhibitory level for the success of biological systems in proper leachate treatment.
As ammonia stripping is simple and less expensive than other physico-chemical methods
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