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

After the chemical cleaning of the membrane during both the times, 99% of the
initial membrane resistance could be obtained. The fouling resistance in the membrane
bioreactor was 20% and 17% during the first and second cleaning, respectively of that of
the total resistance. This indicated that the cake layer resistance was much higher than the
fouling resistance in the membranes. The reduction in flux due to membrane biofouling is
largely affected by physico-chemical characteristics and physiology of activated sludge as
well as membrane materials (Kim, et al., 1998). The factors affecting the membrane
fouling will be discussed in later part of this chapter.
4.4.3 Performance of Ammonia Stripping Coupled Membrane Bioreactor Process
As the performance in terms of COD removal efficiency without ammonia stripping
was not significant with 16 and 24 h HRT, the performance of MBR was evaluated in
terms of both COD and BOD at HRT of 16 h followed by 24 h. Stable biomass retention in
the MBR is effective in BOD removal. The MBR system though effective in BOD removal,
is not easy to remove nitrogen (Ahn, et al., 2002). The optimum conditions derived from
ammonia stripping studies as described in section 4.4.1 were used for ammonia removal.
Ammonia removal was used for nitrogen removal instead of nitrification-denitrification
process because old leachate does not have sufficient degradable organics to supply the
bacteria with carbon needed for growth. The ammonia stripping was done once every three
days to feed the membrane bioreactors. The performance could be evaluated as described
below.
(1) COD Removal Efficiency
The COD of the influent leachate ranged from 7,600 to 8,200 mg/L with 16 and 24 h
HRT. After the ammonia stripping, the leachate was fed into the feed tanks to feed
membrane bioreactors. In both the operational conditions, with 16 and 24 h HRT, the
average MLSS concentration ranged from 11,000 to 12,000 mg/L. The MLSS
concentration was similar to the membrane bioreactors without ammonia stripping. The
variation in the MLSS concentration and the influent COD influent with 16 and 24 h HRT
is given in Figure 4.24 and 4.25, respectively. The advantages of biomass retention in
membrane bioreactor are that, even the slow growing organisms, normally washed off in
conventional process are retained in membrane bioreactor (Ben Aim and Semmens, 2002).
The entire range of data is given in Table G-1 to G-4 of Appendix G.
The fluctuations in the membrane bioreactor treatment in terms of COD removal
with ammonia stripping were found to be lower than that without ammonia stripping. Both
YMBR and BMBR reactor without ammonia stripping, did not show improvement in COD
removal when the HRT was increased, while in both the systems there was slight
improvement in COD removal when the HRT was increased. The nitrogen removal in the
membrane bioreactor was satisfactory. The probable reason for nitrogen removal would
have been denitrification rather than nitrification as there was not any sufficient increase in
oxidized nitrogen compounds (Muller, et al., 1995). The COD removal in the YMBR and
BMBR with the ammonia stripping was the same. Both the membrane reactors showed a
COD removal of 72% at 16 h HRT and 76% at 24 h HRT. When Ahn, et al. (2002) treated
leachate with 1,017 mg/L COD, they found that the MBR system could achieve a COD
97