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

From the data obtained above, it can be seen that 5 days BOD contributes to about
67% of the 20 days COD present in the raw leachate, while in the BMBR and YMBR
effluent; the 5 days BOD contributed only 38 and 25% of the 20 days BOD. This shows
that compared to that of the raw leachate, the effluents of the membrane bioreactors take a
longer time to degrade the organics suggesting the presence of greater amount of slowly
biodegradable organics. In comparison between YMBR and BMBR effluents, slowly
biodegradable organics in YMBR effluent was higher than that in BMBR effluent.
When the BOD/COD ratio for raw leachate was considered, it was found that
BOD5/COD was 0.45 which increased to a BOD20/COD of 0.68 after 20 days. This
suggests that the degradable component in the raw leachate is almost 68%. The
BOD5/COD of both the bacterial and yeast effluent was found to be 0.01. Though, the
bacterial and yeast effluent had a similar BOD5/COD ratio, the BOD20/COD ratio of the
bacterial and yeast effluent varied with a ratio of 0.02 and 0.04, respectively. This also
suggests that the slowly degradable components are more in the yeast effluent in
comparison with bacterial effluent.
4.5.2 Molecular Weight Cut-off
The organic matter present in the leachate varies and is dependent on the waste
composition and degree of degradation. The medium molecular weight compounds with
molecular weight (MW) between 500 and 10,000 Da are dominated by carboxylic and
hydroxylic groups with fulvic acid and humic fraction also contributing to this fraction in
the leachate (Chian and DeWalle, 1976; Harmsen, 1983). They are difficult to degrade and
are termed refractory. The fulvic and humic-like compounds present in leachate are formed
from micobiological processes from the intermediate products of degradation of polymeric
organic compounds such as lignine (Andreux, 1979).
The high molecular weight organics are usually stable to degradation. The
effectiveness of a treatment process can be related to the removal of specific organic
fraction in leachate. Both fulvic and humic substances are inert to biological treatment.
Therefore, fractionating the COD based on molecular weight can act as an indicator to the
removal efficiency and degradation potential of the biological system.
According to the results, the molecular weight distribution or molecular weight cutoff
(MWCO) was computed by measuring the COD concentration of each fraction and the
volume filtered. The transformation of organic substances corresponding to the change of
COD mass is shown in Figure 4.39. Detailed calculation is given in Table H-3 of Appendix
H.
As shown in the figure, the raw leachate contained a higher fraction of high
molecular weight compounds (> 50 k). The low-molecular weight fractions, which include
lower molecular weight compounds, were present at low fraction. Figure 4.40 shows
percent COD contribution of various molecular weight components to the total COD in
raw leachate, stripped leachate, bacterial and yeast effluents.
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