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

(Yoon, et al., 1998). This shows that the complex higher molecular weight compounds
could be degraded effectively using membrane bioreactor systems. For the yeast and
bacteria effluent, the increase of the COD of below 5 k MW fraction could be explained by
the biodegradation of high molecular weight organic substances to compounds below 5 k
MW, as confirmed by the decrease of the COD of the 5 k MW UF retentate. Similar
results were obtained while treating leachate in aerobic and anaerobic system by Gourdon,
et al. (1989). The studies also revealed that recalcitrant organics were non-degradable in
anaerobiosis, while it could be degraded to 50% in aerobic conditions.
The COD removal after the membrane bioreactor treatment was from 7,500 mg/L to
about 1,950 mg/L in both the reactors. Among the COD of 7,500 mg/L, about 5,500 mg/L
was removed by the membrane bioreactor system, either through degradation for energy
consumption or through assimilation.
To further understand the degradable components present in the leachate and their
molecular weight distribution, another sample was analyzed with BOD along with COD
after fractionation. Figure 4.41 and 4.42 gives the COD and BOD contribution of
compounds at different molecular weight. Table H-4 of Appendix H gives the detailed
calculation of the results. In the second sample showed a slight difference from the first
sample. The > 50 k fraction decreased from 91% to 72% and corresponding increase of the
< 5 k from 0 to 18%.
When the analysis of the molecular weight fractions having organic matter below
MW 500 of the leachate was done, it has been shown that they contain synthetic organics
and solvents such as aromatic and alcoholic groups. Phenols, amines and chlorinated
organics were also found in this fraction. As suggested earlier, the fractions in 5 k to 10 k
and higher molecular weight contained humic and fulvic substances, along with products
of municipal dumping and natural fermentation (Slater, et al., 1985). In another study on
leachate sample suggested that relatively high concentrations of carbohydrates could be
observed in a high molecular weight fractions and substantial quantities of aromatic
hydroxyl and carboxylic compounds present in the lower molecular weight fraction (Chian,
1977).
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