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

change in the predominant species while carbon assimilation metabolism with different
substrates.
The yield depends upon the oxidation state of the carbon sources and nutrient
elements, degree of polymerization of the substrate, pathway of metabolism, growth rate
and other physical parameters of cultivation (Tchobanoglous, et al., 2003). The maximum
yield coefficient of the bacterial culture was found to be greater than that of the yeast
culture signifies that bacterial growth is more pronounced than that of the yeast culture.
This is further supported by the evidence that the specific growth rate of the bacterial
culture was 0.42 d -1 compared to that of 0.27 d -1 of the yeast culture.
The growth rate of the bacterial culture was almost 1.53 times the yeast culture at a
maximum substrate concentration of around 40 mg/L COD. Such an observation is in
accordance with the biokinetic studies conducted by Dan (2002) in high saline wastewater,
where the yeast culture showed a lower yield coefficient and specific growth rate in the
yeast system compared to that of the bacterial system.
4.2.3 Toxicity Studies
In addition to many organic and inorganic compounds that are present in the landfill
leachate, the presence of toxic substances also persists. These toxic compounds not only
pose harm to the environment when released but also affect the efficiency of the biological
treatment system. These metals affect the performance of the bioreactors by inhibiting the
bacterial growth. Oxygen consumption in a biological system has been monitored in
several studies to monitor the toxicity of the wastewater from several sources (Solyom, et
al., 1976; Solyom, 1977). For an aerobic organism, toxicity test could be measured by
measuring the oxygen uptake rate (OUR) in presence of the toxicant, which will signify the
inhibitory effect of the toxicant on the microorganism (Chen, et al., 1997).
(1) Ammonia Toxicity
The ammonium concentration in the leachate is usually found to be very high. As
mentioned by Keenan, et al. (1984), the high concentration of ammonia is a challenge for
the biological treatment of leachate as it may brings about toxicity to the organisms. For
better understanding of the effect of the ammonia concentration on the growth of the
organisms used in the present study, toxicity test was done with ammonium chloride as the
source of ammonia. The operational parameters for the toxicity test are described in Table
3.3. The procedure of toxicity test is described in section 3.4.1. The ammonium chloride
concentration used in the study were 70, 1000, 1500 and 2000 mg/L. The substrate
concentration used in the study was 7 mg COD/L for the bacterial system and 5.6 mg
COD/L for the yeast system.
An aerobic biological process contains two major classes of aerobic microorganisms,
namely nitrifying bacteria and heterotrophic bacteria. The heterotrophs represent the
microorganisms responsible for carbonaceous removal. Nitrifying bacteria (Nitrosomonas
and Nitrobactor) are responsible for the oxidation of ammonia to nitrite and nitrate
nitrogen. The optimum pH is 7.5 to 8.6 for Nitrosomonas and 6.0 to 8.0 for Nitrobactor.
The range of free ammonia concentration affecting to Nitrosomonas had been investigated
by some researcher is around 7 to 150 mg/L and Nitrobactor is around 0.1 to 1.0 mg/L
(Barnes, 1983; Abeling and Seyfried, 1992). It was observed by Blum and Speece (1992)
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