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ethanol and lactic acid (1.58±0.25 mM and 3.12±0.56 mM, respectively) was detected.
Moreover, when the proportion of POME in the mixture was increased were 40, 45 and 50% as
well as sole fermentation of POME, less lactic acid was generated. The possible reason might be
due to existing appropriate C/N ratio in the mixture for biohydrogen fermentation as previously
mentioned. Moreover, a study from Kalil et al. (2008) found that proper C/N ratio enhance the
bacteria for more growth as well as substrate utilization. On the contrary the mixture consisting
of high proportion of SLS, lactic acid was the major soluble metabolite products. The possible
cause might be due to relatively high inorganic nitrogen contained in the SLS such as ammonium
nitrate and ammonium sulfate are affected on microbial growth (Kalil et al., 2008). These
inorganic salt will usually produce acid condition due to the ammonium ion is utilized and the
free acid will be liberated. Moreover, they are reported that inorganic nitrogen source contained
in fermentation broth will did not change. This could be affected on microbial growth due to
improper C/N ratio along with fermentation time. Similarly lactic and acetic acids were the
major soluble end-products for initial organic concentration of 21 g-VS added/L are summarized in
Table 3.8 and Fig. 3.5. Moreover, other reason is that relatively high initial ammonia content in
SLS of 1213±81 mg/L which was significant factor affecting on microbial growth, resulting shift
in metabolic pathway to lactic acid formation pathway.

O-Thong et al. (2008) used Thermoanaerobacterium-rich sludge as inoculum for
producing biohydrogen from POME; however, it was collected from a palm oil mill wastewater
treatment plant that the same source of seed sludge used in this experiment. They are reported
lactic acid was produced through glucose fermentation. Furthermore, hydrogen was generated
from acetic acid, butyric acid and ethanol fermentations, not from propionic acid and lactic acid
fermentations (Chan and Holtzapple, 2003; Angenent et al., 2004). This is a drawback for using
anaerobic mixed cultures from palm oil mill wastewater treatment plant as inoculum to produce
biohydrogen. However, these VFA can be further converted to acetic and following methane in
the second methane stage.