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respectively. The pH of H 2-CSTR reactor was maintained within the optimal favorable pH for
hydrogen producing bacteria ranged 5.0-5.5 (O-Thong et al. 2012) using NaHCO 3 2 g/L as
shown in Fig. 4.2. The optimal mixing ratio of SLS to POME at 55:45 (%v/v) supplemented
with NaHCO 3 2 g/L was completely mixed by magnetic stirrer at 90 rpm during semi-continuous
fed into H 2-CSTR reactor.

At HRT of 2.25 days and OLR of 20 g-VS/L reactor d, the average hydrogen content in
biogas was 30±2% (Fig. 4.3) with lowers of both hydrogen production rate and hydrogen
production yield was 95±8 mL H 2/L reactor d and 231±18 mL H 2/L substrate were obtained under
steady state conditions at 5 times of HRT as shown in Fig. 4.4. One explanation for low
hydrogen production yield obtained under the HRT of 2.25 days is that microbial are inhibited by
high substrate supplied which facilitate microbial population shift to other metabolic pathways
such as lactate formation pathway. Moreover, POME is a concentrated substrate with high
concentration of oil and grease which preferred longer time to degrade this substance. Thus,
operating shorter HRT of 2.25 days is may be not adequate on biodegradability, resulting in low
hydrogen production yield was achieved.

Meanwhile, the highest hydrogen production rate and hydrogen production yield was
341±19 mL H 2/L reactor d and 1533±88 mL H 2/L substrate, respectively achieved under HRT of 4.5
days with OLR of 10 g-VS/L reactor d, which is just only 7% of hydrogen theoretical yield (498
mL H 2/g-VS) (Boe, 2006). Although the extension of HRT as well as decreasing of OLR,
resulting increase in hydrogen production yield, even so lactic acid was still found dominated
soluble metabolite products (Fig. 4.5). Thus, this is one of the possible reasons for lower
hydrogen production yield was obtained. Moreover, the other possible reason is high hydrogen
content in biogas accumulated in the headspace of the H 2-CSTR reactor which could be
dissolved in the fermentation broth, causes deactivation of hydrogenase (Kongjan, 2010). A
study from Batstone et al. (2002) found that the solubility of hydrogen gas in water is 3.3%
under the temperature ranged 35-50°C.

At HRT of 4.50 days and OLR of 5 g-VS/L reactor d, BA medium was used to reinforce the
microbial growth at the volumetric mixing ratio of substrate and BA medium of 1:1. The result
shows that the hydrogen production yield is still lower (699±15 mL H 2/L substrate) which is just
only 6% of hydrogen theoretical yield. On the other hand, lactic acid produced and accumulated
nd
in the H 2-CSTR reactor is gradually decreases and a relatively stable after the 12 of operating
time as shown in Fig. 4.5.

In final run was operated under HRT of 4.50 days with OLR of 10 g-VS/L reactor d to assert
the performance of the H 2-CSTR reactor. The hydrogen production yield was gradually increases
and eventually be similar to the hydrogen production yield achieved under the same previous
operational conditions. These results indicated that the operating under the optimal conditions
provides higher hydrogen production ability, the possible reasons is that the hydrogen producing
bacteria could utilize the carbohydrates more efficiency to generates hydrogen. Nevertheless,