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that hydrogen and total gas (H and CO ) production rate ammonia and sulfate in SLS were diluted and C/N ratio in
2
2
increased slightly when the mixing ratio of POME was the mixtures was increased. In this experiment the hydrogen
increased up to 60 (%v/v) because ammonia and sulfate content ranged from 15.1±1.4 to 27.1ฑ0.8%. The highest
in SLS were diluted and C/N ratio in the mixtures was hydrogen content (27.1±0.8%) was obtained using
increased. Cumulative hydrogen production decreased co-digestion of SLS and POME with mixing ratio of 75:25
slightly when the mixing ratio of POME was increased to (%v/v). The hydrogen production yields ranged from
40 (%v/v) with cumulative hydrogen production of 2.2±0.1 to 23.8±1.3 mL H /g-COD as shown in Fig. 4.
added
2
39.7±1.1 mL H . Hydrogen and total gas production rate The highest hydrogen production yield (23.8±1.3 mL H /
2
2
decreased dramatically when the mixing ratio of POME g-COD ) was obtained using co-substrate with mixing
added
was higher than 40% because anaerobic mixed microf- ratio of SLS to POME of 65:35 (%v/v). The hydrogen
lora used in this experiment was not previously acclimated production yield achieved from co-fermentation was 1 and
with POME. Moreover, phenol and phenolic compounds, 5 times higher than that achieved from individual fermen-
which could pass antibacterial and phytotoxic properties, tations of SLS and POME, respectively. Comparing with

were reported previously that they are contained at high other reports, hydrogen production yields obtained from
concentration, 200-1000 mg/L, in POME (38). The lowest this study are considerably low. This could be due to
cumulative hydrogen production at high substrate con- several reasons such as no external nutrients adding, no
centration indicated inhibition caused by the substrate initial pH adjustment, and high initial substrate concentra-
overload (28). The highest hydrogen production rate was tion (30 g-VS/L) and/or microbial toxicants of sulfate and
obtained using co-digestion of SLS and POME with phenolic compounds contained significantly in SLS and
mixing ratio of 60:40 (%v/v) with hydrogen production POME, respectively. Therefore, further optimizations on
rate of 22.7±1.4 mL H /d, corresponding to biogas those mentioned impacts are needed in order to improve
2
production rate with high biogas production rate of hydrogen productivity from co-fermentation of SLS and
102.6±4.6 mL/d as shown in Fig. 2. The lowest hydrogen POME.
production rate was obtained from using fermentation of There are mainly four fermentation types in the
only POME with hydrogen production rate of 6.9ฑ0.1 mL anaerobic acidogenesis of organic matters (e.g. glucose),
H /d, corresponding to biogas production rate with the namely acetic acid fermentation, propionic acid type
2
lowest biogas production rate of 28.2±0.5 mL/d. Not only fermentation, butyric acid type fermentation, and ethanol
lower hydrogen production rate but also the lower hydrogen type fermentation (29-31). Many microbial communities
content was obtained when using more than 45% of POME exhibit acetic acid fermentation with acetic acid as the
as shown in Fig. 3. Due to fermentation mechanism, the major product (Reaction (1)) (32, 33). The major products
different hydrogen content in each mixing ratio changed of propionic acid type fermentation are propionic and
to the pathways that produce more other gases compo- acetic acids (Reactions (1) and (2)), while the products of
nent such as carbon dioxide. The lowest hydrogen butyric acid type fermentation include butyric and acetic

content was obtained using POME with hydrogen content acids (Reactions (1) and (3)). As for ethanol type fermen-
of 9.7±0.2%. The hydrogen content increased slightly tation, ethanol and acetic acid are the primary fermenta-
when increasing the mixing ratio of POME because tion products (Reactions (1) and (4)) (30, 31).