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methane production yield was 392 mL CH 4/g-VS (O-Thong et al., 2012). In our previous study
was performed using anaerobic co-digestion of skim latex serum and palm oil mill effluent of
thermophilic batch two stages process, satisfactory results of hydrogen and methane production
yield was 85.7±4.9 mL H 2/g-VS and 418±10 mL CH 4/g-VS, respectively was achieved. The
hydrogen and methane production yield achieved from co-digestion of SLS and POME which
was 3 and 2 times greater than that achieved from sole fermentation of SLS. These results
indicate that it would be possible to be applied in a commercial scale.

Deliberation of reactor types that will be used to generate biogas mostly is determined by
substrate characteristics. In this work was carried out by using SLS and POME as substrate,
POME is a slurry, relatively viscous fluids and high colloid suspension substrates which was
necessary to treat by using continuously stirred tank reactor (CSTR). Besides substrate and
biomass in the reactor was mixed well, it was provided to a more convenient method for the
engineering commissioning. At the same time, organic soluble wastes which were treated in the
acidogenic stage mostly are further fed into fixed film reactors such as upflow anaerobic sludge
blanket (UASB) and expanded granular sludge bed (EGSB) reactors. Among them, there are
several advantages of UASB over EGSB reactors such as more stable of methane production and
lower of VFA accumulation in the reactor. In order to develop a two stages anaerobic digestion
process to generates biohydrogen and biomethane from co-digestion of SLS and POME in a
commercial scale which is necessary to be applied foreseeable future.

In the light of these concerns, in this work aims to investigate the effect of hydraulic
retention time (HRT) on the sequential productivity of hydrogen and methane from the optimal
mixing ratio of SLS to POME in a continuously stirred tank reactor (CSTR) and an upflow
anaerobic sludge blanket reactor (UASB), respectively.

4.3 Materials and methods

4.3.1 Inoculum preparation

The anaerobic seed sludge originating employed in this research was collected from
mesophilic wastewater treatment pond of Palm Pattana Southern Border Co, Ltd., Pattani
Province; Southern Thailand was used as inoculum for produced hydrogen and methane. The
sludge used for produced hydrogen was treated by load shock method by using basic anaerobic
(BA) medium (Angelidaki and Sanders, 2004) with sucrose 30 g/L to inactivate methanogens
and enrich for hydrogen producing bacteria. Meanwhile, the sludge used for produced methane
was enriched for methanogens by using BA medium with sucrose 3 g/L. Both hydrogen and