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4.4.2 CH 4-UASB experiments

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During CH 4-UASB reactor start-up and acclimation period (from the 1 to the 34 days
of operation), BA medium supplemented with sucrose 2 g/L was applied during the first 5 days
of operating time under the HRT of 18 days in order to enrich and reinforce the methanogens
growth. The result shows that high methane content in biogas of 85±3% (Fig. 4.6) was achieved,
which indicates that the methanogens immobilized in CH 4-UASB reactor was enriched and could
functioned. Then, CH 4-UASB reactor was switched to acclimate using the mixture consisting of
the homogenized effluent obtained from H 2-CSTR reactor under steady state conditions which
was operated under HRT of 4.50 days and BA medium supplemented with sucrose 2 g/L at a
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volumetric mixing ratio of 1:1 was continuously fed from the 6 to the 10 days of operating
time. Under this operating, the methane content in biogas was still high ranged 84±3% and lower
concentration of VFAs produced found in the reactor. Subsequently, the mixture consisting of
effluent obtained from H 2-CSTR reactor and BA medium at the mixing ratio of 50:50 (%v/v)
was continuously fed into CH 4-UASB reactor, resulting dramatically decreased in methane
content in biogas from 87 to 29%, corresponding to VFAs produced is gradually increase (Fig.
4.7). The possible cause for having low methane content in the biogas is that due to the enriched
methanogens was inhibited by substrate fed compositions which having high content of VFAs
produced in H 2-CSTR reactor especially lactic acid. To prevent the process failure as well as to
reinforce the methanogens growth, BA medium supplemented with sucrose 2 g/L was applied
from days 18-34 of operating time. The result shows that the methane content in biogas is
progressively increased along with gradually decreased in VFAs produced in the reactor. After 7
days of acclimation period (from days 35-41), the methane content in biogas slightly decreased,
which indicates that the enriched methanogens immobilized in CH 4-UASB reactor acclimated
and could tolerate under high lactate content in the substrate (Fig. 4.5). Corresponds to methane
production yield which was 4 times greater than that achieved from the previously acclimated
(from days 11-17).

After acclimation period, it was started by using the homogenized effluent achieved from
H 2-CSTR reactor as previously mentioned supplemented with NaHCO 3 2 g/L. Under this
condition, the methane content in biogas and methane production rate is progressively decrease
and stable after 7 days of operating time. Corresponding to methane production yield was
1587±140 mL CH 4/L substrate or 54±7 mL CH 4/g-VS added (Fig. 4.8), whereas, acetic acid was still
found in very high concentration up to 103±25 mM (Fig. 4.7). Although high VFA content was
produced and/or accumulated in CH 4-UASB reactor as well as in the substrate fed. The CH 4-
UASB reactor was still maintained pH within the optimal favorable pH for methanogenesis stage
ranged 6.5-8.0 (Nielsen, 2004) (Fig. 4.9). Therefore, further operating under this condition
would be very much risky to the process failure due to imbalance of metabolite products
produced and accumulated in the reactor. Thus, the mixture consisting of effluent from H 2-CSTR
reactor and BA medium at the volumetric mixing ratio of 1:1 was applied from days 66-76 of