Product formation (stoichiometry) linked to reactor operation
Operation at different SRT values, long settling times and biogas
recirculation resulted in distinct differences in product spectrum in
this work. At an SRT of 50 d a product spectrum consisting of propionate
and acetate reached an overall VFA yield of 77%
(gCOD·gCODglucose-1) obtained during a
cycle measurement. At lower SRT values the product spectrum contained
less propionate and additionally butyrate was measured in the product
spectrum at the end of a cycle. During the 1-2 d SRT there was transient
production of lactate observed during glucose consumption. The product
spectra obtained at 1-2 d SRT and 10-20 d SRT were comparable to those
obtained in a previous study operated at a pH of 5.5 and using granular
sludge (J. Tamis et al., 2015). The difference between the studies are
the ratio in which the main fermentation products were obtained and
possible H2 production as shown in table 2. In this
study more acetate was produced compared to butyrate which was the main
fermentation product in (J. Tamis et al., 2015) The COD-balances as
measured in the cycle measurements were 0.94, 0.92, 0.88
gCOD·gCODglucose-1 for respectiveley
1-2 d, 10-20 d and 50 d SRT. This indicated that the lion-share of the
fermentation products were found.
Despite the high SRT, methane production was effectively repressed
probably due to the relatively low operational pH. As opposed to
previous studies, in this work at an SRT of 40-50 d a relatively higher
percentage of the product spectrum consisted out of propionate compared
to operating at lower SRT. Namely, 0.57
gCODPro·gCODglucose and 0.14-0.18
gCODPro·gCODglucose respectively for the
40-50 d and 1-2 d & 10-20 d SRT. Tamis et al., (2015) operated an
anaerobic sequential batch reactor using granular sludge reactor at a pH
of 5.5 and at an SRT of 1-2 d, around 0.05-0.10
(gCOD·gCOD-1) of the product spectrum contained
propionate. Chemostat studies have found varying results. (De Kok et
al., 2013) demonstrated that by increasing the dilution rate in a
glucose-fed chemostat from 0.05 to 0.125 h-1 the
COD-based propionate yield decreased from 0.23-0.29 to 0.02-0.03
gCOD·gCOD-1. These results indicate that the SRT plays
an important role in the product spectrum and high SRT favour propionate
production at the expense of butyrate production. The answer to question
why elevated SRT-values seem to favour propionate production in this
study remains to be elucidated.