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.