Correlation between system performance and bacterial composition under varied mixing intensity in thermophilic anaerobic digestion of food waste

Abstract

This study examines the stability and efficiency of thermophilic anaerobic digesters treating food waste under various mixing velocities (50-160 rpm). The results showed that high velocities (120 and 160 rpm) were harmful to the digestion process with 18-30% reduction in methane generation and 1.8 to 3.8 times increase in volatile fatty acids (VFA) concentrations, compared to mild mixing (50 and 80 rpm). Also, the removal rate of soluble COD dropped from 75 to 85% (at 50-80 rpm) to 20-59% (at 120-160 rpm). Similarly, interrupted mixing caused adverse impacts and led to near-failure conditions with excessive VFA accumulation (15.6 g l), negative removal rate of soluble COD and low methane generation (132 ml gVS). The best efficiency and stability were achieved under mild mixing (50 and 80 rpm). In particular, the 50 rpm stirring speed resulted in the highest methane generation (573 ml gVS). High-throughput sequencing of 16S rRNA genes revealed that the digesters were dominated by one bacterial genus (Petrotoga; phylym Thermotogae) at all mixing velocities except at 0 rpm, where the community was dominated by one bacterial genus (Anaerobaculum; phylum Synergistetes). The Petrotoga genus seems to have played a major role in the degradation of organic matter.

Impact of mixing intensity and duration on biogas production in an anaerobic digester: a review

“…These species produced hydrogen from cellulose, and further hydrogenotrophic methanogens are required to convert hydrogen to methane, but they were not found in sufficient quantity in agitated digesters, and a retardation of methane production was noted. Similar results were obtained by Ghanimeh et al [18] in which the Petrotoga genus (phylum of Thermotogae) proliferated under mixing conditions, whereas in non-mixed digesters its absence was noted. It was also stated that the genus Petrotoga played an important role in the degradation of organic matter.…”

“…Ghanimeh et al [18] carried out microbial analysis at various mixing speeds ranging between 80 and 160 rpm. It has been observed that the agitated digester was dominated by Thermotogae phylum (89% at 80 rpm, 87% at 50 rpm and 85% at 160 rpm), which was gradually replaced by Synergistetes.…”

“…At an organic loading rate of 1.9 g VS/l/day, the peak methane content was 5.29 l/ day and 5.10 l/day for digesters "A" and "B" respectively. In further studies, Ghanimeh et al [18] compared mixing intensities from 50 to 160 rpm. 26 À 41% higher methane content was obtained at 50 rpm, than at 80rpm mixing intensity.…”

“…Ghanimeh et al [18] noted that the digestion process was most stable at mixing speeds of 80 and 50 rpm as the VFA concentration was below 2 g/l. On the other hand, vigorous mixing at 120-160 rpm displayed an instability in the process along with reduction in removal efficiencies as the VFA concentrations were 3.3 and 1.8 times higher than the slower mixing.…”

Latest research progress on food waste management: a comprehensive review

“…Stepwise feeding mode without dilution had a lower performance and instability because of insufficient contact between substrate and microorganisms and decrease of effective volume. Ghanimeh et al [6] put the research on varied mixing intensity strategy in thermophilic FW anaerobic digestion. They found that high mixing velocities (120 and 160 rpm) were harmful to the digestion process with 18-30% reduction in methane yield and 1.8 to 3.8 times increase in VFA concentrations which has inhibitive effect on AD process when VFA content is overmuch.…”