Epsilon Archive for Student Projects

Abstract

As the global demand for sustainable energy rises, optimizing biogas production and minimizing post-digestion GHG emissions are crucial. This study compared two serially connected continuously stirred anaerobic reactors (CTRSs) with different volume configurations (65:35% and 35:65%) with a conventional single-step reactor (B1). The study aimed to investigate the effects of system configuration on biogas production, degradation, process efficiency, and stability, while also evaluating the possibility of mitigating post-digestion methane emission during storage and improving digestate quality. All systems were fed with agricultural substrate (manure and crop residue), with an overall organic loading (OLR) for the study of 2.2 L/day, with a retention time of 55 days. The study was repeated for a 1.3 hydraulic retention time (72 days) at a mesophilic temperature of 37°C.

Methane production was highest in the 65:35% serial configuration (C-system), producing 2.3-4.8% more specific methane than the single system and the 35:65% serial configuration (D system). In both serial systems, 70-90% of the methane was produced by the first reactors, with a small quantity, 20-30 % from the second reactors. Process stability was high for all reactors despite an initial volatile fatty acids (VFA) accumulation in the first 30 days, indicating a temporary microbial inhibition, which later decreased due to improved VFA conversion and microbial adaptation.

The serial C system had the highest volatile solids (VS) reduction (86%), indicating a higher substrate degradation, followed by the single-stage B1 (85%) and the D system (84%). Ammonium levels increased across all systems, showing mineralisation of proteins, with the serial systems having slightly higher levels than the single stage B1.
These findings suggest that serial digestion could enhance methane production and degradation efficiency, specifically with a larger to smaller volume configuration. However, its effectiveness depends on retention time, system configuration, and microbial adaptation with implications for improving digestate quality and reducing residual methane emissions during post-digestion storage.