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Biochemical conversion of dairy manure as mono- and co-substrate in a single- and two-stage anaerobic digestion systems

<span style="color: #1f1f1f">The energy potential in manure is not fully utilized during conventional anaerobic digestion (AD). Fibrous material, when discharged as digestate, still contains residual energy that can be extracted. This study aimed to maximize manure energy recovery using single-stage (1S) and two-stage (2S) AD configurations. Unlike traditional 2S systems that separate biochemical phases, the proposed design spatially separates the fiber-rich solid fraction from the liquid fraction. Solids are digested in a CSTR, while the liquid, containing diluted intermediates, is fed into a packed-bed reactor. The experiments were conducted without intentional first-stage acidification, as manure digestion is optimal at neutral to slightly alkaline pH, contrasting with previous studies that often used conditions unsuitable for this substrate. Both systems were operated at 38&nbsp;°C and 55&nbsp;°C, with identical hydraulic retention times (25&nbsp;&nbsp;days) and organic loading rates (1.3–2.5&nbsp;g</span><sub style="color: #1f1f1f">VS</sub><span style="color: #1f1f1f">&nbsp;L</span><sup style="color: #1f1f1f">−1</sup><span style="color: #1f1f1f">&nbsp;d</span><sup style="color: #1f1f1f">−1</sup><span style="color: #1f1f1f">), enabling a direct comparison of specific methane yields (SMY). Results showed that the 2S configuration outperformed the 1S configuration, especially during thermophilic mono-digestion, achieving an SMY of 0.156&nbsp;</span><span style="color: #1f1f1f"></span><span style="color: #1f1f1f">&nbsp;g</span><sub style="color: #1f1f1f">VS</sub><sup style="color: #1f1f1f">−1</sup><span style="color: #1f1f1f"><span>&nbsp;</span>compared to 0.089&nbsp;</span><span style="color: #1f1f1f"></span><span style="color: #1f1f1f">&nbsp;g</span><sub style="color: #1f1f1f">VS</sub><sup style="color: #1f1f1f">−1</sup><span style="color: #1f1f1f"><span>&nbsp;</span>in 1S, representing a 75&nbsp;% increase. The 2S system recovered approximately 87&nbsp;% of its methane potential, versus 55&nbsp;% in the 1S. Co-digestion with sugar beet seepage (SBS) was also studied to improve SMY; however, the enhancement was negligible, as the readily degradable SBS was almost completely converted in the first stage. Overall, spatial separation of substrate fractions in a 2S system significantly enhances methane recovery from lignocellulosic manure, improving manure-based biogas system productivity and viability.</span>