In this study, farm-scale biogas plants in Sweden, were evaluated regarding process performance to provide important insights into operational patterns, methane yield and nutrient outcomes. In total, 38 plants were included in the survey and grouped depending on substrate category, i.e . cattle (C) or pig manure (P), in some cases including co-substrate (CO and PO, respectively). The analysis demonstrates that operational conditions, particularly organic loading rate (OLR) and hydraulic retention time (HRT), were key drivers of performance, efficiency and residual methane potential (RMP). A significant negative relationship was observed between HRT and RMP, demonstrating that insufficient retention time increased the risk of incomplete degradation and elevated methane emissions from digestate during storage. Also, higher OLR was associated with higher RMP across plant types, and the results indicated that higher load intensified methane production but compromised overall conversion efficiency. Co-digestion significantly increased specific and volumetric methane production compared with mono-digestion, especially in plants dominated by pig manure, even at relatively low OLR. The results clarified that balanced operation with adequate HRT in relation to OLR is essential to maximise methane yield and minimise emissions, and that volumetric methane production alone is not a reliable indicator of yield, process efficiency or climate performance. Microbial composition and diversity were mainly shaped by manure type rather than operational parameters. All plants appeared to have sufficient levels of trace metals. Plants operated with pig manure had higher levels of ammonium, potentially increasing the risk of process instability compared with cattle-manure-fed plants.
Perman et al. (Fri,) studied this question.