Abstract
Abstract Background The use of biogas as renewable resource of energy is of growing interest. To increase the efficiency and sustainability of anaerobic biogas reactors, process failures such as overacidification, foaming, and floating layers need to be investigated to develop sufficient countermeasures and early warning systems to prevent failure. Methods Chemical, rheological, and molecular biological analyses were conducted to investigate a stirring disruption in a full-scale biogas plant. Results After the agitation system was disturbed, foaming and floating layer formation appeared in a full-scale biogas plant fed with liquid manure and biogenic waste. Rheological characterizations and computational fluid dynamics (CFD) revealed a breakdown of the circulation within the reactor and a large stagnation zone in the upper reactor volume. Molecular biological analyses of the microbial community composition in the floating layer showed no differences to the digestate. However, the microbial community in the digestates changed significantly due to the stirring disturbances. Foam formation turned out to be a consequence of overloading due to excessive substrate supply and disturbed mixing. The subsequent increase in concentration of both acetic and propionic acids was accompanied by foaming. Conclusions Effective mixing in full-scale biogas plants is crucial to avoid foaming and floating layers and to enhance sustainability. Disturbed mixing leads to process imbalances and significant changes in the microbial community structure. Additionally, controlled feeding might help prevent foam formation due to overloading.
Highlights
The use of biogas as renewable resource of energy is of growing interest
Biogas plants are often affected by process failures such as overacidification or foam and floating layers that reduce the efficiency of plants
Foam formation related to substrate overloading Formation of foam was observed in both methanogenic reactors on day 12 in the first month after high organic loading and agitator break-down
Summary
The use of biogas as renewable resource of energy is of growing interest. To increase the efficiency and sustainability of anaerobic biogas reactors, process failures such as overacidification, foaming, and floating layers need to be investigated to develop sufficient countermeasures and early warning systems to prevent failure. Most of the studies regarding the filamentous bacteria were done in activated sludge treatment plants and anaerobic digesters fed by sewage sludge In these systems, mainly Gordonia spp. and Microthrixparvicella were identified as foam causers [8,9]. Several studies on foam formation in anaerobic digesters treating activated sludge were published [8,12], formation of floating layers and foam in anaerobic digesters fed with biogenic waste and the impact of mixing procedures in full-scale biogas plants as well are still rarely investigated. The objective of the study presented in this paper was to investigate the alterations in rheological, chemical, and microbiological parameters during an agitator disruption accompanying floating layer and foam formation in a full-scale biogas plant treating biogenic waste. The results of this study give further insight into the improvement of the efficiency of the biogas production process as well as understanding of the complex microbial community composition
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