Process upsets in a full-scale anaerobic digestion bioreactor: over-acidification and foam formation during biogas production

Background: Biogas plays a worthwhile role as a renewable, storable energy source. Anaerobic digestion (AD) is the core process in biogas production. The two most common process upsets that occur during this biotechnological process in practice are over-acidification and foam formation. Several research projects have been carried out so far to identify the causes of these upsets and to develop early warning indicators. However, the relevant laboratory results have not yet been verified in practice. Accordingly, the aim of this work was to study the practical application of the published tools in the case of a full-scale biogas plant. Methods: A full-scale AD plant utilizing cattle manure and energy crops was sampled over a period of 12 weeks. During this time, over-acidification and foam formation occurred in the primary digester. Results: The sum of acetate, propionate and butyrate (VFAS) and the ratios of VFAS to magnesium (VFAS/Mg), calcium (VFAS/Ca), and phosphorus (VFAS/P) were found to be good predictive tools to identify over-acidification for the given AD system. Their values increased by factors of 6 (VFAS/Mg and VFAS/Ca) and 13 (VFAS, VFAS/P) in the early over-acidification phase. In contrast, the VFA/TIC ratio, which is a common indicator in practice, increased only by a factor of 2 during this phase. An ammonium-nitrate-urea solution proved to be an effective agent for foam suppression when sugar beet was used as a substrate. Its application showed no negative effect on the daily electricity production and the achieved long-term foam elimination. Conclusions: Several parameters were verified to be able to serve as an early warning indicator of over-acidification, but not one was found to be able to serve as an early warning indicator of foam formation in AD. Further research is needed to identify parameters with predictive potential for indicating foaming in AD.