Additional biofilms improve the anaerobic digestion of food leftovers Frank R. Bengelsdorf 1 , Susanne Langer 1 , Martin Kern 2 , Manuel Zak 2, 3 , and Marian Kazda 2 Addresses 1 Institute of Microbiology and Biotechnology, Ulm University, Albert-Einstein-Allee 11, D- 89081 Ulm, Germany 2 Institute of Systematic Botany and Ecology, Ulm University, Albert-Einstein-Allee 11, D- 89081 Ulm, Germany 3 Present address: Renergon GmbH, Kalmenbrunnenstraße 2/1, D-89129 Langenau, Germany Introduction The effect of additional biofilm carriers on process stability and biogas yield was investigated in different studies (Bengelsdorf et al., submitted; Kazda et al., 2013; Langer et al., 2013). The main focus of these studies was the improvement of the anaerobic digestion process supplied with food leftovers or organic waste as substrate. Food leftovers or dissolved air flotation (DAF) sludge have high energy potential but are low in structural compounds and if supplied, the biogas-forming process is often impaired by accumulation of volatile organic acids. The biofilm mode of life offers advantages as syntrophic interactions due to the physical vicinity of microorganisms within biofilms. Syntrophism is a special case of cooperation between two metabolically different types of microorganisms, which depend on each other for degradation of a certain substrate, typically through transferal of one or more metabolic intermediate(s) between the partners. Due to syntrophic interactions, the pool size of the shuttling intermediate can be kept low, resulting in an efficient cooperation. Further, microorganisms attached to a biofilm carrier form an extracellular polymeric substance (EPS) matrix that offers protection. The diffusion barrier also prevents the entry of harming substances into the biofilm. Thus, cells within biofilms are less affected than suspended cultures from changes in environmental conditions such as temperature, pH, nutrient concentrations, metabolic products, and toxic substances. Studies and results Digestion of food leftovers was performed in a lab-scale experimental approach using four parallel-driven batch digesters at two levels of organic load. One of the digesters with the same load of food leftovers was experimentally enriched with chopped plant material (straw) as additional biofilm carrier. The four 12 L digesters (operating volume 9 L, periodically mixed at 38 °C) were on-line monitored for pH inside the digester, biogas volume, and methane concentration. Such monitoring allowed direct comparison of biofilm-enriched digesters with control digesters missing structural amendment. The amendment of surfaces for biofilm formation significantly increased the methane production, which also started earlier than in the control reactors. Furthermore, specific methane yields were consistently higher in reactors containing additional biofilm carriers. In summary, it has been found that slight amendment of straw for biofilm formation significantly stabilized methane production compared to control digesters (Kazda et al., 2013). Based on these experiments, straw was applied in a full-scale biogas reactor predominantly supplied with biowaste such as sanitized food residues and stale bread (Bengelsdorf et al., 2013). Straw (0.6 g l -1 d -1 ) was added to the biogas reactor (day 0) in order to increase the amount of fiber structures. Reactor performance was analyzed 100 days before and after supplementation with straw. In this way, the anaerobic digestion process was improved while the organic loading rate