High precision of measurement of methane potential is important for the economic operation of biogas plants in the future. The biochemical methane potential (BMP) test based on the VDI 4630 protocol is the state-of-the-art method to determine the methane potential in Germany. The coefficient of variation (CV) of methane yield was >10% in several previous inter-laboratory tests. The aim of this work was to investigate the effects of inoculum and the digestion system on the measurement variability. Methane yield and methane percentage of five substrates were investigated in a Hohenheim biogas yield test (D-HBT) by using five inocula, which were used several times in inter- laboratory tests. The same substrates and inocula were also tested in other digestion systems. To control the quality of the inocula, the effect of adding trace elements (TE) and the microbial community was investigated. Adding TE had no influence for the selected, well- supplied inocula and the community composition depended on the source of the inocula. The CV of the specific methane yield was <4.8% by using different inocula in one D-HBT (D-HBT1) and <12.8% by using different digestion systems compared to D-HBT1. Incubation time between 7 and 14 days resulted in a deviation in CV of <4.8%.

Anaerobic digestion (AD) produces bioenergy and bio-fertilizer. However, weed seeds can contaminate the substrates for biogas production. This work assesses seed viability of four weeds species origin from Germany and China under AD conditions at 37, 42 and 52 °C with different retention times. All investigated seeds were inactivated after 12 h at 52 °C, 128 h at 42 °C and 512 h at 37 °C under AD treatment. Inactivation duration varying between species and a global test showed that the countries of seeds origin have a significant effect on inactivation times. For two species, by contrast, short treatment times increased the germination rate which increases the risk of weed dispersal. An anoxic water bath treatment with identical temperatures resulted in longer deactivation times, indicating that other factors also play a role in the weed seeds elimination. The results indicate that the risk of weed seed spreading through digestate application can be minimized by an adequate retention time.

Efforts to integrate biogas plants into bioeconomy concepts will lead to an expansion of manure-based (small) biogas plants, while their operation is challenging due to critical characteristics of some types of livestock manure. For a better process understanding, in this study, three manure-based small biogas plants were investigated with emphasis on microbiome diversity. Due to varying digester types, feedstocks, and process conditions, 16S rRNA gene amplicon sequencing showed differences in the taxonomic composition. Dynamic variations of each investigated biogas plant microbiome over time were analyzed by terminal restriction fragment length polymorphism (TRFLP), whereby nonmetric multidimensional scaling (NMDS) revealed two well-running systems, one of them with a high share of chicken manure, and one unstable system. By using Threshold Indicator Taxa Analysis (TITAN), community-level change points at ammonium and ammonia concentrations of 2.25 g L-1 and 193 mg L-1 or volatile fatty acid concentrations of 0.75 g L-1were reliably identified which are lower than the commonly reported thresholds for critical process stages based on chemical parameters. Although a change in the microbiome structure does not necessarily indicate an upcoming critical process stage, the recorded community-level change points might be a first indication to carefully observe the process.

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