Anaerobic digestion (AD) has been commercially operated worldwide in full scale as a resource recovery technology underpinning a circular economy. However, problems such as a long start-up time, or system instability, have been reported in response to operational shocks. These issues are usually linked to the dynamics of the functional microbiota in AD. Exploring the microbiota-functionality nexus (MFN) could be pivotal to understand the reasons behind these difficulties, and hence improving AD performance. Here we present a systematic MFN study based on 138 samples taken from 20 well-profiled lab-scale AD reactors operated for up to two years. All the reactors were operated in the same lab within the same period of time using the same methodology to harvest physio-chemical and molecular data, including key monitoring parameters, qPCR, and 16S sequencing results. The results showed a core bacterial microbiota prevailing in all reactor types, including Bacillus, Clostridium, Bacteroides, Eubacterium, Cytophaga, Anaerophaga, and Syntrophomonas, while various methanogens dominated different communities due to different inocula origins, reactor temperatures, or salinity levels. This core bacterial microbiota well correlated with biogas production (Pearson correlation coefficient of 0.481, p < 0.0001). Such strong correlation was even comparable to that between the biogas production and the methanogenic 16S rRNA gene content (Pearson correlation coefficient of 0.481, p < 0.0001). The results indicated that AD performance only modestly correlated with microbial diversity, a key governing factor. AD microbiota was neither functionally redundant nor plastic, and a high variety in communities can exhibit a strong difference in reactor performance. Our study demonstrates the importance of a core bacterial microbiota in AD and supports inspiring considerations for design, bioaugmentation, and operational strategies of AD reactors in the future.

Sewage fine sieved fraction (FSF) is a heterogeneous substrate consisting of mainly toilet paper fibers sequestered from municipal raw sewage by a fine screen. In earlier studies, a maximum biodegradation of 62% and 57% of the sewage FSF was found under thermophilic (55 °C) and mesophilic (35 °C) conditions, respectively. In order to research this limited biodegradability of sewage FSF, this study investigates the biodegradation of different types of cellulosic fibers-based hygiene papers including virgin fibers based toilet paper (VTP), recycled fiber based toilet paper (RTP), virgin pulp for paper production (VPPP) as a raw material, as well as microcrystalline cellulose (MCC) as a kind of fiberless reference material. The anaerobic biodegradation or digestibility tests were conducted under thermophilic and mesophilic conditions. Results of the experiments showed different biomethane potential (BMP) values for each tested cellulose fiber-based substrate, which might be associated with the physical characteristics of the fibers, type of pulping, presence of lignin encrusted fibers, and/or the presence of additive chemicals and refractory compounds. Higher hydrolysis rates (K_h), higher specific methane production rates (SMPR) and shorter required incubation times to achieve 90% of the BMP (t_90%CH₄), were achieved under thermophilic conditions for all examined substrates compared to the mesophilic ones. Furthermore, the biodegradability of all employed cellulose fiber-based substrates was in the same range, 38-45%, under both conditions and less than the observed FSF biodegradability, i.e. 57-62%. MCC achieved the highest BMP and biodegradability, 86-91%, among all cellulosic substrates.

Production of biogas from different organic materials is a most interesting source of renewable energy. The biomethane potential (BMP) of these materials has to be determined to get insight in design parameters for anaerobic digesters. A workshop was held in June 2015 in Leysin Switzerland to agree on common solutions to the conundrum of inconsistent BMP test results. A discussion covers actions and criteria that are considered compulsory ito accept and validate a BMP test result; and recommendations concerning the inoculum substrate test setup and data analysis and reporting ito obtain test results that can be validated and reproduced.