Anaerobic digestion is among the most complicated and unknown biological processes in the environment (Schink 1997). Different aspects attract operational, chemical and biological criticisms. Moreover, these aspects are strictly interconnected with one another. A wide number of papers in this field have been published in recent years (Khalid et al. 2011). Most of these studies, however, didn't include methanogens characterization or they have been based on a metagenomic approach in which a small subunit of ribosomal RNA was used (Pycke et al. 2011; Supaphol et al. 2011). Methanogen studies using the mcrA-based method have become more common in recent years (Narihiro and Sekiguchi 2011).
Over 90% of the detected methanogenic Archaea in the mesophilic reactor fed swine slurry belonged to the hydrogenotrophic methanogens. These were predominantly Methanobacteriales followed by Methanomicrobiales (Zhu et al. 2011). On the other hands always in mesophilic biogas plant but fed with cattle manure, 84% of all detected methanogens were affiliated with the Methanomicrobiales, whereas only 14% belonged to the Methanosarcinales and 2% to the Methanobacteriales (Bergmann et al. 2010a, b) and in other plant always running on cattle manure, the methanogen community presented the following composition: 41.7% of clones were affiliated with Methanomicrobiales, 30% with Methanosarcinales, and 19% with Methanobacteriales; at temperatures lower than 25°C, the Methanomicrobiales became most prevalent (> 90%) (Rastogi et al. 2008).
In reactor fed leachate and OFMSW, various orders of hydrogenotrophic methanogens belonging to Methanomicrobiales and Methanobacteriales were identified (Cardinali-Rezende et al. 2009). However, during mesophilic digestion of wastewater sludge, Methanosarcina and Methanosaeta were most abundant, comprising up to 90% of the total Archaea present or more (Narihiro et al. 2009; Das et al. 2011). This data confirms the results of our work and the ability of Methanosarcina species to form multicellular aggregates that may resist inhibitions in the reactor (Vavilin et al. 2008).
Despite the data variability such bio-molecular approach can improve the available knowledge of anaerobic digestion, as demonstrated in this work, the biogas production efficiency is significantly and positively correlated to two methanogen groups (Methanosarcina and Methanosaetaceae). Most importantly, this method can represent a way to introduce useful bioindicators into the reactors for early diagnosis of an unbalance or a sufferance situation in the microbiologic community. Establishing an efficiency cut-off during the anaerobic digestion process - optimal production that for our set up is around 0.6 CH4 m3/kg SVadded - it makes possible to observe a role for certain groups of methanogens, primarily the Methanosarcina as useful Archaea bioindicators in the digestion process. On the other hands the produced data shows a clear advantage in the pressure-extrusion respect to turbo-mixing pre-treatment as production rate moreover also the cost of the two pre-treatment plants are very different, against the pressure-extrusion. After a validation process with different digestion processes, the definition of a threshold of alarm seems to be possible.
Finally, it is critical that this kind of approach be utilised and that knowledge in this scientific field be increased. The methanogen diversity in the reactor is widely influenced by the feeding. During anaerobic digestion in which input is mainly cattle manure, the presence of hydrogenotroph methanogens is favoured. However, when other feedings are involved, as in this experimental activity, the methanogen community structure differs in terms of the prevalence of Methanosarcineae such as Methanosarcina and Methanosaeta. This family presents a prevalent acetoclastic methane production. A closer examination is needed for substrate and product analysis. A profile of the substrates, such as butyrate, propionate, H2 and CO2, could be useful in understanding the microbiologic dynamics and the consequent methanogen modulations.