Optimization of methane fermentation of agricultural waste

Abstract

Accumulation of environmentally hazardous agricultural waste, in particular cow manure without the proper treatment, creates an ecological threat to the environment. During the energy crisis, the search for alternative pathways to obtain energy is also one of the most urgent issues today. Cow manure is promising as a substrate for anaerobic fermentation with the production of a valuable energy carrier — methane. Fermentation of cow manure as a sole substrate is not spread. It takes place due to the low efficiency of the process. We hypothesized that low efficiency is caused by the inhibition of the microbiome of cow manure by the end products of fermentation in the gastrointestinal tract of cows and non-optimal conditions for microbial growth. Thus, the goal of the work was to develop the approach of methane fermentation of cow manure using a conversion succession for its effective degradation with the obtaining of a valuable energy carrier. The following methods were used: potentiometric measurement of pH and Eh, chromatographic determination of the composition of the gas phase. To study the patterns of methane fermentation of cow manure, the following treatments were analyzed: 1. fermentation of manure by the autochthonous microtome without regulation of the pH of the medium; 2. fermentation of manure with the addition of fermented sludge of methane tank as an inoculum without pH regulation; 3. via adding fermented sludge of methane tank and with pH regulation. As a result, the fermentation of cow manure by the autochthonous microbiome was shown to be inefficient. In this case, the methane yield was only 20 L/kg of substrate. The inhibition of the microbiome was suggested to take place due to the accumulation of the end products of fermentation. Conversion succession, i.e., the replacement of the autochthonous microbiome, due to the introduction of biomass of fermented sludge of methane tank, provided the increase in the yield of methane to 230 L/kg of substrate. This result testifies the application of another methanogenic microbiome is useful to increase the efficiency of the process. In this case, compounds that inhibited the autochthonous microbiome serve as the substrate for the introduced one. The regulation of the pH allowed creating of the optimal growth conditions for microorganisms and increased the yield of methane to 280 L/kg of substrate. So, the change of the microbiome, i.e. conversion succession, and the regulation the pH made it possible to 14-fold increase in the efficiency of methane fermentation of cow manure. This approach is promising for the effective degradation of cow manure with the production of a valuable energy carrier, methane. The obtained results can be the basis for the further development of environmental and energy microbial biotechnologies.