Feasibility of successive hydrogen and methane production in a single-reactor configuration of batch anaerobic digestion by simultaneous stimulation of hydrogenase activity and direct interspecies electron transfer

The growing energy needs of mankind lead to the need to search for and develop technologies for renewable energy sources. Along with this, the issue of disposal of a large amount of organic waste generated as a result of human activity remains relevant. Methods of biodegradation of wastes with simultaneous production of bio-fuels based on anaerobic digestion processes have become widespread. Recently, a process of direct interspecies electron transfer (DIET) between syntrophic bacteria and methanogenic archaea has been discovered, which does not depend on metabolic intermediates. It also became known that it is possible to stimulate the transfer of electrons between microorganisms using electrically conductive materials of an abiogenic nature, increasing the overall yield of biomethane. Previous studies have shown the possibility of increasing the efficiency of two-stage anaerobic digestion by adding various stimulating materials. In this work, we studied a model of two-stage anaerobic digestion with time-separated stages in one reactor due to the enrichment of the microbial community with the thermophilic hydrogen-producing bacterium Thermoanaerobacterium thermosaccharolyticum SP-H2. The medium was supplemented with 10 mg/L of ferrous sulfate (II) to stimulate hydrogen production, and 10 g/L of granular activated carbon (GAC) was added for activation of direct interspecies electron transfer (DIET). This work aimed to test the method of sequential production of hydrogen and methane in one reactor by simultaneously maintaining a low pH to reduce the activity of methanogens, and adding soluble iron (II) sulfate and GAC to activate hydrogenases and DIET, respectively. The experiment was carried out in sealed glass vials with a volume of 120 ml at a temperature of 55°C (thermophilic temperature regime). The thermophilic digested sewage sludge obtained from the Lyubertsy and Cherepovets treatment facilities (methanogenic) and the culture of anaerobic thermophilic bacterium T. thermosaccharolyticum SP-H2 (hydrogenogenic) were used as inoculums. The successive production of hydrogen and methane was observed in the batches that contained both GAC and ferrous sulfate (II). The efficiency of the process was estimated using the modified Gompertz equation. According to the data obtained by scanning electron microscopy, active biofouling was observed on the particles of granular activated carbon taken from the GAC-Fe vials, covering all cavities and protrusions of the surface. Syntrophic conversion of metabolite products along with positive effect of ferrous sulfate (II) on hydrogenase activity can be assumed based on the obtained data. Thus, the tested original strategy of sequential production of hydrogen and methane in one reactor due to the separation of stages in time is promising for further study and application, including with the use of a real substrate.

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