Thermophilic two-phase anaerobic digestion of source-sorted organic fraction of municipal solid waste for bio-hythane production: effect of recirculation sludge on process stability and microbiology over a long-term pilot-scale experience

A two-stage thermophilic anaerobic digestion process for the concurrent production of hydrogen and methane through the treatment of the source-sorted organic fraction of municipal solid waste was carried out over a long-term pilot scale experience. Two continuously stirred tank reactors were operated for about 1 year. The results showed that stable production of bio-hythane without inoculum treatment could be obtained. The pH of the dark fermentation reactor was maintained in the optimal range for hydrogen-producing bacteria activity through sludge recirculation from a methanogenic reactor. An average specific bio-hythane production of 0.65 m3 per kg of volatile solids fed was achieved when the recirculation flow was controlled through an evaporation unit in order to avoid inhibition problems for both microbial communities. Microbial analysis indicated that dominant bacterial species in the dark fermentation reactor are related to the Lactobacillus family, while the population of the methanogenic reactor was mainly composed of Defluviitoga tunisiensis. The archaeal community of the methanogenic reactor shifted, moving from Methanothermobacter-like to Methanobacteriales and Methanosarcinales, the latter found also in the dark fermentation reactor when a considerable methane production was detected.