Acetogenic Inoculum Selection for Acetate Production from Waste Biomasses via Thermal Shock Treatment

Abstract

Innovative treatment and utilization of waste biomass streams are crucial for increase environmental sustainability of human activities. Sewage sludge from the biological degradation of biomass can be valorized for the selection of biocatalysts capable to convert CO₂ into valuable products. Indeed, chemoautotrophic microorganisms, like methanogens and acetogens, respectively, are able to convert CO₂ into CH₄ or acetate by using hydrogen as electron donor, i.e., their utilization for several bio-based CO₂ reutilization processes has been widely proposed by several authors. Chemoautothrophic acetogens are widely present in waste streams deriving from the organic matter degradation, however, due to the syntrophic relationship between acetogens and acetoclastic methanogens in anaerobic environments, autothropihc acetate results immediately converted into methane. Therefore, the selection of an acetogenic inoculum which allow to obtain CO₂ reduction into acetate, requires methanogens inhibition. Among the different methanogen’s inhibition strategies, the most common method is the use of BES (bromo-ethane sulphonate) which results a not scalable technique for large scale application. A most promising and sustainable approach is offered by the adoption of a thermal treatment which allows to the selection of an acetogenic inoculum, thanks ot the sporogenous capacity of acetogenic bacteria. This work presents the results obtained in the thermal pre-treatment of different type of waste biomasses coming from pilot and full-scale biological processes for the selection of an acetogenic inoculum able to convert CO₂ into acetate. Each waste biomass was treated by a thermal shock procedure that consisted in the treatment of the dried biomass at 120°C for 2 hours. Acetogenic inoculums obtained by the thermal pre-treatment of an acidogenic fermentate, an activated sludge and a mesophilic anaerobic digestate, were tested under hydrogenophilic conditions in comparison with blank tests and raw inoculums. The results clearly indicate the effectiveness of the thermal pre-treatment in the selection of the acetogenic microorga