In this study, we investigated thermophilic (55 °C) anaerobic digestion (AD) performance and microbial community structure, before and after hydrogen addition, in a novel hybrid gas-stirred tank reactor (GSTR) implemented with a partial immobilization of the microbial community and fed with second cheese whey (SCW). The results showed that H₂ addition led to a 25% increase in the methane production rate and to a decrease of 13% in the CH₄ concentration as compared with the control. The recovery of methane content (56%) was reached by decreasing the H₂ flow rate. The microbial community investigations were performed on effluent (EF) and on interstitial matrix (IM) inside the immobilized area. Before H₂ addition, the Anaerobaculaceae (42%) and Lachnospiraceae (27%) families dominated among bacteria in the effluent, and the Thermodesulfobiaceae (32%) and Lachnospiraceae (30%) families dominated in the interstitial matrix. After H₂ addition, microbial abundance showed an increase in the bacteria and archaea communities in the interstitial matrix. The Thermodesulfobiaceae family (29%)remained dominant in the interstitial matrix, suggesting its crucial role in the immobilized community and the SHA-31 family was enriched in both the effluent (36%) and the interstitial matrix (15%). The predominance of archaea Methanothermobacter thermoautrophicus indicated that CH₄ was produced almost exclusively by the hydrogenotrophic pathway.

This study aimed at evaluating the economic performances of and carbon footprint associated with innovative systems for the energetic valorization of second cheese whey (SCW), a by-product of whey cheese manufacture, through anaerobic digestion processes. Three systems were modeled: a conventional single-stage anaerobic digester (FAD), located at about 50 km from the dairy factory; an on-site conventional single-stage anaerobic digester (CAD), located at the dairy industry; and an on-site two-stage anaerobic digester (TAD). The TAD technology enables the simultaneous production of hydrogen and methane on site. The biogases produced were combusted in combined heat and power plants (CHP), but only the onsite systems provided process heat to the dairy factory. In the specific conditions assumed, TAD configuration exhibited a higher energy output, which led to a GHG emission reduction of about 60% compared to FAD, mostly thanks to the additional hydrogen (H₂) production and the improved engine performances. A detailed cost analysis confirmed the results of the environmental analysis, pointing to the TAD solution as the most economically viable, with a payback period of 9 years, while the CAD had a payback time of 12 years. The results here presented aim at providing the dairy industry with a robust economic analysis on the opportunity of building an innovative system for SCW valorization, as well as providing policymakers with environmental reliable data to support the promotion of this technology.

This study evaluated different strategies to increase gas–liquid mass transfer in a randomly packed gas stirred tank reactor (GSTR) continuously fed with second cheese whey (SCW), at thermophilic condition (55 °C), for the purpose of carrying out in situ biogas upgrading. Two different H₂ addition rates (1.18 and 1.47 L_H2 L_R⁻¹ d⁻¹) and three different biogas recirculation rates (118, 176 and 235 L L_R⁻¹ d⁻¹) were applied. The higher recirculation rate showed the best upgrading performance; H₂ utilization efficiency averaged 88%, and the CH₄ concentration in biogas increased from 49.3% during conventional anaerobic digestion to 75%, with a methane evolution rate of 0.37 L_CH4 L_R⁻¹ d⁻¹. The microbial community samples were collected at the end of each experimental phase, as well as one of the thermophilic sludge used as inoculum; metanogenomic analysis was performed using Illumina-based 16S sequencing. The whole microbial community composition was kept quite stable throughout the conventional anaerobic digestion (AD) and during the H₂ addition experimental phases (UP1, UP2, UP3, UP4). On the contrary, the methanogens community was deeply modified by the addition of H₂ to the GSTR. Methanogens of the Methanoculleus genus progressively increased in UP1 (47%) and UP2 (51%) until they became dominant in UP3 (94%) and UP4 (77%). At the same time, members of Methanotermobacter genus decreased to 19%, 23%, 3% and 10% in UP1, UP2, UP3 and UP4, respectively. In addition, members of the Methanosarcina genus decreased during the hydrogen addition phases.

In situ Bio-Methanation (BM) is a recently developed biogas upgrading technique which finds application also in the Power to Gas (P2G) field. In this study a novel configuration of BM digester, the randomly packed Gas Stirred Tank Reactor (GSTR), was modelled. A 49 L reactor, in thermophilic conditions (55 °C) and at atmospheric pressure, was filled up with random packing on which the microbial populations could adhere. The feedstock used was Second Cheese Whey (SCW), liquid waste of cheese factories, rich in lactose (38 g/L), and its flowrate was chosen to obtain a Hydraulic Retention Time (HRT) of 30 days. The process was analyzed for different hydrogen inlet flowrates of 10 mL/min and 50 mL/min. The produced biogas was also recirculated in the reactor in order to transfer, into the liquid phase, as much hydrogen as possible. The model parameters were estimated by means of stationary state information of the reactor working without hydrogen injection, while a dynamical fitting was necessary to evaluate the value of the hydrogen mass transfer coefficient during BM. The model well described the reactor behavior and, by means of a dimensionless analysis in which the numbers of Stanton (St) and β were defined, it was found out that the mass transfer coefficient is the limiting step of the process.