Thermal post-treatment of digestate in order to increase biogas production with simultaneous pasteurization.

Biogas production by anaerobic digestion (AD) of organic wastes is important for the transition to fossil free fuels in both the transport sector, industries and shipping. The aim of this study was to target the residual organic matter in the outgoing residue from the AD process, so called digestate, with different thermal treatment methods in order to improve digestate degradability and biogas potential upon post-digestion. The thermal treatment was performed at 55 °C in 24 h, 70 °C in 1 h and by thermal hydrolysis process (THP; 165 °C, 8 bar in 0.33 h), and were carefully selected to offer a simultaneous possibility for pasteurization of the digestate according to the regulations in Sweden. Digestates from ten full-scale biogas plants were collected, with different substrate profiles including wastewater treatment plant (WWTP), food waste digestion, agriculture digestion and manure digestion. The results showed that all thermal treatment methods caused increased dissolved organic carbon concentration (DOC). Four of the thermal treated digestates with the highest increase in DOC were subsequently tested for the bio-methane potential. Thermal treatments at 70 °C and THP, respectively, resulted in the highest increase in bio-methane potentials, with an increase of 15-39% for one WWTP, 38 - 40% for digestate from an agriculture digestion plant and 20 - 22% for digestate from a co-digestion plant treating food waste. Interestingly, the bio-methane potential from digestate treated with the energy-intense THP method, did not show any significant difference compared to thermal treatment at 70 °C for 1 h. The outcomes of this study suggest that placing a pasteurization unit between a main digester and a post digester, when applying two-step digestion allows for a combined pasteurization and increased biogas production.

Post-treatment of dewatered digested sewage sludge by thermophilic high-solid digestion for pasteurization with positive energy output.

This study investigated the possibility to use thermophilic anaerobic high solid digestion of dewatered digested sewage sludge (DDS) at a wastewater treatment plant (WWTP) as a measure to increase total methane yield, achieve pasteurization and reduce risk for methane emissions during storage of the digestate. A pilot-scale plug-flow reactor was used to mimic thermophilic post-treatment of DDS from a WWTP in Linköping, Sweden. Process operation was evaluated with respect to biogas process performance, using both chemical and microbiological parameters. Initially, the process showed disturbance, with low methane yields and high volatile fatty acid (VFA) accumulation. However, after initiation of digestate recirculation performance improved and the specific methane production reached 46 mL CH4/g VS. Plug flow conditions were assessed with lithium chloride and the hydraulic retention time (HRT) was determined to be 19-29 days, sufficient to reach successful pasteurization. Degradation rate of raw protein was high and resulted in ammonia-nitrogen levels of up to 2.0 g/L and a 30% lower protein content in the digestate as compared to DDS. Microbial analysis suggested a shift in the methane producing pathway, with dominance of syntrophic acetate oxidation and the candidate methanogen family WSA2 by the end of the experiment. Energy balance calculations based on annual DDS production of 10000 ton/year showed that introduction of high-solid digestion as a post-treatment and pasteurization method would result in a positive energy output of 340 MWh/year. Post-digestion of DDS also decreased residual methane potential (RMP) by>96% compared with fresh DDS.

Effects of trace element addition on process stability during anaerobic co-digestion of OFMSW and slaughterhouse waste.

This study used semi-continuous laboratory scale biogas reactors to simulate the effects of trace-element addition in different combinations, while degrading the organic fraction of municipal solid waste and slaughterhouse waste. The results show that the combined addition of Fe, Co and Ni was superior to the addition of only Fe, Fe and Co or Fe and Ni. However, the addition of only Fe resulted in a more stable process than the combined addition of Fe and Co, perhaps indicating a too efficient acidogenesis and/or homoacetogenesis in relation to a Ni-deprived methanogenic population. The results were observed in terms of higher biogas production (+9%), biogas production rates (+35%) and reduced VFA concentration for combined addition compared to only Fe and Ni. The higher stability was supported by observations of differences in viscosity, intraday VFA- and biogas kinetics as well as by the 16S rRNA gene and 16S rRNA of the methanogens.