Energy recovery from syngas and pyrolysis wastewaters with anaerobic mixed cultures

The anaerobic digestion of aqueous condensate from fast pyrolysis is a promising technology for enhancing carbon
and energy recovery from waste. Syngas, another pyrolysis product, could be integrated as a co-substrate to improve
process efficiency. However, limited knowledge exists on the co-fermentation of pyrolysis syngas and aqueous
condensate by anaerobic cultures and the effects of substrate toxicity. This work investigates the ability of mesophilic
and thermophilic anaerobic mixed cultures to co-ferment syngas and the aqueous condensate from either sewage
sludge or polyethylene plastics pyrolysis in semi-batch bottle fermentations. It identifies inhibitory concentrations
for carboxydotrophic and methanogenic reactions, examines specific component removal and assesses energy recov-
ery potential. The results show successful co-fermentation of syngas and aqueous condensate components like phe-
nols and N-heterocycles. However, the characteristics and load of the aqueous condensates affected process perfor-
mance and product formation. The toxicity, likely resulting from the synergistic effect of multiple toxicants, depended
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on the PACs’ composition. At 37 °C, concentrations of 15.6 g$_{COD}$/g$_{VSS}$ and 7.8 g$_{COD}$/g$_{VSS}$ of sewage sludge-derived aqueous condensate inhibited by 50% carboxydotrophic and methanogenic activity, respectively. At 55 °C, loads between 3.9 and 6.8 g$_{COD}$/g$_{VSS}$ inhibited by 50% both reactions. Polyethylene plastics condensate showed higher toxicity, with 2.8 g$_{COD}$/g$_{VSS}$ and 0.3 g$_{COD}$/g$_{VSS}$ at 37 °C decreasing carboxydotrophic and methanogenic rates by 50%. At 55 °C, 0.3 g$_{COD}$/g$_{VSS}$ inhibited by 50% CO uptake rates and methanogenesis. Increasing PAC loads reduced methane production and promoted short-chain carboxylates formation. The recalcitrant components in sewage sludge condensate hindered e-mol recovery, while plastics condensate showed high e-mol recoveries despite the stronger toxicity. Even with challenges posed by substrate toxicity and composition variations, the successful conversion of syngas and aqueous condensates highlights the potential of this technology in advancing carbon and energy recovery from anthropogenic waste streams.