Energy recovery from syngas and pyrolysis wastewaters with anaerobic mixed cultures

Abstract

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 recovery potential. The results show successful co-fermentation of syngas and aqueous condensate components like phenols and N-heterocycles. However, the characteristics and load of the aqueous condensates affected process performance and product formation. The toxicity, likely resulting from the synergistic effect of multiple toxicants, depended 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.

Keywords: Carbon capture; Energy recovery; Nitrogen heterocycles; Open cultures; Phenolics; Polyethylene; Pyrolysis wastewater; Sewage sludge; Volatile fatty acids.

Fig. 1

Effects of increasing SS-PAC loading on the metabolism of mesophilic (a, c, e) and thermophilic (b, d, f) mixed cultures. The graphs a and b show the kinetic rates of CO, H₂, CH₄, formate, acetate, propionate and butyrate for each load of SS-PAC. The colours highlight increasing e-equivalence of the metabolites detected. Negative values indicate consumption. The bar-graphs c and d show the removal efficacies of some SS-PAC compounds clustered for each load of SS-PAC. The bar-graphs e and f illustrate the e-mol recoveries from substrates (syngas fed and PAC) into products (methane, formate, acetate, propionate, butyrate). Error bars represent standard deviation among replicates (n = 3)

Fig. 2

Total ammonium nitrogen (TAN) production rates and nitrate removal rates for mesophilic (a) and thermophilic (b) SS-PAC experiments. TAN production rates are red while nitrate removal rates are blue. The left Y-axis of graph a represents the scale for both M-SS-PAC and T-SS-PAC experiments. The right Y-axis of graph b represents the scale for both M-SS-PAC and T-SS-PAC experiments. Error bars represent standard deviation among replicates (n = 3)

Fig. 3

Effects of increasing SS-PAC loading on the metabolism of mesophilic (a, c, e) and thermophilic (b, d, f) mixed cultures. The graphs a and b show the kinetic rates of CO, H₂, CH₄, formate, acetate, propionate and butyrate for each load of PE-PAC. The colours highlight increasing e-equivalence of the metabolites detected. Negative values indicate consumption. The bar-graphs c and d show the removal efficacies of some PE-PAC compounds clustered for each load of PE-PAC. The bar-graphs e and f illustrate the e-mol recoveries from substrates (syngas fed and PAC) into products (methane, formate, acetate, propionate and butyrate). Error bars represent standard deviation among replicates (n = 3)