Two-stage conversion of syngas and pyrolysis aqueous condensate into L-malate

Abstract

Hybrid thermochemical-biological processes have the potential to enhance the carbon and energy recovery from organic waste. This work aimed to assess the carbon and energy recovery potential of multifunctional processes to simultaneously sequestrate syngas and detoxify pyrolysis aqueous condensate (PAC) for short-chain carboxylates production. To evaluate relevant process parameters for mixed culture co-fermentation of syngas and PAC, two identical reactors were run under mesophilic (37 °C) and thermophilic (55 °C) conditions at increasing PAC loading rates. Both the mesophilic and the thermophilic process recovered at least 50% of the energy in syngas and PAC into short-chain carboxylates. During the mesophilic syngas and PAC co-fermentation, methanogenesis was completely inhibited while acetate, ethanol and butyrate were the primary metabolites. Over 90% of the amplicon sequencing variants based on 16S rRNA were assigned to Clostridium sensu stricto 12. During the thermophilic process, on the other hand, Symbiobacteriales, Syntrophaceticus, Thermoanaerobacterium, Methanothermobacter and Methanosarcina likely played crucial roles in aromatics degradation and methanogenesis, respectively, while Moorella thermoacetica and Methanothermobacter marburgensis were the predominant carboxydotrophs in the thermophilic process. High biomass concentrations were necessary to maintain stable process operations at high PAC loads. In a second-stage reactor, Aspergillus oryzae converted acetate, propionate and butyrate from the first stage into L-malate, confirming the successful detoxification of PAC below inhibitory levels. The highest L-malate yield was 0.26 ± 2.2 mol_L-malate/mol_carboxylates recorded for effluent from the mesophilic process at a PAC load of 4% v/v. The results highlight the potential of multifunctional reactors where anaerobic mixed cultures perform simultaneously diverse process roles, such as carbon fixation, wastewater detoxification and carboxylates intermediate production. The recovered energy in the form of intermediate carboxylates allows for their use as substrates in subsequent fermentative stages.

Keywords: Carbon capture; Detoxification; Energy recovery; Gas fermentation; Mixed cultures; Pyrolysis wastewater.

Fig. 1

Fermentation profile of the mesophilic process. Top x-axis marks the PAC loading, bottom x-axis shows the elapsed fermentation time (EFT). The red bar indicates the period of weekly re-inoculations. a Consumption and production rates of gaseous compounds. Negative values indicate consumption. b Formate, acetate, propionate and butyrate (SCCs) and ethanol concentrations in the fermentation broth. Acetate_{theoretical, PAC} is the theoretical acetate concentration from PAC. c Daily e-mol recovery into products from syngas and PAC fed; daily ratio of e-mol of PAC in the feed to total e-mol of syngas and PAC fed (brown). d Removal efficacies of selected PAC components. Negative efficacy values indicate production. e Relative abundance of the enriched microbial genera (based on 16S rRNA amplicon sequencing variants)

Fig. 2

Spearman’s rank correlations between relative abundance of dominant amplicon sequencing variants (ASVs) and process parameters for the mesophilic semi-continuous STR enrichment. The strength of the correlation is represented by the size of the circle and intensity of the color. Blue circles indicate positive correlations. Red circles indicate negative correlations. p values are shown for non-significant correlations (p > 0.05)

Fig. 3

Fermentation profile of the thermophilic process. Top x-axis marks the PAC loading, bottom x-axis shows the elapsed fermentation time (EFT). Red arrows point to re-inoculation events, the red bar indicate the period of weekly re-inoculations. a Consumption and production rates of gaseous compounds. Negative values indicate consumption. b Formate, acetate, propionate and butyrate (SCCs) and ethanol concentrations in the fermentation broth. Acetate_{theoretical,PAC} is the theoretical acetate concentration from PAC. c Daily e-mol recovery into products from syngas and PAC fed; daily ratio of e-mol of PAC in the feed to total e-mol of syngas and PAC fed (brown). d Removal efficacies of selected PAC components. Negative efficacy values indicate production. e Relative abundance of the enriched microbial genera (based on 16S rRNA amplicon sequencing variants)

Fig. 4

Spearman’s rank correlations between relative abundance of amplicon sequencing variants (ASVs) and process parameters for the thermophilic semi-continuous STR enrichment. The strength of the correlation is represented by the size of the circle and intensity of the color. Blue circles indicate positive correlations. Red circles indicate negative correlations. p values are shown for non-significant correlations (p > 0.05)

Fig. 5

L-Malate yields calculated for the highest L-malate concentrations per SCCs consumed. Bars represent mean values with standard deviations (n = 3)