Co-Fermenting Pyrolysis Aqueous Condensate and Pyrolysis Syngas with Anaerobic Microbial Communities Enables L-Malate Production in a Secondary Fermentative Stage

Abstract

The pyrolytic conversion of lignocellulosic biomass into fuels and chemicals is a promising option for the valorization of agricultural and forestry residues. However, technological developments are still needed to maximize product recovery and carbon fixation of the pyrolysis process. The pyrolysis aqueous condensate (PAC), a pyrolysis by-product, has a high water content and is highly toxic, hampering its use. The anaerobic digestion of PAC from different biomasses has been proven a viable technology for PAC valorization and detoxification, but its toxicity limits the methanogenic potential. Alternatively, methanation or VFA production from syngas by anaerobic mixed cultures are technologies of scientific interest. This study investigates the potential of a two-stage process to convert the carbon and energy in syngas and PAC into L-malate. PAC and syngas were co-fermented by two mixed cultures at 37 and 55 °C, identifying kinetic inhibitions and the effects of increasing PAC concentrations on the product pool. The media from selected mixed culture fermentations were then inoculated with Aspergillus oryzae for L-malate production. The results show that mixed cultures can perform simultaneous syngas fermentation and PAC detoxification. While PAC concentrations above 2% completely inhibited methanogenesis, CO consumption was inhibited at PAC concentrations above 5%, regardless of the temperature. In fermentations where PAC inhibited methanation, the mixed cultures channelled the carbon and electrons from syngas and PAC to volatile fatty acids or acetate/H2 production, depending on the incubation temperature. Substantial detoxification of PAC was observed under PAC concentrations up to 10% independently of the rates of syngas metabolism. PAC detoxification enabled the further valorization of the acetate produced via syngas and PAC fermentations into L-malate, achieving yields up to 0.17 mM/mM. These results are promising for the development of an integrated process that simultaneously detoxifies and recovers value from gaseous and aqueous waste streams originating from pyrolysis.

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

“…The anaerobic sludge was collected at a biogas reactor treating cow manure and handled as described in a previous work [ 48 ]. The total solids, total fixed solids and total volatile solids of the anaerobic sludge were 79.6 ± 0.5 g/L, 26.1 ± 1.1 g/L, and 53.5 ± 0.7 g/L, respectively, as determined following Method 1684 [ 70 ].…”

“…The pellet was dried for 24 h at 80 °C and used to determine the total suspended solids. The concentrations of formate, acetate, propionate, n- butyrate (from here onwards defined as short-chain carboxylates, SCCs), L-malate and ethanol together with the concentrations of few selected PAC components (furfural, phenol, guaiacol, and o- , m- , p -cresol) were determined by a high-performance liquid chromatography (HPLC) device run as described previously [ 48 ].…”

“…However, when dealing with biological processes to treat toxic wastewater, the success of the second fermentation stage depends on the detoxification achieved in the first stage. Besides improving toxicant removal rates in the first stage, the selection of appropriate microorganisms for the second stage might affect process performance and carboxylate conversion rates [ 48 ]. Fungi have been reported to be the microorganisms most tolerant to the oil fraction of the condensates from pyrolysis [ 56 ], and the tolerance of Aspergillus oryzae to PAC and some selected PAC components has been characterized before [ 48 , 57 , 58 ].…”

“…L-Malate production from PAC was viable only after an extensive physiochemical detoxification process [ 57 , 69 ]. Biological detoxification via anaerobic mixed cultures, on the other hand, has been proven to be a valid alternative to reduce PAC components’ toxicity below inhibitory levels while producing carboxylic acids [ 48 ].…”

Aspergillus oryzae as a Cell Factory: Research and Applications in Industrial Production

“…Alberto Robazza et al utilized pyrolysis waste derived from lignocellulose as a culture substrate for L-malic acid production through inoculation of A. oryzae, achieving yields of up to 0.17 mM/mM [121]. Apart from malic acid, organic acids found in plants are also popular regenerative products.…”