Biorefinery for a circular carbon paradigm: process benefits to the use of dryland CAM crops for anaerobic volatile fatty acid production.

Background: Anaerobic digestion (AD) or acidogenic fermentation (AF) of biomass can generate either biogas fuel or C ‒ C volatile fatty acids (VFAs) as feedstocks for synthesis of other petrochemical products. Typical AD feedstocks require large amounts of land that could otherwise be used for food production. Unlike these traditional bioenergy crops, plants using the crassulacean acid metabolism pathway (CAM), such as cacti and succulents, may be cultivated on degraded or semi-arid land that cannot support conventional agriculture. This could allow significant biorefinery feedstock to be sourced with minimal impact on existing agriculture or biodiversity. Several economically important CAM crops (e.g. pineapple, agave, prickly pear) are cultivated globally, with waste biomass that could be valorised as a biorefinery feedstock.

Results: Here, we investigate the fermentation kinetics of this novel feedstock class (CAM plants) against traditional bioenergy crops with two contrasting inocula: AD sludge and rumen fluid. Fermentations were performed under the influence of a methanogenesis inhibitor (bromoethane sulfonate) to isolate the acidogenic fermentation processes. CAM and non-CAM substrates in this study demonstrated distinct degradation kinetics (yields and degradation rates). We demonstrate that regardless of the inoculum type, CAM crops show higher hydrolysis rates for VFA production. Moreover, yields of VFAs from three CAM crops (0.41 ± 0.01 - 0.48 ± 0.02 g/g) were higher than for the three non-CAM crops (0.21 ± 0.01 - 0.38 ± 0.01 g/g) when AD sludge was used as the inoculum. This superior performance appeared to correlate with a higher abundance of soluble material and lower structural carbohydrate content in CAM biomass.

Conclusions: At industrial scale, the observed kinetic advantages of VFA production from CAM-plant feedstocks could translate into process enhancements that would greatly improve the cost-competitiveness of anaerobic biorefinery. Assuming comparable biomass productivities of CAM and non-CAM crops, this high yield could allow higher VFA production per unit of cultivated land, improving the environmental credentials of CAM biorefinery.