Biogas digestate as a sustainable phytosterol source for biotechnological cascade valorization

Abstract

Every year, several million tonnes of anaerobic digestate are produced worldwide as a by‐product of the biogas industry, most of which is applied as agricultural fertilizer. However, in the context of a circular bioeconomy, more sustainable uses of residual digestate biomass would be desirable. This study investigates the fate of the sterol lipids β‐sitosterol and cholesterol from the feedstocks to the final digestates of three agricultural and one biowaste biogas plants to assess if sterols are degraded during anaerobic digestion or if they remain in the digestate, which could provide a novel opportunity for digestate cascade valorization. Gas chromatographic analyses showed that feedstock sterols were not degraded during anaerobic digestion, resulting in their accumulation in the digestates to up to 0.15% of the dry weight. The highest concentrations of around 1440 mg β‐sitosterol and 185 mg cholesterol per kg dry weight were found in liquid digestate fractions, suggesting partial sterol solubilization. Methanogenic batch cultures spiked with β‐sitosterol, cholesterol, testosterone and β‐oestradiol confirmed that steroids persist during anaerobic digestion. Mycobacterium neoaurum was able to transform digestate sterols quantitatively into androstadienedione, a platform chemical for steroid hormones, without prior sterol extraction or purification. These results suggest that digestate from agricultural and municipal biowaste is an untapped resource for natural sterols for biotechnological applications, providing a new strategy for digestate cascade valorization beyond land application.

Mesophilic, Anaerobic Digestion in a Full-Scale, Commercial Biogas Reactor Kills Seeds More Efficiently than Lab-Scale Systems

“…In (organic) agriculture, digestate is considered an environmentally friendly alternative to mineral fertilizers to close nutrient cycles (e.g., [4,5]). Furthermore, digestate is being discovered for material recovery and use, such as peat replacement in growing media [6], fiber for composites [7,8] and biochemical production [9,10]. Beyond feedstock selection, effective digestate valorization must become a priority to successfully integrate AD into a circular bioeconomy in which biogas plants process residues from other production systems and return them to the biomass cycle.…”