Fungal Pretreatments on Non-Sterile Solid Digestate to Enhance Methane Yield and the Sustainability of Anaerobic Digestion

Abstract

Fungi can run feedstock pretreatment to improve the hydrolysis and utilization of recalcitrant lignocellulose-rich biomass during anaerobic digestion (AD). In this study, three fungal strains (Coprinopsis cinerea MUT 6385, Cyclocybe aegerita MUT 5639, Cephalotrichum stemonitis MUT 6326) were inoculated in the non-sterile solid fraction of digestate, with the aim to further (re)use it as a feedstock for AD. The application of fungal pretreatments induced changes in the plant cell wall polymers, and different profiles were observed among strains. Significant increases (p < 0.05) in the cumulative biogas and methane yields with respect to the untreated control were observed. The most effective pretreatment was carried out for 20 days with C. stemonitis, causing the highest hemicellulose, lignin, and cellulose reduction (59.3%, 9.6%, and 8.2%, respectively); the cumulative biogas and methane production showed a 182% and 214% increase, respectively, compared to the untreated control. The increase in AD yields was ascribable both to the addition of fungal biomass, which acted as an organic feedstock, and to the lignocellulose transformation due to fungal activity during pretreatments. The developed technologies have the potential to enhance the anaerobic degradability of solid digestate and untap its biogas potential for a further digestion step, thus allowing an improvement in the environmental and economic sustainability of the AD process and the better management of its by-products.

Digestate Post-Treatment Strategies for Additional Biogas Recovery: A Review

“…The pH of the digestate after sterilization was 11.4, while the optimal reported pH for P. ostreatus and S. rugosoannulata are 6.5-8.7 and 6.5-7.5, respectively [13]. In the studies by Zanellati et al (2020 and [71,72], however, the pH of SFD was lower, between 8.4 and 9.4 and a slight acidification was observed after treatment. The C/N of the SFD studied by Brémond et al (2020) [13] was 23.6, which might not have been high enough to promote enzymes excretion.…”

“…Low C/N ratios negatively affect ligninolytic enzymes excretion, which occurs during the secondary metabolism of WRF under nutrients starvation, usually at nitrogen limitation [75]. High pH could be detrimental for fungal growth and enzymatic activities [72]. In the study conducted by Brémond et al (2020) [13], for instance, pH and C/N ratio were not optimal and might have negatively influenced the fungal growth and secondary metabolism.…”

“…Interestingly, Zanellati et al (2020 and 2021) [71,72] reported successful fungal treatments over 10-20 days even when employing non-sterilized SFD. Methane yields increments ranged between 72% and 214% when treating SFD with Cephalotrichum stemonitis, Coprinopsis cinerea, and Cyclocybe aegerita.…”

“…For instance, the optimal C/N ratio for S. rugosoannulata is 50-100 [13]. The C/N ratios from the studies by Zanellati et al (2020 and [71,72] were not reported, but an ammonia stripping step prior to fungal inoculation was performed, which should have lowered N levels, allowing the C/N ratios to approach optimal ranges. The potential of fungi as a post-treatment method is therefore still not clear, and will require future tests at conditions optimal to fungal growth.…”

Food Waste-Assisted Metal Extraction from Printed Circuit Boards: The Aspergillus niger Route

“…Therefore, the present approach could help to divert fermentescible organic wastes from landfills and to guarantee that high-quality secondary raw materials are produced. The remaining waste could then be used for agricultural compost production [25] or as a fungal pre-treated feedstock for anaerobic digestion to produce renewable energy [26]. Due to the predominance of CA in the biological solution, a classic titration was carried out in order to determine the concentration of CA.…”