Human activities produce vast amounts of organic waste, which poses environmental, social, andeconomic challenges. To address these issues, various treatment methods have been explored totransform waste into valuable resources. Anaerobic digestion (AD) offers a versatile andenvironmentally sustainable waste management solution, particularly in comparison to traditionalmethods, such as composting, incineration, and landfilling. Although AD is well established forbiogas production, its potential to generate other valuable products such as hydrogen and volatilefatty acids (VFAs) is of increasing interest. Part of this thesis aimed to investigate VFA productionfrom manure using AD-incorporating membrane bioreactors (MBRs) for in situ VFA recovery. Theresulting VFA-rich effluents, containing short-chain carboxylic acids (C-2 to C-6) and essentialnutrients, such as nitrogen and minerals, demonstrated potential as substrates for filamentous fungalbiomass cultivation. Filamentous fungi are economically important because of their diversemetabolisms and adaptable cultivation methods. They are utilized in various industries, and theirbiomass can be used for food and feed or for the extraction of valuable products. Therefore, theprimary focus of this thesis is to explore a novel and sustainable approach to further convert VFAsinto high-value fungal biomass. However, owing to the antimicrobial properties of VFAs, theirinhibitory effects were also investigated.

In this thesis, semi-continuous immersed membrane bioreactors (iMBRs) were applied for efficientproduction and in situ recovery of VFAs from the AD of cow and chicken manure. The processesyielded particle-free, VFA-rich effluents containing up to 18 g/L of VFAs along with essentialnutrients. Initial experiments on VFA utilization focused on optimizing fungal growth byinvestigating the effects of nutrient supplementation, pH, and acid concentrations. The resultsindicated that the optimal pH range for fungal growth was between 6 and 7, with Aspergillus oryzaedemonstrating tolerance to acetic acid concentrations of up to 9 g/L. Cultivation trials utilizing VFAeffluents from various organic waste sources confirmed the potential for biomass production, witheffluents rich in nitrogen and acetic acid promoting fungal growth and yielding biomass containingup to 47% crude protein. However, microbial growth is often hindered by the inhibitory effects ofvarying acid concentrations in VFA effluents. Detailed analysis of the inhibitory effects of individualVFAs (acetic, propionic, and butyric acids) and their mixtures on fungal strains, including A. oryzae,revealed that propionic and butyric acids substantially inhibited fungal growth. To mitigate acidinhibition and improve VFA utilization, fed-batch feeding strategies were applied. This resulted in athree-fold increase in biomass yield compared with batch cultivation, demonstrating enhanced acidconsumption. Furthermore, an upscaled fed-batch system was implemented, leading to acceleratedfermentation and improved biomass yields with a high crude protein content of nearly 50%.

In conclusion, utilizing VFA-rich effluents recovered from the AD of organic residues as sole carbonand nutrient sources proved promising for producing edible fungal biomass, offering an attractivestrategy for organic waste valorization within a circular bioeconomy framework.