## 4 Waste Streams as a Resource for VFA Production
Dedicated energy crops, such as switchgrass (_Panicum virgatum_), miscanthus (_Miscantthus_\(\times\)_siganteus_), and giant reed (_Arundo donax_), are cultivated specifically for biomass production. These crops are characterized by high biomass yields and can be  
Figure 4: Main sources of residual biomass and key factors influencing the production of VFAs through acidogenic fermentation. grown on marginal lands, reducing competition with food crops and enhancing land-use efficiency. Switchgrass has been investigated for its ethanol production potential, which indirectly relates to VFA production as intermediates in the fermentation process. Switchgrass can theoretically yield between 4800 and 7900 L of ethanol per hectare annually, depending on cultivation conditions [107]. This corresponds to a theoretical VFA production in a similar range, estimated to be between 4900 and 8000 kg per hectare annually, depending on the initial ethanol yield and the efficiency of the conversion process. Recent studies have explored the potential of switchgrass for VFA production through anaerobic digestion demonstrating that pretreated switchgrass can yield significant concentrations of VFAs, with acetic acid being the predominant component [108]. Various organic waste streams, including food waste, animal manure, and industrial byproducts like glycerol from biodiesel production, serve as viable feedstocks for VFA production. Food waste, for instance, contains high concentrations of proteins, lipids, and carbohydrates, making it a promising substrate for VFA production [109]. Studies have demonstrated that the acidogenic fermentation of food waste can yield significant VFA concentrations. [PERSON] et al. [110] demonstrated that untreated food waste effectively supports targeted butyric acid production under psychrophilic fermentation conditions (17 \({}^{\circ}\)C), achieving concentrations of 0.5 g/L after 7 days. In contrast, thermal-alkaline pre-treatment reduced butyric acid yields, emphasizing the importance of intrinsic food waste microflora. These findings highlight psychrophilic fermentation as a potential strategy for VFA production from food waste. [PERSON] et al. [14] investigated the scale-up of VFA production from food waste via anaerobic digestion and compared it with biomethane generation. At lab scale (5 L), optimal conditions of pH 7 and an OLR of 10 g COD/L/d achieved a VFA yield of 36.6% COD-VFA/g COD substrate and a production rate of 3.9 g COD-VFA/L/d. These results were successfully replicated in a semi-industrial pilot plant (1.5 m\({}^{3}\)), maintaining similar performance metrics. Acetic acid was the predominant VFA (50% mass basis), followed by propionic and butyric acids (20% each). The economic analysis revealed that VFA production yields significantly higher revenues (22.9 EUR/t food waste) compared to biomethane production (11.7 EUR/t food waste), highlighting the economic advantage of VFA recovery from food waste. Animal manure is a typical co-substrate in VFA production processes. [PERSON] and [PERSON] [111] explored the anaerobic co-fermentation of sludge and chicken manure, highlighting the impact of pH on VFA production and microbial dynamics. Alkaline conditions increased VFA concentrations by 1.4 times, with acetic and propionic acids dominating. pH influenced NH\({}_{4}\)\({}^{+}\)-N and PO\({}_{4}\)\({}^{3-}\)-P levels, metabolic pathways, and microbial communities, with Firmicutes prevailing and Bacteriodota and Proteobacteria abundant at pH 9. A strategy to achieve high concentrations of free VFAs involves regulating pH during the anaerobic digestion of nitrogen-rich animal manure. In a 150-day study of high-solid chicken manure digestion, the process initially stabilized at pH 6.0 with total ammonia nitrogen at 7.0 g/L, yielding a low free VFA concentration of 3.1 g/L. On day 70, hydrogen chloride was added to lower the pH to 5.5, eventually dropping further to 4.3, with ammonia decreasing to 2.3 g/L.