## 4 Waste Streams as a Resource for VFA Production  
Currently, the commercial production of VFA primarily relies on petrochemical-derived substrates or refined carbohydrate sources such as glucose, sucrose, and molasses. Acetic acid, for example, is commonly produced via methanol carbonylation or fermentation of glucose, while propionic and butyric acids are typically obtained through the fermentation of starch, sugar beet molasses, or whey permeate. However, these raw materials compete with food and feed applications, making the transition to organic waste streams an environmentally and economically attractive alternative. Selecting an appropriate feedstock is crucial for VFA production from biomass, primarily due to variations in composition and availability. Biomass sources differ significantly in their carbohydrate, lignin, and other constituent levels, directly influencing their suitability for fermentation processes. Since VFAs are predominantly produced through carbohydrate fermentation, feedstocks with higher carbohydrate content are generally more favorable. For instance, simpler substrates like starch and fruit wastes have been shown to produce approximately 50 mmol/L of VFAs, whereas more complex substrates such as grass yield around 25 mmol/L, highlighting the impact of substrate complexity on VFA production efficiency [103]. Beyondcomposition, the availability of biomass feedstocks is vital for sustainable and large-scale VFA production. Factors such as geographic location, climate, and agricultural practices significantly affect the accessibility and consistency of biomass supply [104]. For industrial applications, selecting feedstocks that are not only rich in fermentable carbohydrates but also readily available in substantial quantities is essential to ensure economic viability and continuous operation. To provide a general overview of the factors influencing the conversion of residual biomass into VFAs, Figure 4**summarizes the main feedstock sources and the operational parameters that affect VFA production efficiency. **  
Various biomass types have been extensively studied for their potential in VFA production, considering factors such as composition, availability, and environmental impact. Some recent works are shown in Table 3. However, in most of the studies included in this table, fermentation was performed using uncharacterized mixed cultures, typically anaerobic sludge from operating digesters. Therefore, specific microorganisms used as inoculum are not reported. Lignocellulosic wastes, such as agricultural residues like corn stover, wheat straw, rice husks, and sugarcane bagasse, as well as forest residues including wood chips and sawdust, present a valuable resource for VFA production due to their abundance and potential for waste vaporization. With an estimated global production of over 4 billion tons annually, these residues offer a substantial feedstock for bioconversion processes [105]. To optimize VFA yields from lignocellulosic biomass, it is crucial to carefully regulate key operational parameters. [PERSON] et al. [105] emphasized the importance of maintaining particle size within an acceptable range and operating under mesophilic conditions (35-40 \({}^{\circ}\)C). Additionally, a carbon-to-nitrogen (C/N) ratio between 20 and 40, an OLR below 12 g of volatile solids (VSs)/(L-d), and a HRT of 8-12 days have been recommended as optimal conditions to maximize VFA production. These parameters ensure efficient fermentation by creating an environment conducive to microbial activity while preventing process inhibition. Among these types of biomass, rice straw has shown remarkable potential for VFA production through altered anaerobic digestion processes. [PERSON] et al. [106] reported a VFA yield of 0.63 g/g COD, with a productivity of 6.4 gCOD/L-d. These results were achieved under specific conditions, including an organic OLR of 12 gCOD/L-d and HRT of 1 day.