## 4 Waste Streams as a Resource for VFA Production
This adjustment significantly increased free VFA concentration to 12.6 g/L, demonstrating that targeted pH regulation can enhance free VFA production in nitrogen-rich substrates [112]. Municipal wastewater sludge is another valuable substrate for VFA production due to its high organic content. [PERSON] et al. [113] demonstrated that pretreatment methods, such as thermal hydrolysis at 150 \({}^{\circ}\)C or alkali addition to adjust pH to 10, significantly increased solubilized organic matter by up to 40%, enhancing VFA yields. Optimal operational conditions included a HRT of 10 days and mesophilic temperatures (35 \({}^{\circ}\)C), which resulted in VFA concentrations of approximately 8.5 g/L. Additionally, co-digestion with food waste was shown to balance carbon-to-nitrogen ratios, further improving process efficiency and VFA production. The examples discussed underline how the choice of feedstock significantly shapes the efficiency and feasibility of VFA production. From carbohydrate-rich agricultural residues to complex organic streams like sludge and food waste, the diversity of substrates demands tailored approaches to optimize yields. Strategies such as co-digestion and specific pretreatments play a key role in enhancing substrate performance, demonstrating the potential for sustainable and scalable VFA production across a variety of waste streams. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Substrate** & **Operational Setup** & **Yield** & **VFA Composition (\%)** & **Reference** \\ \hline \multirow{4}{*}{Citrus residues} & Batch fermentation: pH 6, 37.5 \({}^{\circ}\)C, S:I ratio 1:1, duration 14 days & 0.79 g-VFA/g-VS & Acetic: 40,  Propionic: 5.3,  Butyric: 18.3,  Others: 36.4 & [114] \\ \hline \multirow{4}{*}{Potato peels} & Batch fermentation: pH 7, 37 \({}^{\circ}\)C, duration 5 days & 0.63 g-COD/g-VS & Acetic: 45.8,  Propionic: 28.5,  Butyric: 24.2,  Others: 1.5 & [115] \\ \hline \multirow{4}{*}{Brewery spent grain} & Batch fermentation: pH 6.5, 30 \({}^{\circ}\)C, duration 10 days & 0.68 g-VFA/g-VS & Acetic: 50,  Propionic: 20,  Butyric: 25,  Others: 5 & [116] \\ \hline \multirow{4}{*}{Rice straw hydrolysate} & Anaerobic digestion; pH 9.0; 37 \({}^{\circ}\)C; 1 day HRT & 0.63 g VFA/g COD & Acetic: 45,  Propionic: 30,  Butyric: 25 & [106] \\ \hline \multirow{4}{*}{Food waste} & Semi-continuous  membrane bioreactor: uncontrolled pH, OLR 2 g-VS/L-d & 0.54 g-VFA/g-VS & Acetic: 20–30,  Butyric: 3–10,  Butyric: 14–23,  Others: 35–65 & [117] \\ \hline \multirow{4}{*}{Food waste} & Leach bed fermentation: pH 7, 22 \({}^{\circ}\)C, S:I ratio 25:1, duration 6 days & 0.65 g-COD/g-VS & Acetic: 27.9,  Propionic: 12.9,  Butyric: 33.7,  Others: 25.5 & [118] \\ \hline \multirow{4}{*}{Food waste} & Anaerobic digestion: pH 7.0; 35 \({}^{\circ}\)C; 20 days & 0.4–0.6 g VFA/g VS & Acetic: 50–60,  Propionic: 15–25,  Butyric: 10–20 & [119] \\ \hline \multirow{4}{*}{Food waste} & Batch fermentation: initial pH 10 (alkaline condition), inoculum acclimated to food waste, 35 \({}^{\circ}\)C, 15 days & 21.98 g COD/L (highest VFA concentration reported) & Acetic: 38–57,  Propionic: 25–30 & [120] \\ \hline \multirow{4}{*}{Dairy manure} & Semi-continuous anaerobic membrane bioreactor: uncontrolled pH, OLR 4.7 g-VS/L-d, 114 days & 0.41 g-VFA/g-VS & Acetic: 53–89,  Propionic: 4–15,  Butyric: 1–12 & [121] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Waste streams for VFA generation.