## 2 Mechanisms of Acidogenic Fermentation
### Technical and Operational Challenges
The enhanced C/N ratio supported optimal microbial activity, highlighting the effectiveness of this approach in maximizing VFA yields from biowaste [70]. Similarly, [PERSON] et al. [68] demonstrated the impact of varying C/N ratios on VFA production during the acidogenic fermentation of sucrose. Their study revealed that lower C/N ratios, such as 5, resulted in the highest VFA concentrations (26.08 g L\({}^{-1}\)), underscoring the importance of nitrogen availability for efficient acidogenesis. As the C/N ratio increased to 100, VFA production decreased significantly, accompanied by reductions in volatile solid degradation and carbohydrate utilization rates. Acetic acid remained the dominant VFA across all conditions, with its proportion rising at higher C/N ratios, while butyric acid production declined. These results reinforce the pivotal role of C/N ratio optimization in enhancing VFA yields and tailoring acid profiles, aligning with findings that nitrogen balance is critical for sustaining microbial activity and process efficiency. Co-digestion studies, such as those by [PERSON] et al. [80], demonstrated that mixing sewage sludge (COD/TN of 72) with organic waste (COD/TN of 57) increased valeric acid concentrations in the VFA spectrum from 11% to 22%, due to higher nitrogen content in the organic waste. This observation aligns with [PERSON] et al. [73], who found that co-fermenting carbohydrates and proteins altered acidification pathways, favoring the production of protein-derived VFA. Substrate concentration is another influential factor in acidification. While higher substrate concentrations generally yield more VFA, excessive acid accumulation can inhibit microbial activity unless separated efficiently [70]. [PERSON] et al. [21] observed that substrate concentration significantly influences VFA production, identifying a concentration of 200 g/L as optimal for both total VFA and n-butyrate production using food waste saccharified residue. At this concentration, the highest VFA yield was 280.87 mg COD/g VS, with n-butyrate comprising over 90% of the VFA composition. The efficiency of acidogenesis reached 82.39% (VFA/solubleCOD). Chain elongation, primarily driven by _Clostridium_ Sensu Stricto 12, contributed 43.93% to n-butyrate formation, while 38.47% of the organic matter was further utilized. This demonstrates the critical role substrate concentration plays not only in determining VFA yields but also in influencing the metabolic pathways and microbial dynamics involved in the acidogenic process. When considered alongside other parameters, such as the C/N ratio and substrate composition, substrate concentration further underscores the need for tailored operational strategies to maximize VFA production. The interplay of these factors highlights the complexity of acidogenic fermentation and the importance of precise control over conditions to achieve optimal results. By addressing these interconnected variables, it is possible to refine acidification processes and enhance the efficiency of VFA production systems. The microbial also community plays a critical role in determining the efficiency and composition of VFAs during acidogenic fermentation. Anaerobic mixed cultures, commonly used in these processes, offer several advantages, such as robustness, stability, and the ability to adapt quickly to diverse substrates without requiring feedstock sterilization [66]. However, the diversity of these microbial consortia often results in complex syntrophic interactions and unpredictable metabolic networks, making it challenging to control the production of specific acids in downstream processes. Bacillota, Bacteroidota,and Pseudomonaspromoadota are frequently reported as key phyla associated with VFA production [81]. [PERSON] et al. [66] found that higher VFA yields were associated with an increased relative abundance of _Clostridium_, a genus within the Firmicutes _phylum_ known for its role in butyric acid production.