## 2 Mechanisms of Acidogenic Fermentation
### Technical and Operational Challenges  
Converting organic waste and wastewater into valuable VFAs presents a promising avenue for resource recovery. However, this process faces significant challenges, such as slow substrate hydrolysis, low yields, unstable acid compositions, and the complexity and expense of purification and post-processing methods [11]. To overcome these obstacles, optimizing operational strategies is essential. This includes enhancing substrate hydrolysis, managing factors that influence fermentation processes, improving VFA yields and profiles, and mitigating process inhibition. As a direct consequence, the selective production of VFAs from diverse feedstocks is an emerging research focus. It seeks to address the challenges posed by the complex composition of organic-rich waste streams, the dynamic nature of mixed microbial communities, the unpredictability of product types and concentrations, and the demand for efficient extraction and purification techniques [66]. Among the key parameters that govern acidogenic fermentation performance are pH, temperature, HRT, and organic loading rate (OLR). These variables shape the activity of microbial communities and determine both the rate and spectrum of VFA production. pH plays a central role in directing metabolic pathways. Acidogenic bacteria typically perform best under mildly acidic conditions (pH 5.0-6.5), which inhibit methanogenesis and favor VFA accumulation [65]. More acidic environments can hinder microbial activity due to proton accumulation, while near-neutral or alkaline pH may promote competing pathways such as methanogenesis or solventogenesis [67]. Temperature affects microbial kinetics and community structure. Mesophilic conditions (30-37 \({}^{\circ}\)C) are the most widely used due to their operational stability and balance between activity and robustness [21]. Thermophilic regimes (50-55 \({}^{\circ}\)C) may improve the hydrolysis and solubilization of complex substrates but often reduce microbial diversity and increase sensitivity to process fluctuations [68]. HRTs determine the contact time between biomass and substrate. Short HRTs (2-5 d) are frequently applied to wash out slow-growing methanogens and favor acidogenic pathways, although excessively short HRTs can limit substrate conversion and result in acidification inefficiencies [69]. The OLR defines the daily input of organic matter and strongly influences microbial stress and product accumulation. Moderate OLRs (2-6 g VS/L-d) are commonly used in acidogenic systems to balance productivity and microbial inhibition [70]. High OLRs may cause rapid pH drops and VFA accumulation, leading to process inhibition if not adequately buffered. Together, these parameters must be carefully balanced to maintain stable and efficient acidogenic conditions. In this section, we focus on the general  
Figure 3: VFA production pilot plant on Valladolid, Spain. operational challenges associated with real organic waste streams. Specific strategies for directing VFA composition by tuning these parameters are addressed in Section 5. One of the major challenges in VFA production is the rate-limiting step in the hydrolysis of biopolymers. This phase, catalyzed by enzymes secreted by anaerobic acidogenic bacteria, plays a crucial role in determining process efficiency. The hydrolysis rate depends on substrate characteristics, particularly the available surface area for enzymatic activity [71]. Enzymatic interactions follow the induced-fit model, where substrate binding induces changes in the enzyme's active site, enabling catalysis. Bacteria either secrete exoenzymes into the bulk liquid for subsequent adsorption onto the substrate or colonize the substrate surface, maximizing enzymatic activity through localized interactions [72]. For particulate substrates, hydrolysis is often described using first-order kinetics, with the rate influenced by substrate concentration and surface availability.