## 3 Results and Discussion
### Organic Loading Effect  
According to Sections 3.1 and 3.2, the experiment that was carried out at pH 6.0 was the optimal in terms of H2 production, whereas acetate and ethanol were produced as main end products. Two additional tests (runs F and G) were performed at pH 6.0, under different organic loading values. More specifically, run F tested the substrate without any prior dilution (125 g COD/L), whereas the following one was performed after dilution until the organic loading of 80 g COD/L. As shown in Figure 4, both experiments of runs F and G exhibited similar behavior. No biogas was detected, and the production of the other metabolites remained at low levels, about 400 mg/L, indicating unfavored conditions for the efficient growth of microorganisms and biomass decomposition. For the same reason,  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Substrate** & **Reactor Type** & **Temperature** & **H2 Yield** & **Other Metabolites** & **Reference** \\ \hline Cheese whey & batch & 39 \(\pm\) 1 \({}^{\circ}\)C & 162 L/kg TOC & Max lactate yield & [48] \\ \hline Swine wastewater pretreated & batch & 36 \({}^{\circ}\)C & 7.0 mL H2/g VSS & Acetate \(>\) Propionate \(>\) & [49] \\ with thermophilic bacteria & batch & 37 \({}^{\circ}\)C & 4.75 mmol H2/g & Butyrate \(>\) Propionate \(>\) & [50] \\ (Preteated 90 \({}^{\circ}\)C for 10 min) & batch & 37 \({}^{\circ}\)C & COD\({}_{\text{influent}}\) & Acetate & [28] \\ \hline Expired food products and used disposable nappies hydrolysate & batch & 37 \(\pm\) 0.2 \({}^{\circ}\)C & 1.33 mol H2/mol glucose & Mainly Acetate and butyrate & [28] \\ \hline Paddy straw after acid & batch & mesophilic & 1.03 mol H2/mol glucose & Final VFAs of 3.45 g/L & [51] \\ \hline OMW-Cheese whey-liquid & batch & 37 \({}^{\circ}\)C & 0.642 mol H2/mol glucose & Butyrate \(>\) Propionate \(>\) & [52] \\ cow manure (55:405 \(\
icefrac{{}^{\circ}}{{}^{\circ}}\)/\(\
icefrac{{}^{\circ}}{{}^{\circ}}\)) & & & & Acetate & [52] \\ \hline Hydrothermal preteated & CSTR & 55 \(\pm\) 1 \({}^{\circ}\)C & 10.38 mL g\({}^{-1}\) vs. h\({}^{-1}\) Gas & Butyrate \(>\) Acetate & [53] \\ food wastes & food wastes & & & & \\ \hline Fruit/vegetable wastes and disposable nappies hydrolysate & CSTR & 37 \(\pm\) 0.5 \({}^{\circ}\)C & 1.93 \(\pm\) 0.17 L H2/L\({}_{\text{feed}}\) & 11.7 \(\pm\) 3.4 g VFAs/L & [24] \\ \hline OMW & batch & 55 \(\pm\) 0.5 \({}^{\circ}\)C & 0.7 mol H2/mol glucose & Acetate \(>\) Butyrate \(>\) Ethanol & this study \\ \hline \hline \end{tabular}
\end{table}
Table 5: Optimal yields of various substrates under DF experiments, at pH value of 6.0.  
insignificant changes in phenolic compounds content were also detected. Such experiments underline the importance of organic loading not only as a DF parameter, but also as a major inhibitor. Indeed, the results in Figure 4 indicate the significance of operating the DF reactors at an initial substrate concentration lower or equal to 50 g COD/L [25]. According to [PERSON] et al. [55], low organic loading increased the H2 yield but decreased VFA production for a mixture of pig manure, cocoa mucilage, and coffee mucilage. However, their best results were achieved for an organic load 10 g VS/L, whereas our experiments operated successfully for an average of 30 g VS/L (run C- dilution for 40 g COD/L). These findings are not in line with our study, where both VFAs and H2 were significantly decreased with the increase in organic loading, but such behavior could be attributed to the combination of organic loading and the presence of inhibitory compounds (phenols).  
The organic loading barrier could be managed not only by dilution, but also by the implementation of pretreatment methods for organic load reduction and inhibitors' elimination or removal, such as several advanced oxidation processes [56], membrane filtration, extraction [57], or coagulation [58]. Such pretreatment strategies could be applied to the OMW, prior to DF, in order to further evaluate the coupled process for operational yield improvement and efficiency enhancement.