## 3 Results and Discussion  
### Effect of pH  
The main metabolic products were in, all cases, acetic and butyric acid, with the former always preceding the latter. Using just fruits and vegetables and DNs hydrolysate in similar experiments [5], the same main metabolic products were observed at the range of pH examined. The substrate carbohydrates were cellulosic fibers (cellulose and hemicellulose) [30] from the nappies, cellulose, and soluble sugars from fruit and vegetables [4; 31], and starch [31], due to the presence of bakery products in the mixture. According to [PERSON] et al. [32], when using sugars and disaccharides such as glucose, fructose, and maltose, the production of butyric acid is favored, followed by acetic acid. Depending on the inoculum used, the presence of starch can lead to different metabolic pathways [32]. In the case of cellulose, the products are mostly acetic and butyric acid, followed by low concentrations of propionic acid, exhibiting a behavior similar to the results recorded in this study for the mixture of substrates.  
As it can be observed in Figures 2 and 3, acetic acid concentration increased independently of the hydrogen production rate for pH 5 and 7 (Figures 2a and 3b,f), while butyric acid production followed the hydrogen production pattern (Figures 2 and 3a-f). Similar behavior for acetic acid production during the dark fermentation of food waste has been reported in literature, with butyric acid production diminishing with increasing pH [33]. Some metabolic products, like caproic and propionic acid, where detected towards the end of the experiment, and not as intermediary products that were converted during the process. Small concentrations of ethanol were also detected (pH 5.5 and 6), as well as valeric acid (pH 4.5 and 5), in agreement with literature [33].  
Figure 2: (**a**) Normalized cumulative H\({}_{2}\) production per L of reactor, (**b**) H\({}_{2}\) production yield and carbohydrates removal during the batch dark fermentation of used DNs hydrolysate and EFPs at different pH values.  
Figure 3: Concentrations of VFAs and ethanol during the batch dark fermentation of used adult diapers hydrolysate and expired food products, at (**a**) pH 4.5, (**b**) pH 5, (**c**) pH 5.5, (**d**) pH 6, (**e**) pH 6.5, and (**f**) pH 7.  
The components typically consumed during dark fermentation for hydrogen production are carbohydrates. Around 52% of the total substrate COD originated from carbohydrates, with 1.7-5.6% of total COD being converted to hydrogen and 25-36% to VFAs and ethanol at the pH range examined. Around 10% of the carbohydrates were not consumed during the process, probably because they were present in a form difficult to biodegrade, like cellulose. The 89-92% consumption of carbohydrates (Figure 3b) is in good agreement with relevant literature [33].  
The pH of the process appears to strongly affect the products, with hydrogen production peaking near pH 6 (Figure 2). The production of hydrogen at pH 4.5 (0.97 mol H\({}_{2}\)/mol equivalent glucose) is also worth mentioning, as it is not typically observed [34]. Concerning the efficiency of the process, the maximum yield (1.33 mol H\({}_{2}\)/mol equivalent glucose) was observed at pH 6.0. In a previously published work by the authors [5], where fruit and vegetables and DNs hydrolysate were used as substrate, the maximum yield was 1.12 mol H\({}_{2}\)/mol equivalent glucose at pH 7.5. It is also worth mentioning that the hydrogen yield of EFPs and DNs hydrolysate at pH 5.5 was estimated at 1.12 mol H\({}_{2}\)/mol equivalent glucose while the hydrogen yield of the same substrate mixture at identical pH and temperature conditions in CSTR reactors remained unchanged (1.17 mol H\({}_{2}\)/mol equivalent glucose) [19]. Regarding the total VFAs concentration, the maximum amounts were observed at pH 6.5 and 7, at 13.44 and 13.37 g/L, respectively.