## 3 Results and Discussion
### Volatile Fatty Acids  
Figure 9 shows the concentrations of VFAs, namely acetic acid, propionic acid, and butyric acid, in the influent and effluent of the bioreactor, for each of the tested HRTs. The VFA contents present in the substrate were different during the three experimental assays. This may have been due to the residence times of the OFMSW inside the pre-processing unit (waste sorting and classification; pulping; mixture with anaerobic sludge; mixture with process water) in the full-scale AD plant. This may lead to the fermentation of the organic substrates and increase in VFAs in the feedstock when the residence times are longer in the pre-processing unit. The main VFAs present in the effluent of the DF bioreactor were acetic acid and butyric acid. These VFAs are related to the most efficient pathways for the production of bioH\({}_{2}\), especially by _Clostridium_ sp. [25; 35]. As regards propionic acid, this organic acid was  
Figure 8: Average concentrations (\(n=3\) samples per HRT) of total phosphorus in the influent (tP influent) and effluent (tP effluent) of the bioreactor, and the respective removal rates (error bars: standard deviation) (concentrations: on the left \(y\) axis; removal percentages: on the right \(y\) axis). Figure 9: Average concentrations (\(n=3\) samples per HRT) of VFAs in the influent and effluent of the DF bioreactor for each of the tested HRTs (error bars: standard deviation). detected in very low concentrations in the HRTs of 5 and 6 d, in comparison with the HRT of 4 d.  
The OFMSW used in the HRT4 assay was characterized by a high content of VFAs, especially acetic acid (6.413 g.L\({}^{-1}\)), which may indicate that this OFMSW sample was already in an advanced stage of fermentation, probably due to a high retention time in the pre-processing unit at the full-scale AD plant. Consequently, it may not have offered favorable conditions for bioH\({}_{2}\) production. Additionally, the concentrations of acetic acid were much higher than those reported in the literature (1.7 g.L\({}^{-1}\)[35]; 1.4 g.L\({}^{-1}\)[30]) for food wastes. The molar ratio between butyric acid and acetic acid (BA/AA) is a useful quantitative indicator of the stability of metabolic pathways associated with the production of bioH\({}_{2}\)[36; 37]. Table 5 shows the BA/AA molar ratios obtained for the different tested HRTs as well as the optimum molar ratio defined by [PERSON] et al. [37]. According to [PERSON] et al. [37], the BA/AA molar ratio is directly proportional to the bioH\({}_{2}\) yield. The authors established this direct relation during the DF of sucrose, under mesophilic conditions, with an OLR in the interval of 10-60 g COD.L\({}^{-1}\).d\({}^{-1}\), at pH 5.5 and an HRT of 12 h. These authors [37] also indicated that a BA/AA molar ratio greater than 2.6 presents an efficient production of bioH\({}_{2}\), despite the fact that other authors have stated that more work is needed to support this optimum value. The BA/AA molar ratios obtained in the present work were much lower than the optimum value indicated in the literature [38; 39], demonstrating that the high concentration of acetic acid may have limited the production of bioH\({}_{2}\), not contributing to the maximization of the biOH\({}_{2}\) yield. In another study by [PERSON] et al. [24], BA/AA molar ratios between 0.9 and 1.2 were recorded during the co-digestion of OFMSW under thermophilic conditions. These values are lower than the optimum value defined by [PERSON] et al. [37], however, even here, the BA/AA molar ratios obtained in the present work were lower than the ratios obtained by [PERSON] et al. [24]. As shown in Figure 9, the increase in OLR produced a significant increase in the concentration of VFAs, particularly when the HRT decreased from 5 to 4 d. The VFA content substantially increased as the OLR increased from 12.58 g TS.L\({}^{-1}\).d\({}^{-1}\) (VFAs = 6.7 g.L\({}^{-1}\)) (HRT of 6 d) to 26 g TS.L\({}^{-1}\).d\({}^{-1}\) (VFAs = 10.8 g.L\({}^{-1}\)) (HRT of 4 d).