## 3 Results and Discussion
### VFAs Production Using Immersed Membrane bioreactor  
The highest total VFAs concentration was obtained at pH 12 for both food waste slurry and excess sewage sludge without pretreatment, although obtaining pH 12 resulted in substantial usage of  
Figure 9: Biogas production from (**a**) excess sewage sludge and (**b**) food waste slurry in batch reactors for the different treatment methods at different pH levels. chemicals (in this case NaOH) and a higher risk of corrosion and other extra operational costs. Moreover, since the purpose of producing VFAs is using them as the carbon source for the denitrification step in the wastewater treatment process, the final product must contain a high soluble C/N ratio. The highest final soluble C/N ratio for the fermentation with the non-pretreated excess sewage sludge was 15.24, which occurred at pH 5. For the fermentation with the non-pretreated food waste slurry, the C/N ratio at pH 5 was 48.45, which was similar to that at pH 12. As the final C/N ratio was higher in pH 5 and the final total VFAs concentration was close to values at pH 12, and considering the long-run effect of the high alkalinity of the medium on chemical degradation of the polymeric membrane, the iMBR fermentation was conducted at initial pH 5. A stable fermentation system was established even after a considerably long period of operation of more than 3 months in total. The initial VFAs concentrations were 6.98 g/L and 11.01 g/L for excess sewage sludge and food waste slurry, respectively. During the whole period of feeding both reactors with OLR 1 gVS/L/day, the VFAs concentration declined, which indicates that the recovery rate of VFAs from the system was higher than the production rate. To boost the VFAs production, the OLR was increased to 3 gVS/L/day at day 31. As seen in Figure 10, at an organic loading rate of 3 gVS/L/day, the VFAs concentration in the reactor feeding with excess sewage sludge was ramped up till day 44 and reached 9.79 g/L. Thereafter, it leveled off until the last day of fermentation. A similar observation regarding a boost in acid production by increasing the OLR was reported by [PERSON] et al. [27]. An average VFAs concentration of 9.8 g/L was achieved from the excess sewage sludge at a yield of 0.38 gVFA/g VS\({}_{\text{added}}\) (533 mgCOD/g VS\({}_{\text{added}}\)), which is higher than the yield reported for uncontrolled pH systems and is comparable with systems with pH control. [PERSON] et al. [18] reported a VFAs yield of 252 mg COD/g VS\({}_{\text{added}}\) for the system without any pH control and 398 mg COD/g VS\({}_{\text{added}}\) for the system with pH control at 10 using caustic soda. Moreover, the composition of VFAs produced by this iMBR is well suited for the denitrification process. It is important to analyze the composition of VFAs as different carbon sources have different denitrification efficiencies. For instance, the denitrification rate of acetic acid is double the value of propionic acid. The order of VFAs based on the denitrification rate from high to low is as follows: acetic acid, butyric acid, valeric acid, and propionic acid [17]. In the MBR fermentation solution, the most abundant VFA produced was acetic acid (53.90% of total VFAs) followed by propionic acid (15.45% of total VFAs), while iso-valeric acid, butyric acid, iso-butyric, and valeric acid comprised 12.18%, 10.90%, 3.84%, and 3.69% of total VFAs, respectively. Figure 10: The concentration profiles of VFAs recovered from membrane bioreactors (MBRs) fed with excess sewage sludge and food waste slurry. On the other hand, using food waste as the substrate for AD MBR, a VFAs concentration of about 5.5 g/L at a yield of 0.34 gVFA/g VS\({}_{\text{added}}\) was obtained. Regarding food waste AD, our previous research has shown that high yields of up to 0.54 gVFA/g VS\({}_{\text{added}}\) can be reached when the process conditions (mainly pH and OLR) are optimized in the MBR [9].