## 3 Results and Discussion
### VFAs Production Using Immersed Membrane bioreactor
The average composition of the VFAs was as follows: acetic acid 42.63%, propionic acid 17.29%, iso-butyric acid 1.89%, butyric acid 19.55%, isovaleric acid 15.26%, and valeric acid 3.34% of total VFAs. The composition of the VFAs is linked to the microbial structure in the bioreactor and the established microbial pathways [8]. In the prevailing operating conditions, the dominating bacteria community seemed to favor the biosynthesis of mainly acetic, butyric, and isovaleric acids. The presence of acetic and butyric acids is associated with pathways that result in hydrogen production [9]. On the other hand, the higher presence of isovaleric acid compared to valeric acid could have been caused by the bacterial dynamics in the bioreactor, although this phenomenon requires further investigation. With the considered process parameters, the AD of food waste resulted in excessive bio-hydrogen production (244 NmL/g VS\({}_{\text{added}}\)) due to the favorable pH (about 5.2) compared to the pH condition maintained by the sludge, which was more suitable for VFAs production (about 5.9). The main gas by-product from the sludge fermentation was methane but at a low yield of approximately 93.6 NmL/g VS\({}_{\text{added}}\). The composition and volume of gas produced are presented in Figure 11. Regarding the C/N ratios, the average values were 43.77 for food waste compared to 15.88 for sludge. The concentration of suspended solids in the system can reflect the effectiveness of hydrolysis and acidification. Moreover, it directly affects the membrane filtration efficiency thorough membrane fouling. The viscosity of the fermentation broth is proportionate with the concentration of SS including the biomass and unhydrolyzed substrate. The high viscosity of the medium makes the membrane more susceptible to fouling [26]. During the feeding with OLR 1 gVS/L/day, the average concentration of suspended solids for the excess sewage sludge reactor and the food waste slurry reactor was 20.9 g/L and 13.8 g/L, respectively. During that period the accumulation rate of SS in the reactor was 0.25 g/L/day for the excess sewage sludge reactor while it was 0.09 g/L/day for the food waste slurry reactor. After increasing the OLR to 3 gVS/L/day, in the excess sewage sludge reactor, the suspended solids concentration was increased to 30 g/L in the first 10 days after changing OLR, however, thereafter, the suspended solids accumulation rate was decreased to 0.05 g/L/day till the end of fermentation (Figure 12). Interestingly, the stable concentration level of VFAs was observed in this period. On the other hand, for the food waste slurry reactor, the concentration of suspended solids did not experience any sudden increase after increasing OLR to 3 gVS/L/day, however the solids accumulation rate was raised to 0.29 g/L/day. The membrane performance was flawless and flux deterioration was negligible regardless of daily increases in suspended solids. The recovery of VFAs from the system consisted of a filtration cycle of about 1 to 3 h (210 s forward flow followed by 30 s backwash) and around 21 h relaxation. The average flux for the excess sewage sludge reactor was 5.53 L/m\({}^{2}\)/h and it was stable during the most of fermentation time, although the obtained flux was significantly lower than the other study in this group [27] operating at the similar level of suspended solids. Lower flux may result in lower productivity; however, during the whole 3 months of fermentation (including the preparation time), chemical cleaning service was necessary only once, which indicated that the mentioned strategies were effective for mitigating membrane fouling. Moreover, lower frequency of chemical cleaning lessens the process downtime, chemical waste, and provision, which is favorable for an industrial process. For the food waste slurry AD MBR, the average flux was 16.18 L/m\({}^{2}\)/h and, similarly to the excess sewage sludge reactor, only a single membrane chemical cleaning was needed.