## 1 Introduction
The problem confronted here is that, as most of the organic carbon source has been depleted in the initial BOD removal stage, external organic carbon sources should be provided to the process. The external organic carbon sources, such as methanol, ethanol, and glucose, impose external loads on a wastewater treatment plant, resulting in it being a far from self-sustained independent process [12]. Other drawbacks associated with external carbon source provision include the extra purchase and transfer costs, occasional and seasonal changes in the price and availability, and the environmental unsustainability of the wastewater treatment process in the case of the application of fossil-fuel-derived carbon sources such as methanol. Moreover, safety and storage regulations should be imposed if hazardous and flammable sources such as methanol and ethanol are to be used [13]. Thus, a sustainable alternative for the external carbon source is needed in order to change the wastewater treatment process into a self-sustained process. VFAs could be alternative carbon sources for the denitrification process and, based on their production from waste streams, they can relieve the dependency on fossil-based carbon sources. Moreover, the mixture of VFAs can be used directly as a carbon source for the denitrification process without downstream purification demands. Many studies have tested the effectiveness of VFAs as a carbon source for the denitrification process [13; 14; 15; 16; 17; 18]. In this regard, VFAs higher denitrification rates have been reported compared to methanol. Moreover, due to their continuous on-site production, extra costs such as transportation will be eliminated. Although there has been extensive work on the production of VFAs from waste streams [19], the process cannot easily be scaled up to industrial level due to challenges faced during the recovery of VFAs from the system [10]. The recovery of VFAs from the bioreactor is crucial to the downstream processing, and production stability and efficiency [20]. Due to the complex physiochemical nature of the fermentation broth and low concentration of acids in the system, separating the VFAs from the fermentation broth is a challenging task [21]. Different separation techniques have been used for the recovery of the VFAs, such as distillation [22], liquid-liquid extraction [23], centrifugation [24], electrodialysis [25], and membrane filtration [9; 21]. The membrane-assisted process seems to be a cost-effective and stable recovery method among the current separation methods, with low energy demands and a small footprint. Another study in our group [9] has reported an effective membrane-assisted process for production and in situ recovery of VFAs from synthetic food waste, resulting in high VFAs yield and productivity. Figure 2: Anaerobic digestion of excess sludge and other organic wastes for the production of volatile fatty acids (VFAs) instead of biogas. Membrane-assisted cell retention systems benefit from a selective synthetic membrane to retain cells and specific compounds in the bioreactor while allowing other soluble compounds to pass freely through the membrane. This separation occurs either due to the convection caused by pressure difference over the membrane as the driving force or diffusion of specific compounds due to concentration gradient built over the membrane layer. The selective behavior depends on the membrane characteristics such as pore size and morphology, membrane charge, affinity, and hydrophobicity. Although membrane bioreactors (MBRs) have long been used for wastewater treatment purposes, integration of MBR technology in biological processes is not fully developed at the industrial scale. By the application of MBRs in bioconversion processes, high concentrations of active microorganisms can be retained in the bioreactor and cell washout can be eliminated during continuous processes. This ease of continuous operation guarantees higher productivity rates [26].