## 3 Results and Discussion
### VFAs Production Using Immersed Membrane bioreactor
Figure 11: Composition and volume of gas produced from (**a**) excess sewage sludge and (**b**) food waste slurry. Figure 12: Suspended solid profiles of excess sewage sludge and food waste slurry iMBR. In this section, the feasibility of the application of the developed iMBR set-up for the production of VFAs that can be further applied for denitrification purposes in WWT process is evaluated. As presented previously in other literature [15; 16; 35], the VFA solution produced from the anaerobic digestion of different substrates such as sludge can positively contribute to the denitrification stage during a wastewater treatment process by providing the required carbon source. Using the carbon source produced sustainably from renewable residual sources, such as WWT excess sludge and other organic wastes (e.g., food waste) in the form of VFAs, provides the opportunity for a WWTP to employ circular economy measures. By integrating (or retrofitting) the iMBR set-up used in this research into the WWT process, the provision of VFAs from the above-mentioned sources is facilitated, which helps the realization of a WWT process independent of a fossil fuel source. In this regard, two hypothetic scenarios were considered for the application of excess sludge and food waste as the substrate for VFAs production using the iMBR (Figures 5 and 6). The base processing conditions in the assumed WWTP are presented in Table 3.  
As seen in Figure 5, scenario one considers the provision of the VFAs required for denitrification from an iMBR fed only with food waste. Based on the acquired results using the lab-scale iMBR, a VFAs COD equivalent of 9 g/L can be achieved. The process conditions and the overall performance of this scenario are summarized in Table 4. In this condition, the VFA solution obtained as permeate is fed as an external carbon source to the denitrification stage. In the second scenario, the provision of the VFAs equivalent to required methanol from the anaerobic digestion of excess sludge in the iMBR was considered (Figure 6). The process parameters \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Parameter** & **Value** \\ \hline \multirow{6}{*}{Scenario 1} & Total MBR volume & 11,700 m\({}^{3}\) \\  & VFA production & 298 Kg/h (440 KgCOD/h equivalent) \\  & Membrane area & 6510 m\({}^{2}\) \\  & Flow rate of VFA solution required for denitrification & 0.5\% of the total denitrification flow (49 m\({}^{3}\)/h) \\  & Estimated price of VFA produced & \(\sim\)238 €/h \\  & Estimated cost of VFA production & \(\sim\)12-14 €/h \\ \hline \multirow{6}{*}{Scenario 2} & MBR volume & 7520 m\({}^{3}\) (same biogas reactor or new reactor) \\  & VFA production & 319 Kg/h (440 KgCOD/h equivalent) \\ \cline{1-1}  & Membrane area & 4185 m\({}^{2}\) \\ \cline{1-1}  & Flow rate of VFA solution required for denitrification & 0.3\% of the total denitrification flow (29.5 m\({}^{3}\)/h) \\ \cline{1-1}  & Estimated price of VFA produced & \(\sim\)255 €/h \\ \cline{1-1}  & Estimated cost of VFA production & \(\sim\)8–10 €/h \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Process conditions and the overall performance of scenario one and two. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline In flow-denitrification (m\({}^{3}\)/s) & 2.5 \\ Denitrification (kgN/h) & 91.2 \\ C/N ratio (kg COD methanol/kgN) & 4.8 \\ Methanol COD consumption (kgCOD/h) & 439.4 \\ Methanol consumption (kg/h) & 298.9 \\ Methanol cost (€/kg) & 0.467 \\ Methanol cost (€/kgN) & 1.094 \\ Cost of methanol used per hour (€/h) & 139.6 \\ COD equivalent of methanol (tom COD/ton methanol) & 1.47 \\ NO\({}_{3}\) out through the effluent (mg/L) & 1.3 \\ VFA solution flow to denitrification inflow ratio & 0.0054 \\ \hline Cost of equivalent VFA COD production per hour (€/h) & 11.88 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The base processing conditions taken into account for the denitrification stage. were assumed to be roughly those considered for lab-scale experiments.