## 3 Results and Discussion
### Proposed AD-MES Integrated System  
Results indicate that the electrochemical treatment of reject water can be used as a biogas upgradation technology. The optimised cathode potentials and pH provide proof-of-concept for electrochemical CO\({}_{2}\) reduction. Figure 9 describes the assembly of a food waste (solid lines) and/or wastewater treatment plant (dashed lines) and suggests implementation of an integrated MES unit. The inlets follow through a series of processing steps until the slurry is pumped into a biogas tank. The biogas generated is collected from the top and is sent either for upgrade or low value usage. Digested sludge is then centrifuged to separate the solids and liquid fraction otherwise called reject water. Reject water can contain a COD concentration ranging from 1000 to 8000 mg/L [31] depending on the type of feed and efficiency of biogas tank.  
In food waste treatment plants, reject water is generally recycled for reducing total solids (TS) in the inlet feed, such that the mixture can be pumped without hindrance. It is also a common practice to use fresh water (make-up water) to achieve the desired TS and control inlet ammonium concentration. While in a WWTP, reject water goes back to the main wastewater inlet, that could be in the order of 1/100 of the total inlet flow and can, in many cases, causes instabilities in the main treatment train (coagulation-sedimentation); it therefore necessitates a reject water treatment system in existing WWTPs [32].  
Figure 8: Comparison of pH and alkalinity in open circuit and MES mode operations.  
The proposed MES unit integration in a generalized biogas process as shown in Figure 9 has several advantages in addition to the benefit of biogas upgradation, such as the potential to reduce COD, ammonium and sulphide concentrations of reject water. An improved quality of the reject water will result in overall process optimization. It could completely eliminate the use of fresh water for food waste treatment plants and avoid variations in the main treatment train of typical WWTPs. Furthermore, there is a possibility of feeding biogas generated from the anaerobic digester to be upgraded to methane via electrochemical reduction of CO2 already present in the biogas. This can be achieved by reducing the pH of reject water such that the CO2 in biogas is made to dissolve in the feed. At the same time, the design of the MES reactor should allow the dissolved CO2 to react with the electrons from the cathode.  
Existing food waste or wastewater treatment plants could be refitted with the MES process as it neither interferes with the infrastructure present in the treatment plant nor is it a limiting capital investment. The system does not require extra feed as it generates more (methane, less pollution) from what is already available in the process and in this way, it is able to generate much more value than the investment it demands. However, research at this stage lacks information on mass balance, control experiments, hydraulic retention time and other waste feeds. Also, an economic analysis of integrated anaerobic digesters and microbial electrosynthesis systems is required to completely understand the scalability of such an integrated system.