The identification of decentralized urine treatment processes is a crucial issue considering source separating sanitary systems (NoMix technology). In this work the suitability of urine treatment with microbial fuel cells (MFCs) has been tested in the laboratory. MFCs hold great promises for urine treatment, since organic substances and nitrogen can be eliminated at the anode and cathode respectively.
First, a MFC was started based on synthetic wastewater containing acetate in the anodic and nitrate in the cathodic influent to eliminate COD using a denitrifing cathode. Unfortunately, the denitrification process at the cathode never started. Instead of nitrate oxygen was the electron acceptor, although oxygen was only present in small amounts of approximately 0.3 mgO₂/L .
In a second experiment the anode was fed with 30-,10- and 3-times diluted synthetic urine solutions, corresponding to urine after storage, again containing acetate as COD. At the cathode oxygen was supplied as electron acceptor by aeration of the catholyte. The cell was characterized in each operation state by means of electrode measurements, current interruption tests, cell polarization and power curves.
For a reference measurement taken when the anode was fed with synthetic wastewater prior to the changeover to 30-times diluted synthetic urine, the maximum power point (MPP) was 11.6 W/m³_NAC. In the transition state from synthetic wastewater to 30-times diluted synthetic urine, a MPP of 16.0 W/m³_NAC was reached. At close to steady state conditions with 30-times diluted synthetic urine as anolyte, the MPP was 14.3 W/m³_NACC. With a MPP of 11.8 W/m³_NAC, the maximum attainable power in the case of a 10-times diluted synthetic urine, went back to a level similar to the reference case. The reduction in maximum attainable power with decreasing dilution can be explained with a higher internal resistance of the cell, compensating the positive effect of increasing pH on the anodic reaction reduction potential. Furthermore, an increased overpotential at the anode was calculated for a 10-times diluted synthetic urine anolyte. When a 3-times diluted solution was fed to the anode, the power density collapsed and could not be recovered by going back to a 10-times diluted solution.
These findings suggest that anodic bacteria grown in synthetic wastewater are not able to sustain COD elimination even in diluted synthetic urine.