Process parameters of a Microbial Electrolysis Cell (MEC)
Decentralized systems which treat separated wastewater streams according to their specific properties are quite promising according to the newest research results. The use of a microbial electrolysis cell (MEC) to treat urine and produce hydrogen gas is such a novel technology. The MEC consists of two chambers which are connected via a cation exchange membrane. By applying voltage, the oxidation of organic substrate (assisted by exoelectrogens) at the anode can be coupled with a proton reduction at the cathode. EAWAG takes part in the investigations and set a MEC into operation in February 2009. The goal of this Master project is to understand and quantify the influence of some of the most important process parameters such as applied voltage, temperature and pH.
The chronoamperometry measures the current at stepwise higher applied voltages. The influence of the applied voltage on the system performance was linear before reaching slowly a limit. The needed minimum voltage was determined to 238 mV. Those chronoamperometric curves gave indications that the system got diffusion limited with time. The coulombic efficiency represents the ratio between produced hydrogen according to the measured current and the theoretical possible hydrogen production calculated with a COD balance. The low coulombic efficiency of 57.7% leads as well to the assumption, that other bacteria degrade the organic substrate. Once the MEC was opened and a biofilm consisting of two layers could be observed: the wanted exoelectrogens and undesired other bacteria. By analyzing samples from the anode and the cathode chamber the sulphate degrading bacteria could be quantified to contribute 3% to the COD-consumption. Additional measures such as installing an oxygen-sensor to judge the possibility of aerobic growth or off-gas measurements to quantify the methanogenic growth should be considered. After classifying the bacteria, parameters to damage them uniquely should be investigated.
The temperature experiment presented a linear increase in performance of 2.3% per degree. As the system is probably diffusion limited, the effect was caused by a bigger diffusive flux due to a temperature-dependent diffusion coefficient. The reaction at the cathode was not limiting at a pH between 6 and 9. When the pH was changed and the anode potential was fixed, the measured current increased 25% per pH unit. The growth curve of the exoelectrogens depends therefore linearly on the pH. If the applied voltage was constant, the current increase reached a limit, as due to the higher pH, the anode potential decreased. Sample analysis from the anode and cathode chamber proved, that the proton transport via the membrane is insignificant. The pH is therefore controlled in both chambers by adding sodium hydroxide solution or hydrochloric acid. If some day a perfect proton-exchange membrane is developed, proton-gradients might play a role and the before mentioned results from the pH-experiments might change. Nowadays, instead of adding chemicals, the effluent of the anode could be connected to the influent of the cathode. At the same time, this would unfortunately decrease the hydrogen purity. Some improvements are still needed, before the first full-scale reactor can be built. Another problem will be that resistances get bigger in a full-scale reactor. The measured resistances during the experiment decreased linearly with temperature and plotted against the applied voltage, a minimum resistance at 650 mV was found. For a full-scale reactor it would pay off to choose the operation parameters such that resistances get minimized.
So there is still a long way to go before the first full-scale reactor will be set into operation. The biggest problems for the analyzed reactor at the moment are the growing competition and the insignificant proton-diffusion.