Microbial electrolysis cell (MEC) is a novel process for the renewable and sustainable production of biogas(H₂, CH₄) or chemicals from waste organic materials. In a MEC, exoelectrogenic bacteria oxidized organic matters and released electron to an anode, and then electrons were transferred to cathode to produce methane. In comparison with anaerobic digestion (AD) that is a traditional process for waste organic materials treatment, the MEC has a higher biogas recovery and organic matter removal. This study describes the effect of operating temperature and electrode spacing on the performance of a single-chamber MEC treating waste activated sludge (WAS) with applied voltage of 0.3 V between electrodes.
The indicators such as methane production, pH, alkalinity, organic matter removal rate and current density were used as evaluation index. In the electrode spacing batch test, The pH and alkalinity of the WAS in MEC after experiment were not significantly affected with different electrode spacing. The methane yield increased by 3∼15% from 122.9 L·CH₄/kg·VSSre with 32 mm of electrode spacing to 127.6∼143.2 L·CH₄/kg·VSSre with 16 mm of electrode spacing. Organic matter removal rate from both MECs were in the range of 33.7∼40.3% for TCOD and 34.0∼38.4% for VSS. TCOD and VSS removal rate from a MEC with 16 mm of electrode spacing slightly increased by 1.3∼5.8% and 3.3∼5.3% respectively. However The current density increased by 103∼105% with the decrease of electrode spacing (3.04±0.03∼3.74±0.05 and 1.5±0.01∼1.8±0.03 A/m³ for 16 mm and 32 mm, respectively). The results imply that the decrease of electrode spacing between 16 and 32 mm is able to enhance the performance of MEC for methane production and current density, but it does not improve efficiently the rate-limiting step on MEC system.
After the electrode spacing test, the operating temperature was varied from 30℃ to 40℃ with 16 mm of electrode spacing. The highest methane yield achieved 89.1±4.1 L·CH₄/kg·VSSre at 35℃, 10.3% and 23.1% higher than at 30℃ and 40℃, respectively. For organic matter removal, the increase of operating temperature enhanced the VSS removal rate from 35.1% to 45.8%, but not COD removal rate, which meant the activity of microorganism in MEC could be accelerated by temperature. On the other hand, Seeing that the sudden decrease of current density from one of electrode in MEC at 40℃ was observed, the activity of exoelectrogenic bacteria on anode was affected by temperature. These results showed that the operating temperature of 35℃ could be proper condition for methane production with MEC system, since the function of electrode might be started to drop above 40℃ of operating temperature.