Exploring key operational factors for improving hydrogen production in a pilot-scale microbial electrolysis cell treating urban wastewater

Bioelectrochemical systems (BES) are becoming popular technologies with a plethora of applications in the environmental field. However, research on the scale-up of these systems is scarce. To understand the limiting factors of hydrogen production in microbial electrolysis cell (MEC) at pilot-scale, a 135 L MEC was operated for six months under a wide range of operational conditions: applied potential [0.8–1.1 V], hydraulic residence time [1.1–3.9 d], and temperature [18–30 °C], using three types of wastewater; synthetic (900 mg CODs L⁻¹), raw urban wastewater (200 mg CODs L⁻¹) and urban wastewater amended with acetate (1000 mg CODs L⁻¹). The synthetic wastewater yielded the maximum current density (1.23 A m⁻²) and hydrogen production (0.1 m³ m⁻³ d⁻¹) ever reported in a pilot scale MEC, with a cathodic recovery of 70% and a coulombic efficiency of 27%. In contrast, the use of low COD urban wastewater limited the plant performance. Interestingly, it was possible to improve hydrogen production by reducing the hydraulic residence time, finding the optimal applied potential or increasing the temperature. Further, the pilot plant demonstrated a robust capacity to remove the organic matter present in the wastewater under different conditions, with removal efficiencies above 70%. This study shows improved results compared to similar MEC pilot plants treating domestic wastewater in terms of hydrogen production and treatment efficiency and also compares its performance against conventional activated sludge processes.