Scaling-Free Electrochemical Production of Caustic and Oxygen for Sulfide Control in Sewers

Caustic shock-loading and oxygen injection are commonly used by the water industry for biofilm and sulfide control in sewers. Caustic can be produced onsite from wastewater using a two-compartment electrochemical cell. This avoids the need for import and storage of caustic soda, which typically represents a cost and a hazard. An issue limiting the practical implementation of this approach is the occurrence of membrane scaling due to the almost universal presence of Ca²⁺ and Mg²⁺ in wastewater. It results in a rapid increase in the cell voltage, thereby increasing the energy consumption of the system. Here, we propose and experimentally demonstrate an innovative solution for this problem involving the inclusion of a middle compartment between the anode and cathode compartments. Caustic was efficiently produced from wastewater over a period of 12 weeks and had an average Coulombic efficiency (CE) of 84.1 ± 1.1% at practically relevant caustic strengths (∼3 wt %). Neither membrane scaling nor an increase in the cell voltage was observed throughout the experiments. In addition, dissolved oxygen was produced in the anode, resulting in continuously oxygenated wastewater leaving the three-compartment cell. This membrane-scaling control strategy represents a major step forward toward practical implementation of on-site simultaneous electrochemical caustic and oxygen generation for sulfide control in sewers and also has the potential to be applied to other (bio)electrochemical systems receiving wastewater as source for product recovery. © 2015 American Chemical Society.