An electrochemical advanced oxidation process for the treatment of urban stormwater

Recharge of urban stormwater has often been limited by the high cost of land and concerns about contamination of groundwater. To provide a possible solution, we developed an electrochemical advanced oxidation system (UV/H₂O₂) that is compatible with high-capacity stormwater recharge systems (e.g., drywells). The system employed an air-diffusion cathode to generate a H₂O₂ stock solution (i.e., typically around 600 mM) prior to the storm event. The H₂O₂ stock solution was then metered into stormwater and converted into hydroxyl radical (•OH) by an ultraviolet lamp. The energy consumption for H₂O₂ generation was optimized by adjusting the applied current density and adding an inert salt (e.g., Na₂SO₄) to stormwater. H₂O₂ in the stock solution was unstable. By mixing the basic H₂O₂ containing catholyte and the acidic anolyte, the stability increased, enabling generation of the H₂O₂ stock solution up to three days prior the storm event with loss of less than 20% of the H₂O₂. Lab-scale experiments and a kinetic model were used to assess the feasibility of the full-scale advanced oxidation system. System performance decreased at elevated concentrations of dissolved organic carbon in stormwater, due to enhanced light reflection and backscattering at the water-air interface in the UV reactor, competition for UV light absorption with H₂O₂ and the tendency of organic matter to act as a •OH scavenger. The proposed system can be incorporated into drywells to remove greater than 90% of trace organic contaminants under typical operating conditions. The electrical energy per order of the system is estimated to range from 0.5 to 2 kWh/m³, depending on the dissolved organic carbon concentration.