A hybrid system for groundwater denitrification using electrocoagulation and adsorption

The treatment of nitrate-contaminated groundwater was studied using a hybrid system comprising an electrocoagulation unit and a zeolite adsorption reactor. In the electrocoagulation (EC) process, aluminum alloy electrodes were used in an undivided cell. Experiments in the laboratory-scale reactor were carried out in unregulated temperature conditions to treat synthetic groundwater solutions containing initial nitrate concentrations of 10-100 mg NO₃^--N·L^-1 in batch mode and without using additional pH buffers. Various operating variables, such as applied current density (about 20 mA cm^-2 to 80 mA cm^-2), concentration of NaCl electrolyte (0.0-1.0 g L^-1) and treatment time (up to 120 min), were tested for their effects on nitrate removal. Results showed that initial NO₃^--N concentration, current density and electrolyte concentration, play important roles in EC. For all initial NO₃^--N concentrations and current densities tested, the highest NO₃^--N removal rates (up to 2.374 g L^-1·d^-1) were achieved without additional electrolyte and/or with the lowest electrolyte concentration of 0.1 g L^-1. In these experiments, EC reduced NO₃^--N to below the standard limit of 10 mg L^-1 after 10-60 min of electrolysis. A significant quantity of by-products, ammonium and dissolved aluminum, formed during the process, however these were successfully removed by zeolite adsorption in the post-treatment step. The electrochemical reactor using the specific anode/cathode combination and an environmentally-friendly post-treatment step such as zeolite adsorption, can be used to efficiently remove nitrate from groundwaters because of its high efficiency.