Catalytic reduction of nitrate in reverse osmosis concentrate by using Pd-Cu/activated carbon felt

The reverse osmosis concentrate generated during the water reuse process contains a high concentration of nitrate but a low amount of biodegradable organic carbon for heterotrophic denitrification. Catalytic reduction of nitrates using Pd-Cu is one of the most promising technologies to achieve complete removal of nitrate; however, the effect of a range of experimental factors on the nitrate removal rate and N-2 selectivity is still an ongoing concern. Two kinds of supporting materials, alumina and activated carbon felt, were used to immobilize the Pd-Cu catalyst. The alumina-based catalyst was used to establish reference conditions for further experiments, and the effect of pH control was evaluated for both supporting materials. It was observed that pH has a direct influence on the nitrate reduction rate as well as the N-2 selectivity. Nitrate reduction efficiency was low at acidic conditions while the highest N-2 selectivity was obtained at the acidic conditions. The optimal pH condition for Pd-Cu/activated carbon felt was determined as pH 4, showing the highest total nitrogen removal as N-2 gas. Finally, the feasibility of catalytic nitrate reduction for reverse osmosis concentrate was evaluated by investigating the effects of organic and inorganic components commonly present in reverse osmosis concentrate. The organic and inorganic components did not show a significant inhibitory effect on catalytic nitrate reduction, while a high concentration of salt significantly decreased the nitrate reduction rate as well as the N-2 selectivity. The filter type morphology of the Pd-Cu/activated carbon felt would be beneficial for field application compared to the conventional catalyst with powder form.