Electrocatalytic synthesis of ammonia (NH₃) from nitrate (NO₃⁻) is an effective approach for reducing nitrate pollutants in the environment and a promising method for ammonia synthesis under mild conditions. However, current catalyst syntheses are costly, and their application in real wastewater remains underexplored. Herein, we prepared high-efficiency copper-based single-atom-nanoparticle synergistic sites on carbon felt (Cu_SA/NP@C) from copper-containing wastewater. Trace Cu²⁺ in wastewater enables efficient nitrate reduction reaction (NO₃RR), and the effect of different Cu²⁺ concentrations was also investigated. The optimized Cu_SA/NP@C, with synergy between single atoms and nanoparticles, shows excellent activity: a maximum Faradaic efficiency of ~ 100% (at −0.3 V vs. reversible hydrogen electrode (RHE)) and a NH₃ yield of ~ 10 mg·cm⁻². Theoretical calculations reveal that Cu single-atoms and nanoparticles synergistically act on the NO₃* and H* intermediates to promote nitrate hydrogenation to NH₃. A membrane electrode assembly with the Cu_SA/NP@C cathode achieves the NH₃ synthesis at an industrial current density of 200 mA·cm⁻² for over 90 h. This “treating waste with waste” strategy offers new paths for nitrate wastewater recycling and green NH₃ synthesis advancement.