Non-sedimentation nanoparticles containing alkaline wastewater as effluent from the cleaning process of car-painting lines can be purified by sedimentation after adding of mineral acids to adjust the pH below 5. Using 10 % aq. sulfuric acid to coagulate and filtrate the colloid particles, and adjusting the effluent pH with 10 wt. % aq. NaOH to the original value (~9), the wastewater can be recycled with ca. 0.3 % salt content into the cleaning process of the painting line (rinsing) technology.