Removal of chromium(VI) from wastewater by combined electrocoagulation-electroflotation without a filter

A combined electrocoagulation and electroflotation process was designed to reduce Cr6+ to Cr3+ first and then to remove the total Cr from wastewater to a value below 0.5 mg/L. Acidic condition was employed in the reduction of Cr6+ and neutral conditions were found to be beneficial for the coagulation of the precipitates of Cr(OH)3 and Fe(OH)3. The formation of Fe(OH)3 was ensured by sparging compressed air in the coagulation unit through a draft tube. The air not only oxidizes Fe2+ produced electrically, but also helps to mix the water for a better coagulation of the particles. The two-stage electroflotation arrangement can separate the solids from the wastewater to a value of less than 3 mg/L with total Cr less than 0.5 mg/L. The residence time required is about 1.2 h. The optimal conditions for the treatment are: charge loading about 2.5 Faradays/m3 water, pH value in the coagulation unit is 5-8. The power consumption is less than 1 kW h/m3 water at the conductivity of 1.5 mS/cm. When aluminum ions are either added or produced in situ in the coagulation unit, the treated wastewater can be discharged without any filtration.