Electrochemical treatment of wastewater from the process of surface protection of metals

The paper investigated a possibility of implementing electrochemical methods (electroreduction, electrocoagulation, indirect anodic oxidation) by using iron and aluminium electrode sets, with or without the addition of electrolytes for treatment of wastewater generated in the process of surface protection of metals. The wastewater is characterized by high values of zinc (2029 mg dm(-3)) and iron (78.4 mg dm(-3)) as well as increased values of Cr(VI) (5.4 mg dm(-3)) and chemical oxygen demand (820 mg dm(-3)) that exceed limit values for discharge into the environment by 1014, 39, 56 and 6.6 times, respectively. In order to optimize the process, the paper investigated the impact of electrode type (iron, aluminium), contact time (5-30 minutes), electrolyte mass (0.5-1.5 g NaCl) and amperage (20-50 A) on the removal level of the stated four indicators. Of the two tested electrode types, iron electrodes proved to be significantly more efficient in the removal of all measured indicators, in particular Cr(VI) and COD. A statistically significant improvement of the results in COD removal for both electrode types was achieved by the addition of electrolytes and increased amperage from 20 to 40 A. The majority of inorganic pollution is removed in the first 5 minutes of treatment. The best results are achieved by a combination of the two electrode types, with the addition of chloride as an electrolyte. After 15 minutes of electroreduction / electrocoagulation / indirect anodic oxidation by iron electrodes, 15 minutes of electrocoagulation by aluminium electrodes and 20 minutes of coagulation / flocculation and oxidation by air, 99.79% Cr(VI), 99.97% Fe, 98.23% Ni, 99.23% Cu, 99.999% Zn, 98.00% Pb and 86.46% COD were removed. All measured indicators complied with the conditions for discharge into the environment whereas all output concentrations of heavy metals ranged from 0.009 to 0.023 mg dm(-3).