Synthetic wastewater of pH 1.9, consisted of Zn, Cd, Mn and Mg at concentrations of 450, 150, 1,085 and 3,154 mg/l respectively. It was found that the optimum pH for chemical coagulation and precipitation by lime treatment was more than 9.5, which could decrease the concentration of heavy metals pertinent to the Wastewater Standard of the Ministry of Industry. Moreover, if 0.5 mg/l of polymer was added, it could decrease the concentration of heavy metals further. In addition when sodium sulphide solution was used in secondary treatment, it was also found that the optimum pH by lime in primary treatment was more than 9.5 and the optimum concentration of sodium sulphide was more than 250 mg/l. (C) 1999 Published by Elsevier Science Ltd on behalf of the IAWQ. All rights reserved.
