Evaluation of Pollution Indices in Some Wastewater Treatment Plants Emphasizing on the Concentration of Heavy Metals

The aim of this study was to investigate the quality of wastewater and to determine the concentration of heavy elements. For this purpose, wastewater samples were collected before and after four wastewater treatment systems in the villages around Zarand named Dehmilan, Hatkan, Sarbagh and Sekukan, each in 10 stages and at one-week intervals; a total of 80 samples were prepared. Different parameters including BOD, COD, TOC, EC, TSS, TDS, DO, TKN, TP, pH, Temperature, turbidity, and alkalinity and also the concentrations of Cd, Zn, Pb, Ni and Mo were measured using the standard methods and were compared with the permissible concentration of Iran Department of Environment. The results showed that after treatment in all the studied systems, the concentrations of Cd and Pb were reduced from 17.5 and 37.4 to 0.5 and 60.0%, respectively, whereas the concentration of Ni increased 8.7-59.7%. The amount of BOD and COD in the effluent of most of the studied treatment systems were up to 283.9 and 271.3% higher than the permissible levels, respectively. Furthermore, in all the treatment plants except Dehmilan, TSS decreased to less than the permissible level. Moreover, in comparison with the standard levels, the amount of TP and DO increased while pH and turbidity decreased. In the effluent of all treatment plants, the concentrations of Cd, Zn, Pb and Ni were less than the permissible limits (up to a maximum of 58.0, 82.4, 97.7 and 85.2%, respectively), whereas the concentration of Mo was higher than the permissible level.