Heavy metals concentration in sewage treated at the collective wastewater treatment plant

This paper focuses on the analysis of the concentration of heavy metals in sewage treated at the municipal collective two-stage wastewater treatment plant (WWTP). The quality of raw sewage, mechanically treated sewage and totally treated sewage were tested. Heavy metals concentrations in sewage were in order Zn > Mn > Cu > Ni > Pb > Cr > Cd. A subsequent treatment stages gradually reduced metals concentrations, but the role of a mechanical treatment was not significant. The order of percentage reduction as a result of the total mechanical-biological treatment was Cr > Cd > Zn > Cu > Pb > Ni > Mn, which means that it was not consistent with the order of metals concentrations. It was found that totally treated sewage were characterised by greater variability in metal concentrations than raw sewage and mechanically treated sewage. Among the tested elements, the exception was Mn, with not only the lowest percentage reduction, but also with the most even concentrations level (low or average variability), while Cd and then, Cr, were characterised by the greatest differences in concentrations. Additionally, the following dependence was found: the greater WWTP hydraulic load, the greater heavy metals concentrations. Importantly, sewage met the legal requirements regarding to the permissible levels of heavy metals concentrations.