Long-term performance of two free-water surface wetlands for metallurgical effluent treatment

Abstract

The aim of this study was to evaluate the efficiency of two constructed wetlands (CW1 and CW2) for the wastewater treatment of metallurgical industries and determine if contaminants are retained by the sediment or the plants, key knowledge for suitable wetland management. Both systems are free water surface CWs and were designed to treat industrial wastewater (with high pH and salinity containing Cr, Ni and Zn) and sewage together. Wastewaters receive a primary treatment before reaching the wetlands. CW1 and CW2 have been operating for 14 and 7 years, respectively. Wastewater, sediment and plants were sampled monthly in the inlet and outlet zones of both CWs. In both CWs, removal efficiencies were satisfactory. T. domingensis, the dominant species, is tolerant to metallurgical wastewaters and efficient in metal accumulation. Metals and P were efficiently removed in both CWs, being metals retained mainly in sediment, and P was retained in sediment and plants of the inlet zone. Metal concentration in macrophyte tissues is related to influent concentration while metal concentration in sediment depends not only on influent concentration but also on the time of operation of the CWs. Metals are bound to sediment fractions that can be considered steady under chemical and environmental conditions of the wetlands. Since the conditions for metal removal are largely provided by the influents (high pH, alkalinity, Fe, Ca and ionic concentrations), the sediment will continue retaining metals as far as the composition of the influents remain the same.