Macrophyte role and metal removal in constructed wetlands for the treatment of effluents from metallurgical industries

Abstract

Ecological technologies for wastewater treatment have attracted attention as alternative non-conventional solutions. Constructed wetlands (CWs) are one of the most widely used ecological technologies due to their high contaminant removal efficiencies and reduced costs for maintenance and operation. CWs, also known as treatment wetlands or wetland systems, are engineered systems designed and constructed to utilize natural processes to remove contaminants from water. The removal of contaminants in CWs is complex and depends on a variety of removal mechanisms, including sedimentation, filtration, precipitation, volatilization, adsorption, plant uptake, microbial processes, etc. CWs can be classified in a variety of ways. The most used is according to the flow regime: Free Water Surface (FWS) wetlands, composed by sediment and floating, submerged and/or emergent macrophytes, similar in appearance to natural marshes; Horizontal subsurface flow (HSSF) wetlands that employ a gravel bed planted with emergent macrophytes, where the water flows horizontally from the inlet to the outlet; and Vertical Flow (VF) wetlands, where water is discharged as rain on all the wetland surface and flows vertically across different layers of sand and/or gravel bed planted with emergent vegetation. The most suitable CW type, substrate, plant species, etc. to be used are chosen according the volume and chemical composition of the effluent to be treated. In Argentina, despite the environmental conditions are favorable with a great land availability, CWs are not widely implemented for sanitary effluents; however, two CWs have been designed for the treatment in metallurgical industries, where the critical contaminants are metals. The removal of metals using CWs has been applied worldwide to mine effluents, the pH of which is acid. In the case of effluents from metallurgical industries, the pH is alkaline and the salinity is high. Such is the case of the two CWs for final effluent treatment at metallurgical industries (CW1 and CW2). Free-water surface CWs were used because they are the most convenient CW type for metal removal. Despite the importance of sediment, macrophytes are the main biological component of CWs. In the case of nitrogen or phosphorous, main contaminants of domestic sewage, agricultural, industrial, urban runoff, etc., plants assimilate them as nutrients. However, most metals are toxic for plants. CW1 and CW2 have been operating for 14 and 7 years respectively. As the chemical composition of the wastewaters and the volumes to be treated are different, CWs have different design characteristics. Wastewater from the industrial processes and sewage from the factory are treated together after a primary treatment. The idea of treating sewage was based on the fact that sewage composition is rich in organic matter and nutrients. High nutrient concentrations could improve macrophyte tolerance to metals. The studied cases of the CWs for the treatment of metallurgical effluents are explained in this chapter.