Wetland constructed for metal removal faced with a depredation event.

MAINE, M. A.; HADAD H. R.; SANCHEZ, G.C.; MUFARREGE, M. M.; DI LUCA, G. A.; CAFFARATTI, S. E.; PEDRO, M. C.

Perth

Conferencia; 13th International Conference Wetland Systems for Water Pollution Control; 2012

A free water surface wetland was constructed at a metallurgic plant to treat wastewater of high pH and conductivity containing metals (Cr, Ni, Zn and Fe). Industrial wastewater and sewage (100 m3 d-1, both with a previous primary treatment) were treated together. The CW has been in operation since 2003. Locally common macrophytes were transplanted. Vegetation was initially dominated by Eichhornia crassipes (water hyacinth), but later the emergent macrophyte Typha domingensis was the dominant species during 5 years. Efficient metal and nutrient retentions were registered during this period. In June 2009, aboveground parts of plants were depredated by capybaras (South American amphibious rodents). The CW looked like a non-vegetated pond. However, the plant roots and rhizomes were not damaged. After stopping the depredation, T. domingensis showed a luxuriant growth, reaching a cover of 60 % in 30 days. The objective of this work was to evaluate the CW sustainability faced to an extreme event, comparing the removal efficiency of the system planted with T. domingensis during normal operation (February 2005-May 2009), the same system without aboveground parts of plants, during the eventual event (June-November 2009) and the subsequent recovering period (Dec. 2009-Sept. 2010). Samplings of the influent and effluent, sediment and macrophytes were performed monthly. The CW efficiently retained contaminants during all the studied period; however, the best efficiencies for most of the parameters were registered during the normal operation stage. There were not significant differences between the performances of the CW in the two last stages, except for BOD. The mean removal, expressed as % during normal operation/depredation event/recovery stage, were: 84.9/73.2/74.7 % Cr, 66.7/48.0/51.2 % Ni, 97.2/91.0/89.4 % Fe, 50.0/46.8/49.5 % Zn, 81.0/84.0/80.4 % NO3-, 98.4/93.4/84.1 % NO2-, 73.9/28.2/53.2 % BOD and 75.4/40.9/54.6 % COD. Phosphate and ammonium presented low removal efficiencies. T. domingensis showed an excellent response regarding growth and propagation, consequently the period without aerial parts lasted a few months and the CW could recover its normal operation. Plants continued retaining contaminants in their roots and the sediment increased its retention capacity, equilibrating the operating capacity of the system. This was probably due to the CW had reached its maturity, with a complete root-rhizome development that may require 3-5 years. These results demonstrated that faced to an eventual problem, this mature CW was capable to maintain its efficiency and to recover its vegetation, demonstrating the robustness of these treatment systems.