Sustainability of a constructed wetland faced to 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.

Conferencia; 9th. Intecol. International Wetlands Conference; 2012

The application of constructed wetlands (CW) for industrial wastewater treatment is a promising/growing alternative in Argentina, because the central and northern areas of the country have mild winters and the low population density determines the availability of marginal land around cities. A 2,000 m2 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 (both with a previous primary treatment) were treated together. The CW has been in operation since 2003. An assemblage of locally common macrophytes was 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 retention during this period was registered. 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 DBO. The mean removal, expressed as % during: normal operation / depredation event/ recovery stage, were 84.9/73.2/84.7 % Cr, 66.7/52.0/51.2 % Ni, 97,2/91.0/89.4 % Fe, 50.0/36.8/39.5 % Zn, 81.0/84.0/80.4 % NO3- and 98.4/93.4/84.1 % NO2-, 73.9/28.2/53.2 % BOD and 75.4/40.9/44.6 % COD. Phosphate and ammonium presented low removal efficiencies. T. domingensis showed an excellent response regarding growth and propagation, 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 system operating. This was probably due to the CW had reached its maturity, with the complete root-rhizome development. A complete root-rhizome development may require 3-5 years and the CW performance improves with its maturity. These results demonstrated that faced to an eventual problem, this CW was capable to maintain its efficiency and to recover its vegetation, demonstrating the robustness of these treatment systems.