Macrophyte growth in a pilot-scale constructed wetland for industrial wastewater treatment
Introduction
The construction of artificial wetlands for the treatment of wastewater has been developing fast over the last decades and it represents now a widely accepted and an increasingly common treatment alternative. Macrophytes are assumed to be the main biological component of wetlands. They not only assimilate pollutants directly into their tissues, but also act as catalysts for purification reactions by increasing the environmental diversity in the rhizosphere, promoting a variety of chemical and biochemical reactions that enhance purification (Jenssen et al., 1993). Initially, constructed wetlands were mainly used for nutrient retention in municipal sewage, storm water and agricultural runoff (Adcock and Ganf, 1994, Greenway, 1997, Merlin et al., 2002). The accelerating industrialization in developing countries with an enormous consumption of metals is a growing environmental contamination hazard. Therefore, the application of constructed wetlands for industrial wastewater treatment represents a promising alternative. However, current experience in Argentina remains largely unreported. In principle, conditions are favorable since populations of a low density are found together with a large availability of marginal land surrounding cities. The central and northern part of the country has mild winters, allowing extended growth periods for the vegetation.
Bahco metallurgic factory constructed a small-scale experimental wetland to assess the feasibility of treating wastewater from its plant in Santo Tomé (Argentina). The effluent presented high pH and conductivity, and contained Cr, Ni and Zn.
Knowledge of the functioning of constructed wetlands is not as advanced as to provide detailed predictive models, since they depend on biological characteristics such as interspecific competition and tolerance to a residual liquid of changing characteristics (Cole, 1998). Regionally abundant macrophyte species are adapted to the local climatic and edaphic conditions. Their performance under the environmental conditions imposed by wastewater was unknown. Thus, it was decided to transplant an initial assemblage of three floating and eight emergent species, representative of the macrophytes that attain a remarkable presence in wetlands and ponds from the nearby Middle Paraná River floodplain in Santo Tomé (Table 1). This paper describes the growth and nutrient and metal removal of vegetation in the experimental wetland.
Section snippets
Wetland design
The system was a free water surface wetland (FWSW) constructed at Bahco Argentina S. A. metallurgic plant, located in Santo Tomé, Argentina (S 31° 40′ 01, 9″; W 60° 47′ 06, 9″). The wetland measures were 6 × 3 m. A polyethylene impermeable film was placed at the bottom and a soil layer of 30 cm depth was added.
Eleven different macrophyte species were transplanted (Table 1). Floating species Salvinia herzogii, Pistia stratiotes and Eichhornia crassipes were chosen because they had showed high uptake
Pilot-scale wetland
Table 2 summarizes the variables measured in the influent and effluent of the wetland and the estimated removal percentages. Water temperatures ranged from 13 to 26 °C in most samplings, dropping to 11 °C coincidentally with a cold storm. Temperatures in nearby undisturbed wetlands and ponds were 9–10 °C from May to July. As samplings were not performed in coincidence with storms, temperatures plausibly dropped below the recorded range in natural environments during cold weather. Dissolved oxygen
Discussion
Decreased COD and BOD in the outlet points to a large organic matter mineralization within the wetland. In turn, organic matter mineralization was likely to cause the observed decrease of pH and dissolved oxygen in the outlet. Nitrate and nitrite were depleted along the flow through the wetland. Since the amounts stored in the macrophyte biomass and in the sediment pool represented a small proportion of the removed nitrate and nitrite, intensive denitrification presumably occurred. N15 balances
Conclusions
The composition of vegetation depends on the interaction among competitive skills, availability of resources and disturbance grade. Our study represents an experiment in which an assemblage of an initial composition similar to natural wetlands developed in an environment of a high availability of resources (large nutrient load) and a very drastic disturbance grade represented by the toxicity of the incoming wastewater. It was experimentally shown that floating macrophytes were unable to develop
Acknowledgements
The authors thank Bahco Argentina S. A. for allowing them to develop this pilot-scale wetland on the grounds of their plant in Santo Tomé, Santa Fe, the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and Universidad Nacional del Litoral, CAID+D Project for making this work possible.
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