Phosphate and metal retention in a small-scale constructed wetland for waste-water treatment.

MAINE A,; SUÑE N,; HADAD H,; SÁNCHEZ G,; BONETTO, C.

Phosphates in sediments

Backhuys

Lugar: Leiden; Año: 2005; p. 21 - 31

Abstract A small-scale wetland was constructed for assessing the feasibility of treating effluents containing metals (Cr, Ni and Zn) mixed with nutrients coming from a tool industry factory. The constructed wetland was 6 m long, 3 m wide, and 0.6 m deep. It was isolated from the ground by a nylon sheet, covered with about 0.3 m of the surrounding soil. Eleven emergent and floating macrophyte species common in the area were transplanted into the wetland. A mean flow-through discharge of 1,000 l d-1 was assayed, resulting in a mean water residence time of about a week. The inlet waste-water had a high conductivity (3.3-8.0 mS cm-1), pH (8.0-12.5), nutrient (Tot-P: 0.17-4.7 mg l-1 and NO3: 5-35 mg l-1 of N) and metal concentrations (Cr: 5-589 µg l-1, Ni: 3-750 µg l-1 and Zn:  40-210 µg l-1). Nutrient and metal concentrations in the inlet and outlet were measured throughout 14 months and its mass balances estimated. The wetland efficiently removed nutrients (74 and 88 % of Tot-P and NO3-, respectively) and metals (81, 66 and 59 % of Cr, Ni and Zn, respectively) from the inlet waste-water. Although several macrophytes showed an initial growth, at the end of the first year the wetland became a monospecific stand of Typha dominguensis (cattail), which attained a high biomass (1.9 kg m-2). The total P concentration in the bottom sediment increased by a factor of two close to the inlet but did not change in the outlet after one year. Cr, Ni and Zn showed large increments in the sediment close to the inlet but smaller close to the outlet. The total P concentration in T. dominguensis tissue roughly doubled the initial concentration and metal concentrations were strongly increased.