Application of constructed wetlands to treat some toxic wastewaters under tropical conditions

Abstract

This study evaluated the potential of a free water surface constructed wetland system in treating some toxic wastewaters (i.e. phenolic and heavy metals). In a temperature range of 22–30°C, the constructed wetland units, whose hydraulic retention times (HRT) were 5–7 days, could remove more than 99% of the input phenol when they were operated at or below the organic loading rates (OLR) and influent phenol concentrations of 270 kg COD/(ha.d) and 400 mg/l, respectively. The effluent dissolved oxygen (DO) levels were 4–7 mg/l at OLR of 40–140 kg COD/(ha.d), but these DO levels decreased to 0.2–0.3 mg/l when the OLR were increased to 165–270 kg COD/(ha.d). Under similar operating conditions, the constructed wetland units could remove more than 99% of the applied chromium (Cr) and nickel (Ni), when either the Cr or Ni influent concentrations were 1–50 mg/l. The phenolic and heavy metal compounds were found to accumulate mostly at the roots of Typha, followed by the stems and leaves.