Removal of heavy metals in a horizontal sub-surface flow constructed wetland.

Constructed wetlands with horizontal sub-surface flow designed for the treatment of municipal sewage have been monitored extensively with respect to removal of organics (BOD5, COD), suspended solids, nutrients (N, P) and microbial pollution. However, the information on the removal and distribution of heavy and other metals in these systems are very limited because heavy metals do not constitute a major problem in wastewater from small villages with no industry. The measurements were carried out in a constructed wetland with sub-surface flow at Morina near Prague. Removal of metals in constructed wetland at Morina near Prague varied widely among studied metals. The highest removal was recorded for aluminium (> 98.9%), zinc (94.1%) and chromium (92.8%). Iron and manganese increased their concentration in the outflow suggesting that these metals are actually washed out of the system. This phenomenon is attributed to the fact that in the absence of dissolved oxygen and nitrate manganic Mn and then ferric Fe are used by microorganisms as the terminal electron acceptors during respiration. It has been shown that substantial amounts of metals were retained in pretreatment units.