Redox potential dynamics in a horizontal subsurface flow constructed wetland for wastewater treatment: Diel, seasonal and spatial fluctuations

Redox potential was measured in a treatment bed of a constructed wetland (CW) with subsurface horizontal flow used for municipal wastewater treatment. CW was situated in the Czech Republic, designed for 150 person equivalents; it was planted with Phragmitesaustralis and put into operation in 2001. The system was very efficient in removing organic pollution (BOD5 82% and COD 74.0%), nitrogen (62.6%), phosphorus (75.4%) and suspended solids (52%). Redox potential (EH) was measured in two depths (0.2 and 0.5 m) of the treatment bed at regular distances from inflow using platinum electrodes, continuously, every 15 min during a more-than-2-year period (27 months) from 2002 to 2004. EH ranged from −400 to +800 mV. The fluctuations were the most intense in the upper layer (0.2 m) at the furthest distance from inflow (13 m) and EH changed within very short time periods (hours) by several hundred millivolts. Among the factors affecting EH concentrations of pollutants, pore water temperature and water flow rate were determined.Vegetated treatment beds of subsurface horizontal flow constructed wetlands filled with mineral substrate can be extremely dynamic systems, in which oxidation–reduction status changes within short time from anaerobic to aerobic and vice versa. Anaerobic conditions do not prevail necessarily in the upper layer of treatment bed, in which plant roots are present.