The dynamics of low-chlorinated benzenes in a pilot-scale constructed wetland and a hydroponic plant root mat treating sulfate-rich groundwater

A rarely used hydroponic plant root mat filter (PRMF, of 6 m²) and a horizontal subsurface flow constructed wetland (HSSF CW, of 6 m²), operating in continuous flow and discontinuous outflow flushing modes, were investigated for treating sulfate-rich and organic carbon-lean groundwater contaminated with monochlorobenzene (MCB); 1,2-dichlorobenzene (1,2-DCB); 1,4-dichlorobenzene (1,4-DCB); and 2-chlorotoluene. Whereas the mean inflow loads ranged from 1 to 247 mg m⁻²days⁻¹, the range of mean inflow concentrations of the chlorobenzenes recorded over a period of 7 months was within 0.04 and 8 mg L⁻¹. A hydraulic surface loading rate of 30 L m⁻²days⁻¹was obtained in both systems. The mean load removal efficiencies were found to vary between 87 and 93 % in the PRMF after a flow path of 4 m, while the removal efficiencies were found to range between 46 and 70 % and 71 to 73 % in the HSSF CW operating in a continuous flow mode and a discontinuous outflow flushing mode, respectively. Seasonal variations in the removal efficiencies were observed for all low-chlorinated hydrocarbons both in the PRMF and the HSSF CW, whereby the highest removal efficiencies were reached during the summer months. Sulfide formation occurred in the organic carbon-lean groundwater particularly in summer, which is probably due to the plant-derived organic carbon that fostered the microbial dissimilatory sulfate reduction. Higher redox potential in water was observed in the PRMF. In conclusion, the PRMF could be an option for the treatment of water contaminated with compounds which in particular need oxic conditions for their microbial degradation, such as in the case of low-chlorinated benzenes.