Benzene removal with vertical-flow constructed treatment wetlands

Background: Twelve vertical-flow experimental wetlands have been constructed using different compositions, and were operated in batch-flow mode to reduce pumping costs. Six wetlands were located indoors and six outdoors. Benzene was used as a representative example substance to assess the removal of low molecular weight petroleum compounds. Results: Findings indicate that the constructed wetlands remove benzene (inflow of approximately 1.3g L^-1) from hydrocarbon-contaminated wastewater streams with better indoor (controlled environment) than outdoor treatment performances. Overall mean removal efficiencies for the experimental rig placed outside were as follows: benzene 85%, chemical oxygen demand (COD), 70%; ammonia-nitrogen, 83%; nitrate-nitrogen, 88%; orthophosphate-phosphorus, 58%. In comparison, removal efficiencies for the experimental rig placed indoors were higher: benzene 95%, COD, 80%; ammonia-nitrogen, 90%; nitrate-nitrogen, 94%; ortho-phosphate-phosphorus, 66%. Benzene removal was predominantly due to volatilization after 1 day of retention time. Conclusion: The use of aggregates (sand and gravel) and the presence of Phragmites australis (Cav.) Trin. ex Steud. resulted in no significant difference in terms of benzene, redox potential, dissolved oxygen, 5-day at 20 °C N-allylthiourea biochemical oxygen demand, COD and nutrients removal. Statistical differences were assessed by analysis of variance and Tukey HSD tests (P<0.05).