Removal of organics in constructed wetlands with horizontal sub-surface flow: A review of the field experience

Constructed wetlands with horizontal sub-surface flow (HF CWs) have successfully been used for treatment various types of wastewater for more than four decades. Most systems have been designed to treat municipal sewage but the use for wastewaters from agriculture, industry and landfill leachate in HF CWs is getting more attention nowadays. The paper summarizes the results from more than 400 HF CWs from 36 countries around the world. The survey revealed that the highest removal efficiencies for BOD5 and COD were achieved in systems treating municipal wastewater while the lowest efficiency was recorded for landfill leachate. The survey also revealed that HF CWs are successfully used for both secondary and tertiary treatment. The highest average inflow concentrations of BOD5 (652 mg l− 1) and COD (1865 mg l− 1) were recorded for industrial wastewaters followed by wastewaters from agriculture for BOD5 (464 mg l− 1) and landfill leachate for COD (933 mg l− 1). Hydraulic loading data reveal that the highest loaded systems are those treating wastewaters from agriculture and tertiary municipal wastewaters (average hydraulic loading rate 24.3 cm d− 1). On the other hand, landfill leachate systems in the survey were loaded with average only 2.7 cm d− 1. For both BOD5 and COD, the highest average loadings were recorded for agricultural wastewaters (541 and 1239 kg ha− 1 d− 1, respectively) followed by industrial wastewaters (365 and 1212 kg ha− 1 d− 1, respectively). The regression equations for BOD5 and COD inflow/outflow concentrations yielded very loose relationships. Much stronger relationships were found for inflow/outflow loadings and especially for COD. The influence of vegetation on removal of organics in HF CWs is not unanimously agreed but most studies indicated the positive effect of macrophytes.