First stage of the French vertical flow constructed wetland system: experiments with the reduction of surface area and number of units

Small vertical-flow constructed wetland units comprising the first stage of the French system were studied in Brazil for the treatment of raw sewage. Planted and unplanted units and different feeding strategies were tested. In the first phase, hourly batches of a daily flow of 13 m³ d⁻¹ were applied over three alternating units, resulting in an average hydraulic loading rate (HLR) on the full system of 0.15 m³ m⁻² d⁻¹. A second phase, aimed at reducing land requirements, kept the same daily flow and batch frequency, but used two alternating units, resulting in a HLR on the full system of 0.22 m³ m⁻² d⁻¹. Removal efficiencies were very good when the system operated with three units, with mean values of 82% for biochemical oxygen demand (BOD), 81% for chemical oxygen demand (COD), 85% for total suspended solids (TSS) and 59% for NH₄⁺-N. With two units, the equivalent values were 74% for BOD, 59% for COD, 67% for TSS and 51% for NH₄⁺-N. There were significant differences in the median removal efficiencies of COD and TSS. No significant differences were found between planted and unplanted units for most constituents. In both phases, the overall good performance and the simplicity of the system make this treatment process a very attractive alternative for developing countries.