Resilience and reliability of compact vertical-flow treatment wetlands designed for tropical climates.

Most of the tropical areas have sanitation problems to contend with. The French system of vertical-flow treatment wetlands (FS-VFTW) fed with raw wastewater could be a good water and sludge management solution. The purpose-adapted tropical design can reduce area requirement to below 1 m2/population equivalents (p.e.). The Taupinière FS-VFTW on Martinique Island was built according to this design, with one stage but with a saturated layer at the bottom of the filter and a simplified trickling filter (TF) added for further treatment to meet the high performances targeted. Unsaturated/saturated vertical-flow filters (US/S FS-VFTW) have shown improved performances on total nitrogen, carbon and suspended solids removal in temperate climates, but the performances in tropical conditions remain unknown. Here, we report on real-world-operation in the French Overseas Territories (FOT), the reliability and performances of this VFCW tropical-design. The system experienced loading conditions ranging from 30% to 165% of nominal carbonaceous biological oxygen demand (BOD5), as well as tropical rainstorms that brought over 7 times the nominal hydraulic load. Over a period of 3 years, 29 campaigns collected 24-h flow-proportional samples at each treatment stage (raw wastewater, FS-VFTW outlet, TF outlet). When applied loads were close to nominal values, the US/S FS-VFTW itself guarantees 85/90/60/50% removal and 125/25/40/50 mg/L at the outlet for chemical oxygen demand (COD)/total suspended solids (TSS)/total Kjeldahl nitrogen (TKN)/total nitrogen (TN), respectively. By comparison with US/S systems in mainland France, it appears that the warmer tropical-climate temperatures facilitate both nitrification and denitrification kinetics. Performances in overload conditions confirm that the US/S FS-VFTW remains robust and reliable although COD and TKN removal are impacted, especially after strong tropical rain events. By adding a simple compact trickling filter to a US/S FS-VFTW, the treatment system delivers high-level performances (>95% removal for BOD5, COD, TSS and TKN) at less than 1 m2/p.e.

Which plants are needed for a French vertical-flow constructed wetland under a tropical climate?

Plants are essential in the functioning of constructed wetlands. When setting up systems in tropical areas, Phragmites australis is not always a good choice because of its invasiveness. In vertical-flow constructed wetlands (VFCWs) fed with raw wastewater, the main role of plants is their mechanical action, which helps prevent clogging of the deposited organic matter. Various species have already been used in some tropical climate studies, but generally not for such systems, and no attempt has been made to screen large numbers of alternative species. Here we describe a method to select species among a hundred studied, along with promising plants tested in batches, and at full scale. Species of the order Zingiberales showed good adaptation to the main stresses generated by VFCWs. They have long vegetative cycles, which may require weed growth control after plantation, but low harvesting frequency. Root systems with long rhizomes such as Heliconia psittacorum should take priority to ensure even growth and avoid clumps. To limit the phytosanitary risk with Musaceæ (banana tree), Canna indica or Canna glauca are preferable. Species of the genus Cyperus also demonstrate good adaptation, and could be of interest, especially when a high stem density is required (e.g. planted sludge drying beds).