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 m 3 d −1 were applied over three alternating units, resulting in an average hydraulic loading rate (HLR) on the full system of 0.15 m 3 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 m 3 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 NH 4 + -N. With two units, the equivalent values were 74% for BOD, 59% for COD, 67% for TSS and 51% for NH 4 + -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.
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.
Development of a new generation of reed-bed filters in France: First results
Since 1992, Reed-Bed Filters (RBF) wastewater treatment plants have been built to meet the needs of 15 rural authorities and, on an experimental basis, 2 farms to treat milking parlour washwater using this method. This paper presents results obtained at a plant designed for 200 p.e. after 15 months of operation, organic and hydraulic loads being 67% and 115% of the nominal loads respectively. With an overall area of the order of 2.2 m2 per person equivalent, the plant produces an effluent which easily complies with the following water quality criteria : 90 mg l−1 COD, 30 mg l−1 TSS and almost 10 mg l−1 TKN. Phosphorus removal is normally very low. A farm-located experimental plant, receiving milking parlour washwater (COD between 700 and 1400 mg l−1), is providing promising results which will certainly be improved by extending the current RBF areas. RBFs are capable of directly treating raw sewage by applying procedures involving batch feeding and alternating several units operating in parallel; in this way aerobic conditions are maintained and physical surface clogging is controlled with the aid of rooted macrophytes. RBFs used for complementary treatment seem to accept continuously high hydraulic loads.