The study of wastewater treatment series based on the use of macrophyte beds, in accordance with a design initially developed in Germany by Dr. Seidel, was initiated by CEMAGREF in 1983. The number of plants constructed was deliberately limited and they were subject to regular assessments and improvements. It was thus shown that only reed bed filters that were drained and alternately fed, were able to handle a pollutant load of the order of 80 g COD m-2 day-1 with satisfactory treatment reliability over a period of several years. The Gensac la Pallue plant, in the Charente department, is original since it combines this type of filter in the first treatment stage with three microphyte ponds designed with a low theoretical residence time of 21 days (compared with the usual residence time of 30 days in tertiary treatment). The whole treatment series only occupies an effective surface area of approximately 6 m2 per p.e. (about 1 m2 and 5 m2 for filters and ponds respectively).
After 5 years of operation and monitoring, it has been clearly established that filters dosed with raw wastewater at a rate of 65 g of COD and 30g of TSSm-3 per day, regularly achieve a removal rate of 70 to 85 % of these same elements without clogging. The daily hydraulic load that the filter can withstand in service regularly exceeded 2 metres in the month of November 1992.
The ponds finish the treatment and this article limits itself to analysing results relating to the elimination of nutrient, which reaches 35% for nitrogen and 55% for phosphorus. Residence times that are too short in winter, in conjunction with a large part of the sewerage system being of combined type, can sometimes temporarily lead to an increase in the amounts of nitrogen and phosphorus discharged to the natural environment.