In order to preserve the River Seine against point discharge of secondary effluent, the purification plant of Achères is building a tertiary step to nitrify and polish secondary effluent by biofiltration, the dry weather flow of which reaches 2 100 000 m3/day.

Two biofilter prototypes, Biofor and Biostyr, have been created to test their performances in a wide range of operating conditions, especially in the case of storm water. First results show that a removal of 0.8 kg N-NH4/m3.d, corresponding to a Global Oxygen Demand removal of 5.5 kg O2/m3.d, can be achieved by Biofor at a temperature of 14°C.

The Biostyr prototype brought later into service has not yet enough results to draw clear conclusions. Nevertheless the first data during May 1993 (temperature 18.5°C) are also very promising.