MECHANISMS FOR THE PURIFICATION OF EFFLUENTS WITH HIGH NITROGEN-CONTENT BY A PLANT COVER OF WATER HYACINTHS (EICHHORNIA-CRASSIPES)

The mechanisms for the purification of three effluents containing a high level of ammoniacal nitrogen by water hyacinths (Eichhornia crassipes) have been studied. The pollutants (nitrogen, phosphorus, COD and suspended solids) were eliminated by means of the creation of a favourable ecosystem. The liberation of oxygen and the root support provided by the rhizosphere allow a nitrifying bacterial flora to develop. Denitrification can occur when there is no more available nitrogen, namely at the bottom of the pond or when a high BOD mobilises all the available oxygen in the pond. Ammoniacal nitrogen is eliminated by plants (< 10%), by nitrification and denitrification (31-68%), sedimentation (2-28%). Phosphorus is eliminated by plant assimilation (3-38%) and precipitation; COD is eliminated by biodegradation and sedimentation (depending on the initial biodegradation of the effluent); the suspended solids (SS) are eliminated (30-90%) by sedimentation and trapping in the roots (26-36%). The purification of effluents containing a high level of ammoniacal nitrogen differs from that of urban effluents in that plants have a lesser role for the elimination of nitrogen.