DENITRIFICATION OF SOILLESS TOMATO CROPS RUN-OFF WATER BY HORIZONTAL SUBSURFACE CONSTRUCTED WETLANDS

Soilless tomato crops runoff contains high nutrient concentrations, particularly nitrates, with concentrations ranging from 150 to 500 mg NO₃-N.L^-1. Biological denitrification with horizontal subsurface flow wetlands planted with common reed (Phragmites australis) and bulrush (Scirpus lacustris) has been evaluated to treat soilless crops effluent. Because of the high level of NO₃-N, a carbon source was required to support denitrification with a good activity of denitrifying bacteria. Vinery sewages have been choosen because they are easily and freely available in France. The trials were carried out in 20 small horizontal flow wetlands (1.8 m x 0.6 m x 0.67 m water depth). The first results show that the level of nitrate nitrogen can be lowered by 70 to 100% when carbon source supply is sufficient (COD/N = 2 to 4) and with Hydraulic Residence Times of 5 to 10 days depending on the season.