For the first year, in contrast with results obtained by Mandi et al. (1998) and Stober et al. (1997), no seasonal differences were found in the performance of BOD, COD and TSS. This could be due to the fact that the sampling period in this year ranged from June to November, since plants were transplanted in April and analysis began 2 months laterwhen they were well established. During the second year, significantly lower BOD removals were found in winter, independently of the hydraulic application rate or the type of plant. Generally poor removals are attributed to microbial breakdown of carbonaceous compounds being limited by low oxygen availability. Consequently, the decrease of BOD removals observed in winter could be due to the fact that there was no above-ground biomass, since the plants were harvested in autumn and, as a consequence, the available oxygen in the root zone diminished.