This work delineates the role of plants in free surface flow constructed wetland systems through the
direct comparison of two parallel pilot-scale systems, one with plants (Typha latifolia) and one without.
The basins were fed with real municipal wastewater at retention times ranging from 27.6 to 38.0 days.
The variation of the water volume in each basin was monitored and rainfall and evaporation rates were
calculated. The difference of the water volume between the basins due to the water uptake by the plants
was compared with the predictions of evapotranspiration using the REF ET calculation software. Plant
harvesting data were used to assess the overall nitrogen uptake rates by the plants. The main difference
in the two systems was that in the system with plants, a significant amount of water was removed
through evapotranspiration, with the plants acting in essence as pumps. A mathematical model
simulating the behavior of the systems was developed for each basin, describing the basic
physicochemical and biochemical processes using AQUASIM. The key microbial processes accounted
for in the modeling included ammonification, aerobic heterotrophic growth, nitrification and algal
growth. A strong seasonal dependence of each system behavior was observed. The model developed in
the first year of experimentation was validated in the second year, by comparing the predicted with the
measured values of the key wastewater characteristics. The BOD and nitrogen removal efficiencies were
60% and 69% on the average respectively, for the basin without plants and 83% and 75%, respectively for
the basin with plants. The developed model may be used for the design and simulation of full-scale
artificial free superficial flow constructed wetlands