A pilot-scale study for modeling a free water surface constructed wetlands wastewater treatment system

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.