Estimation of water dynamics in a vertical-flow constructed wetland with a growing plant species

Purpose Vertical-flow constructed wetland (VFCW) is a promising technique for wastewater treatment comparable to conventional wastewater treatment plants. The physical, chemical, and biological processes and interactions in a VFCW are highly coupled with water movement, and thus the performance of a VFCW to remove contaminants hinges on a better understanding of its water dynamics. The aim of this study was to develop a model for estimating water dynamics in an artificial VFCW with a growing plant system. Materials and methods The model was developed using the commercial available package STELLA (Structural Thinking, Experiential Learning Laboratory with Animation) in conjunction with our greenhouse experimental data. The water dynamics included in the model are: (1) application of wastewater to the VFCW, (2) evaporation and transpiration, (3) formation of water column due to the surface ponding, (4) uptake of water by roots, (5) clogging of substrate pores by wastewater, and (6) flow of water through the VFCW under variably saturated conditions. The resulted model was applied to estimate water dynamics in an artificial VFCW(i.e., a large cylindrical column) under a wetting-to-drying ratio of 1: 3, i.e., recharging the column for 8 h followed by drying the column for 24 h. Results and discussion A simulation scenario was performed to investigate the daily water dynamics in the presence of the Cyperus alternifolius species under a wetting-to- drying ratio of 1:3. Results showed that surface water ponding occurred periodically and increased with time due to a decrease in the column infiltration rate resulting from the pore clogging by the wastewater (with a suspended solid content of 99 mg/L). Root water uptake and leaf water transpiration showed a typical diurnal pattern, i.e., increased during the day and decreased to near-zero at night as well as increased as the C. alternifolius grew. A 15% decrease in porosity due to the pore clogging resulting in 65% decrease in wastewater inflow rate for the simulation conditions used in this study. Conclusions A good agreement was obtained between the model predictions and the greenhouse measurements during the model calibration. The model was quite successfully applied to estimate water dynamics in an artificial VFCW with a growing C. alternifolius wetland plant. Further study is warranted to extend the model for estimation of coupled water movement and contaminant removal in the VFCWs.