Simulation of total dissolved solids removal from domestic wastewater in vertical and horizontal subsurface constructed wetland

Introduction
Population growth and changes in the pattern of needs in accordance with economic growth and lifestyle changes have increased the need for suitable water sources. Treatment and reuse of wastewater is very important in order to reduce the water crisis and prevent the pollution of surface and underground water sources and ecological destruction caused by the discharge of sewage and preserv human health. Conventional wastewater treatment methods have many implementation limitations, including high cost, complex operations and maintenance, etc. For this reason, it is not possible to implement conventional and high-tech methods everywhere. While natural wastewater treatment systems have lower technology and less need for trained labor and at the same time have high efficiency. Among the solutions for green wastewater treatment, we can mention phytoremediation and artificial wetland system. An artificial terrestrial wetland that removes pollutants by creating a saturated porous environment and providing the conditions for the occurrence of physical, chemical and biological processes. Phytoremediation is a biological treatment method in which a plant reduces the pollutants by selective absorption of pollutants and accumulation of them in its tissues. In this method, various factors such as retention time, pollutant concentration, environmental factors (acidity, temperature) and plant characteristics (species, root system, etc.) are important. Since the artificial wetland is similar to a black box whose processes cannot be predicted, simulation models are used to design them with the aim of achieving the highest pollutant removal efficiency. The HYDRUS-2D model is an advanced two-dimensional model related to the simulation of water movement and solute and heat transfer in saturated and unsaturated porous media. In this model, Richards and dispersivity-diffusion equations have been used respectively to simulate water movement and solute transport, as well as features such as spatial distribution of plant roots, water absorption by plant roots, various equations of hydraulic properties of porous media and various initial and boundary conditions, the possibility to provide the simulation of an artificial wetland with plants.
 
Methodology
Experimental site and Measuring TDS
To collect the information from the model, six artificial wetland systems, including three vertical subsurface flow systems and three horizontal subsurface flow systems were constructed at the sewage treatment plant located in Fakhb, Rasht. From the three systems that were constructed, one system was considered without plants and the other two systems contained Reed and Typha plants.  The wetlands were filled with Gravel in diameter of 5 to 25 mm. To adapt Plants with the cultivation environment and wastewater quality, the plants were irrigated by wastewater about three months. After the three months, the main data collection was done by sampling and checking the performance of the systems. Collecting data was for 9 months. During experiment, the raw wastewater sample was entered into the wetland systems and after the hydraulic retention time (about one month in winter and one week in spring and summer), were sampled from the outlet of the wetlands and the parameter of total dissolved solids measured.
 
 Calibration and Validation HYDRUS-2D
In this research, the HYDRUS-2D model was used to simulate the processes governing the movement of water and transport of solutes and its absorption by plants. S-ship model was used to estimate water absorption by Reed and Typha plants. The effective parameters of water movement and transport of solutes, including saturated hydraulic conductivity, longitudinal and transverse dispersivity, and diffusion of the wetland bed, were estimated using the inverse solution method, respectively, using the output flow data and the total dissolved solids of the output wastewater. Statistical indices were used to evaluate the accuracy of the model in the simulation of the purification process in the wetland. The data of days 112, 125, 131, 140, 146 and 152 from the start of the experiment were used for calibration and the data of days 187, 208, 215 and 222 from the start of the experiment were used to validate the wetlands.
 
Results and discussion
Comparison of the saturated water conductivity value estimated by the HYDRUS-2D model with Sheykhan et al.'s research (2019) showed that the model was able to estimate the hydraulic properties of the bed properly. The longitudinal and transverse dispersivity coefficients in the horizontal wetland were found to be almost half of the vertical wetlands, which is in line with the lower saturated water conductivity and as a result, the lower velocity of the wastewater in the pores in these wetlands compared to the vertical wetland. Dispersion coefficient was a more important factor in solute transport than dispersivity coefficients. On average, in vertical wetlands, the model was able to estimate the amount of reduction of total dissolved solids with about 2% less than the values measured in wetlands under plant cultivation, while in the horizontal wetland, this amount was between 3 and 5 percent.
 
Conclusions
According to the results of statistical indices, the estimated values of the total dissolved solids of the artificial wetland system under Reed cultivation were more consistent with the measured values than other wetlands. The results showed that the model has a suitable ability to simulate the movement of solutes and total dissolved solids, which means that it can be used in the design process of treatment in the wetland.