Modeling the impacts of plants and internal organic carbon on remediation performance in the integrated vertical flow constructed wetland

Abstract

The integrated vertical flow (IVF) constructed wetland consists of two or more chambers with heterogeneous flow patterns and strong aeration capability, possesses favorable remediation performance. The Constructed Wetland Model No.1 (CWM1) embedded in the OpenGeoSys # IPHREEQC was applied to investigate the wetland plant effects on treatment efficiency. Two fundamental functions of the plant roots (i) the radial oxygen loss (ROL) and (ii) exudation of internal organic carbon (IOC), are developed and implemented in the model to simulate the treating processes of planted laboratory-scale IVF wetlands fed by the synthetic wastewater. The good agreement between simulated results and measurements of the planted IVF wetland and the unplanted filters mimicking wetland demonstrates the combined effects of ROL and IOC and the model reliability. In summer the ammonia (NH4-N) and total nitrogen (TN) removals are high as above 90% in both IVF wetlands, and in winter they decline significantly to around 55% and 45% in unplanted wetland, contrastively to about 85% and 78% in the planted wetland. The nitrogen removal - COD/N ratio relation curves of IVF wetlands are proposed and obtained by modeling to evaluate organic carbon loading status. Based on the curves, the COD/N ratios of unplanted and planted wetlands are about 3∼7 and 3∼10 gCOD/gN for high TN removal respectively. Planted wetlands can tolerate a wider range of COD/N ratio influents than unplanted ones. The ROL in the unplanted wetland promotes COD and NH4-N removal, while may inhibit denitrification under low-temperature conditions. The single addition of IOC enhances the oxygen-consuming and restrains the nitrification under the full loaded COD condition. Summing up all organic carbon releases from substrate and roots as IOC, the quantification of IOC acts on nitrogen treatment was simulated and compared with the external organic carbon (EOC) loading from influent. IOC performs higher efficiency on TN removal than EOC at the same organic loading rates. The results provide the thoughts of the solution for low TN removal in the carbon deficient constructed wetlands.