Inorganic particle accumulation promotes nutrient removal of vertical flow constructed wetlands: Mechanisms and implications.

Abstract

Vertical flow constructed wetlands (VF CWs) are widely applied for treating eutrophic water due to prominent advantages in economy and ecology. Natural inorganic particles are ubiquitous in contaminated water and the accumulation of inorganic particles takes place spontaneously in VF CWs. To reveal how the accumulation of inorganic particles affects the transport and transformation of phosphorus (P) and nitrogen (N) in VF CWs, column experiments with and without inorganic particle loading were conducted for over 180days. The morphology and mass balance of P and N, microbial community structure and hydraulic characteristics of VF CWs were investigated. The average total phosphorus (TP) and total nitrogen (TN) removal efficiencies in VF CWs with inorganic particle loading were steady at 90.4±1.9% and 87.8±2.3%, respectively. Inorganic particle accumulation improved TP removal mainly via adsorption and plant uptake, while enhanced TN removal was mainly attributed to higher plant uptake and microbial degradation. Of particular interest was that plant biomass production was doubled by the concentrated nutrients (e.g., bioavailable P and N) in the rhizosphere, accompanied by the accumulation of inorganic particles up to 9.5gL-1. Accumulated particles increased the bacterial abundance by 7.7-fold, and the diversity of the bacterial community associated with P and N transformations was significantly enhanced (p<0.05). 31P NMR and P fractionation revealed that the elevated P proportion in the substrate was mainly in the form of iron-bound inorganic P. Moreover, inorganic particle accumulation decreased the substrate hydraulic conductivity, while it showed limited effect on the reduction of the hydraulic retention time.