Nutrient removal in gravel- and soil-based wetland microcosms with and without vegetation

In this study, four lab-scale microcosms, including gravel-beds with and without plants, and soil-beds with and without plants, were used to conduct the nutrient removal tests. The influent used in the tests is primary treated sewage, while the plant selected was Napier grass (Pennisetum purpureum). The purpose of this study is to compare the removal efficiencies of nitrogenous and phosphorus nutrients among these four microcosm wetland systems based on statistical analyses. Three factors, namely, with/without vegetation (F-1), medium types of gravel/soil (F-2), and time period for the test run of first/second stage (F-3), and four combined effects of factors, F-1 by F-2 (F-1,F-2), F-1 by F-3 (F-1,F-3), F-2 by F-3 (F-2,F-3), and F-1 by F-2 by F-3 (F-1,F-2,F-3), were run by an ANOVA model to analyze the relationships between the amounts of nutrient removed from the wetland systems and these seven factors. We found that the removals of ammonia (NH3-N), nitrate (NO3--N), and soluble reactive phosphorus (SRP) were related to these factors and combined effects of the factors. It was found that the main removal mechanism for NH3-N was nitrification, which could be enhanced by the root zone effect in the vegetated gravel-bed wetland systems, while NO3--N was removed mainly by denitrification and plant uptake in vegetated systems. However, the main removal mechanism for SRP was chemical adsorption in the unsaturated soil-bed systems. The effect of plant litter was also a significant mechanism affecting nutrient removal in the surface flow pattern soil-bed wetland systems without harvest. (C) 2001 Elsevier Science BY. All rights reserved.