Nutrient accumulation and removal in two novel constructed wetlands (CW) : duplex-CW and roof-CW

Constructed wetlands (CW) are wastewater treatment systems with low energy consumption, no negative effects to the environment and some limitations (e.g. bed clogging, high surface area requirements and poor nutrient removal). In order to overcome the area requirement limitation, two novel setups (Duplex-CW and Roof-CW) were developed and constructed. The Duplex-CW consists of a vertical flow CW (planted with Phragmites australis) on top of a horizontal flow filter, fed with primary effluent, while the Roof-CW is a shallow horizontal flow CW installed in the roof of a building treating wastewater produced in an office. In order to properly develop a technology it is important to comprehend the ""black box"", hence this research was aimed to understand and assess the nutrient flow in both systems. Additionally, the hydrology of the Roof-CW was investigated in order to comprehend the treatment effects under exposure to the atmospheric conditions. Both systems are functioning since July 2012. The study of the Duplex-CW was divided in three periods, each applying different total nitrogen (TN) concentrations. Three different configurations of the Duplex-CW with different retention times of the vertical flow CW were tested: Fill&Drain, Stagnant Batch and Free Drain. Above-ground biomass of Phragmites australis in the Duplex-CW (all configurations) removed from 1 to 13% of the TN load and 3-10% of the total phosphorus (TP) load. However, both plant weight and its contribution to the nutrient removal decreased along the study period. Moreover, potential denitrification rates were estimated along the depth of vertical flow CW compartment. It was found to be in the range 5-21 mg/kg/h, which was higher than in similar CWs.Pollutants removal in the Roof-CW was 80%, 95% and 96% for COD, TN and TP, respectively. Water flow characteristics during dry and wet periods were studied and revealed that the water, during the dry days, was retained longer only in the inlet and outlet, while it was middle and outlet zone for rainy days. The total runoff reduction was 71% as effluent was only 29% of the influent volume. However, that raised a question of ways in which nutrients are removed. Substratum was analysed in five points along the bed length. Nitrification and denitrification potential rates were in the ranges 0-1.5 and 50-70 mg/kg/h, respectively, while substratum nutrient content was found to be 100-700 and 200-600 g/m3 for TN and TP, respectively. Both of them, however, were in a correlation with water flow during the dry days as they were highest in the inlet and outlet zones.Results showed that the two novel systems can treat wastewater to the satisfactory level and be considered as a valuable asset in smaller communities. Moreover, it was concluded that although plant uptake of TN and TP is significant, the major role has to be assigned to the denitrification for TN and substratum adsorption for TP