Kinetics of pollutants removal in vertical and horizontal flow constructed wetlands in temperate climate

This paper reports a comparative study on kinetics of organic matter expressed as BOD₅ and nitrogen removal in constructed wetlands operated in Poland. Analyzed data were collected at eight wetland systems, composed of subsurface flow beds: horizontal flow (HF) and vertical flow (VF), in different number and sequences. The analysis involved particularly mass removal rates (MRR) and first-order removal rate coefficients of BOD₅ and total nitrogen (k_A and k_v for VF and HF filters, respectively, and k₂₀ as a parameter averaged for a temperature of 20 °C). It was found that the higher the load of pollutants applied to the beds, the higher MRR values were obtained. The average k-rates in analyzed systems were mostly lower than those reported in the literature, especially in the case of total nitrogen. Its removal obtained in horizontal flow beds was k_v = 0.002-0.042 d^-1, while in vertical flow systems k_A varied from 0.007 m d^-1 to 0.0037 m d^-1. According to data given by previous studies, first-order reaction rates for nitrogen removal varied in range from k_v = 0.048 d^-1 to k_v = 0.19 d^-1 and k_A from 0.007 to 0.1 m d^-1 in HF and VF beds, respectively. Regarding BOD₅ shown in literature, removal rate k_v for HF beds varied from 0.071 to 6.11 d^-1, and k_A for VF beds varied from 0.019 to 1.0 m d^-1, while in this study lower k-rates were obtained: k_v = 0.005-0.085 d^-1 and k_A = 0.015-0.130 m d^-1. Relatively long monitoring period, for some of constructed wetland up to 16 years, resulted in good data set and enables creation of the graphs, which could be helpful in evaluation and designing of constructed wetlands for PE bigger than 50, in moderate climate conditions.