Abstract
Nutrient losses from agricultural fields have long been a matter of concern worldwide due to the ecological disturbance this can cause to surface waters downstream. In this paper a new design concept, which pairs a surface-flow constructed wetland (SFW) with a woodchip bioreactor (WB), was tested in relation to its capacity to reduce both nitrogen (N) and phosphorus (P) loads from agricultural tile drainage water. A nutrient mass balance and a comparative analysis were carried out together with statistical regressions in order to evaluate the performance of four SFW+WBs under different catchment conditions. We found marked variations between the systems in regard to hydraulic loading rate (0.0 to 5.0 m/day) and hydraulic retention time (1 to 87 days). The paired system worked as nutrient sinks throughout the study period. Total N and total P removal efficiencies varied from 8% to 51% and from 0% to 80%, respectively. The results support the use of the new design concept for nutrient removal from tile-drained agricultural catchments in Denmark as part of national management plans, with the added advantage that smaller areas are needed for construction (0.1% to 0.2% of the catchment area) in comparison to standalone and currently used SCWs (~1%).
Highlights
Most agricultural areas represent a major source of nitrogen (N) and phosphorus (P) to surface waters [1,2,3]
The surface-flow constructed wetland (SFW)+WBs have the following 1-m deep components in the order of the water flow having a diameter of approximately 95 mm (Øland Entreprenørforretning, Aabybro, Denmark), and direction: (i) a sedimentation basin (SB), (ii) a woodchip bioreactor (WB) consisting of raw woodchip (iii) a clarification basin (CB)
The Surface-flow constructed wetlands paired with woodchip bioreactors (SFW+WB) in Ondrup was equipped from September 2017 with an additional flowmeter, which measured any discharge (Qoverflow ) bypassing the system through a parallel pipe connecting the inlet with the outlet
Summary
Most agricultural areas represent a major source of nitrogen (N) and phosphorus (P) to surface waters [1,2,3]. Current national efforts are searching for effective measures at the landscape scale These can intercept the drainage networks and reduce the nutrient load to surface waters, in order to prevent eutrophication and meet the environmental objectives of the EU Water Framework Directive. Water 2020, 12, 1891 some studies reported SFWs as P sources when receiving P predominantly in dissolved form [20,21] This is generally attributed to removal processes for nitrate being superior to those for phosphate [22]. This study aimed to assess the annual and seasonal performance in the removal of N and P by a newly developed design concept, consisting of paired systems (SFW+WB) receiving agricultural tile drainage water in Denmark. The results presented will help authorities to evaluate the viability of the paired systems as part of management plans to reduce nutrient loads from Danish agricultural catchments to surface waters
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