Field Test Results for Nitrogen Removal by the Constructed Wetland Component of an Agricultural Water Recycling System

Abstract

<abstract><title><italic>Abstract. </italic></title> <underline>W</underline>etland <underline>R</underline>eservoir <underline>S</underline>ub<underline>i</underline>rrigation <underline>S</underline>ystems (WRSIS) are innovative agricultural water recycling systems that can provide economic and environmental benefits. A constructed wetland is a main component of WRSIS, and one important function of this constructed wetland is drainage water treatment of nitrogen. Four field tests were therefore conducted at a northwest Ohio WRSIS wetland to evaluate nitrate-nitrogen (NO₃^--N), ammonium-nitrogen (NH₄⁺-N), and total nitrogen (TN) removal effectiveness. Tests 1 and 2 had lower inflow water volumes, shorter effective retention times, and smaller nitrogen fertilizer input loads. Test 3 had an intermediate inflow volume, intermediate effective retention time, and an intermediate nitrogen input load, while Test 4 had a high inflow volume, long effective retention time, and a large nitrogen input load. Based on nitrogen mass balance calculations, wetland processes reduced NO₃^--N, NH₄⁺-N, and TN by 11.0%, 61.7%, and 13.2%, respectively, during Test 1, 10.8%, 41.7%, and 11.7%, respectively, during Test 2, 44.0%, 87.5%, and 44.9%, respectively, during Test 3, and 15.6%, 81.1%, and 16.1%, respectively, during Test 4. Tests 1, 2, and 3 were run consecutively, and if considered as a single test having three water inflow periods, three water outflow periods, and three nitrogen fertilizer inputs, then the NO₃^--N, NH₄⁺-N, and TN reductions for Test 1→2→3 were 36.8%, 78.1%, and 38.1%, respectively. Test 1→2→3 and Test 4 combined had NO₃^--N, NH₄⁺-N, and TN reductions of 27.7%, 79.2%, and 28.5%, respectively. For the three tests conducted in May and June, 2009, Test 3 had the longest effective retention time, which may account for the better nitrogen reduction results achieved with Test 3 as compared to Tests 1 and 2. Cooler temperatures for Test 4 during October and November 2009 may have decreased denitrifying bacterial activity, in turn suppressing the amount of NO₃^--N and TN reduction obtained with Test 4. Overall results of this study indicate that WRSIS wetlands are capable of providing water quality benefits by removing substantial amounts of nitrogen present in drainage waters.