Nitrogen Removal in Permeable Woodchip Filters Affected by Hydraulic Loading Rate and Woodchip Ratio.
Unregulated and event-driven agricultural tile drainage discharge poses several challenges that potentially limit the nitrate (NO) removal performance of woodchip-based wetlands constructed to intercept subsurface tile drain flows. Laboratory column tests were conducted to evaluate the biogeochemical response of mixed reactive media (woodchips-seashells and woodchips-Filtralite mixtures) at two woodchip ratios to changes in hydraulic loading rate (HLR). The tests involved continuous loading of aerated artificial drainage water spiked with NO-N and tritium (HO) breakthrough experiments. Flow-normalized NO reduction rates ranged from 0.35 to 3.97 g N m L, corresponding to N removal efficiencies of 5 to 64%, depending on HLR and filter mixtures. At high HLRs, oxic conditions prevailed in the woodchip filters, resulting in reduced N removal. At low HLRs, progressively lower pore-water velocities extended the period for consumption of terminal electron acceptors, increasing N removal. When increasing the content of mineral material, N removal declined, probably due to a lower denitrifying biomass at lower woodchip mass. The effect of woodchip ratios on solute transport characteristics was difficult to assess. However, woodchip media including a mineral fraction of crushed seashells demonstrated the highest N removal rates and efficiencies, most likely due to the alkalizing effect of the seashells. In conclusion, filter mixtures consisting of woodchips and seashells were the most effective material for N removal in subsurface flow-constructed wetlands treating agricultural drainage water.
Jacob
J
Bruun
Carl Christian
CC
Hoffmann
Charlotte
C
Kjaergaard
D058440:Denitrification; D009566:Nitrates; D009584:Nitrogen; D014865:Waste Disposal, Fluid; D014874:Water Pollutants, Chemical; D018508:Water Purification; D053833:Wetlands
D009566:Nitrates; D014874:Water Pollutants, Chemical; D009584:Nitrogen