Nitrogen Removal in Permeable Woodchip Filters Affected by Hydraulic Loading Rate and Woodchip Ratio

Abstract

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.

Nitrogen Removal in Woodchip‐based Biofilters of Variable Designs Treating Agricultural Drainage Discharges

“…Further, HRT ranging from 0.1 to >4 d covers the HRT distribution relevant for woodchip‐based CWs targeting agricultural drainage water. The major influence of water temperature (Volokita et al, 1996; Cameron and Schipper, 2010; Hoover et al, 2016) and HRT (Chun et al, 2009; Bruun et al, 2016a; Hoover et al, 2016) on N removal efficiency of woodchip‐based biofilters is widely accepted.…”

“…Based on laboratory experiments (Bruun et al, 2016a), six subsurface flow constructed wetlands (CW1–CW6) were constructed in autumn 2012 on a field site in central Jutland, Denmark (56.214294° N, 9.743437° E). All six CWs received tile drainage water from a 78‐ha drainage catchment.…”

“…Crushed seashells were added as a structural component to help maintain porosity. Filter physical parameters were measured in the laboratory and reported by Bruun et al (2016a) (Table 1).…”

Microbiome Structure and Function in Woodchip Bioreactors for Nitrate Removal in Agricultural Drainage Water

“…The woodchip/seashell ratio (v/v) was 50:50 for BR1 and 75:25 for BR2. Seashells were used as a material to improve the physical matrix stability and enhance water conductivity (Bruun et al, 2016). Dimensions of BR3 (55.988 • N, 10.081 • E) were 8 × 31 × 1.2 m and it was filled with willow woodchips.…”

“…Here we used metagenomics to obtain a more complete overview of the taxonomic composition and functional potential of woodchip bioreactor microbiomes. We report the microbiome composition at a given time point in three stable operating fullscale woodchip bioreactors (BR1-BR3), where NO 3 − removal efficiencies were previously monitored (Bruun et al, 2016;Carstensen et al, 2019;Audet et al, 2021) and where concurrent metadata on water chemistry were available. The aim was to determine if the microbiomes differed among the bioreactors due to different design parameters and location in different agricultural settings.…”