Performance of a Woodchip Bioreactor for the Treatment of Nitrate-Laden Agricultural Drainage Water in Northeastern Germany

Lennart Gosch,Haojie Liu,Bernd Lennartz

doi:10.3390/environments7090071

Abstract

Reactive barriers, such as denitrifying bioreactors, have been identified as a clean-up option for nutrient-laden agriculture runoff. Here we tested a 20 m long, 3.75 m wide and 2.2 m deep woodchip bioreactor receiving tile drainage water from a 5.2 ha field site, aiming at testing the hydraulic functioning of a dual-inlet system and quantifying its impact on nutrient loads (nitrogen, reactive phosphorus, organic carbon) in a region with a drainage season taking place in the hydrological winter (November to April). The hydraulic conditions in the dual-inlet bioreactor system developed differently than expected; asymmetric flow rates led to long average hydraulic retention times and a highly dispersed residence time distribution, which was revealed by a bromide tracer test. With a nitrate load reduction of 51 to 90% over three drainage seasons, the woodchip bioreactor proved at the same time to be very effective under the winter conditions of northeastern Germany. The bioreactor turned from an orthophosphate source in the first year of operation into an orthophosphate sink in the second and third year, which was not expected because of anoxic conditions (favorable for denitrification) prevailing within the woodchips. Besides an efficient nutrient retention, the woodchip bioreactor contributed to the total organic carbon load of receiving waters, which impairs the overall positive role of bioreactors within intensively agriculturally used landscapes. We consider this promising low-maintenance biotechnology particularly suitable for single drainage pipes with high discharge and high nitrate concentrations.

Highlights

The nitrogen load of agricultural tile drainage water can contribute to eutrophication of surface waters, in naturally nitrogen-limited aquatic ecosystems [1,2]
To make sure a woodchip bioreactor works hydraulically as planned, it is recommended to choose a single inlet with high inflow and high nitrate concentrations to load the bioreactor
A high nitrate removal efficiency was obtained with a woodchip bioreactor for agricultural drainage water in northeastern Germany indicating the usability of this measure type in regions with drainage seasons in the hydrologic wintertime

Summary

Introduction

The nitrogen load of agricultural tile drainage water can contribute to eutrophication of surface waters, in naturally nitrogen-limited aquatic ecosystems [1,2]. The total export of nitrogen from field sites via the tile drainage pathway is a function of both the fertilization regime and the specific hydrological situation of a given year. In order to overcome the dependence of nutrient export from agricultural land from the hydrological regime, constructed wetlands for the treatment of nitrate-laden tile drainage water have been proposed [4]. In case of limited available space, a denitrifying bioreactor, a rather technical variant, which falls into the category of reactive barriers, may be an option. It consists of an artificial ditch filled with organic substrates (e.g., woodchips) through which the drainage water is passed before it discharges to the surface water. The anoxic and carbon-enriched conditions in the Environments 2020, 7, 71; doi:10.3390/environments7090071 www.mdpi.com/journal/environments

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