Document Type

Thesis - Open Access

Award Date

2022

Degree Name

Master of Science (MS)

Department / School

Agricultural and Biosystems Engineering

First Advisor

John McMaine

Keywords

conservation drainage, denitrifying bioreactor, nitrate reduction, subsurface drainage, woodchip bioreactor

Abstract

Denitrifying woodchip bioreactors are a critical tool for mitigating nitrate loading to downstream water bodies. The properties of the woodchip are a key factor in the design of the bioreactor which are usually designed to optimize that retention time of the water being treated. Retention time is affected by active flow volume, porosity, and flow rate. As a bioreactor ages, the carbon material will break down and, in some cases, sedimentation will occur within the bioreactor. Both processes will affect the porosity and hydraulic performance of the bioreactor. When flow through the bioreactor is significantly different than the original design, nitrate removal performance will be negatively affected. A denitrifying woodchip bioreactor was installed in 2014 near Hartford, South Dakota. This bioreactor was monitored since installation and has demonstrated a decline in concentration reduction performance. Since the installation in 2014, the Hartford bioreactor has also been affected by external factors that were not accounted for at the time of installation. Heavy rains and unprotected soil upgradient of the bioreactor led to increased sediment loading and flooding in and around the inflow control structure. In 2021, the bioreactor was excavated, and particles were characterized for particle size distribution, bulk density, drainable porosity, and total porosity at six transects along the length of the bioreactor and three depths within each transect. This study was conducted to characterize woodchip and sediment particles within the bioreactor to assess the likely causes of failure. Woodchip degradation and sedimentation the two main mechanisms of failure within this system. These mechanisms led to reduced pore sizes in affected areas changing the hydraulic properties within the bioreactor. A bioreactor was installed in 2012 near Baltic, South Dakota. Weekly inlet and outlet samples have been collected and analyzed for nitrate and E. coli. Results from these samples show that treatment performance has diminished, and the bioreactor may be reaching the end of its useful life. Hydraulic performance of the aged bioreactor was assessed with a bromide pulse tracer study in July 2021. The objective of the tracer study is to assess hydraulic performance metrics including time to peak, residence time distribution and the mean residence time of the tracer. These factors will be used to determine the primary flow type, indicate dead zones and short circuiting, and overall hydraulic efficiency. This project will provide guidance for maintenance and recharge methods for denitrifying woodchip bioreactors.

Number of Pages

69

Publisher

South Dakota State University

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Rights Statement

In Copyright