Nutrient Removal in Agricultural Drainage Ditches

Principal Investigator: Jeffrey Strock
Co-Investigator(s): David Mulla, Kurt Spokas and Gary Feyereisen
Organization(s): University of Minnesota and USDA-ARS
Sponsor: Clean Water Fund
Award Amount: $404,112
Start Date: 4/1/2013 | End Date: 12/31/2016
Project Manager(s): Heather Johnson (Heather.Johnson@state.mn.us)

FINAL REPORT is available in the Minnesota Water Research Digital Library

A project aimed at improving bioreactor design to increase nutrient removal efficiency and minimize the need for maintenance and management, while leaving land in production.

Background

Nitrogen (N) and phosphorus (P) pollution of surface waters from non-point agricultural sources is a problem nationwide, as well as in the Midwestern US, including Minnesota. Hypoxia in the Gulf of Mexico has been a serious problem since the mid 1980’s, and is largely attributed to nutrient enrichment of marine waters by N and P entering the Gulf from the Mississippi River. A federal task force has recommended a 45% reduction in N and P loads entering the Gulf of Mexico from the Mississippi River in order to reduce the long-term average area of the Gulf hypoxic zone to 1,930.5 mi2 (5,000 km2) or less.

Much of the nitrogen and phosphorus that pollutes surface waters in the Upper Midwestern region flow through agricultural drainage ditches (roadside ditches). These ditches receive water from subsurface drain tiles and over land (surface) agricultural runoff. Currently, the primary function of drainage ditches is to remove excess water quickly without any treatment. Drainage ditches discharge large quantities of N and P into creeks, streams and rivers. New technologies that remove nutrients from drainage ditches offer the possibility of supplementing traditional on-farm or edge-of-field best management practices (BMPs) without taking land out of crop production.

This project offered the potential to remove significant quantities of N and P from agricultural drainage ditches. A bioreactor which removes N and P was installed parallel to an agricultural ditch, rather than at the outlet of subsurface tile drains as has been the case with most bioreactors. The bioreactor removes both N and P, which has not been the case with previously studied bioreactors that were designed to remove only N at the end of subsurface drains, or only P in regions where overland flow occurs. 

Research Objectives

1. Evaluate physical and chemical characteristics of materials that have potential use in a bioreactor
2. Evaluate the efficiency of selected N - denitrifying and P-sorbing materials in a laboratory setting
3. Construct a two phase bioreactor and evaluate N and P removal in agricultural runoff under field conditions

Design

There were three phases for this project:

1. Initial laboratory screening of various materials for abiotic nitrate and phosphorus removal to select six optimum materials for column testing,
2. Laboratory column evaluation of the six optimum materials for their combined biotic and abiotic removals, and
3. Field installation and monitoring of the biofilter in a drainage ditch in Lamberton, Minnesota using the material selected through the laboratory column testing. 

Key Findings

• The results of this study show that the variety of the biochar used impacted the N removal rate –biochar varieties should not be assumed to result in equal N removal efficiency. Once the biochar was combined with soil, there was not an observed N removal benefit.
• The addition of acetate, a readily available carbon source and electron donor, to wood chips resulted in the highest removal of nitrate-N.
• Nitrate removal was related to the retention time in the bioreactor coupled, with the addition of acetate. The longer the retention time or length of time the water was in the bioreactor, the greater the rate of nutrient removal.
• Results indicated consistent reduced conditions (i.e. improved conditions for denitrification) within the bioreactors when acetate was added to the subsurface drainage water.

Potential areas for future research

• Additional research is needed to determine the longevity of P and N sorbing materials.

• Microscopy and EDS point analysis indicate that P removal was likely due to microbial uptake. However, further research is needed to confirm this.

• Research should also be done to determine if DRP is susceptible to biological release from bioreactors.

• Further research on the use of soluble carbon should be conducted in field-scale bioreactors.