Evaluation of Feed Storage Runoff Water Quality and Recommendations on Collection System Design

Why Study Silage Leachate?

Silage storage is required for many livestock and poultry facilities to maintain their animals throughout the year.  While feed storage is an asset which allows for year round animal production systems, they can pose negative environmental impacts due to silage leachate and runoff.  Silage leachate and runoff have high levels of oxygen demand and nutrients (up to twice the strength of animal manure), as well as a low pH posing issues to surface waters when discharged.  Although some research exists which shows the potency of silage leachate and runoff, little information is available to guide the design of collection, handling, and treatment facilities to minimize the impact to water quality.  Detailed information to characterize the strength of the runoff through a storm is needed to develop collection systems which segregate runoff to the appropriate handling and treatment system based on the strength of the waste. 

What did we do?

In order to evaluate collection designs, we evaluated six bunker silage storage systems in Wisconsin.  Runoff from these systems was collected using automated samplers throughout one year to assess water quality for nutrients (nitrogen and phosphorus species), oxygen demand, total solids, and pH.  Flow rate for each system was also recorded along with weather data including precipitation information.  Feed quantity and quality was also recorded at each site to have a better understanding of the impact of silage management on water quality.  Data was analyzed to determine flow weighted average runoff concentrations for pollutants measured, seasonality and feed impacts to water quality, storage design impacts, the presence or absence of first flush conditions, total loading, and evaluated to make collection design recommendations.

What have we learned?

Flow rate, timing of ensiling of forage, site bunker design, and amount of litter present were determined to influence silage runoff concentrations.  Leachate collection played a significant role in water quality as the runoff from the site without leachate collection had a lower average pH (4.64) and higher COD values (5,789 mg L-1) than the sites with leachate collection (6.09 and 5.54 pH, and 1,296 and 3,318 mg L-1 COD).  Nutrients were also higher for the site without leachate collection TP (83 mg L-1), NH3 (68 mg L-1), and TKN (222 mg L-1) compared to TP (29 and 63 mg L-1), NH3 (25 and 48 mg L-1), and TKN (184 and 215 mg L-1) for the sites with leachate removal. Time of ensilage also played an important role in water quality with increased losses occurring within two weeks of ensilage.  The most important finding for the design of treatment systems was that the water quality parameters (including nutrients) were found to be negatively correlated with flow.   The resulting effect is that the storms hydrograph has a significant impact on the pollutant loading to the surrounding waterways.  It was also found that loading was relatively linear throughout each storm event indicating that there is no first flush phenomenon which is found to occur with urban runoff systems.  Therefore designing systems to collect the initial runoff from a system is not an efficient way to capture the greatest pollutant load.  It was found that low flows throughout a storm have high pollutant concentrations and collecting low flows throughout a storm would result in the greatest load collected per unit volume.

Future plans

The next phase of this research will be to develop loading recommendations to filter strips for sizing and minimizing impact to the environment.

Corresponding author

Rebecca Larson, Assistant Professor and Extension Specialist, Biological Systems Engineering, University of Wisconsin-Madison ralarson2@wisc.edu

Mike Holly, Eric Cooley, Aaron Wunderlin

Additional information

Published paper is currently in review and will be available within the next year.


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