Effects of Mixing Duration on Biogas Production and Methanogen Distribution in Dairy Manure Anaerobic Digesters

Why Did We Study Mixing Duration?

Mixing is an important parameter for anaerobic digesters for both design and operation. This is especially true for digesters that treat diary manure, which is a mixture of feces, urine, blood, food wastes, and bedding. Many of the solids fed to the digester have no or low degradability, and some of the large fibers can clog pumps or pipes in the transfer system. Mixing also plays an important role in maintaining a uniform environment for biological processes. However, the energy input for operating mechanical mixers requires a large part of the total energy for a biogas plant. Previous studies have suggested that optimum biogas production does not require continuous active mixing. It is essential to evaluate the mixing duration in order to balance energy inputs and biogas production rates.

What Did We Do?

The study was designed as a one factor (mixing duration) experiment with three levels and was carried out in triplicates. Three pilot scale anaerobic digesters were used to compare the impact of different mixing duration (continuous – CON, intermittent – INT, and no mixing – NO) on the performance of biogas production from dairy manure, Figure 1.

Figure 1. a laboratory scale digester

Figure 1: Digester Design

During the experiment, all digesters were fed 8 kg of dairy manure daily (total solid content = 5.5%), and operated at 37oC with a hydraulic retention time (HRT) of 21 days.  The mixing and feeding were controlled by an automated computer system, Figure 2. Mixers were set at a speed of 450 revolution per min (RPM). Biogas production was recorded three times a week and the percentage of methane content was analyzed weekly. Manure samples were collected weekly from the feed tank, top, middle & bottom of the digester, and from the effluent tank for volatile solids (VS), chemical oxygen demand (COD), pH, carbon to nitrogen ratio (C/N), and violate fatty acids (VFAs).  The concentration of archea and five common methanogens found in digesters was also determined using quantitative polymerase chain reaction (qPCR).

Figure 2. A closeup of the top of the lab-scale digester

Figure 2: Digester Mixing System

What Have We Learned?

The cumulative biogas production from digesters with no mixing was statistically significantly higher than intermittent and continuous (CON) mixing at the same loading rate. However, CON had a higher methane production rate per kg of volatile solids destroyed, Figure 3.

Figure 3: Methane Production Rate per Volatile Solids Destroyed

Future Plans

We are currently exploring the settling of manure solids in digesters with different mixing duration through a tracer study. The impact of different settling time on the hydraulic retention time (HRT), would help us better understand the performance of the anaerobic digesters with different mixing duration.

Authors

Hui Wang, Graduate Research Assistant, University of Wisconsin-Madison, hwang355@wisc.edu

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

Acknowledgements

This material is based upon work supported by the National Institute of Food and Agriculture, United States Department of Agriculture, under ID number WIS01693.

Institute for Environmentally Integrated Dairy Management (Marshfield, WI), United States Dairy Forage Research Center

The authors are solely responsible for the content of these proceedings. The technical information does not necessarily reflect the official position of the sponsoring agencies or institutions represented by planning committee members, and inclusion and distribution herein does not constitute an endorsement of views expressed by the same. Printed materials included herein are not refereed publications. Citations should appear as follows. EXAMPLE: Authors. 2015. Title of presentation. Waste to Worth: Spreading Science and Solutions. Seattle, WA. March 31-April 3, 2015. URL of this page. Accessed on: today’s date.

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.

Acknowledgements

Wisconsin Discovery Farms

Manure Irrigation: Airborne Pathogen Transport and Assessment of Technology Use in Wisconsin

This presentation will outline the completed research on manure irrigation pathogen transport including field data, transport models, and a quantitative microbial risk assessment.  Details will also be provided on the workgroup recommendations for use of this technology in Wisconsin.

Why Study Irrigation of Manure?  

Manure irrigation is of increasing interest to producers in Wisconsin as it allows for multiple application of manure throughout the growing season. This can reduce application costs while providing nutrients to a growing crop as opposed to a single manure application in the spring or fall. With increasing interest and potential for practice expansion many communities were concerned with the potential human health (pathogens), odor, and environmental issues associated with the practice.

What did we do?

The University of Wisconsin-Extension formed an 18 person workgroup representing many stakeholders and experts to review the practice of manure irrigation for impacts to odor, water quality, air quality, and human health among others. The workgroup developed recommendations for the practice which will be available in early May 2015 at http://fyi.uwex.edu/manureirrigation/. In addition, a research team evaluated manure pathogen drift in the field to assess concentrations at increasing distance away from the source. These results were used to develop an air dispersion model as well as develop a quantitative risk assessment. These models and assessment were used to evaluated practice recommendations and to determine if there are reasonable setback distances which reduce risk to a level deemed acceptable by the workgroup.

What have we learned?        

There are a number of concerns and benefits that may be realized when using manure irrigation. There may be scenarios in which manure irrigation is a beneficial practice, but there may be locations in which it is not suitable due to sensitive environmental factors or proximity to neighbors. Like many manure system components management of the system is key, and if improperly manged can lead to negative impacts. Detailed recommendations of the workgroup will be available in May 2015.

Future Plans

The workgroup intends to complete the report by May 2015 to be made available to interested parties on the webpage. The research team is currently evaluating expanding the measurement of pathogens to other areas of the farm and additional land application techniques for comparison.

Authors

Dr. Becky Larson, University of Wisconsin, ralarson2@wisc.edu

Susan Spencer, Tucker Burch, Yifan Liang, Chris Choi

Additional information            

http://fyi.uwex.edu/manureirrigation/

Acknowledgements      

Wisconsin Department of Natural Resources and the USDA ARS in Marshfield, Wisconsin.

The authors are solely responsible for the content of these proceedings. The technical information does not necessarily reflect the official position of the sponsoring agencies or institutions represented by planning committee members, and inclusion and distribution herein does not constitute an endorsement of views expressed by the same. Printed materials included herein are not refereed publications. Citations should appear as follows. EXAMPLE: Authors. 2015. Title of presentation. Waste to Worth: Spreading Science and Solutions. Seattle, WA. March 31-April 3, 2015. URL of this page. Accessed on: today’s date.

Anaerobic Digestion: Co-Digestion and Operational Issues

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Abstract

A study was conducted to assess the performance of various mixing regimes on methanogen biomass content in anaerobic digesters.  Methane production in anaerobic digesters is directly related to the methanogens within the system.  Current systems involve mixing to increase biogas production and system efficiency, however little is known about the underlying mechanisms of this relationship.  In this study three pilot scale anaerobic digestion systems with three different mixing regimes were run with replication to examine the impacts to methanogen biomass content and biogas production.  The results will provide insight for operational recommendations as well as the basic microbial processes with digestion systems which are critical for optimization.

Authors

Rebecca Larson, University of Wisconsin-Madison            ralarson2@wisc.edu

Purpose

To evaluate various feedstocks and operational parameters for anaerobic digesters, including impacts to biogas production, quality, and operational issues.

What Did We Do?

Evaluated numerous co-feedstocks with manure in laboratory and large scale systems to identify biogas production impacts and potential operational issues associated with each.

What Have We Learned?

Analysis of ffedstocks is critical for determination of digester fundtioning.  Constituents can significantly impact the quantity and quantity of biogas produced.

Future Plans

To evaluate scale up to determine if small scale biomethane potential analyses can be used to determine full scale biogas production.

Authors

Rebecca Larson, Assistant Professor, University of Wisconsin – Madison

Corresponding author email address   ralarson2@wisc.edu

Asli Ozkaynak, Post-Doctoral Researcher, University of Wisconsin – Madison

Additional Information

Data is to be published

Acknowledgements

 

Funded by the USDA

 

The authors are solely responsible for the content of these proceedings. The technical information does not necessarily reflect the official position of the sponsoring agencies or institutions represented by planning committee members, and inclusion and distribution herein does not constitute an endorsement of views expressed by the same. Printed materials included herein are not refereed publications. Citations should appear as follows. EXAMPLE: Authors. 2013. Title of presentation. Waste to Worth: Spreading Science and Solutions. Denver, CO. April 1-5, 2013. URL of this page. Accessed on: today’s date.