Co-Digestion: A Primer on Substrate Utilization and Project Considerations

Why Study Co-Digestion?

An overwhelming percentage of farm-based, anaerobic digestion projects practice co-digestion for improved business models that result from revenues enhanced by tipping fees and extra biogas production. This presentation utilizes over a decade of research and practical experience available within the Pacific Northwest regarding co-digestion, highlighting its benefits, potential pitfalls, and project considerations. Throughout, specific industry examples, made available through a scientific survey of experts, are used to relay information.

What did we do?

To provide an insider’s look at design and management considerations, five individuals with extensive experience in co-digestion at dairy digesters were interviewed. Interviewees included a project developer who has successfully implemented co-digestion at a number of dairy digesters, a dairy farmer who owns and operates a co-digestion project, a scientist with in-depth knowledge of AD and co-digestion, and two system engineers who have designed numerous digesters. The sample size was relatively small, but few individuals have technical expertise in co-digestion in the US, and not all individuals with expertise were willing to be interviewed. Several of these individuals work primarily in the Pacific Northwest where authors are located; however, to the extent possible, individuals with broader experience throughout the US were included.

costs and revenue streams for codigestion compared to baseline manure only digestion

Figure 1. Costs and revenue streams for codigestion compared to baseline manure only digestion

What have we learned?

Co-digestion can provide a significant economic boost to AD operations at dairies. However, after talking with numerous experts in the field of co-digestion, it is clear that careful consideration and planning is required to successfully incorporate substrates. Substrates should be chosen to complement existing waste streams, and should be carefully screened to avoid inhibition. In most cases, the selection of a substrate will be limited by location and volume attainable, and project developers may need to invest considerable time and effort into developing and maintaining the necessary relationships for acquiring substrates. Regulatory restrictions and nutrient management implications are also important. A solid understanding of these issues can contribute to successful implementation of co-digestion.

Successful co-digestion depends on multiple factors including but not limited to type of substrate, hauling costs, location of digester compared to substrate, local substrate competition, tipping fees, and nutrients. Before beginning co-digestion, developers need to first determine whether co-digestion makes economic sense at a particular dairy operation. Otherwise, co-digestion may turn into an economic burden for project developers that are already economically strained by high AD capital costs and low received electrical rates. If a sound business plan is developed and implemented, co-digestion can provide additional profit to project owners.

Future Plans

In the US, most post-consumer food scrap recycling is currently achieved via composting. For example, in western Washington State, many residents of Seattle and King County have their food scraps recycled along with yard waste into saleable compost. While this effectively diverts food scraps from landfilling, AD could capture the energy within food scraps and use it to replace fossil-derived energy, providing additional benefits. When linked with nutrient recovery, the process could also produce saleable fertilizers. If dairy farmers are located near post-consumer food scrap sources, they may be able to position themselves well as an environmentally conscious (lower odor production) and less expensive (shorter hauling distances and lower tipping fees) recycling option.

Existing barriers to co-digestion of post-consumer food wastes include current regulations excluding these wastes from AD, and the extensive pretreatment required so that these wastes could be viably fed to digesters. However, if solutions to these issues could be found, it could be a win-win scenario for food waste diversion and AD projects looking to remain viable.

Authors

Jingwei Ma, Research Associate at Washington State University mjw@wsu.edu

Nick Kennedy, Associate in Research at Washington State University, Georgine Yorgey, Research Associate at Washington State University, Chad Kruger, Director of CSANR, Craig Frear, Assistant Professor at Washington State University

Additional information

https://pubs.extension.wsu.edu/considerations-for-incorporating-codigestion-on-dairy-farms

Acknowledgements

This research was supported by funding from USDA National Institute of Food and Agriculture, Contract #2012-6800219814; National Resources Conservation Service, Conservation Innovation Grants #69-3A75-10-152; Biomass Research Funds from the WSU Agricultural Research Center; and the Washington State Department of Ecology, Waste 2 Resources Program.

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 a Continuously-Mixed Farm-Based Anaerobic Co-Digestion System Following the U.S. EPA Protocol for Quantifying and Reporting on the Performance of Anaerobic Digestion Systems for Livestock Manures – Final Project Results

This paper compliments another paper proposed for this conference “Lifecycle analysis of greenhouse gas (GHG) emissions from a New York State dairy farm anaerobically co-digesting manure and food waste.”

Purpose 

New York State’s largest manure-based anaerobic co-digestion facility was evaluated continuously for a 2-year period following the U.S. EPA Protocol for quantifying and reporting on the performance of anaerobic digestion systems for livestock manures. Overall, we assessed and determined the system’s performance with respect to the: 1) conversion of biomass to biogas, 2) conversion of biogas to useful energy, and 3) system’s economics. The information developed by this project can be used to compare performance information developed from other manure-based anaerobic digestion systems. Related: Treatment Technologies for Livestock Manure

What did we do? 

After initial system evaluation and monitoring plan development, the farm was visited monthly for 24 months to collect data. In addition to the digester influent and effluent samples taken during each monthly sampling date, on-site measurements were taken and data were manually recorded from equipment and plant logs. A particularly important log, were the imported feedstocks brought on-site for inclusion to the AD. This log recorded the date and time feedstock was delivered, the type of feedstock, and the volume delivered. The specific data collected/measured are shown in Table 1.

Table 1. Data collected/measured on-site at each sampling date.

Item
1. Date and time of readings
2. Methane (CH4), Carbon dioxide (CO2), Oxygen (O2), and Hydrogen sulfide (H2S) concentrations in biogas after digester
3. CH4, CO2, O2, and H2S concentrations in biogas after bio-scrubber
4. Engine-generator set run time
5. Cumulative electricity purchased and sold
6. Daily animal populations since previous sampling event
7. Logs of imported feedstocks
8. Problems occurred during period

Further, data (Table 2) from the system’s supervisory, control, and data acquisition (SCADA) unit were downloaded, compiled and analyzed for each period. SCADA data were generated from an array of sensors and meters originally installed by the company that designed and built the digester, i.e., Bigadan A/S.

Table 2. Data obtained from the SCADA system for each period.

Parameter

1. Total influent to pasteurization
2. Food waste to pasteurization
3. Manure to pasteurization
4. Biomass from pasteurization to digester
5. Effluent digester to storage tank
6. Biogas production digester
7. Biogas to generator
8. Generator electrical energy output
9. Generator thermal energy recovered
10. Digester vessel upper temperature
11. Digester vessel lower temperature

Overall, digester influent and effluent samples were collected with the goal of obtaining representative samples. To do this, grab samples were collected directly from both the digester influent and effluent lines over a period of approximately 30 min during a pumping sequence, to develop a 5-gallon composite, master-sample. The entire volume of this sample was then agitated using a paint mixer powered by a portable electric drill until visibly determined to be homogenized. A 1-liter composite sample was immediately taken and stored on ice, and subsequently frozen before being sent for laboratory analysis. Samples were taken in this fashion approximately every 30 days over the 24-month monitoring period. Additionally, samples coming from the raw manure receiving tank and from the combined imported feedstocks tank were also obtained for two sampling dates at the beginning of the monitoring project to characterize the individual influent streams to the digester.

All samples collected during the 24-month monitoring period were sent for analysis to Certified Environmental Services’ (CES) laboratory in Syracuse, NY, approved by the New York State Department of Health, Environmental Laboratory Approval Program (NYSDOH-ELAP #11246). All samples were analyzed in triplicate for: total solids (TS), total volatile solids (VS), chemical oxygen demand (COD), pH, and total volatile acids as acetic acid (TVFA). In addition, the following nutrients were determined in triplicate: total phosphorus (TP), ortho-phosphorus (OP), total Kjeldahl nitrogen (TKN), ammonia-nitrogen (NH3-N) and potassium (K). CES followed the appropriate testing methods outlined in Table 3 for each parameter measured.

Table 3. Standard analytical methods used by CES laboratory for sample analyses.

Parameter Standard
Total Solids (TS) EPA 160.3
Total Volatile Solids (VS) EPA 160.4
Fixed Solids (FS) EPA 160.4
Volatile Acid as Acetic Acid (TVFA) SM18 5560C
Chemical Oxygen Demand (COD) SM18 5220B
pH SW846 9045
Total Kjeldahl Nitrogen (TKN) EPA 351.4
Ammonia-Nitrogen (NH3-N) SM18 4500F
Organic-Nitrogen (ON) By subtraction: TKN – NH3-N
Total Phosphorous (TP) EPA 365.3
Ortho Phosphorous (OP) EPA 365.3
Total Potassium (K) EPA SW 846 6010

Methane (CH4), carbon dioxide (CO2), hydrogen sulfide (H2S), and oxygen (O2) concentration in biogas, were measured on-site during monthly visits using a Multitec 540 (Sewerin GmbH, Germany), a portable hand-held gas measuring device equipped with infra-red/electrochemical sensors.

What have we learned? 

For the entire monitoring project, an average of 1,891±62 lactating cows per day from Synergy Dairy contributed manure to the digester. The average daily loading rate of the digester was 80,408±19,266 gal, where the average percent of imported waste (mostly food-grade residues) co-digested with manure was 25±6% on a volume-to-volume (v/v) basis. The average reduction of organic matter thru the monitoring project was 42% with respect to the influent, while 75% of the odor-causing volatile fatty acids were reduced. In comparison, a previous monitoring study reported by the authors in five manure-based co-digestion operations showed a reduction in organic matter and volatile acids between 36% and 53% and 85% and 91%, respectively. The average daily digester biogas production for the entire monitoring project was 495±78 ft3 per 1,000 lbs of total influent added to the digester, or 173±34 ft3 per cow contribut ing to the digester. The engine-generator set produced an average of 23±7 MWh of electricity per day, from which the average daily parasitic load of the AD system was 3±1 MWh, accounting for approximately 14% of the electricity generated by the plant. Overall, the average capacity factor and online efficiency of the anaerobic digester system during the entire monitoring project were 0.66±0.22 and 80±23%, respectively. The electrical energy generated translated into an overall thermal conversion efficiency of 42±4%. Also, an additional 13±5% of the total energy in the biogas was recovered by the engine as hot water. Thus, an overall 55% (electrical + thermal) of the total energy contained in the input biogas was recovered by the engine-generator set during the monitoring project.

The majority of the challenges experienced by the Synergy AD system were of mechanical origin, whereas 20% were related to the biological process; only 8% of the downtime was due to scheduled systems maintenance. Some of the problems were related to the extreme cold conditions experienced in the Northeast during the period from December 2013 to February 2014. According to NOAA’s National Climatic Data Center, this period was the 34th coldest for the contiguous 48 states since modern records began in 1895, with an average temperature of 31.3F, 1.0F below the 20th century average (NOAA, 2014).

Future Plans 

This manure-based anaerobic digester is the 8th New York State digester we have extensively monitored and reported on. Near-term future planned work includes monitoring a lower cost horizontal plug flow digester on a 2,000-cow farm. This digester uses high density polyethylene (HDPE) material heat welded together as the digester vessel.

Authors

Curt Gooch, Senior Extension Associate, Cornell PRO-DAIRY Program cag26@cornell.edu

Rodrigo Labatut

Additional information 

A full report, written for the project sponsor, can be found on the Cornell PRO-DAIRY dairy environmental systems website, https://prodairy.cals.cornell.edu/environmental-systems/.

Acknowledgements

First and foremost, we wish to thank the Synergy Dairy Farm, Synergy Biogas, and CH4 Biogas for their collaborative efforts that made this project possible. We also like to thank the project sponsor, the Wyoming County (New York) Industrial Development Agency.

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.

Lifecycle greenhouse gas (GHG) analysis of an Anaerobic Co-digestion Facility Processing Dairy Manure and Industrial Food Waste in NY State

While the theoretical benefits of anaerobic digestion have been documented, few studies have utilized data from commercial-scale digesters to quantify impacts.  Previous studies have analyzed a range of empirical studies to constuct emission factors for a generic European AD plant processing source separated municipal solid waste.  However, most U.S. studies have applied reporting protocols and have been based upon theoretical assumptions.  Furthermore, GHG analyses of U.S. co-digestion facilities are limited to one scenario in protocol based analysis of community digester options. 

Purpose          

We are not aware of any peer-reviewed studies of US anaerobic co-digestion. Several case studies have presented calculations of impacts using GHG reporting protocols, however significant portions of the lifecycle have been neglected such as the feedstock reference case emissions, digestate storage emissions and fertilizer displacement impacts. Furthermore, they have often been modeled using general theoretical assumptions such as number of cows rather than empirical data on feedstock volume and characteristics and digester operation.

What did we do? 

A lifecycle GHG analysis was performed based upon data reported on a farm-based anaerobic co-digestion system in New York State, resulting in an 71% reduction in GHG impact relative to conventional treatment of manure and food waste.

The objective of this study was to provide a comprehensive analysis of GHG emissions based upon a NYS digester that co-digests manure and industrial-sourced food waste. Empirical data on feedstock (t-km transport, avoided disposal, TS, VS, TKN), digester operation (m3CH4, KWh, exhaust emissions) and effluent properties (TS,VS,TKN) were combined with regional parameters (i.e., climate, soil type and management practices) to represent a state-of-the-art, anaerobic co-digestion facility in NYS. This data was combined with information collected through interviews in order to model a reference case, representing the business-as-usual food waste disposal and manure management practices en lieu of the anaerobic co-digestion system.

What have we learned? 

Displacement of grid electricity provided the largest benefit followed by avoidance of food waste landfill emissions and reduced impacts associated with storage of digestate vs. undigested manure. Nominal land application N2O emissions were offset by inorganic fertilizer displacement and carbon sequestration in both cases. The higher volume of digestate increased net land application emissions as did increased transportation distance to the fields and lower carbon sequestration. Digestate is a by-product of the co-digestion process and its treatment must be considered in an LCA. Modeling of land application impacts are highly uncertain and can be significant.

The largest source of direct emissions was CH4 emissions. N2O emissions were larger in the land application phase than during storage. Direct fossil fuel emissions had a minor impact. Emissions were offset by displacement of grid electricity and fossil based fertilizers along with carbon sequestration.

Future Plans    

More empirical research is needed to measure emissions and to provide emission factors that incorporate key variables and characteristics affecting emissions. A whole system, dynamic approach is necessary to incorporate complex interdependencies between stages of farm and manure management.

Authors

Jennifer L. Pronto, Research Assistant, Cornell University jlp67@cornell.edu

Ebner, Jackie      jhe5003@rit.edu              Rochester Institute of Technology

Rodrigo A. Labatut, Matthew J. Rankin, Curt A. Gooch, Anahita A. Williamson, Thomas A. Trabold

Additional information               

www.manuremanagement.cornell.edu

Figure 1: Contributional analysis of GHG impacts for the reference and anaerobic co-digestion cases.

Figure 1: Contributional analysis of GHG impacts for the reference and anaerobic co-digestion cases.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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.

Dairy Manure Digestion Influenced by Wasted Milk from Milking Operations

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Why Study Waste Milk in Anaerobic Digestion?

Anaerobic digestion has many advantages both environmentally and economically. First, it produces renewable energy in the form of methane, a renewable energy source, which leads to a steady increase in the number of anaerobic manure digesters in the United States. According to the report from the World Dairy Expo held in Madison, Wisconsin in 2009 (Expo’09, 2009), the US dairy industry is taking the lead in adopting anaerobic technology because the majority (over 75%) of operating US manure digesters is installed on dairy farms. It is anticipated that this trend will continue as the country has determined to reduce its reliance on ever diminishing fossil-based energy resources.

Second, the technology can significantly reduce the polluting strength of the treated waste materials, such as chemical oxygen demand (COD), thus ameliorating their pollution potential to the environment when discharged. Due to the nature of dairy operations, a tangible amount of milk coming from the milking parlor wastewater is often discharged to the bulk manure, which can dramatically increase the COD level of such waste streams. The high COD content (190,000 mg/L) of milk makes the common practice of land applying the milk contaminated manure dangerous due possibly to the potential of causing severe contamination of surface and ground waters from runoff and leaching. Such practice is therefore drawing increased scrutiny from the public and environmental regulatory agencies. Fortunately, with the number of dairy producers willing to adopt anaerobic digesters on their farms continuing to grow, the concern for such pollution could be tempered.

However, a remaining question of this remedy is whether the added milk has any impact on the overall digestion process in terms of biogas production and pollutants removal.

What Did We Do?

In this project, the overall response of co-digesting dairy manure with milk added at 7 different levels, i.e., 1, 3, 5, 7, 9, 14, and 19%, using lab-scale batch anaerobic digesters was investigated. The co-digestion performance was evaluated based on total biogas volume production, methane concentration, and its volume in the biogas generated. The changes and/or reductions in COD of the treated liquid were also presented.

What Have We Learned?

  1. Cumulative Biogas Production Affected by Different Milk Content

First, increasing milk content could increase the cumulative biogas production during the operation, with the total volume of biogas produced being 5260, 5790, 6300, 7010, 7480, 8960, and 10150 mL for the milk treatments of 1, 3, 5, 7, 9, 14, and 19%, respectively, as opposed to the control (4980 mL). Second, higher milk content could significantly raise the initial biogas production rate. Third, the presence of milk appeared to have some influence on the stability of the digestion process, as evidenced by the fluctuation of biogas production at high milk concentrations. For instance, the treatments having milk content up to 7% demonstrated a similar trend. But for milk content of 9, 14, and 19%, the fluctuation in biogas production volume became progressively conspicuous. Especially for the 19% milk treatment, the biogas volume produced first jumped from 190 mL at 6 hour to 1190 mL at 12 hour after the digestion started, followed by a relatively moderate production period before it jumped again after 8 days of digestion. Considering the results from this study, it may be concluded that milk can increase biogas production when co-digested with dairy manure.

  1. Cumulative CH4 Volumes Affected by Different Milk Content

The performance of different treatments in cumulative CH4 production indicates that adding milk to dairy manure digestion will promote the volumetric production of both biogas and methane. However, the CH4 content in the produced biogas deteriorated as the milk content increased (from 66.5% for the control to 63.5% for 19% milk treatment). It can thus be inferred that although the volumes of total biogas and methane were increased by increasing the milk content in the digester, the increase in methane volume was not in tandem with that in total biogas volume, implying that a significant amount of CO2 was concurrently produced. Apparently, the presence of milk in the digestion substrate is the only legitimate cause for the increasing production of CO2. In addition, although the effect of milk on lowering the CH4 content in biogas is observed for all milk treatments, the extent of such an effect is different. The milk impact on CH4 content in biogas was not significant for manure containing milk up to 3% (v/v), but it turned significant at 5%. Summarizing the above discussions leads to an intuitive suggestion that in order to avoid production of a substantial amount of CO2 due to the spilled milk in the digestion process, dairy producers should manage to control the milk content in the digester liquid ≤ 3%.

  1. COD, TKN, and C/N Ratio Changes in Digestates From the Digestion of Dairy Manure with Milk

The added milk substantially increased the digester content COD as the amount of milk increased. However, at the end of the experiment, the final COD concentration in most digester effluent samples reached a fairly similar level, suggesting that the digestion process for the majority of the treatments was completed properly. In addition, since all the experiments were run on the same time schedule, the COD degradation efficiency obviously increased with increasing milk addition from 49.7% for the control to 77.8% for the 19% milk treatment. The improved COD removal efficiency in company with the increasing milk content could be attributed to the gradually elevated C/N ratio due to the added milk (from 5.19 for the control to 10.7 for the 19% treatment) because it is recognized that the optimum C/N ratio for anaerobic digestion is around 20/1 to 30/1, which could explain the continuous increase in COD removal as the C/N ratio increased. At the end of experiment, the effluent C/N ratio averaged 2.75, which was very close to the value for the digested dairy manure (2.83). As for TKN, the removal efficiency is almost negligible, which is the typical behavior commonly observed for anaerobic digestion, indicating that the digestion operation was carried out successfully. Based on the information obtained from this study, it may be concluded that milk content up to 19% (v/v) in dairy manure may have little negative impact on COD removal efficiency in the anaerobic digestion process.

Future Plans

Two stage digestion process to produce hydrogen and methane may be studied with milk addition.

Authors

Jun Zhu, Professor, University of Minnesota, zhuxx034@umn.edu

Wu, X., Postdoc, University of Minnesota

Dong, C., Associate Professor, Zhejiang Gongshang University, Hangzhou, China

Yao, W., Postdoc, University of Kentucky

Additional Information

Wu, X., C. Dong, W. Yao, J. Zhu. 2011. Anaerobic digestion of dairy manure influenced by the wasted milk from milking operations. Journal of Dairy Science 94(8): 3778-3786.

Acknowledgements

The authors wish to thank the Minnesota Legislature Rapid Agricultural Response Fund for providing financial support to this project.

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.