Anaerobic Co-digestion of Agro-industrial Feedstocks to Supplement Biogas Produced from Livestock Manure


Anaerobic digestion (AD) is commonly used in agriculture to break down livestock manure and produce a sustainable source of energy by producing biogas, which is predominantly methane. Digestion of livestock manure can be supplemented with additional agricultural or industrial organic waste, potentially adding sources of revenue to the farm or digestion facility through tipping fees and additional biogas production. However, quantifying the anticipated impact on digester performance and operation is challenging, particularly as some potential feedstocks have not been studied previously. Understanding how a feedstock might impact a digester’s performance is critical, as digester upsets can lead to loss of revenue or even digester failure.

What Did We Do?

We conducted a set of mono-digestion biomethane potential experiments of several feedstocks currently in use at an agricultural AD facility that accepts mixed industrial waste streams in addition to digesting beef manure. The mono-digestion studies used triplicate 1-L working volume batch digesters which ran for 30-38 days. We tested beef manure, off-spec starch from food manufacturing, slaughterhouse wastewater treatment sludge, waste activated sludge from a corn processing facility, soap stock from glycerin refining, filter press slurry from a food grade water treatment facility, and food waste dissolved air flotation sludge. We also included a treatment for the effluent from the digester’s ammonia recovery system and a mixture of all the feedstocks at the same time. A blank (inoculum only) and positive control (cellulose with inoculum) digester were included as controls. This set of studies is described here as Experiment 1 (E1).

We then conducted a set of co-digestion biomethane potential tests combining the manure pairwise with some of the industrial feedstocks, specifically starch, slaughterhouse waste, soap stock, and filter press slurry (Experiment 2 or E2). These combinations were made at two different ratios of the two feedstocks. The first set of treatments combined the manure and an additional substrate at a 1:1 ratio on a volatile solids basis. The second set of treatments combined the feedstocks proportional to the amounts commonly used in the AD facility providing the materials. A final treatment pairing starch and soap stock at a 3:1 ratio was also included. These co-digestion treatments were conducted in triplicate alongside a single mono-digestion treatment of each feedstock for comparison. Finally, we examined the potential synergistic or antagonistic impacts of these combinations on methane yield and production rate. This was done by comparing the measured methane production at each time point compared to the expected methane production if the feedstocks each contributed additively to the methane production.

What Have We Learned?

Figure 1 shows the cumulative specific biogas production on a volatile solids basis for the mono-digestion experiment (E1). Some feedstocks, such as soap stock and slaughterhouse waste, experienced a substantial lag phase at the beginning of the experiment, which may have been due to the high levels of lipids and proteins.

Figure 1: Average biogas production of all treatments during mono-digestion experiment (Experiment 1).

During the co-digestion experiment (E2), we observed both total yield and kinetic synergy in all treatments. Only two digesters (one of the replicates from the starch and manure proportional treatment and one from the starch and soap stock treatment) produced substantially less (<30%) methane than would be expected for an additive effect for more than one day. This effect can be seen in Figure 2, which shows the cumulative methane curves (corrected for inoculum contribution and averaged over the three replicates) of the mono-digestion digesters for manure and starch individually and the curves for both co-digestion treatments using both manure and starch. Figure 3 shows the same curves for the co-digestion of manure and slaughterhouse waste. These co-digestion treatments show that combining the feedstocks causes an increase in methane production at a faster rate. They also show that co-digestion alleviates the lag phase experienced by the slaughterhouse waste.

Figure 2: Cumulative specific methane production for manure (F1) and starch (F2). F1 + F2 Eq = 1:1 ratio of VS; F1 + F2 Pr = ratio of VS is proportional to what full-scale digester receives.
Figure 3: Cumulative specific methane production for manure (F1) and slaughterhouse waste (F3). F1 + F3 Eq = 1:1 ratio of VS; F1 + F3 Pr = ratio of VS is proportional to what full-scale digester receives.

Future Plans

We plan to continue exploring the impact of co-digestion on methane yield and production rate by using additional combinations of these feedstocks and exploring the impact of macromolecular composition (percentages of carbohydrates, proteins, and lipids) on synergistic effects. These results will help inform current or future agricultural AD operators regarding the use of co-digestion feedstocks for optimal energy production and best practices in selecting new feedstocks for co-digestion.


Jennifer Rackliffe, Graduate Research Fellow, Purdue University

Corresponding author email address

Additional authors

Dr. Ji-Qin Ni, Professor, Purdue University; Dr. Nathan Mosier, Professor, Purdue University

Additional Information:


This material is based upon work that is supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under agreement number 2020-38640-31522 through the North Central Region SARE program under project number GNC21-334. USDA is an equal opportunity employer and service provider. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture. We also thank Purdue’s Institute for Climate, Environment and Sustainability for supporting the dissemination of this work. Finally, we acknowledge the assistance of Gabrielle Koel, Kyra Keenan, Amanda Pisarczyk, and Emily McGlothlin in conducting the laboratory work.


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. 2022. Title of presentation. Waste to Worth. Oregon, OH. April 18-22, 2022. URL of this page. Accessed on: today’s date.

Greenhouse gas impacts resulting from co-digestion of dairy manure with community substrates


The US Dairy industry established a voluntary environmental stewardship goal to achieve greenhouse gas (GHG) neutrality by 2050 among farmers and processors collectively. Manure management and enteric emissions combined account for approximately 70% of the GHG footprint of the US dairy industry, with nearly equal contributions from each (Thoma, 2013). There are multiple manure management systems used by dairy farmers in the Northeast and Upper Midwest that substantially impact GHG emissions. Quantification of GHG emissions for different manure management systems is necessary to compare options and strategies that can be applied to reduce GHG, especially methane, to move toward sustainability and reach the targets set by industry and governments.

Methane is the primary GHG emitted from the long-term storage of dairy manure, a water quality best management practice employed by many dairy farms today. Landfills are also a significant source of methane emission primarily due to degradation of organic waste, notably pre- and post-consumer food wastes (community substrates). Methane is a highly potent GHG that impacts warming by 25 – 28 times as much as carbon dioxide (CO2) on a 100-year global warming potential (GWP) time scale (US EPA). However, because methane has a lifespan in the atmosphere of around 12 years, it has been accounted for on a 20-year GWP scale (84 times the impact of CO2) by the State of New York (Climate Leadership and Community Protection Act). Manure management systems that substantially reduce methane, such as the co-digestion of manure with food waste, can achieve significant reductions of the GHG emissions associated with milk production.

What Did We Do?

The GHG emissions resulting from the anaerobic co-digestion of raw dairy manure and community substrate (i.e., food processing waste mixture diverted from landfilling) in an equal mass of each (total mass basis) were calculated as part of a larger study comparing eight different manure management systems. The community substrate was modeled as 50% ice cream and 50% dog food by mass. Methane and nitrous oxide emissions were calculated with equations that use the mass flow of volatile solids (VS) and nitrogen through the co-digestion manure management system that included digestate solid-liquid separation using a screw press and the long-term storage of separated liquid. Carbon dioxide and methane associated with system energy use and energy production as pipeline-quality renewable natural gas (RNG), as well as landfill organics diversion were also calculated. The parasitic energy use (heat and electricity) of the digester and related manure management and biogas upgrading equipment was supplied on an average annual load basis by a portion of the biogas produced. The total net GHGs were summed using a CO2-equivalent (CO2e) methodology (both GWP100 and GWP20 were computed) and normalized on a per lactating cow per year basis. A sensitivity analysis of eleven variables was conducted to quantify the impact of each on the net GHG result.

What Have We Learned?

The co-digestion system net annual GHG impact was calculated to be −16 metric tons (MT) CO2e cow-1 (GWP100) and −43 MT CO2e cow-1 (GWP20). For the co-digestion mixture analyzed (50% liquid dairy manure, 25% ice cream, and 25% dog food), the anaerobic digester biogas production was 4 times greater than the biogas production for manure alone (on a per lactating cow basis). This significant energy production potential contributed an offset of 3.9 MT CO2 cow-1 year-1, assuming the net RNG after supplying the system’s parasitic energy usage displaced the CO2 emissions from combusting approximately 380 gallons of diesel. In comparison, a methane leakage (or loss) of 2% from the digester to RNG system was equivalent to 18% of the energy offset at GWP100 (0.7 MT CO2e cow-1 year-1) and 62% at GWP20 (2.4 MT CO2e cow-1 year-1). Despite the greater contribution of methane leakage at GWP20 on a CO2e basis, the methane offset from landfilling the community substrate also substantially increased, resulting in just a 5 – 6% increase in the net annual GHG (remaining net negative) when methane leakage was varied from 1 to 3% under both GWP time scales. The methane leakage amount was also the most sensitive variable studied for the co-digestion system and the relatively low impact on total net GHG indicates the effectiveness of this type of manure management system as a tool to reach net GHG neutrality.

Future Plans

A next step in the assessment of co-digestion of dairy manure and food waste diverted from landfills is to continue improvement of our Cornell Dairy Digester Simulation Tool that predicts biogas production from a variety of food wastes combined in different quantities with dairy manure. This tool will also allow for the economic feasibility analysis of different co-digestion system sizes and substrate mixtures, inclusive of tipping fee variation and energy generation options (electricity and RNG) and associated values. This work will help farmers assess the feasibility of implementing or participating in a co-digestion system for manure management.

In future work contingent on funding, we plan to conduct comprehensive field measurements of methane emissions from the long-term storage of raw manure, separated manure liquid, and digested effluent. The equations that calculate methane are gross and depend on volatile solid content and degradability of the stored material, as well as temperature and retention time. Verification of these equations and inputs will give more confidence in utilizing bottom-up calculations of GHGs from manure management practices.


Lauren Ray, Extension Support Specialist III, Cornell PRO-DAIRY Dairy Environmental Systems Program

Corresponding author email address

Additional authors

Curt A. Gooch, Sustainable Dairy Product Owner, Land O’Lakes – Truterra; Peter E. Wright, Extension Associate, Cornell PRO-DAIRY Dairy Environmental Systems Program

Additional Information

More information on related work can be found on the Cornell University PRO-DAIRY website under Environmental Systems:

Thoma, G., J. Popp, D. Shonnard, D. Nutter, M. Matlock, R. Ulrich, W. Kellogg, D. S. Kim, Z. Neiderman, N. Kemper, F. Adom, and C. East. (2013). Regional analysis of greenhouse gas emissions from USA dairy farms: A cradle to farm-gate assessment of the American dairy industry circa 2008. Int. Dairy J. 31:S29–S40.

US EPA, Accessed 2/24/2022.

Climate Leadership and Community Protection Act. 2020. New York State Senate Bill S6599.


The Coalition for Renewable Natural Gas and the New York State Department of Agriculture and Markets provided a portion of the financial resources to support the development of this work.


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. 2022. Title of presentation. Waste to Worth. Oregon, OH. April 18-22, 2022. URL of this page. Accessed on: today’s date.

Nitrogen and Phosphorus Cycling Efficiency in US Food Supply Chains – A National Mass-Balance Approach

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Assessing and improving the sustainability of livestock production systems is essential to secure future food production. Crop-livestock production systems continue to impact nitrogen (N) and phosphorus (P) cycles with repercussions for human health (e.g. secondary particle formation due to ammonia emission and drinking water contamination by nitrate) and the environment (e.g. eutrophication of lakes and coastal waters and exacerbation of hypoxic zones). Additionally, P is a limited resource, and sustaining an adequate P supply is a major emerging challenge. To develop strategies for a more sustainable use of N and P, it is essential to have a quantitative understanding of the flows and stocks of N and P within the society. In this study, we developed detailed national N and P budgets to assess nutrient cycling efficiency in US (livestock) food supply chains, to identify hotspots of nutrient loss and to indicate opportunities for improvement!

What did we do? 

1. National nutrient mass-balance

A mass-balance framework was developed to quantify nutrient flows within the US. In this framework, the national US system is represented by 9 major sectors are relevant in terms of nutrient flows: mining (relevant for P only), industrial production, agriculture, food & feed processing industry, retail, households and other consumers, energy and transport, humans, and waste treatment. These sectors can exist of several sub-sectors. For example, the agricultural sector consists of several secondary sub-systems including pasture, agricultural soil, livestock and manure management (WMS – waste management system).

Different livestock categories can have distinct environmental impacts and nutrient use efficiencies (e.g. (Hou et al. 2016), (Eshel et al. 2014), (Herrero et al. 2013)), we therefore distinguish six livestock categories (dairy cattle, beef cattle, poultry (meat), poultry (layers), sheep, hogs) and

 their associated food commodities (dairy products, beef from dairy cattle, beef, poultry, eggs, lamb, pork).

For each sub-system, we identify and quantify major flows to and from this compartment. All flows are expressed in a common unit, i.e. metric kiloton N per year (kt N/yr) for nitrogen and metric kiloton P per year (kt P/yr) for phosphorus. Quantified flows include nutrient related emissions to the environment and waste flows.

At present, the waste sectors and environmental compartment are outside the system boundaries, that is, we quantify flows to these compartments, but we do not attempt to balance these sectors. We do, however, keep track of the exact chemical species (e.g. emission of N2O-N to air instead of N to air) emitted as far as possible. The municipal waste treatment (MSW) and municipal waste water treatment (WWTP) are treated in more detail: major flows from and to these compartments are quantified. These sub-sectors are treated in more detail because of their role in nutrient recycling through e.g. sewage sludge application on agricultural soils.

Data were collected in priority from national statistics (e.g. USDA NASS for livestock population) and peer-reviewed literature, and were supplemented with information from industrial reports and extension files if needed. If available, data were collected for the years 2009 to 2012 and averaged, when unavailable, we collected data for the closest year.

2. Scenario analysis

In the scenario analysis, we test the opportunity for dairy livestock production systems to contribute to a more efficient nutrient use through anaerobic co-digestion of dairy manure and organic food waste. Recently, Informa Economics assessed the national

 market potential of anaerobic digester products for the dairy industry (Informa Economics 2013). Next to a reduction in greenhouse gas emissions, anaerobic co-digestion of dairy manure and organic food waste can contribute to improve nutrient cycling efficiency (Informa Economics 2013). Dairy manure contains high levels of nitrogen and phosphorus, which can be used as a natural crop fertilizer, if recuperated from manure. Presently, non-farm organic substrates such as food waste are typically disposed of in landfills, which causes greenhouse gas (GHG) emissions and also results in a permanent removal of valuable nutrients from the food supply chain (Informa Economics 2013). By anaerobic co-digestion, a part of the nutrien! ts contai ned in dairy manure and food waste can be recovered. These nutrients can be used to fertilize crops and substitute synthetic fertilizer application. In the scenario analysis, we test to what extent anaerobic co-digestion of dairy manure and food waste can contribute to improve nutrient cycling efficiency, particularly by substituting synthetic fertilizers. We develop the scenario based on data provided in the InformaEconomics report.

What have we learned? 

In general, our results show that livestock production is the least efficient part of the total food supply chain with large losses associated with manure management and manure and fertilizer application to crops. In absolute terms, the contribution of the household stage to total and N and P losses from the system is small, approximately 5 and 7% for N and P, respectively. However, households ‘waste’ a relatively large percentage of purchased products, (e.g. 16% and 18% of N and P in dairy products end up as food waste), which presents an opportunity for improvement. A scenario was developed to test to what extent anaerobic co-digestion of dairy manure and food waste can contribute to improving nutrient cycling efficiency on a national scale. Results suggest that 22% and 63% of N and P applied as synthetic fertilizer could potentially be avoided in dairy food supply chains by large scale implementation of anaerobic co-digestion o! f manure and food waste.

Future Plans     

Future research plans include a further development of scenarios that are known to reduce nutrient losses at the farm scale and to assess the impact of these scenarios on national nutrient flows and losses.

Corresponding author, title, and affiliation        

Karin Veltman, PhD, University of Michigan, Ann Arbor

Corresponding author email

Other authors    

Carolyn Mattick, Phd, Olivier Jolliet, Prof., Andrew Henderson, PhD.

Additional information                

Additional information can be obtained from the corresponding author: Karin Veltman,


The authors wish to thank Ying Wang for her scientific support, particularly for her contribution in developing the anaerobic co-digestion scenario.

This work was financially supported by the US Dairy Research Institute.


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. 2017. Title of presentation. Waste to Worth: Spreading Science and Solutions. Cary, NC. April 18-21, 2017. URL of this page. Accessed on: today’s date.