Full proceedings paper at: http://www.extension.org/72812 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.

1. Co-digestion: A primer on substrate
utilization and project considerations
Nick Kennedy, Georgine Yorgey, Chad Kruger, Craig Frear
Jingwei Ma
Center for Sustaining Agriculture and
Natural Resources
Washington State University
Photo: Andgar

2. Outlines
1. Introduction
2. Substrate considerations
3. Capital cost considerations
4. Operational cost considerations
5. Revenue considerations
Photo: DVO

3. Introduction
Photo: Jim Jensen

4. Anaerobic Digestion
• Remove 90% of odors1.
• Destroy 99% or >2.0 log of
indicator pathogens in
manure2.
• Reduce GHG emissions by
3-4 MT CO2eq/cow year
compared to baseline2.
• Produce renewable energy
at a rate of 0.25 KW/cow/yr
or 167 DGE/cow/yr2.
• Stabilize volatiles, reduce
solids, separate fiber, shift
nutrients towards inorganic
form2.1 Wilkie 2000 2 Frear et al, 2011
4

5. Advantage of co-digestion
• Improved overall
process economics
(higher biogas
production, tipping
fees, renewable
credits)
• GHG emissions
reductions
• Improved fertilizer
value of the digestate
• Synergistic effects
leading to improved
carbon to nitrogen
(C/N) ratio, nutrient
balances, and other
parameters

6. Disadvantage of co-digestion
• Biological inhibition and
process upsets occur from
constituents within the co-
digestion waste
• Inorganic material impact
the digester performance
via clogging
• Excessive scum from
flotation and mixing
problems due to FOG
• Higher production of biogas
contaminants
• Increased nutrients (N, P, K)
• Additional regulations
6

7. Substrate considerations
Photo: Jim Jensen

8. Methane output from different substrate
648.5
502.3
423.6
380.3
296.1
242.7
216.2
122.2
0 100 200 300 400 500 600 700
Used Vegtable Oil
Ice Cream
Cheese Whey
Dog Food
Corn Silage
Raw Manure
Meat Pasta
Switchgrass
ML METHANE PER GRAM TOTAL VOLATILE SOLID ADDED
FEEDSTOCK
Industry Tip:
It is beneficial to have an estimate of the biogas production potential of a
substrate when co-digested with manure prior to adding it to a digester.
Existing estimates, available for most substrates via reports and scientific
publications, can give a good indication of how a substrate will impact a
digester project prior to performing tests or introducing it into the digester.

9. Biological, chemical and physical inhibition
• Biological inhibitors
• FOG: cause sludge flotation, digester foaming, blockage of
pipes/pumps, and clogging of gas handling systems
• Ammonia: TAN, pH, presence of ion, temperature
• Chemical inhibitors
• Antibiotics
• Antifungals
• Physical inhibitors
• Straws, sawdust: high lignocellulosic content
• Struvite formation in pipes, Inert material
Industry Tip:
Tests such as an anaerobic toxicity assay (ATA) and biochemical methane
potential (BMP) can be performed by laboratories and third party contractors to
help identify inhibitory potential before a new substrate is introduced to a
commercial digester.

10. Feedstocks choosing consideration
• Local availability and cost
• Moisture and total solids
• Mixing and particle size
• Inhibitors that suppress bacteria
• Nutrient balance (C:N ratio)
• Digester capacity
• Permitting
• pH/alkalinity
Industry Tip:
It is helpful to think of the microbes in a digester as being similar to those in
the stomach of ruminant livestock. You need to create conditions that are
optimal for microbial growth so they can grow well and perform their job
efficiently. A balanced diet, achieved with careful choice of substrates, will
help ensure that the digester is performing at an optimal level.

11. Capital cost considerations
Photo: Jim Jensen

12. Screening and pre-treating wastes
Substrates can contain unwanted material such as plastics and bottles (left
image); prescreening facilities (right image) use separation equipment to
remove inorganics from organics prior to AD (images courtesy of American
Biogas Council)
Industry Tip:
The amount of pre-processing (e.g. source separation and pre-treatment)
and post-processing (managing nutrients) that is required for each co-
digestion substrate should be carefully considered. The question is whether
the revenue from additional biogas and tipping fees will outweigh the costs
to manage the substrate and AD effluent.

13. Mixers, macerators
Mechanical mixers are usually added to receiving pits to maintain consistency
(left image) and mechanical mixers can be used to mix together different
substrates such as food waste and manure (right image) (images courtesy of
Jim Jensen and WSU Energy)
Macerators are all-purpose grinding machines that reduce the size of
materials. Macerators can be used to decrease the size of substrates,
allowing for easier introduction to the digester.
Mechanical agitators (stirring devices) are often used in continuous stirred
tank reactors (CSTR) to maintain mixing and high rate AD. Agitators can
also be added to receiving pits to homogenize substrates into slurry prior
to introduction to the digester.

14. Buffer tanks, Receiving pits, metering systems
Automated covers can be placed on top of receiving pits and opened for
substrate addition (left image) and closed to minimize odor release (right image)
(photo courtesy of Andgar Corporation)
Industry Tip:
Often a buffer tank with a pumping system will be utilized when introducing
high-energy liquid substrates, while direct dumping from the hauling truck is
typically done when bringing in high-energy solid substrates.

15. Modifications to generator sets
Three additional engines were added at a co-digestion facility in Reynolds, Indiana to
handle additional biogas production from substrate addition; total output is 6.3 megawatts
(image courtesy of DVO, Inc.)
Generator sets may need to be added or altered in order to generate
electricity from the higher biogas output and capture higher revenues.

16. Nutrients management
-20
-10
0
10
20
30
40
50
60
Total Ammonia Total Kjeldahl Nitrogen Total Phosphorus Total Potassium
PERCENTCHANGE
Percent change in nutrient content due to the addition of 16% substrates
(egg waste, fish breading, artificial crab sauce, and ravioli sauce)on a dairy
in northwest WA (modified from Frear et al. (2011))
Industry Tip:
Even when NMPs must be altered and additional nutrients hauled off-site,
the profits from co-digestion can be substantial. However, careful
consideration of these costs is warranted.

17. Operational cost considerations
Photo: Jim Jensen

18. Hauling
Trucks hauling substrates substrate receiving pits have been provided plenty of
space, reducing congestion between haulers (image courtesy of DVO, Inc.)
One of the largest operating costs for co-digestion is normally an
increase in hauling costs. These hauling costs are incurred both to get
substrates to the digester (See Figure 8), and in many cases, to haul
effluent (treated or untreated) away from the digester for land application.

19. Monitoring and laboratory testing
Under co-digestion, it should be assumed that there will be some ongoing
increase in the time required for system maintenance, as it is likely that issues
will more frequently arise. These needs will be most significant when new
substrates are introduced to the digester, though appropriate laboratory test can
help minimize issues
Industry Tip:
One of the problems with substrates is a lack of consistency. Over time,
inconsistent substrates can have dramatic effects on a digester, good or bad.
• Total solids
• Volatile solids
• Total phosphorus
• Total ammonia nitrogen
• Biochemical methane potential (BMP)
• Anaerobic toxicity assay (ATA)

20. Revenue considerations
Photo: Jim Jensen

21. Revenue changes
• Cost changes
• Prescreening/mixing
• Effluent hauling
• Extra engine set
• Revenue changes
• Tipping fees
• Electricity and Renewable Natural Gas
• Tax credits, carbon credits, renewable energy
certificates, and renewable identification numbers
• Fiber, bedding, and other co-products
Industry Tip:
Even incorporating a small
amount of high-energy
substrates to a manure-only
digester can shorten the
project’s payback period and
increase the return on
investment.
Co-digestion and manure-only revenue scenarios (Frear et al., 2011)

22. 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.
Acknowledgements

23. Contact Information
Craig Frear, PhD
Assistant Professor
Washington State University
PO Box 646120
Pullman WA 99164-6120
509-335-0194
cfrear@wsu.edu
Jingwei Ma
Research Associate
Washington State University
PO Box 646120
Pullman WA 99164-6120
mjw@wsu.edu