Co-digestion: A primer on substrate

utilization and project considerationsNick Kennedy, Georgine Yorgey, Chad Kruger, Craig Frear

Jingwei Ma

Center for Sustaining Agriculture and

Natural Resources

Washington State University

Photo: Andgar

Outlines

1. Introduction

2. Substrate considerations

3. Capital cost considerations

4. Operational cost considerations

5. Revenue considerations

Photo: DVO

Introduction

Photo: Jim Jensen

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

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

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

Substrate considerations

Photo: Jim Jensen

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

FE

ED

ST

OC

K

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.

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.

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.

Capital cost considerations

Photo: Jim Jensen

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.

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.

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.

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.

Nutrients management

-20

-10

0

10

20

30

40

50

60

Total Ammonia Total Kjeldahl Nitrogen Total Phosphorus Total Potassium

PE

RC

EN

T C

HA

NG

EPercent 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.

Operational cost considerations

Photo: Jim Jensen

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.

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)

Revenue considerations

Photo: Jim Jensen

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)

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

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