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Co-digestion considerations for anaerobic digestion systems

Ryan Holzem and John Katers Published on 30 September 2015

One option often considered for dairy farmers currently operating an anaerobic digestion system is the co-digestion of off-farm substrates.

Although accepting high-strength substrates from off-farm generators can seem like an attractive option, great care should be taken when evaluating the overall impacts of these substrates on the operation of an anaerobic digestion system and the overall operation of the farm.

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This article will summarize the advantages and disadvantages of co-digestion, describe some of the most common substrates that have been utilized for co-digestion and identify critical questions that should be addressed prior to entering into a contract to secure off-farm substrates for co-digestion.

By its very nature, dairy manure is not one of the highest-strength substrates available for anaerobic digestion, primarily because dairy manure has already been partially digested as it passed through the cow’s stomach. Therefore, co-digestion of dairy manure along with other substrates can provide several significant advantages, which include:

1. The ability to collect a tipping fee for the substrate

2. The production of additional biogas

3. The potential to form regional collaborative partnerships on waste management issues

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The first two advantages are clearly the most important from the financial perspective of a farm and are also closely related to each other.

First, we should start off by saying that in most circumstances, the main goal of implementing co-digestion should be to select high-value substrates that provide the highest biogas generation potential.

However, sources and subsequent substrate values will vary widely by location, and high-value substrates may not always be available. As a result, tipping fees will generally need to be higher for low-value substrates (i.e., those with less biogas generation potential) and lower for substrates that offer higher biogas generation potential.

In areas with multiple anaerobic digesters, or as the number of digesters installed on farms continues to increase, competition for high-value substrates could occur.

This could potentially result in a reversal where the farm would need to pay a fee to acquire the substrates, which is referred to as a substrate acquisition fee.

It is especially important in these instances that the return on investment be taken into account, whether the return is through the additional revenue generated through a power purchasing agreement with a utility or by decreasing energy purchases by using the additional heat and electricity on the farm.

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There are also several potential disadvantages of co-digestion that need to be considered, including:

1. Additional regulatory approvals and oversight

2. Additional infrastructure in the form of receiving stations, storage tanks, pumps, etc.

3. Lack of consistent quantities and quality of substrates

4. An absence of materials that will not disrupt the operation of the anaerobic digester or pose any threat to the farm, particularly from a biological, pathogenic or toxicity standpoint

These disadvantages can present substantial barriers to the implementation of co-digestion and highlight the fact that long-term contracts are likely necessary to provide consistent substrates for co-digestion.

Without these long-term contracts, farms will be reluctant to make the required capital investments to accommodate the additional substrates, which could potentially disrupt the operations of the anaerobic digestion by overloading or shocking the system, resulting in failure.

Both the advantages and disadvantages of co-digestion show that selection of substrate source and type is the most important when considering co-digestion, as this will have an impact on potential financial gains and the overall operation and management of anaerobic digestion system and farm.

There is a range of substrates that have been co-digested (Figure 1), including used fats/oils/greases, energy crops (maize and grasses), food waste, dairy waste and distillery waste.

substrate biogas yieldsFats, oils and greases have the highest biogas generation potential (14,000 to 28,000 cubic feet per ton), which is substantially more than dairy manures (880 to 1,200 ft3 per ton).

Energy crops can range from 3,900 ft3 per ton for grasses to 7,100 ft3 per ton for maize. Food waste, dairy waste and distillery waste have biogas potentials of approximately 1,200 ft3 per ton, 1,900 ft3 per ton and 2,800 ft3 per ton, respectively.

As noted previously, the tipping fee (or substrate acquisition fee) will be closely related to the biogas generation potential.

However, these fees could also be related to the cost of other management and disposal options available for these substrates, such as landfilling, composting or treatment at a municipal wastewater plant. Thus, when the farm accepts these substrates for co-digestion, they become responsible for their management and disposal.

Furthermore, the physical, chemical and biological characteristics of the substrate directly impact operation and management of the anaerobic digestion system and farm differently.

For example, the liquid volume of low-solids substrates like dairy wastes (i.e., whey, etc.) will not appreciably change during the anaerobic digestion process, which can result in additional liquid waste and the associated cost of management or disposal (e.g., land application).

Similarly, the presence of solid refuse (e.g., trash), which is not necessarily tied to the solids content of the manure but is a concern nonetheless, can harm pumps, mixers and other equipment.

Aside from the solids content of the substrate or the presence of solid refuse, attention should also be given to the biological or chemical composition of the substrates.

Of particular concern is the chemical composition which, if toxins (i.e., antibiotics, copper, etc.) are present, could have an adverse impact on the operation of the anaerobic digester or the land application of the digestate.

Therefore, before accepting materials for co-digestion, an analysis of the substrate should be requested, with some of the most common parameters that need to be considered being pH, total solids, volatile solids, chemical oxygen demand, volatile fatty acids, alkalinity and ammonia.

Similarly, the biological composition of the substrate is also critical, particularly due to the increased scrutiny and regulations associated with food security.

The presence of pathogens, which may not be deactivated through digestion, could pose a potential risk to humans and animals during operation and maintenance of the digesters or disposal of the solids.

Because of the complexity of co-digestion and the potential impacts on the anaerobic digestion system and the farm, a number of questions should be addressed prior to initiating co-digestion. Some of the most important questions would include the following:

1. What additional permits, infrastructure or management is necessary?

2. What is the overall economic impact of the substrate when considering the tipping fee/payment, increased biogas generation, disposal costs, etc.?

3. How will the additional biogas be utilized, and is capacity available under existing power purchase agreements if the biogas will be used for electrical generation?

4. Do the substrates pose any biological or chemical risk to the operation of the anaerobic digestion system or the farm?

5. Will the substrates be available on a consistent, long-term basis to allow for steady operations of the anaerobic digestion system?

By addressing these questions early on in the process of considering co-digestion, many of the potential issues discussed here can be avoided, and the possibility that the co-digestion process will be successful from both an economic and environmental perspective will improve greatly.

As the number of farm-based anaerobic digestion systems continues to increase, there will be more consideration given to co-digestion of off-farm substrates. With proper planning and coordination, co-digestion can provide benefits to both the farm and the off-farm substrate generator.  PD

John Katers is a professor in the Department of Natural and Applied Sciences at University of Wisconsin – Green Bay. He can be reached by email.

  • Ryan Holzem

  • Assistant Professor
  • Department of Engineering Technology
  • University of Wisconsin – Green Bay
  • Email Ryan Holzem

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