Exploring Co-digestion? Learn the Secrets of Success From Industry Insiders

An industry workshop explores strategies for designing site-specific co-digestion programs that are mutually beneficial for utilities and the communities.

Exploring Co-digestion? Learn the Secrets of Success From Industry Insiders

Co-digestion in digesters like these enables utilities to enhance energy recovery during wastewater treatment.

Co-digestion of food waste with wastewater residuals holds substantial promise as a way to increase renewable energy, reduce costs and create environmental benefits. The question is how to design co-digestion projects to maximize the benefits while working with existing wastewater treatment infrastructure.

Last October, the Water Environment & Reuse Foundation and the Water Research Foundation held a workshop called Advancing Anaerobic Digestion of Wastewater Solids and Food Waste for Energy and Resource Recovery: Science and Solutions – A Framework for the Practice of Co-digestion.

At the workshop, sponsored by the National Science Foundation, speakers and participants discussed existing co-digestion programs, ways to improve on their success, and recommendations and research needs for the industry.

Co-digestion is defined as the anaerobic digestion of two or more feed materials, including the addition of food and other organic wastes to wastewater solids. Workshop participants highlighted a pressing need to identify and characterize the drivers for co-digestion programs in specific communities or at individual water resource recovery facilities.

Co-digestion using existing infrastructure has great potential to generate more renewable energy (as biogas), divert organic wastes from landfills, and recycle food waste sustainably and at a competitive cost. Here is a summary of commentary and recommendations from the workshop.

Selecting Feedstocks

Define types

Rather than be based on a standard design and implementation structure, a co-digestion program should be unique to the community’s or utility’s attributes. Feedstock quality may determine the level of processing necessary. Feedstocks suitable for co-digestion need to have high energy value, and the facility must have sufficient digester capacity. Fats, oils, and grease are favorable for co-digestion due to low solids content, high degradability and energy content, and generally greater consistency than other food wastes. There are three categories of suitability:

  • Unsuitable under any conditions (waste that has no energy content).
  • Suitable with preprocessing (waste containing certain compounds that could inhibit digestion, end up in residuals, or contain high levels of contaminants).
  • Suitable under all conditions.

Characterization methods

Workshop participants agreed that some materials perform better than others but that the ideal feedstock also depends on the facility’s capabilities. Feedstock choice can be influenced by digester capacity, gas handling capacity, landfill tipping fees, biosolids storage space, the digester’s ability to handle the feedstock, pH control and more. There also can be risks downstream from digestion. Metrics and standards for acceptability need to be established for each facility. Facilities should locate waste sources close by and determine upfront what pretreatment is needed.

Improving and Optimizing Performance

Factors that influence performance

Digester capacity, feedstock type, and delivery options all drive the size and arrangement of waste receiving and storage facilities. Preprocessing requirements depend on the capacity of the co-digestion system. Material can be preprocessed at a solid waste management facility by removing contaminants and blending materials to create a high-quality slurry.

The optimal feeding pattern depends on the facility’s goals. For example, a facility may use different loading rates based on whether the aim is to maximize methane production or stabilize co-digestion. Utilities have found that a change in the feed rate can upset digesters, but that continuous feeding generally does not.

Improving modeling

Participants noted that modeling and monitoring parameters can help characterize feedstock and maintain quality. Modeling of plant nitrogen and phosphorus load returns can suggest initial techniques and so can visiting other plants that have met their permitting requirements with co-digestion programs. Advance modeling can help in predicting upsets and optimizing performance. Online monitoring can provide immediate performance results, but that must be balanced with lab samples for better data and reliability, since co-digestion programs can have considerable variability.   

Operational strategies

At a minimum, participants agreed that co-digestion programs should monitor total solids, volatile solids and chemical oxygen demand. Facilities can also implement practices to protect process integrity and identify potential problems. These could include adding waste screens, interviewing suppliers to characterize their wastes, requesting safety data sheets, and including feedstock characterization in the contract with the hauler.  

Feed rates depend on pumping strategies, substrate makeup and co-digestion objectives. To help prepare operators and maximize stability, individual facilities must define what continuous feeding practices mean for their site. Consistency of feed is also important; feedstocks should be well mixed before digestion and cleaned of any contaminants.

Creating Guidelines

Contracts for feedstock

Workshop participants stressed the need for an agreement with feedstock providers on minimum specifications and standards for suitable materials. Facilities can protect process integrity by including feedstock specifications in a contract, inspecting material as it is received, and reserving the right to sample trucks and take daily composite samples.

Feedstock providers and utility staff need a clear understanding of the application and the contracting process. Agreements need clear metrics on what feedstock can be brought in, what the utility can accept and the defined tolerance levels for key site parameters. 

Hauler agreements

A receiving program that is manageable and cost-effective is important to the success of co-digestion projects. Suppliers should be under contract for a consistent feedstock supply. Haulers need specific delivery times that align with the facility’s need for feedstock. Twenty-four-hour access for haulers is preferred, but not all facilities have that capability. Proper training for delivery drivers and receiving station personnel is essential for programs to function efficiently. 

Tipping fees

The cost to treat waste is an important consideration. Tipping fees function as a cost-recovery mechanism. The water industry has no standard tipping fees and no single tool for setting such fees. Some utilities use a tiered system based on waste strength, while others use a flat rate but keep flexibility to adjust on a case-by-case basis. 

Viable funding sources 

Public-private partnerships or other partnerships can help facilities leverage funding for co-digestion programs. This includes partnerships that have a design-build component.   

Identifying benefits

An effective approach is to meet the facility’s needs by working toward an integrated system that covers power generation, heat production, biosolids management and direct biogas use. Projects can be mutually advantageous to the facility and its community for benefits such as:

  • Diverting material from landfills
  • Improving efficiency at reduced cost
  • Producing renewable energy and reducing dependence on fossil fuels
  • Producing marketable commodities
  • Reducing electricity or natural gas costs
  • Reducing carbon emissions in line with community-wide greenhouse gas goals
  • Delivering social benefits and public relations value.

Examining Different Business Models 

The benefits of co-digestion can be maximized by promoting local businesses, expanding the tax base and increasing green jobs. The mission of a co-digestion program can include a business-case justification. Selecting the best end use for the produced biogas depends on factors like environmental sustainability, social justice and economics. 

Managing Competing Goals

There are often competing goals between energy projects and other utility initiatives, or within energy recovery projects themselves. Alignment of the co-digestion program’s mission with overall utility goals, compliance and financial considerations will make such projects more appealing to decision-makers. This also means managing goals within existing facility operations. Operators need to understand why co-digestion is important to the utility; it requires a shift in mindset from discarding waste to taking in resources to create value. 

Advancing Regulations and Community Goals

Workshop participants emphasized that products of co-digestion programs should be identified and valued based on an accounting of triple bottom line benefits and community sustainability goals. They noted that facilities find it useful to balance their co-digestion programs, the community’s philosophy and sustainability objectives. Products may include energy, digestate, and credits for energy, carbon, and nutrients.

Co-digestion is one process that can be used to create marketable products from municipal wastewater and food wastes. Such programs can be mutually beneficial for utilities and their communities and customers.

About the author

Kelsey Beveridge, kbeveridge@waterrf.org, is a technical writer with the Water Research Foundation.


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