FREQUENTLY ASKED QUESTIONS ON FOODSERVICE PACKAGING IN ANAEROBIC DIGESTION FACILITIES
Why should anaerobic digestion (AD) facilities accept foodservice packaging?
Both paper and bioplastic foodservice packaging, and the food scraps associated with it, contain valuable carbon which is needed to produce biogas. In addition, the digestate resulting from the AD process can be used as feedstock for a composting facility.
What type of digester is best suited to process foodservice packaging?
Each digester and each waste stream is unique, yet three basic AD technologies are predominant in the market: Wet (low-solids) AD, dry (high solids) plug-flow AD, and dry (high solids) batch fermentation AD. Typically, in wet systems, a pre-treatment step sorts out packaging materials, and it may be possible for screened compostable packaging to be sent directly to composting. Dry plug flow systems also require that feedstocks undergo pre-treatment such as shredding/pulping, and packaging materials can enter the digester without significant impact to digester operation. Lastly, dry fermentation digesters are best suited to handle larger items like packaging and don’t generally require pretreatment. Any undigested fractions of compostable materials can be sent to composting along with the digestate solid fraction.
Other factors that may influence the performance of foodservice packaging in a digester include residence time, temperature and digester size. Many digesters run for roughly 28 days, and additional retention time and/or a composting phase may be needed to completely break down compostable packaging. Thermophilic digesters, which run at higher temperatures (above 50°-55°C, or 122°-131°F) may be able to biodegrade compostable packaging more readily than mesophilic digesters, which operate at lower temperatures (typically 30°C, or 86°F). However, this may not always be the case. Today, some mesophilic digesters are processing FSP without issue.
Small AD facilities are frequently built for very specific feedstock recipes and cannot handle much variability that may drastically disturb the microbial balance of the digester. These small systems would most likely not be good candidates for significant quantities of foodservice packaging. Large scale systems often co-digest food scraps with animal waste and sewage sludge, and these may be a better option for FSP. Again, because of different variables, this may not always be the case, and therefore small AD facilities shouldn’t be ruled out.
Will foodservice packaging fully process in an AD facility?
Paper items typically digest more readily under anaerobic conditions than bio-based plastics, however different bio-based plastic resins also behave differently. Other factors such as how the incoming feedstocks (including FSP) are pre-processed have an impact as well. If the digestate exiting a digester is ultimately going to be used as a compost product, further processing the digestate solid fraction at a composting facility is recommended. This will provide greater opportunity for the materials, both the compostable FSP and other organics, to fully biodegrade and be incorporated into the final product.
How can an AD facility determine which FSP will successfully process in their facility?
There are a couple of options to identify which foodservice packaging products are suitable for an AD facility:
Most facilities require digestate solids be subsequently sent to anaerobic composting, so check with the foodservice packaging manufacturer to determine whether the item meets appropriate compostability standards or whether it’s been certified compostable by BPI. If the digestate is not being composted, check to see whether testing has been done according to ASTM D5511, the “Standard Test Method for Determining Anaerobic Biodegradation of Plastic Materials Under High-Solids Anaerobic Digestion Conditions.”
Test the product in your facility. Anaerobic conditions in an AD facility are quite different from aerobic conditions in a composting facility, so it should not be assumed that compostable items will process well in your AD facility.
How can AD facilities maximize recovery and minimize contamination?
Once you determine which foodservice packaging will process successfully in your facilities, it’s important to work with various stakeholders to make sure you get the materials you want – and limit the materials you do not want.
If you plan to accept food scraps and compostable packaging from residential collection programs, work with the municipality or county’s program coordinator to review the list of acceptable materials. If possible, provide input into the educational materials for and outreach to residents. It is advisable to launch resident education and collection of these new materials on a pilot basis (e.g. certain routes or neighborhoods) in order to manage the change in the stream. During the pilot phase, communicate your observations back to the program coordinator and identify any areas that need to be improved before a broad roll-out.
If you plan to accept food scraps and compostable packaging from commercial collection programs (i.e. restaurants and other foodservice establishments), work with these operators and/or their haulers to review the list of acceptable materials. Check whether they will be taking the appropriate steps to minimize contamination by using clearly-labeled compostable items and educating both staff and customers to put only compostable items in the compost bins. You may want to educate them on the importance of receiving only compostable materials and discuss ramifications (if any) of loads contaminated with non-compostable materials.
Can AD systems more easily process materials that are problematic in composting?
Not necessarily. Some large AD operations may screen the incoming feedstock by pulping and separating into fractions according to the natural tendency of the pulped materials to float or become sediment when centrifuged. If appropriate, foodservice packaging may be removed at this stage and sent directly to the subsequent aerobic composting finishing stage.