BioCycle Magazine

Managing Compostable Bags At Anaerobic Digestion Plants

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The highest and best use for treating residential food waste is through source separation, sending organics first to anaerobic digestion (to capture energy), followed by aerobic composting (to return nutrients to the soil). Sending food waste to anaerobic digestion (AD) facilities requires appropriate tools and planning for capturing high volumes of material with low contamination. Compostable bags are widely used to achieve high capture rates. However, further investigation was needed to determine processability of compostable bags in AD systems. What is the impact of compostable bags on various anaerobic digestion technologies, and are they beneficial for the ultimate goal of highest and best use for organics recovery?

This article addresses both the quality of residential organics being collected, as well as the interaction of the compostable bag with pretreatment systems. In the Italian study presented, compostable bags help maximize organics recycling and facilitate recovery of residues that otherwise would be disposed through landfilling or incineration. An analysis was performed to better understand the interaction of compostable bags with hydropulping pretreatment systems in wet AD facilities. The analysis showed interesting results about the composition of the residues generated in those systems, indicating further organics recovery potential with an optimization of the overall performance of the system.

European Guidelines

The European standard EN 13432 aims “to determine the compostability and anaerobic treatability of packaging and packaging materials.” Packaging that complies with EN 13432 can be recovered by organics recycling, i.e., composting and AD. For nonpackaging plastic items the exact same characteristics are defined in the European Standard EN 14995. To comply with both standards, products must show enough biodegradability, disintegratability and absence of ecotoxic effects. Compostable plastic bags for collection of biowaste are one of the most successful applications of EN 13432 certified materials.

The behavior of compostable plastics in anaerobic environments varies depending on temperature (i.e. thermophilic/mesophilic), solids retention time, composition of the material, shape and thickness of the compostable items being tested. EN 13432 and EN 14995 provide guidance on how to measure anaerobic degradation as a further characterization of the tested material. No pass level is fixed, however, because this is considered optional information in the compliance process. If disintegration and biodegradation are not complete during the fermentation process, they will be completed in the subsequent aerobic composting stage.

Different AD technologies need different configurations of the pretreatment stage to properly condition and feed the substrate into the digester. Table 1 shows some of the more common methods found in industrial AD plants treating biowaste.

The pretreatment stage usually generates a certain amount of biogenic and nonbiogenic residue not entering the fermentation process. This amount can be well above 10 percent in weight of the input feedstock. In case of low contamination of the feedstock, these residues are predominantly made of materials of biogenic origin, potentially suitable for composting. When the biowaste is collected in compostable plastic bags, according to the type of pretreatment and fermentation technology, the compostable plastics can take different routes and destinations as shown in Figure 1.

The bags or fragments of them can entirely or partially follow three different paths in an anaerobic digestion plant:

Route 1: Bioplastic fragments fed into the digester along with the biowaste. After digestion, the digestate with possible remains of compostable plastics is composted for aerobic stabilization and mature compost production.

Route 2: Bioplastic fragments sorted out during the pretreatment stage, skipping the digestion stage and rejoining the aerobic composting stage of the digestate.

Route 3: Bioplastic fragments sorted out during the pretreatment stage and sent to disposal because of high contamination of the pretreatment residues by noncompostable materials (e.g. conventional plastics) or because of the lack of a final aerobic stage for compost production.

Route 1 allows for maximum utilization of compostable plastic bags, and additional organic material sticking to them, both in terms of energy and material recovery. Route 2 allows for maximum material recovery of the bags and other pretreatment residues of biogenic origin. Route 3 represents a nonoptimal situation caused by either external factors like the presence of noncompostable plastic contamination impeding optimal material recovery of the pretreatment residues or internal factors like the absence of a composting stage for the stabilization of the digestate.

Switching from Route 3 to Route 1 or Route 2 leads to a higher efficiency in capture of energy and material resources, but is only possible with minimal contamination rates of the input feedstocks. Certified compostable bags help in reducing this contamination (the bag itself is compostable, and residents using them are conscientious about leaving out other plastics) and allow for maximum advantage of the biowaste potential, including the residues normally generated by the pretreatment processes.

Research Study In Italy

Wet AD has traditionally been used for treating sewage sludge (biosolids) at municipal wastewater treatment facilities or by farmers for processing animal manures and more recently also energy crops (e.g. silage). Wet AD works with continuous feeding systems and pumpable substrates that usually must have total solids (TS) content between 12 and 15 percent and particle size less than a half-inch (
This study examines a wet AD plant codigesting agricultural waste and municipal source separated food waste. The facility is owned and run

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