THE ESSENTIALS
- The collection of our organic waste (biowaste), which has been widespread in France since 2024, is a commendable step forward.
- In practice, however, its effectiveness is limited by confusion in sorting: the presence of certain plastics—which are mistakenly considered compatible with anaerobic digestion—slows down the recovery process.
- Research is focusing on the design of materials that are compatible with both the imperfections in collection and the treatment processes.
Peelings, coffee grounds, stale bread, food scraps, grass clippings, dead branches, garden trimmings… our trash cans are overflowing with organic materials that are now collectively referred to as biowaste. Since 2024, France has made source separation of this waste mandatory to divert it from incineration and recover it by transforming it into useful resources.
Of the hundreds of millions of metric tons of waste that France produces each year—across all waste streams—biowaste accounts for nearly one-third, or several million metric tons. Since a large portion of this biowaste consists of water and carbon, incinerating it offers limited benefits.
Collecting it separately, on the other hand, is a sensible way to recover its value. Depending on the region, several solutions coexist: local composting, door-to-door collection, or voluntary drop-off at dedicated stations… The goal is always the same: to transform our kitchen scraps and green waste into compost that enriches the soil or into renewable energy through anaerobic digestion.
This development is good news for the climate and the circular economy, but putting it into practice can sometimes be challenging. Among the difficulties encountered is the persistent presence of plastic waste in organic waste bins.
Plastic in Food Waste
Some sorting mistakes are easy to understand: forgotten packaging, disposable cutlery, or plastic wrap.
Others are more subtle: a simple label stuck on a banana peel or a bag labeled as “biodegradable” may seem compatible with the recycling system, when in fact they are not necessarily so.
Part of the problem stems from common confusion surrounding the term “bioplastic”, which conflates bio-based and biodegradable plastics, even though they refer to very different things. This fuels confusion among the public about what is bio-based, biodegradable, and compostable. However, these terms are not interchangeable.
- A plastic is considered “bio-based” when it is made from biologically derived resources, such as starch, sugars, or vegetable oils. This says nothing about its ability to break down after use.
- A material is described as “biodegradable” when it can be broken down by microorganisms. However, this property depends heavily on environmental conditions: temperature, humidity, the presence of oxygen, microbial activity, and the amount of time available.
- Finally, a compostable material is designed to break down under specific composting conditions. In many cases, this compostability is guaranteed only in industrial facilities, where temperature, humidity, and aeration are carefully controlled.
Just because something is compostable doesn't mean it can be converted into biogas
This confusion sometimes leads to mistakes during sorting: a resident may, in good faith, place a bag or piece of packaging that they believe to be compostable in their food waste bin. However, organic waste does not necessarily go to industrial composting: it may also be sent for anaerobic digestion, a biological energy recovery process that treats various organic materials—livestock manure, agricultural residues, food waste, and even separately collected organic waste.
And contrary to popular belief, not all compostable materials are necessarily suitable for anaerobic digestion. And for anaerobic digestion, the presence of plastics is problematic.
In fact, unlike composting, which is an aerobic process (i.e., carried out in the presence of oxygen), anaerobic digestion is, on the contrary, an anaerobic process that operates in the absence of oxygen. Microbial communities break down the organic matter and produce a biogas rich in methane, which can be used to generate electricity and heat or purified for injection into the grid as biomethane.
Anaerobic digestion also produces a residue called “digestate,” which still contains organic matter and nutrients that can be used in agriculture. Depending on the nature of the waste being treated, a sanitization step may be necessary to ensure the final product is safe for human consumption.
Together, these steps form a technical process in which each stage depends on the quality of the previous one. When the inputs are clean, preparation is simpler, digestion is more efficient, and the resulting products—biogas and digestate—are of higher quality.
The sector's effectiveness is being undermined
The microbial communities, physicochemical conditions, and retention times differ significantly between the two processes (anaerobic digestion and composting). Consequently, packaging that can degrade in an industrial composting facility may prove to be only slightly degradable—or not degradable at all—in an anaerobic digester. It may remain intact for a long time, disrupt equipment, or generate fragments that could contaminate the final products.
In this context, the quality of the collected waste stream is essential. When a municipality authorizes a certain type of plastic bag, this decision generally depends on the local collection system and the treatment process: composting, anaerobic digestion, potential unpacking, etc.
In some facilities, de-packaging equipment is used to separate packaging from the organic fraction in order to extract recoverable material. However, these systems are never perfect: some plastic fragments may persist and continue through the treatment chain.
Outside of this specific context, the introduction of “bioplastics” in no way guarantees better environmental performance than conventional plastics. In theory, therefore, the anaerobic digestion stream would need to be completely separated from the composting stream, but this is difficult to implement.
These plastic fragments found in biowaste have several consequences.
- Upstream, they reduce the amount of material actually available (since some of it is contaminated by plastic) for biogas production and increase the maintenance requirements of the digester as well as preparation costs.
- Downstream, they can degrade the quality of the digestate and even compromise its suitability for land application.
In other words, every undesirable element reduces the overall efficiency of the supply chain.
Toward Compatible Materials?
This situation raises a fundamental question: Since raising awareness alone will not be enough to eliminate all confusion—which, incidentally, is fueled by certain labeling practices—how can we design materials that can be used for biowaste collection while remaining compatible with treatment processes?
To answer this, researchers are currently working to understand which chemical and physical parameters control the biodegradation of polymers in anaerobic digesters. The goal is no longer to produce materials that are biodegradable in the absolute sense, but to develop materials designed for a specific process.
How can this compatibility be measured? What criteria can be used to evaluate the degradation of a polymer in a digester? How can plastics be designed to withstand the demands of the waste management system? These are the questions that research will address. This work paves the way for a new generation of materials designed not only for their intended use but also for their end-of-life.
Victorien Jeux, Assistant Professor of (Bio)Polymer Chemistry, UniLaSalle, and Laura André, Assistant Professor, UniLaSalle
This article is republished from The Conversation under a Creative Commons license. Read the original article.
