A recent study from the University of Alberta [53.5°N, 113.5°W] has improved our approach to processing bioplastics — the so-called “green” plastics designed to break down more easily than their fossil-fuel-based counterparts. But while bioplastics seemed an environmentally-friendly alternative, they don’t always degrade as expected, especially when mixed with food waste in composting and recycling systems.
This research explored a two-step process — high-solids anaerobic digestion (HSAD) followed by hydrothermal liquefaction (HTL) — to break down bioplastics effectively while recovering useful energy in the process. The findings could reshape waste management strategies, ensuring that bioplastics live up to their promise of sustainability.
The Problem: Bioplastics aren’t as Biodegradable as Consumers Think
Many cities encourage the use of biodegradable plastic bags for food waste collection. But when these bags end up in composting or digestion facilities, they don’t always break down completely, but instead:
- Reduce methane production when processed in anaerobic digestion (a common method for converting food waste into biogas).
- Leave behind microplastics that contaminate soil and water.
- Require extra processing, making waste treatment more expensive.
This study aimed to test whether a combination of anaerobic digestion and hydrothermal liquefaction (HTL) could solve these issues — breaking down bioplastics more completely while recovering valuable biofuels in the process.
Key Findings: How Heat and Microbes Can Work Together
The research team tested how well bioplastic bags decompose when mixed with food waste under two treatment methods:
- Anaerobic digestion (HSAD): This method uses microbes to break down waste in the absence of oxygen, producing methane gas.
- Hydrothermal liquefaction (HTL): A high-temperature, high-pressure process that converts organic waste into biocrude oil — a renewable fuel source.
1. Bioplastics Interfere with Methane Production
- When bioplastics were added to food waste in anaerobic digestion, methane production dropped by nearly 50%.
- The plastic broke down into smaller fragments, but much of it remained intact even after 45 days.
- This suggests that bioplastics disrupt the digestion process and may not be fully biodegradable under typical waste treatment conditions.
2. Hydrothermal Liquefaction Successfully Converts Bioplastics into Fuel
- While anaerobic digestion struggled with bioplastics, HTL completely transformed them into biocrude oil, hydrochar, and gas.
- The best results occurred at 330°C, where biocrude yield was highest, recovering up to 38% of the bioplastics’ energy content.
- Unlike traditional plastic waste, which lingers in the environment for centuries, HTL effectively destroyed bioplastics, preventing microplastic pollution.
Why This Matters: A More Sustainable Waste Solution
The study’s findings suggest that cities should rethink their bioplastic waste strategies:
- Bioplastics should not go straight to anaerobic digestion, as they can reduce methane production and leave behind microplastics.
- A combination of digestion and hydrothermal liquefaction could provide a more complete breakdown — recovering both biogas and biofuels while eliminating waste.
- HTL could offer a new way to process food waste and plastic together, reducing landfill use and recovering valuable energy from waste.
For cities investing in circular economy solutions, this research offers a more effective way to handle bioplastics, ensuring they contribute to sustainability rather than becoming just another pollution problem.
Source
Unveiling the Impact of Bioplastic Bags on High-Solids Anaerobic Digestion and Subsequent Hydrothermal Liquefaction of Source-Separated Organics, Process Safety and Environmental Protection, Process Safety and Environmental Protection, 2025-02-26
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