Pyrolysis as a valorization strategy for plastic-contaminated organic waste streams

There is growing interest in the concept of “circular economy”, whereby residues generated in agricultural or industrial operations are converted into value-added materials that can be returned to the original process, and thus enhance the environmental and economic performance of the overall system. A significant barrier to deploying such sustainable systems at industrial scale is plastic contamination, a problem that is particularly challenging in organic waste management associated with agricultural, food processing and post-consumer food waste processes. In many cases, plastic-contaminated organics are deemed unsuitable as feedstocks for composting and anaerobic digestion processes, and such waste streams are often landfilled or incinerated.

We present results from three case studies involving real-world situations where relatively low concentration plastic contamination of organic waste streams presents a barrier to upcycling by conventional composting or anaerobic digestion processes. In Case Study #1, the cafeteria waste from a local school was collected over the course of an academic year and characterized to quantify the proportions of recyclables (bottles, cans, etc.), food waste and non-recyclable plastics and paper. We found that, after removing the recyclable materials, 87% by mass of the remaining waste stream was comprised of food matter and 13% comprised of various plastic and paper packaging, utensils, etc. In Case Study #2, we collaborated with a local farm to characterize the waste materials generated in their agricultural operations, comprised of four main biomass resources (crop residues, wood waste, wooden pallets and cardboard) and agricultural mulch film (AMF) made of low-density polyethylene (LDPE). It was determined that on an annual basis, AMF comprised 5% by mass of the total farm waste generated. In Case Study #3, we partnered with a local mushroom company to develop a valorization strategy for spent mushroom substrate (SMS), the growing medium comprised mostly of saw dust that is contained within a high-density polyethylene (HDPE) bag and generated as a waste stream once the mushrooms are harvested for sale. We determined that the HDPE comprises 4% by mass on a dry basis of the total waste stream. 

For all three Case Studies, slow pyrolysis was applied as an alternative valorization strategy, with biochar being the primary output product of interest. It was found that biochar produced from plastic-contaminated organics under pyrolysis conditions of 500 – 800^oC was indistinguishable from biochar produced from the organics alone, and met the International Biochar Initiative (IBI) standards for applications as soil amendment or materials. The only exception was in the case of post-consumer cafeteria waste where a significant chlorine content was measured, likely resulting from polyvinyl chloride (PVC) based cling films used in clear food packaging.