Assessment of anaerobic degradation of Ingeo™ polylactides under accelerated landfill conditions

Abstract

Ingeo™ polylactide (PLA)11Trademark or registered trademark of NatureWorks LLC in the USA and other countries. biopolymers are used world-wide in a diverse range of applications and, after the useful life of the particular application, they can be recycled (either mechanically or chemically) or disposed of via various end-of-life options, such as composting, incineration and landfilling. The use of compostable materials for food packaging could be an enabling technology to allow the diversion of food waste from landfills into composting facilities. However, despite many new initiatives and existing programs it is still true that a significant part of industrial and household waste still ends up in landfills. Because polylactide polymers are known to be compostable the question is often raised about the behaviour of these materials in landfills.In order to study the behaviour of Ingeo polylactides (PLA) in landfills two studies were performed aimed at generating reliable information on the anaerobic biodegradation of PLA under conditions of extended time and modest temperatures. The first test (under accelerated landfill conditions) was done at 21 °C, and three moisture levels, extending to 390 days and the second test (a high solids anaerobic digestion test under optimal and significantly accelerated conditions) was conducted at 35 °C for 170 days. Each test is meant to represent an accelerated test of what could happen under anaerobic landfill conditions. These two tests each had accelerated the biological degradation sufficiently that they were in some sense equivalent to approximately a century of a “typical” biologically active landfill.The semicrystalline polylactide samples did not produce a statistically significant quantity of biogas during either test. The amorphous PLA did generate a small amount of biogas in the test at 35 °C, but none in the test at ambient temperature. Here it should be noted that the tests were conducted under accelerated, optimal landfill conditions, the biodegradation was observed in a 100 year timeframe and the market volume of amorphous PLA is low. We conclude that semicrystalline PLA (typical of >96 wt% of resin used to manufacture products), under anaerobic biological conditions typical of a landfill at moderate temperatures (where PLA hydrolysis is slow), will not lead to significant generation of methane, and that no significant population of organisms is available under anaerobic conditions to directly degrade high molecular weight PLA. Because there was no direct biological degradation of PLA under the anaerobic conditions, it is likely that any degradation of PLA in a landfill would require a chemical hydrolysis step prior to any biodegradation, which is analogous to the situation in aerobic composting. At 20 °C this process is estimated to take 100+ years, and under those conditions the degradation of the PLA would be extremely low. Additional data on the time/temperature history experienced in landfills will be needed to understand the net effect for disposal of PLA globally.