Bioplastics in the organic fraction of MSW: influence of composting process conditions on biotic and abiotic degradation and fate of residues

The worldwide market of bioplastic products, both film (e.g. biobags) and rigid (e.g. cutleries, dishes, coffee capsules), underwent a significative increase in the last decade. Bioplastics have been considered a suitable alternative to conventional plastics for their cleaner life cycle. They can derive from renewable resources and be treated with the organic waste, to become compost and enter in the chain of food and agriculture. Thus, they are posed as virtuous example of circular economy. Compostable bioplastics, meaning those materials certified to be treatable in aerobic and anaerobic biological processes, should be collected with the organic waste and conferred to aerobic composting possibly preceded by anaerobic digestion. However, some issues have risen in the last years about bioplastic waste management in the industrial treatments for the organic waste. In fact, the conditions set during the tests for standard certifications can largely differ from those of the biological industrial aerobic and anaerobic plants, referring to temperature, humidity and times of the processes. This research aims to stress the effect of the operative conditions on bioplastics degradation, particularly in aerobic composting. It started from the observation of the industrial composting plants in Italy and Europe to carry out lab scale tests with various operative conditions, and developed a synergic approach of methodologies to monitor their effect on the degradation of Mater-Bi®, polybutylene adipate terephthalate, polylactic acid and polyethylene. The study considered different aspect of the degradation, including chemical and physical analyses to assess the level of deterioration at different times of the tests, and microbiological analyses to investigate how the bacterial community can become more specialized in degrading polymers. The obtained results disclosed that some critical operative conditions cannot ensure a complete degradation of the complex bioplastics, raising the issue of microplastics release as part of compost in soil and definitively in water environments. Therefore, lab scale tests were carried out in soil, sand, fresh and saltwater, to explore the behaviour of bioplastic residues derived from incomplete degradation in composting. Unfortunately, it was demonstrated that the outside conditions are not favourable for the degradation process to continue. Finally, it was stressed an important question related to microplastics monitoring in compost and sludge, derived from both aerobic and anaerobic biological treatments. The current legislation set a threshold size of 2 mm for plastics quantification in these matrices, basically for a lack of a standardized protocol allowing to identify and recover items smaller than this size. Therefore, an extraction method was tested on microplastics derived from both conventional plastics and bioplastics, with the objective to make a step more towards the capability to properly monitor and characterize compost and sludge quality.