ANALISI SPERIMENTALE SULLA DEGRADAZIONE ANAEROBICA DI SACCHETTI IN CARTA O IN BIOPLASTICA PER LA RACCOLTA DEL RIFIUTO ALIMENTARE

In Italy, the amount of compostable bioplastics (fulfilling the technical standard UNI EN 13432:2002) collected with the organic waste is constantly growing (+210% in the last three years). The Italian market of biodegradable and compostable bioplastics was originally driven by bags used for food waste collection, for shopping at the groceries and for containing loose foods like fruit and vegetables at the supermarket. In recent years there was a robust growth of the sector of other packages as well as rigid items such as disposable tableware. According to the Italian legislation, these items must be accepted by all biological treatment plants (both aerobic and anaerobic). Anyway, the standard UNI EN 13432:2002 requires only the assessment of the aerobic degradability, while it is generally not necessary to test the behaviour under anaerobic conditions. This aspect is in contrast with the increasing diffusion of anaerobic digestion plants observed in recent years. In particular, in Italy, about 2.9 million tonnes of food waste were sent to anaerobic digestion or integrated (anaerobic/aerobic) plants in the year 2019, compared to only 1.7 million tonnes sent to composting. For this reason, the present research was performed with the aim to assess the anaerobic degradability of bioplastic bags for the food waste collection (typically employed in Italy for this purpose) at the laboratory scale. In detail, BMP (Biochemical Methane Potential) tests were performed under thermophilic conditions on four commercial types of bioplastic bags, including both bags specifically designed for the food waste collection and shoppers, that can be reused for the same purpose after being used for the overall shop at the supermarkets. Both types are made with the Mater-Bi® polymer, a compostable bioplastic according to the UNI EN 13432:2002 standard, whose composition is 70% polybutylene adipate terephthalate, 20% starch, and 10% additives. The tests were performed comparing the behaviour of bioplastic bags to that of a paper bag specifically designed for the food waste collection. Results of BMP tests indicated a maximum anaerobic degradability for bioplastic bags in the range 71% – 93%, resulting for three out of the four tested bags higher than that of the paper bag, equal to 74%. As regards the kinetic behaviour, three out of the four bioplastic bags showed stepped degradation trends, similarly to what typically observed for the anaerobic degradation of starch-based biopolymers. The stepped trend was modelled by means of the combination of two Gompertz models. The degradation kinetic of the fourth bioplastic bag, described by means of a Gompertz model, was also peculiar, since it showed a lag phase lasting up to more than 10 days. As regards the examined paper bag, the kinetic described by a Gompertz model was very fast and characterised by a very short lag phase (about 1 day). The degradation kinetic modelling was useful to estimate the anaerobic degradability of bags when digested under continuous feeding conditions (i.e. conditions similar to the real management at full-scale anaerobic digestion plants). The particular degradation kinetics of bioplastic bags limit their conversion into methane in these conditions. In particular, assuming an anaerobic digestion process characterised by a hydraulic retention time of 21 days (a typical value in full-scale digesters treating the food waste under thermophilic conditions), the degradability (in the range 44% – 69%) resulted sensibly lower than that of the examined paper bag, equal to 70%, in contrast to what observed in terms of maximum degradabilities.

In Italia, la quantità di bioplastiche compostabili (conformi allo standard tecnico UNI EN 13432:2002) raccolte con il rifiuto organico è in costante crescita (+210% negli ultimi tre anni). La legislazione italiana prevede che questi manufatti debbano essere accettati da tutti gli impianti di trattamento biologico, siano essi aerobici o anaerobici. Tuttavia, lo standard richiede la sola valutazione della degradabilità aerobica, mentre non è generalmente necessario testare il comportamento in condizioni anaerobiche. Questo aspetto è analizzato nel presente articolo, con la valutazione della degradabilità anaerobica di uno specifico manufatto in bioplastica, ovvero i sacchetti utilizzati per la raccolta del rifiuto alimentare, il cui utilizzo è ampiamente diffuso in Italia. In dettaglio, sono state eseguite prove di biometanazione (BMP – Biochemical Methane Potential) in condizioni termofile su quattro tipologie di sacchetti in bioplastica, includendo sia quelli specificamente progettati per la raccolta del rifiuto alimentare sia gli shopper, che possono essere riutilizzati allo stesso scopo dopo il primo utilizzo per il trasporto della spesa. Le prove sono state effettuate confrontando il comportamento dei sacchetti in bioplastica con quello di un sacchetto in carta specificamente realizzato per la raccolta del rifiuto alimentare. I risultati delle prove indicano una buona degradabilità (>71%) dei sacchetti in bioplastica. Tuttavia, essi sono caratterizzati da particolari cinetiche di degradazione, con un andamento a gradini o una prolungata fase di latenza iniziale, che ne limitano la conversione in metano nel caso di alimentazione continua. Al contrario, prospettive molto interessanti sono offerte dal sacchetto in carta, che mostra, in aggiunta a una buona degradabilità anaerobica (74%), una cinetica di degradazione molto rapida.