Degradation of some EN13432 compliant plastics in simulated mesophilic anaerobic digestion of food waste

The research looked at the anaerobic biodegradation of 9 different bioplastics, all of which were commercially available and certified in Europe as compostable packaging material compliant with the biodegradation and other requirements of the EN13432 standard. A combination of testing strategies was used to assess the degree of degradation both under batch conditions, and in a simulation in which the plastics and food waste were fed daily to a digester for a period of 147 days. Two non-biodegradable plastics were used as controls, and verified the robustness of the sampling regime and the recovery of the plastic film, with errors of <1% in the final balance. The simulation allowed quantification of the weight loss of the plastics and determination of a decay coefficient for the different materials, which was then used to estimate long-term degradation. Use of a biochemical methane potential (BMP) batch test allowed estimation of the conversion of carbon into gaseous products. There was no evidence that any of the plastic films inhibited the anaerobic digestion process when continuously fed to digesters, although some inhibition occurred when the most readily degradable materials were tested at higher concentrations in batch mode. There were some interesting differences between results from the various measures of plastic degradation in the batch and simulation experiments, with batch testing in most cases suggesting a higher degree of degradation than was achieved in a semi-continuous system at a solids retention time of 50 days. The exceptions to this were two plastics that appeared to show rapid weight loss in the simulation experiment. BMP test results confirmed this was not through biological conversion of the bioplastic to gaseous carbon products, and was therefore probably due to physical disintegration. It was concluded that, of the 9 bioplastics tested, only 4 showed substantial biodegradability under anaerobic conditions. Further evidence to support the mechanism of biodegradation was obtained by microscopy, and photomicrographs using different techniques are included to illustrate the process. Even the most degradable materials would not break down sufficiently to meet the physical contaminant criteria of the UK PAS110 specification for anaerobically digested material, if fed to a digester at 2.0% of the input load on a volatile solids basis.