Potential for anaerobic degradation of polypropylene in thermophilic digester sludge

Microplastic emissions to the environment have gained increasing environmental awareness. Wastewater treatment plants are assumed to be an end of life point for microplastics, and it has been shown that the emission of microplastics from Danish WWTPs to the receiving waters is lower compared to the total load on the plants. Here, we assessed the potential for biodegradation of polypropylene in a thermophilic anaerobic system treating municipal wastewater sludge. Under conditions approximating those of the full-scale parent digester, the polypropylene surface modifications, changes to the chemical structure and oxidation products, as well as the associated microbial communities were analysed after 12 days of incubation. Surface corrosion of the PP coupon surface and microbial attachment was observed with both AFM and SEM microscopy. Formation of the new functional groups at 1536 cm-1 (C–C), 1484 cm-1 (–CH2) and 856 cm-1 (–C=CH2) suggested changes in the amorphous polymeric region as well as chain scissions in crystalline region. NMR spectra from fraction extracts of the PP surfaces suggested that degradation has reduced the average molecular weight of the PP by cleavage of the polymeric chains, leading to the introduction of a terminal double-bond in the PP backbone. Furthermore, formation of double bonds between carbon atoms were identified using Raman spectroscopy. “Candidatus Microthrix”, Gordonia, Romboutsia, Clostridium sensu stricto 1 and Mycobacterium were detected abundantly in the biofilm samples.