Fate and effects of polyethylene terephthalate (PET) microplastics during anaerobic digestion of alkaline-thermal pretreated sludge

Plastics are resilient, hard to degrade materials that can persist in nature for centuries. Microplastics (MPs) exhibit similar tough character and hold the potential to harm marine and terrestrial ecosystems upon their release into the environment. Most modern wastewater treatment plants remove MPs from wastewater with over 90% efficiency but unfortunately concentrate them in sludge. Recent studies have reported MPs' impact on the performance of sludge treatment systems, including anaerobic digesters. Despite its resilience, polyethylene terephthalate (PET) has inherent weaknesses against alkaline and thermal conditions and becomes more prone to further degradation if exposed to such stress conditions. Sludge pretreatment practices aiming to increase biogas production by disrupting floc structure show great similarity with the stress factors mentioned. Thus, this study aims to integrate pretreatment with anaerobic digestion and investigate the fate and effects of PET MPs during these processes. For this purpose, waste activated sludge samples spiked with different doses of PET (0, 1, 3, 6 mg/g TS) in sizes of 250-500 µm were pretreated by 0.5 M alkali for two days and then thermally hydrolyzed at 127 °C for 120 min. Pretreated and unpretreated sludges were digested in a 60-day biochemical methane potential test. The results showed that the spiking of PET MPs into sludge posed a positive impact on the methane yield of unpretreated reactors at statistically significant levels. Integrating pretreatment increased the methane yield by 22.0% and made the impact of MPs on digester efficiency no longer observable. Also, PET exposed to pretreatment and 60-day digestion experienced remarkable changes in surface morphology, crystallinity and carbonyl index, which can further impact their fate and effects on the environment.