Impact of alkaline thermal hydrolysis on anaerobic digestion of mixed sludge contaminated with microplastics

The widespread presence of microplastics in wastewater treatment plants is well-documented and their detrimental impact on different unit processes has been established by the scientific community. This study aims to advance the understanding of how a still little-known process, alkaline thermal hydrolysis process (ATHP) of the mixed sludge (pH = 10, temperature = 125 °C, exposure time = 60 min), affects the subsequent anaerobic digestion in presence of two commonly found microplastics (polyethylene and polyvinylchloride) in the sludge. Biochemical methane potential test (BMP) was performed with sludge samples without and with ATHP and with and without microplastics (MPs) doping for 35 days. Results indicated a partial degradation of the MPs during ATHP according to the Raman spectra. Concerning cumulative methane production, the maximum production was obtained for the samples with microplastics, irrespective of the pretreatment. However, it was observed that ATHP alleviated the decrease in methane production in the digesters. In fact, for the highest polyethylene concentration, the digester with pretreated sludge yielded a 111 % higher methane production that the digester without pretreatment. It can be concluded that ATHP reduces the inhibition of the anaerobic process in presence of MPs. This study addresses the environmental risk posed by microplastics (PE and PVC) in wastewater sludge by evaluating their behaviour during alkaline thermal hydrolysis and subsequent anaerobic digestion. Results show partial degradation of the polymers and a reduction in their inhibitory effects on methane production. These findings highlight a viable strategy to mitigate the environmental impact of persistent contaminants in sludge treatment systems. The proposed approach contributes to improved energy recovery and safer biosolids management, offering a practical solution for wastewater treatment plants facing increasing microplastic contamination. • ATHP enhances anaerobic digestion performance in MP-doped sludge. • Partial degradation of PE and PVC after ATHP was confirmed via Raman spectra. • ATHP mitigates methane inhibition caused by high PE and PVC concentrations. • Methane yield increased by up to 111 % with ATHP at high PE concentrations.