Different Pathways of Microplastics Entering the Sludge Treatment System Distinctively Affect Anaerobic Sludge Fermentation Processes

Microplastics in wastewater inevitably accumulate in waste activated sludge (WAS) via wastewater biological treatment, potentially affecting the subsequent sludge treatment unit. Nevertheless, all previous research studies focused on the impacts of the direct addition of one type of model microplastics on the sludge anaerobic treatment process. This approach actually cannot reflect the real situation where multiple different microplastics simultaneously get into the wastewater treatment unit prior to the sludge treatment unit. Herein, this work innovatively proposed a more realistic method to assess the real toxic influences of microplastics on anaerobic WAS fermentation for short-chain fatty acid (SCFA) production by initially adding four typical microplastics (i.e., polyethylene terephthalate, polystyrene, and polypropylene) to the biological wastewater treatment system. Results showed that four microplastics initially entering the biological wastewater treatment reactor had little influence on the subsequent anaerobic SCFA production since WAS solubilization increased but hydrolysis and acidification decreased. In contrast, when the four microplastics were directly dosed in a WAS anaerobic fermenter, although there was no effect on WAS solubilization, the bioprocess of hydrolysis–acidification was clearly suppressed, ultimately significantly (P = 1.86 × 10–7) inhibiting the maximal SCFA production from WAS by 21.5 ± 0.1% compared to the control without microplastic addition. The excessive oxidative stress and toxic leachates from these typical microplastics reduced the relative abundances of key anaerobes (e.g., Longilinea sp.) involved in the anaerobic fermentation. This work revealed that the different pathways of microplastics entering the sludge treatment system had different impacts on anaerobic sludge fermentation processes and selecting a more realistic and accurate approach was important to evaluate the true toxicity of microplastics on the sludge anaerobic treatment system.