The presence of cationic polyacrylamide attenuated the toxicity of polyvinyl chloride microplastics to anaerobic digestion of waste activated sludge

Abstract

The broad sources of microplastics rendered their inevitable transportation from raw wastewater to wastewater treatment plants. Most of the microplastics are found to be accumulated in waste activated sludge (WAS) and the retention of microplastics is now threatening the subsequent anaerobic digestion of WAS. Therefore, it is urgent to develop viable methods to mitigate the adverse impacts of microplastics on methane production. Here, the feasibility of adding cationic polyacrylamide (cPAM) to alleviate the toxicity of polyvinyl chloride (PVC) microplastics (i.e., 30 mg/g TS) to WAS anaerobic digestion was investigated. As a result, the moderate addition of cPAM (i.e., 3.0 mg/g TS) significantly recovered the methane production from 15.6 ± 1.7% inhibition to 5.8 ± 0.6% inhibition compared with the control reactor containing PVC microplastics (P < 0.01). Moreover, the addition of cPAM inhibited solubilization by increasing mass transfer resistance but facilitated hydrolysis and acidification by reducing inhibition of enzyme activities, thereby providing more bioavailable substrate for methane production. In the long-term operated reactor, the presence of cPAM at 3.0 mg/g TS effectively restored the average COD removal efficiency (63.1% to 76.2%, P < 0.01) and the daily methane production (70.3 mL/d to 83.0 mL/d, P < 0.01) of anaerobic sludge. Aligning with the recovered performance of anaerobic sludge, the total microbial populations and the relative abundances of dominant acidogens and methanogens were elevated with the presence of cPAM. Further mechanism studies revealed that cPAM carried on its positive influence mainly through reducing bisphenol-A (BPA) leaching and efficiency of reactive oxygen species (ROS) entering the cell.