Effect of microplastic on anaerobic digestion of wasted activated sludge

Over 90% of microplastics that enter wastewater treatment plants end in the wasted activated sludge. The effect of microplastic abundance on the activated sludge anaerobic digestion has been rarely reported. This study investigated the methane production performance during anaerobic digestion with different abundance of microplastic doses (0, 1,000, 3,000, 6,000, 10,000, 30,000, 60,000, 100,000 and 200,000 polyester particle/kg activated sludge). The methane production was reduced to 88.53 ± 0.5%, 90.09 ± 1.2%, 89.95 ± 4.7%, 95.08 ± 0.5%, 90.29 ± 0.5%, 93.16 ± 0.8%, 92.92 ± 1.3%, and 92.72 ± 0.6% as compared with control after digestion for 59 days. The methane production of all conditions was fitted with the logarithm model (R² > 0.95) and one-substrate model (R² > 0.99). The predicted and actual methane production values of digestion for 59 days had high correlation in all conditions with R² > 0.95. The analysis based on the biochemical methane potential test model indicated that the methane production potential (B₀) and hydrolysis coefficient (k) decreased at nearly all tested conditions. The reactor digestate with microplastics retained higher organic matter and nutrient concentration and had slightly lower dewaterability than the control. The inhibition of methane production potential could be attributed to the incomplete digestion with the existence of microplastics. The microbial community showed no significant difference with and without microplastics.