Table 3.

Impacts of microplastics on different sludge treatment processes

Process	MPs	Operating conditions (C: Concentration and T:Time)	Major findings	References
AD	PES	C: 0–200,000 PES-particle/kg activated sludge; T: 59 days	Methane production was reduced by 88.53 ± 0.5 to 95.08 ± 0.5% with slight inhibition in AD with improvement in dewaterability	[93]
AD	PA6	C: 5–50 particles/g-TS; T: 45 days	PA6 enhanced methane production by 39.5%	[94]
AD	PVC	C: 20–60 particles/g-TS; T: 45 days	PVC inhibited methane production by 75.8 ± 0.2 to 90.6 ± 0.3%	[95]
AD	PE	C: 100 and 200 particles/g-TS; T: 44 days	12.4–27.5% reduction in methane production during short-term exposure	[96]
Aerobic digestion	PET	C: 15 particles/L of PET MPs; T: 95 days	Inhibition of WAS aerobic digestion by 10.9 ± 0.1% when MPs spiked into wastewater; suppressing WAS aerobic digestion by 28.9 ± 0.1% when MPs were directly added to the digester	[30]
Dewaterability	PS, PE, PVC	C: 1–300 mg/L of MPs; T: 60 days	29.6–47.7% reduction in the sludge dewaterability was observed	[97]
Anaerobic fermentation	PS	C: 30–90 particles/g-TS; T: 28 days	Low PS concentrations (30 particles/g total solid) significantly increased the production of volatile fatty acids by 112.8 ± 2.4%. In contrast, high concentrations of PS (90 particles/g total solid) significantly decreased volatile fatty acids production by 83.01 ± 0.76%	[98]
Anaerobic digestion (UASB)	PP	C: 5 PP-MPs/g-TS to 50 PP-MPs/g-TS	4–58% decrease in methanogenic activity at 18–50 PP-MPs/g-TS	[99]
Alkaline anaerobic fermentation	PET	C: 10, 30, and 60 particles/g-TS of PET MPs; T: 21 days	11.6 ± 0.1 to 29.3 ± 0.1% reduction in hydrogen production from WAS throughout the 21 days test period	[100]