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Microfibers in anaerobic digestion: Effect of ozone pretreatment
Summary
This study found that microfibers from laundry wastewater, which accumulate in sewage sludge, actually increased methane production during anaerobic digestion — with higher fiber concentrations boosting gas output by up to 43%. However, microfibers were not biodegraded during the process and instead accumulated in the resulting biosolids, meaning they will eventually be spread onto land or disposed of in the environment. The findings highlight that wastewater treatment plants are not a solution to microfiber pollution — they simply concentrate and redistribute it.
Wastewater treatment plants receive significant microplastics, which are eventually discharged into the environment. Previous studies indicated that over 90% of microplastics, especially microfibers from laundry wastewater, are retained in primary sludge. The effect of microfibers from household laundry on anaerobic digestion has yet to be fully understood, which is the objective of the present study. The results in this study showed a positive correlation between methane production and the presence of microfibers. Compared to the control, the methane production increased by 2%, 27% and 43% with 20 mg/L, 100 mg/L and 1000 mg/L microfibers spiked into primary sludge, respectively. The present study suggests that microfibers at 20 mg/L insignificantly affected methane production in controlled anaerobic digestion. In contrast, ozone pretreatment of microfibers enhanced gas production by 12% in the same concentration level. Interestingly, ozone pretreatment at a higher concentration (100 mg/L-1000 mg/L) of microfibers did not affect methane production. SEM/EDX results imply that the ozone pretreatment has changed the surface characteristics of the microfibers, which provide more surface area for adsorption. The significant reduction of soluble phosphorus by 58% indicates that microfibers potentially act as a site for adsorption during anaerobic digestion. Overall, the presence of microfibers had a positive effect on anaerobic digestion. However, this work also indicated that the microfibers were not biodegraded during anaerobic digestion. Therefore, microfibers accumulate on biosolids, potentially affecting the final disposal of microfibers.