Behaviour of Microplastics in Methane Fermentation of Sewage Sludge, Japan

Microplastics (MPs) are a kind of pollutants that have attracted widespread attention in recent years. Previous research shows that the vast majority of MPs will be intercepted by sludge after the wastewater enters the wastewater treatment plant (WWTP) 1), and will enter the soil with the subsequent utilization of sludge. Before the sludge enters the soil, it is usually subjected to a series of treatments such as high-temperature composting and methane fermentation. Methane fermentation is an energy conversion of sewage sludge, and its use will expand in the future. Recent study found that the shape, quantity and size of MPs in methane fermentation or high temperature composting of sewage sludge changed 2) but the reason and the MPs' characteristics was not deeply studied. This research study and summarized the characteristics of MPs in the sewage sludge before and after methane fermentation in Japan. We sampled sludge before and after methane fermentation at WWTP A and B in Japan several times in 2022 from June to December. Both WWTP A and B use methane fermentation for sludge treatment. WWTP A uses mesophilic fermentation while WWTP B uses thermophilic fermentation. After organic matter removal using H2O2, ultrasonic treatment, and density separation steps, the sludge sample will be screened for particles suspected to be MP using microscopy and their chemical composition will be determined using μ-FTIR (AIM-9000; Shimadzu.Corp.). This study is focus on non-fibrous MPs with particle size greater than 100 μm. The size (long axis and short axis) and the plastic type of every MPs were recorded, concentration by dry weight and monthly discharge of MPs were calculated. With the above information, the characteristics of MPs before and after methane fermentation of sludge were summarized. In the sludge samples in this research, the concentration of MPs ranged from 0.72×104 to 1.8×104 pcs per kg dry weight. (Figure 1.) Overall, WWTP A had a higher concentration of MPs than WWTP B. The sludge from both WWTPs showed 2.9%-37.1% of decrease in MPs' concentration after methane fermentation. The concentration of MPs in samples from the same WWTP did not show great fluctuations from month to month. If the change in the amount of MPs in the methane fermentation system is calculated from the input and production of sludge, the amount of MPs was reduced by 50.1%-71.1% after the methane fermentation. For the plastic type, PE was found in all of the samples, accounting for 9.1%-21.7%. The detection rates of PP, PP-PE, acrylic and ABS were higher than 75%, PVC, PMMA and EVA were higher than 50%. Some particles with the spectral characteristics of MPs but with an ambiguous spectral information, or with the spectral characteristics of several MPs at the same time were founded. These particles may have been subjected to degradation or may be a mixture of plastics. Although it was not possible to determine the exact type of plastic, these particles were determined to be MPs in this study. The maximum size of MPs observed in this study was 1637.5 µm. 86.5% of the MPs in the sludge were smaller than 500 µm, 8.8% of the MPs were between 500 and 1000 µm, and only 4.7% of the MPs were larger than 1000 µm. (Figure 2.) The particle size of MPs became smaller after methane fermentation in WWTP A. Before digestion, the range of 200-300 µm had the most MPs. After digestion, the range of 100-200 µm had the most MPs. In WWTP B, the range of 200-300 µm had the most MPs but after methane fermentation, the percentage of MPs in this range has decreased significantly. After methane fermentation, there was a decrease in MPs' quantity. The degradation of MPs may have occurred during methane fermentation. Since there is a lower limit of detection of MP size in the analytical and detection methods, the accuracy of analysis decreases near this lower limit, and effective analysis is not possible for MPs smaller than the lower limit of size, 3) the possibility that the decrease in the number and size of MPs was caused by physical effects still cannot be excluded in this study.