Behavior and Flow of Microplastics in Sludge Treatment Systems in Japan

Wastewater treatment plants (WWTPs) were considered to be the biggest microplastics (MPs) source in urban environment (Carr, S.A., Liu, J., Tesoro, A.G., 2016.). Most of MPs in wastewater removed and transferred to sewage sludge (Magnusson, K., Noren, F., 2014.), on the other hand, evidence shown that lager plastic pieces might break apart into small pieces due to the physical or chemical factors in environment (Murphy, F., Ewins, C., Carbonnier, F., Quinn, B., 2016.), some of them will become secondary MPs, therefore, sewage sludge plays the most important role in MPs pollution. In this study, a WWTP in western Japan (we call it as WWTP A here) was the research target. Fig. 1 is the diagram of the sewage sludge treatment system of WWTP A, in this research, all 7 kinds of sludge in WWTP A (marked in red numbers) including primary sludge (raw and thickened), waste activated sludge (raw and thickened), mixed thickened sludge, digested sludge and dewatered sludge were sampled, and all non-fiber MPs larger than 100 μm in those sludges were analyzed. The size, plastic type and color of every MPs were recorded, concentration and annual discharge of MPs were calculated. The sampling will be conducted in some seasons and finally MPs flow in WWTP A was built. Since there are no standard analyzing method of MPs, an analyzing method for sewage sludge was examined and established in this research. Firstly, the pretreatment, if the sludge was in solid state, then dissolved it into milli-Q water until the water content was 96%, after that, all sludge was homogenized by ultrasonic (TOMY, 90% power output of 100 W, 80% duty) for 1 hour. Secondly, every gram (in dry weight) of pretreated sludge was digested by 60 mL of 30% H2O2 in 55 „ƒ for two days, the H2O2 was divided into two parts equally and used in the beginning of each day. After digesting, gravity separation using NaI was used to separate MPs from the solid remains. Finally, micro FTIR (Shimadzu, AIM-9000) was used to analyzing the suspicious MPs. In the summer sample of 2022, Fig. 2(a) shows the MPs concentration in different sludges, MPs concentration was 0.83×103 pcs/kg dry sludge in thickened primary sludge, 2.3×103 pcs/kg dry sludge in thickened waste activated sludge, 1.7×103 pcs/kg dry sludge in mixed thickened sludge, 2.7×103 pcs/kg dry sludge in digested sludge and 3.3×103 pcs/kg dry sludge in dewatered sludge. The MPs concentration in thickened waste activated sludge was much higher than thickened primary sludge, this indicates that more MPs were removed in secondary sedimentation tank in this WWTP, the conclusion here is different in other researches, some report that in some WWTPs, the majority of MPs was removed during primary treatment (Carr et al., 2016; Murphy et al., 2016; Talvitie et al., 2015), while another researcher report that the MPs concentration in primary and secondary treatment was basically the same level (Li et al., 2018.), that's may because the specific treatment method and discharge standard is different between WWTPs. In digesting and dewatering process, the MPs concentration raised 60% and 100% comparing to the mixed thickened sludge, there were two possible explains, one is that lager plastics may break apart into smaller ones due to the physical or microbial factors, the other is the loss of total solid cause the increase of the MPs concentration, combining the MPs amount data in Fig. 2(c), the break apart of MPs might be the major factor since the MPs amount also increased substantially in digesting and dewatering process. Table 1 shows the MPs concentration in this and some other researchers, we can see the MPs concentration varies greatly between WWTPs, the WWTP itself, seasonal reason, analyzing method, target MPs size or other detail cause this difference, overall, the MPs concentration in this Japanese WWTP was in the middle-lower level. Comparing between sludge types, it seems the MPs concentration in primary sludge was the lowest and highly related to the target MPs size, the target MPs size was set smaller, the more MPs would be found, this may indicate the widely existence of smaller size MPs in primary sludge. In waste activated sludge, digested sludge and dewatered sludge, the difference of MPs concentration between WWTPs became even lager and hard to find a pattern, thus it is critical to collect more data to provide some reference for governments to make MPs related policies. Fig. 2(b) shows the particle size of the MPs in different sludges, all MPs found in those sample was smaller than 1,000 μm, the percentage of 100-200 μm MPs was decreasing during the sludge treatment process, those MPs might loss with the separate water during thickening and dewatering. The percentage of 300-400 μm MPs increased significantly, this might because the fermenting bacteria or methanogens are somehow able to decompose MPs partially, it may make lager plastics become fragile and break into 300-400 μm MPs. Fig. 2(c) shows the MPs amount different sludges. The MPs amount in thickened primary sludge was 5.0×109 pcs/y, also much lower than thickened waste activated sludge which was 18.4×109 pcs/y, in mixed thickened sludge the amount was 24.0×109 pcs/y, in digested sludge the amount was 18.6×109 pcs/y and in dewatered sludge the amount was 21.9×109 pcs/y. Different to the concentration, the amount of MPs didn't increased during the sludge treatment process, instead, comparing to the mixed thickened sludge the amount decreased about 23% in digesting and increased 10% in dewatering, the decreased MPs may break apart and become smaller than 100μm thus can not be extract in this research, or possibly drain with the separate water during dewatering. In this research, significant number of MPs was detected in the whole sludge treating process in WWTP A in Japan, evidence also shows that sludge treating process was not able to have a proper treatment of MPs. The final fate of MPs must raise our concern, if those MPs were not treated properly, serious environmental pollution may occur in sludge utilization like compost or agriculture use.