Impact Of Wastewater Treatment Plant On The Accumulation Of Microplastics In The Environment Of Lijiang River Basin

Microplastics(MP,<0.5 mm)as a new type of pollutant have gradually attracted attention at home and abroad in recent years.MPs are divided into primary sources and secondary sources,these sources can be discharged into wastewater treatment plants(WWTPs)along with the wastewater pipe network.Thus,the WWTPs has become an important sink of MP.Because of its small size and low density,MPs are not easy to separate from the wastewater and enter the river ecosystem with tail water.Similarly,the trapped MPs may enter the environment again with the utilization of sludge.Moreover,MP pose an ecological risk due to their large number,wide distribution,and ease of ingestion by aquatic organisms.To date,limited information have quantified MP in the freshwater environment of the Lijiang River.Moreover,the lack of the mechanism to verify MP pollution on the Lijiang freshwater ecosystem.The discussion of the types,sources,abundance,migration and accumulation processes of MP in the freshwater environment of Lijiang River will provide basic data for studying the distribution of MP in the river freshwater environment.A research plan that combined field investigation and laboratory analysis was used in this present study.Samples were taken from the WWTPs along the main stream and main tributaries of the Lijiang River,main stream and main tributaries of the Lijiang River,wastewater sludge composting plants and agricultural soil.MP samples were analyzed and identified by means of digestion,separation,observation and characterizationin the laboratory.The results explored the impact of sludge-based fertilizer application on agricultural soil MP pollution,and assessed the ecological risk of MP pollution in the freshwater environment of the Lijiang River.The present study was conducted to investigate the distribution of MPs in wastewater and wastewater sludge collected from 5 WWTPs in Guilin city.The results showed that the abundance of MP were 0.1-12.8 items/L in influent and 0.007-1.96 items/L in effluent,respectively.The total removal rate was in the range of 54.3%to 95.6%.Thereinto,the flake and fiber type of MPs was observed in the wastewater.The MPs found in sludge was mainly 0.5-5mm.The most of MPs belonged to polypropylene and polyethylene.The abundance of MPs in influent exhibited a close correlation with the servicing population,the tertiary industry.MP abundance in the sludge samples ranged from 0.25×10~3-5.16×10~3 items per kilogram of dry sludge.Thereinto,the flake and fiber type of MPs was observed in the wastewater sludge.The MPs found in sludge was mainly 0.5-5 mm.Polypropylene and polyethylene were the major polymer forms in sludge.Based on the wastewater treatment capacity and wastewater sludge production in WWTP,the average amount of MPs entering into natural environmental was estimated to be 1.4×10~5-1.96×10~8 and 2.6×10~5-3.6×10~8 items per day,respectively.The findings confirmed that WWTP was an important way for MP pollution to enter river freshwater environment.The results showed that the abundance of MPs found in the river water samples ranged from0.13 to 1.50 items/m~3.Among the collected 84 individual fish samples belonging to 4 identified species,81.0%were found of MPs in an average abundance of 0.6±0.6 items/individual.In the surface water samples,flake-shaped,colored and larger-sized(>0.3 mm)dominated the collected MP.In the wild freshwater fish,fiber-shaped,colored and larger-sized(>0.3 mm)dominated the collected MP.PP-PE blends and PE were the major components of the identified particles in surface water.While,cellophane and PET were the main components of the identified particles in biota.A great amount effluent from wastewater treatment plants,local harbors and marinas as well as the associated vessel traffic were identified as the most probable sources of microplastic pollution in the area.The severely different results for plankton nets and pump sampling method underline the need for methodical harmonization and the implementation of sampling devices that are capable of gathering particles smaller than the mesh size.To secure a quality standard regarding sampling,the use of a combination of both the plankton net method and the pump/bulk sample method seems necessary.The results clearly showed that the abundance of MPs in soils exhibited a close correlation with the application rate of sludge-based fertilizers.The total abundances of MPs were 545.9 and87.6 items/kg in soils after annual amendment with 30(field A)and 15 t/ha(field B)of sludge composts,which is significantly higher than that without compost application(field C,5.0items/kg).Correspondingly,MPs were found in earthworms with low quantities of 1.8 and 0.4items/individual in fields A and B,respectively,while no MP was detected in field C.We speculate that sludge composts may act as a vehicle of MPs into soils and then enter soil biota and in turn influence the spread of MPs in the environment.